Master
Reference
Exploring The Presence of N2pc and CDA in Attribute Amnesia
MARTIN, Jessica Lee
Abstract
L'amnésie de l'attribut a lieu quand on rapporte plusieurs fois un attribut d'une cible et on ne se rappelle pas d'un autre de ses attributs quand on nous le demande dans un essai surprise.
Nous avons conduit cette étude EEG sur un groupe expérimental et un groupe contrôle d'étudiants pour savoir si l'amnésie de l'attribut est due à un manque de consolidation en mémoire de travail. Ainsi, nous nous attendions à voir une différence d'amplitude au niveau de la CDA entre les essais pré-surprise et les essais post-surprise, mais pas au niveau de la N2pc. Les résultats ont montré que l'amnésie de l'attribut n'a pas eu lieu et ont montré une N2pc et une CDA de même amplitude entre la phase pré-surprise et post-surprise.
MARTIN, Jessica Lee. Exploring The Presence of N2pc and CDA in Attribute Amnesia. Master : Univ. Genève, 2019
Available at:
http://archive-ouverte.unige.ch/unige:120856
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Exploring The Presence of N2pc and CDA in Attribute Amnesia
Plan d’études
PSYCHOLOGIE CLINIQUE INTEGRATIVE PSYCHOLOGIE COGNITIVE
Choisissez un élément.
PAR
Jessica Lee MARTIN
Directeur du mémoire
Dirk KERZEL & Sabine BORN Jury
Dirk KERZEL Sabine BORN Nicolas BURRA Titre / sous-titre
Lieu, mois, année Genève, le Juin 2019
Université de Genève
Faculté de Psychologie et des Sciences de l’éducation Section de psychologie
RESUME (maximum 150 mots)
L'amnésie de l'attribut a lieu quand on rapporte plusieurs fois un
attribut d'une cible et on ne se rappelle pas d'un autre de ses attributs quand on nous le demande dans un essai surprise. Nous avons
conduit cette étude EEG sur un groupe expérimental et un groupe contrôle d'étudiants pour savoir si l'amnésie de l'attribut est due à un manque de consolidation en mémoire de travail. Ainsi, nous nous attendions à voir une différence d'amplitude au niveau de la CDA entre les essais pré-surprise et les essais post-surprise, mais pas au niveau de la N2pc. Les résultats ont montré que l'amnésie de l'attribut n'a pas eu lieu et ont montré une N2pc et une CDA de même
amplitude entre la phase pré-surprise et post-surprise. Ceci suggère
qu'il n'y a pas eu de différence ni dans le traitement attentionnel ni
dans la consolidation en mémoire de travail entre les deux phases.
Déclaration sur l’honneur
Je déclare que les conditions de réalisation de ce travail de mémoire respectent la charte d’éthique et de déontologie de l’Université de Genève. Je suis bien l’auteur-e de ce texte et atteste que toute affirmation qu’il contient et qui n’est pas le fruit de ma réflexion personnelle est attribuée à sa source ; tout passage recopié d’une autre source est en outre placé entre guillemets.
Genève, le 24 Mai 2019
Prénom, Nom Jessica Lee MARTIN
Signature :
Abstract 2
Introduction 4
Literature Review 5
Attribute Amnesia or Inattentional Blindness? 5
Variations of Chen & Wyble’s Studies 6
Attribute Amnesia and Working Memory Consolidation 8
Linking Attribute Amnesia to Classical Theories of Attention and Memory 10
Present Study 11
Methods 14
Participants 14
Apparatus and Stimuli 14
EEG Recording 15
Dataset Exclusions 16
Procedure 16
Behavioural Results 18
Electrophysiological Results 20
General Discussion 30
Attribute Amnesia and Working Memory 31
N2pc and Task Difficulty 33
Individual Differences and Future Research 34
Conclusion 36
References 37
Annexe A - Consent Form 42
Annexe B - Statistical Analyses for Testing The Presence of Attribute Amnesia 44 Annexe C - Statistical Analyses of N2pc and CDA in Experimental Group 45 Annexe D - Statistical Analyses of N2pc and CDA in Control Group 46 Annexe E - Comparative Analyses between Electrodes, Groups and Conditions 47
I would like to express my sincere appreciation and gratitude to Prof. Dirk Kerzel, Dr. Sabine Born and Dr. Nicolas Burra for their guidance, time and effort in helping me complete this master’s thesis. They have demonstrated a lot of patience in helping me learn the techniques and concepts used in this project and have always been present to answer my very large number of questions.
Thank you.
I would also like to thank my family and friends for supporting me during these two heavy years of work and study. I am grateful to have them by my side.
Abstract
Attribute Amnesia (AA) occurs when participants are asked to report an attribute of a stimulus during several trials and they are then unexpectedly asked to report another one of its attributes during a surprise trial. Most participants are unable to report the unexpected attribute.
We used electroencephalography to test whether AA was due to a lack of consolidation in working memory. We compared the amplitude of N2pc and CDA between pre-surprise and post-surprise trials in one group of 13 students (M = 20.54, SD = 2.15) and on a control group of 12 students (M = 20.09, SD = 2.07), for which we reversed conditions. Unexpectedly, results revealed that AA did not occur, which may be due to our adaptation of the experimental paradigm. Comparative analyses revealed a marginal decrease in the amplitude of N2pc between pre-surprise trials and post-surprise trials for both groups suggesting an effect of confounding variables due to the order of experimental conditions. No difference in the amplitude of CDA was revealed suggesting that there was no difference in working memory consolidation between our conditions.
Introduction
If I asked you to focus your attention on an object, you probably would not doubt your ability to remember its features. I could ask you to look at a red number or at a blue letter and you would probably be certain of your ability to remember these features, even if I removed them from your focused attention just a second before. Indeed, it is generally presumed that a stimulus located in one’s focused attention, as opposed to what is beyond it, can be recalled without too much difficulty (Lamme, 2004). Classic studies on visual attention have shown that the allocation of attentional resources improves the visual processing of a stimulus and its attributes, such as its shape, colour or orientation (e.g., acuity, attribute discrimination, etc.;
Carrasco, Ling, & Read, 2004; Posner, 1980; Posner, Snyder, & Davidson, 1980; Shiu &
Pashler, 1994). One would be tempted to propose that, as a result, attentional focus should also improve recall of its attributes. However, Chen & Wyble (2014, 2015a, 2015b, 2016), amongst others, have shown that sometimes attention does not facilitate recall of all the attributes of a stimulus (see also Chen, Swan, & Wyble, 2016; Jiang, Shupe, Swallow, & Tan, 2016; Swallow, Jiang, & Tan, 2017; Swan, Wyble, & Chen, 2017; Wyble & Chen, 2017). These researchers conducted multiple studies revealing evidence of what they called Attribute Amnesia (AA). It refers to a phenomenon whereby, despite the fact that the observer allocates attentional resources to process a target stimulus, some of its attributes are potentially forgotten when the observer is focused on only one of them. For example, if you ask participants to only report the identity of a target letter (whether it’s an E, a B or an F) shown amongst a number of distractors (which could be numbers) during several trials of an experiment, most of them would be incapable of recalling another attribute of this same target, such as colour, when asked to do so in a surprise trial. In Chen & Wyble’s 2015a study, results showed only a 30%
accuracy rate for reporting colour during the surprise trial compared to a 65-95% accuracy rate in post-surprise trials. This type of result is observed even though participants had just processed the stimulus less than a second ago. It must be clear that if we ask them to report both attributes from the start (with an explicit instruction), they show no difficulty in doing so.
The challenge, thus, resides in the spontaneous recall of a non-target attribute.
According to Chen & Wyble, AA reflects a failure in working memory consolidation.
Working memory refers to the memory system involved in retaining pertinent information necessary for accomplishing a goal and inhibiting interference from impertinent information (Baddeley & Hitch, 1974; Diamond, 2013). In their studies, they reason that AA is revealed when the person doesn’t expect to have to report an attribute. As a result, they do not store it into working memory and cannot recall it. In the present study, we have attempted to extend on Chen & Wyble’s research in understanding the mechanisms behind AA. Is AA purely the
result of a failure in working memory consolidation? Or does it reflect a difference in attentional processing?
We used electroencephalography (EEG) in order to explore these questions. EEG is a non-invasive method for measuring electrophysiological brainwaves. In the field of psychology, we typically extract Event Related Potentials (ERPs) which reflect electrophysiological responses associated with the onset of a perceptual or motor event (Luck
& Kappenman, 2013). In this project, we specifically studied the occurrence of the N2 Posterior Contralateral (N2pc) component, which has been linked to attentional selection of a target and Contralateral Delay Activity (CDA) which has been associated with maintaining objects in visual working memory (Berggren & Eimer 2016; Hickey, Di Lollo & McDonald, 2009; Luck, 2012; Luck, Girelli, McDermott & Ford, 1997; Perez & Vogel, 2012,).
Literature Review
Attribute Amnesia or Inattentional Blindness?
One could argue that Chen & Wyble’s results are just another manifestation of Inattentional Blindness. Indeed, several studies have shown that, when focused on a specific task, other (sometimes very salient) stimuli can occasionally pass through our visual field unnoticed. One study which illustrates Inattentional Blindness was conducted by Rock, Linnett, Grant & Mack (1992). Their experiment involved asking participants to judge whether the vertical or the horizontal line of a fixation cross in the middle of the screen was longer. On certain trials, a blob appeared in one of the four quadrants delimited by the cross. Results showed that when participants were not informed of the presence of the blobs, 25% of them were unable to detect the blob when they concentrated on the line judgment task. This gave rise to the idea of Inattentional Blindness.
Other studies, have shown Inattentional Blindness in a variety of ways. For example, Simons & Chabris (1999) asked participants to watch a video and focus on counting how many times a ball was passed from person to person. In the middle of the video, a man in a gorilla suit walked by, stopped in the middle of the room to pound his fists on his chest and then walked away. Forty-six percent of the participants did not notice him, as they were so focused on the task they were given. More recently, Ward & Scholl (2015) demonstrated a similar effect, by asking participants to focus on counting how many black and white letters crossed the midline of a computer screen. About 30% of participants were once again unable to notice a bright red letter E cross the entire screen as they were focused on their task. Although these
results seem similar to those found in studies on AA, there is a difference. In Inattentional Blindness studies, the stimulus which escapes notice is located outside of the person’s focused attention, whereas in AA studies, the feature which observers fail to report is located within the person’s focus of attention. Classical theories of attention have suggested that what is located outside of one’s focused attention, may not be recalled, but what falls into it should be recalled (Kahneman, Treisman & Gibbs 1992; Lamme, 2004). Hence, the word amnesia is fitting.
Variations of Chen & Wyble’s Studies
Chen & Wyble (2014) first identified AA in a series of experiments they conducted on the difference between the encoding of location and other attributes in working memory. In their first experiment (see Fig.1), participants were asked to identify a target letter which followed a cue (the red bars) sometimes presented at its same location (valid condition) or sometimes at a different location (invalid condition) for several trials. The cue and target could appear at four possible locations (the four corners of the screen) and it was specified that
“sometimes the letter will appear where the red bars were”. They programmed a surprise trial during which only the cue was shown and the participants were suddenly asked to report where the red bars had appeared during the last trial, that is, only 567 milliseconds (ms) before they were asked about it. Importantly, they were not informed in advance that they may have to report the location of the cue. The results showed an accuracy of 60% in recalling the location of the cue even though the question was unexpected, which is significantly higher than chance performance. This suggests that AA did not occur for location. However, in their fourth experiment, Chen and Wyble (2014) explored whether they would find the same result with an attribute other than location. They chose to test colour. The task remained the same, except that the bars were no longer just red, but could randomly appear as either red, blue, yellow, or purple. During the surprise trial, participants were asked to report the colour (not the location) of the cue in the last trial. This time, performance during the surprise trial plummeted to an accuracy rate of 36%, which is just above chance performance. Hence, the authors observed AA for colour. The authors replicated their experiment while slightly altering their design (e.g.
changing the cue, removing the target in the surprise trial, making the colour task-relevant, etc.), but they always arrived at the same result: colour reports decreased in accuracy. Based
on these results, it was concluded that the location, but not the colour was automatically encoded into working memory.
Figure 1. Experimental paradigm used in Chen & Wyble’s (2014) first experiment. The red bars represent the cue indicating where the target will appear. In pre-surprise trials, participants had to report the single target letter which appeared, and on the surprise trial, they had to report the location of the last cue. In their fourth experiment, the colour of the cue (the red bars) varied randomly between red, blue, yellow or purple and this time their task was to report the colour of the last cue during the surprise trial. In this case, they observed AA for colour.
Chen & Wyble (2015a) conducted more experiments in order to test the various conditions in which AA can be observed. They continued with a similar task to that used previously, but this time without a cue: Participants were asked to report the identity of a coloured letter amongst three coloured distractors which were numbers. All of the stimuli (the letter and the numbers) were of different colours, either red, blue, yellow or purple. In this series of experiments, the researchers observed the appearance of AA when increasing the stimulus presentation duration from 150 ms to 250 ms (which is above the 50 ms necessary for conscious perception); when changing the task to detecting an odd number amongst even numbers or vice versa; when increasing the target’s saliency by making it pop-out (with a coloured target and black distractors); and even when reducing the amount of pre-surprise trials
from 155 to 11 (ruling out a habituation effect). AA was not only observed when the surprise task involved the report of the target’s colour, but also when the instructions were switched to request participants to report the colour first and thereafter the identity of the target stimulus during the surprise trial. These findings imply that AA occurs even when the target has been consciously perceived, when the attribute to be reported by surprise is salient and when target selection involves an automatic identification process (as being the case when processing even and odd numbers, as shown by Dehaene, Bossini, & Giraux, 1993; Fias, Brysbaert, Geypens,
& d’Ydewalle, 1996; Reynvoet & Brysbaert, 1999). It is important to specify that none of these effects resulted from impaired working memory or excessive working memory load.
Participants were always asked to attend to one item at a time and they had no difficulty in reporting location by surprise. Furthermore, post-surprise (control) trials were almost always performed whereby the participants were asked to report both the pre-surprise attribute and the surprise attribute and, in this case, they performed well during recall of both attributes. These reasons suggest that their working memory capacities were not deficient.
Born, Puntiroli, Jordan & Kerzel (2019) tested whether AA would be absent if saccadic selection of the target stimulus was enabled. This study was motivated by evidence showing that saccadic selection enhances perceptual representation of the eye movement target and that saccade targets are preferentially processed in working memory in comparison to non-target items (Kowler, Anderson, Dosher, & Blaser, 1995; Puntiroli, Kerzel, & Born, 2015; Bays &
Husain, 2008; Henderson & Hollingworth, 2003; McConkie & Currie, 1996). However, their results revealed that even in this case AA prevailed.
Attribute Amnesia and Working Memory Consolidation
Chen & Wyble’s 2015b study was conducted in an attempt to understand what causes AA. They proposed a hypothesis: AA was the result of a lack of consolidation of the irrelevant attribute into working memory. “By consolidate, [they] mean storing and holding information in a form of memory that is sufficient to survive an interruption such as delay or masking, allowing that information to be used in a subsequent task” (Chen & Wyble, 2015b, p. 2). To test this, they once again performed several experiments, during which they tried to “force”
consolidation of the attribute to be reported during the surprise trial, but without the explicit instruction to recall it. The task was similar to previous ones: during a series of trials, participants were asked to report the location of a target stimulus among three distractors. All four stimuli were of different colours. During a surprise trial, they were asked to spontaneously report the colour of the last target. What differed in this paradigm was that a coloured fixation
cross appeared before the target and distractors, followed by a mask (see Fig. 2). To “force”
consolidation, subjects were asked, during the pre-surprise trials, to report the location of the stimulus which was of the same colour as the fixation cross. With this specification, AA was practically absent in the surprise trial. Chen & Wyble offer these results as being evidence that AA reflects a lack of consolidation in working memory. By forcing participants to be attentive towards the colour of the target before presenting the stimulus array, the colour now became a pertinent information for responding to the task and enough time was also given to store that information (by presenting the coloured cross before the stimulus array).
When reviewing their experiment, they wondered whether their results reflected a familiarity effect of colour. They adapted their experiment by changing the fixation cross from being presented in a single colour to showing all four possible colours that the stimuli could have (see Fig. 2 for illustration). In other words, instead of being a single-coloured cross (red, blue, purple or yellow), each line of the cross was of a different colour with one line presented as longer than the others. Participants were given the directive to report the location of the target which was the same colour as the longest line of the fixation cross and its colour during the surprise trial. AA was once again absent in these conditions, which reinforced their conclusion that AA reflects a lack of memory consolidation.
Lastly, Chen & Wyble (2015b) decided to fool-proof their findings, by replicating their first experiment, but by changing one small detail: the time of presentation of the single- coloured fixation cross. This time, the cross was not presented before the target and distractors, but at the same time. In this situation, the colour was no longer relevant for completing the task and so was not stored in working memory. Thus, AA reappeared. In the two previous experiments, the colour was stored beforehand so that it could be used in the visual search task.
By presenting the fixation cross at the same time as the stimuli, the colour was not stored in working memory, thus preventing recall and supporting their hypothesis that AA is due to a lack of consolidation into working memory.
Figure 2. Experimental paradigm used in Chen & Wyble’s (2015b) study. In their first experiment, they presented a coloured (red in this case) fixation cross before presenting the stimulus array to “force” consolidation of the target’s colour. Participants were instructed to report the letter which was of the same colour as the fixation cross. This paradigm did not allow AA to appear. In a second experiment, the fixation cross represented all four possible colours with one on each of its lines and was also presented before the stimulus array. In this case, AA was also canceled. In their 3rd experiment they presented the coloured fixation cross at the same time as the stimulus array. In this case, AA for colour reappeared as it did not have an opportunity to be consolidated.
Linking Attribute Amnesia to Classical Theories of Attention and Memory
The observation of AA goes against what has been suggested by classical theoretical models of visual attention and memory. For example, Kahneman, Treisman & Gibbs’ (1992) object-file theory suggests that attention to a stimulus leads to the integration of its attributes into a single memory representation of said stimulus which is stored as, what they have named, an object-file. This object-file can be updated if a new, but similar stimulus is perceived at a later time or it can also be reinforced if the same stimulus is perceived in its current state multiple times. Kahneman, Treisman & Gibbs’ object-file theory has been referenced widely and is accepted as a solid basis for understanding the mechanisms of visual attention. However, it does not account for AA. According to this theory, attentional focus leads to an automatic integration of the features of an object which is then stored as a cognitive representation in
memory. In Chen & Wyble’s experiments, participants are asked to attend to a stimulus and report a specific attribute. If an object-file was created, they should have been able to report the surprise attribute as well, but the majority of observers could not do so.
A second example is Lamme’s (2004) model of visual consciousness and its relation to attention. According to this theory, we can distinguish two types of visual consciousness:
access awareness, which refers to the consciousness of an object located directly in one’s focused attention, and peripheral awareness, which refers to the consciousness of an object outside one’s focused attention. This author suggests that the distinction implies that what can be found in access awareness is also accessible in working memory and what remains in peripheral awareness is not. In Chen & Wyble’s studies, the stimulus is located in the participants’ access awareness. It is understood that it should, therefore, find itself in working memory, but the results of their experiments do not support this postulate, at least as far as irrelevant features are concerned. Classic studies on rapid presentation and iconic memory have shown that when we overload the visual system with several items, participants have difficulty in reporting and recalling some of those stimuli (Coltheart, 1980; Lamme, 2004; Potter,1976;
Sperling, 1960). One explanation would be that they are located in the participants’ peripheral awareness. On the other hand, only a few studies support Lamme’s second postulate that what is found in one’s access awareness is automatically encoded in working memory and reportable whereas other studies do not (Cowan, 1998; Oberauer, 2009; Oberauer, 2002; see also Jacobs & Silvanto, 2015). Therefore, we are still uncertain of the relationship between awareness of the stimulus and its efficient recall.
Present Study
In EEG studies, we study the electrophysiological brain responses associated with perceptual and/or motor events. This is done by placing electrodes on the scalp and measuring the electrophysiological activity during a task. Often, we can observe stereotypical reactions in specific areas of the brain after the onset of a stimulus or motor response. These are called Event-Related Potentials (ERP) and will occur at specific times after the event (often only after the lapse of a few milliseconds; Luck & Kappenman, 2013). Different ERPs have been associated with different types of perceptual events. N2pc, as mentioned in the Introduction, is an ERP component involved in attentional selection of a target (Luck, Girelli, McDermott &
Ford, 1997). It is characterised by a negative amplitude difference between posterior electrodes contralateral and ipsilateral to the target and starts at about 175ms after stimulus onset (Luck, 2012). Some studies have suggested that N2pc is a combination of two smaller components,
the first being PD which is involved with the inhibition of distractors and characterised by a more positive amplitude effect contralateral to the distractors (and a more negative effect ipsilateral to the target) and NT which is involved with the processing of the target and characterised by a more negative effect contralateral to the target (and ipsilateral to the distractors; Hickey, Di Lollo & McDonald, 2009). It is interesting to note that N2pc’s amplitude is modulated by factors such as the complexity of the stimulus (e.g. it is smaller for a simple colour patch versus larger for a complex multifeature target) and the number and proximity of the target’s distractors (Luck, 1997).
CDA is an ERP component related to visual working memory storage. It is characterised by a slow negative wave resulting from the difference between posterior electrodes contralateral and ipsilateral to the target. It usually occurs at about 275ms after the stimulus to be remembered is presented and is sustained during the retention period (Berggren & Eimer 2016; Perez & Vogel, 2012). What is particularly important to know about CDA is that some studies have shown that the number of items held in working memory will impact its amplitude, in that the more items stored, the higher the amplitude (Ikkai & Vogel, 2010; see also Luria, Balaban, Awh, & Vogel, 2016). This change in amplitude has also been shown to vary as a function of individual working memory capacity (Vogel & Machizawa, 2004).
In our study, we conducted an experiment during which we asked a first experimental group of participants to perform a task similar to the one used in Chen & Wyble’s studies on AA. We presented a search array with seven distractors of the same colour and a target of a different colour. Subjects were given the explicit instruction to report the identity of the target at the end of each trial and were eventually asked by surprise to report the colour of the last target. After the surprise trial, they were asked to report both the identity and the colour of the target stimulus at the end of each trial. While they did this, we measured their brain activity via EEG paying special attention to N2pc and CDA. To analyse the results, we compared the pre-surprise trials referred to as the single attribute condition and the post-surprise trials referred to as the double attribute condition. Analysis of N2pc and CDA amplitude was not performed in the actual surprise trial, as the results would have been less reliable. The idea was to understand what was different between trials where participants were expected to respond to only one attribute and trials where they were expected to respond to two attributes. Analysis of the surprise trial could not be performed as there was only one surprise trial per participant and EEG analysis in this study required that we extract and process data from several hundreds of trials. An ERP is too insignificant in regard to the whole EEG recording. Therefore, a large number of trials is generally needed to ensure the significance of the ERP’s presence by
reducing the impact of electrical noise and eliminating trials contaminated by ocular artefacts (Handy, 2005).
For these reasons, we hypothesised that if AA occurs because of a lack of working memory consolidation and not a difference in attentional processing, there should be no difference in N2pc amplitude between the pre-surprise single attribute condition and the post- surprise double attribute condition. The amplitude of N2pc should remain the same since the complexity of the target and the number and proximity of the distractors does not vary across conditions. However, since CDA amplitude increases with the amount of items retained in working memory, we expected to observe a difference in its amplitude between both conditions. More specifically, CDA’s amplitude should be lower in the single attribute condition and higher in the double attribute condition, which is coherent with existing evidence showing an increase in the amplitude of CDA as a function of working memory load.
We also included a control group in our experiment in which we reversed the pre-surprise and post-surprise conditions. This time, participants were first asked to report both the identity and the colour during a series of trials before eventually being asked to unexpectedly report the identity alone and not the colour of the target during a “quasi-surprise” trial. Participants were then instructed to only report the identity of the target during the remaining trials. Therefore, in this case, the pre-surprise trials were referred to as the double attribute condition and the post-surprise trials were referred to as the single attribute condition. We did this in order to ensure that the possible differences observed in the experimental group would be due to our experimental conditions and not due to other confounding variables such as experiment duration or habituation to the task. In this experiment, we hypothesised that N2pc’s amplitude should remain the same across conditions, but CDA’s amplitude should be higher in the first pre-surprise double-attribute condition and lower in the second post-surprise single attribute condition.
Both experiments reported in this study were approved by the Ethics Committee of the Faculty for Psychology and Educational Sciences, University of Geneva, Switzerland, and is faithful to the Code of Ethics of the World Medical Association Declaration (Helsinki).
Methods Participants
The participants included in both the experimental and control groups were all undergraduate psychology students from the University of Geneva, Switzerland. They received course credit for participating in this experiment. There were 17 subjects (M = 20.54, SD = 2.15 years, two male) recruited for the experimental group. Two subjects were eliminated due to malfunction during EEG recording. The performance of the remaining 15 was recorded in both conditions of the task. Sixteen (M = 20.09, SD = 2.07 years, one male) new undergraduate psychology students were recruited to take part as the control group. They also participated in order to validate an exam. One subject was eliminated due to malfunction during EEG recording. Performance of the remaining 15 subjects was recorded in both conditions of the task. Every participant had to sign a consent form before the start of the experiment (see Annexe A for consent form).
Apparatus and Stimuli
Stimuli were presented on a 21’’ inch CRT screen with a refresh rate of 85 Hz and a 1280 x 1024pixel resolution. Participants’ responses were entered using a standard computer keyboard. An example of the stimulus array can be seen in Fig. 3. The background of the screen was dark gray (x = 0.30, y = 0.35, L = 10 cd/m2 RGB: [106; 106; 106]) with a fixation cross of 0.3° of visual angle at the center of the screen. Eight stimuli were placed at an eccentricity of 7° of visual angle around the cross. In each trial, seven of these eight stimuli were the distractors and one was the target. The target differentiated itself by its colour. Both colours, the target’s colour and the distractors’ colour, varied randomly across trials. The four possible colours were isoluminant: orange (x =0.52, y = 0.42, L = 30 cd/m2 [RGB : 0,158,255]), green (x = 0.30, y = 0.61, L = 30 cd/m2 [RGB : 0, 185,0]), magenta (x = 0.28, y = 0.16, L = 30 cd/m2 [RGB : 255,86,255]) and blue (x = 0.18, y = 0.18, L = 30 cd/m2 [RGB : 0,185,0]). The target was always one of the four stimuli placed on the diagonal axes (45°) and all four of these stimuli could appear as either an A, B, D or E (so the target was always either one of these four letters). The stimuli placed on the cardinal axes were always either an X, T, U or K. In Fig. 3 the target is the green letter A and the distractors are the purple letters. In another trial, the target could for example be an orange letter D placed where the letter D is in Fig. 3 and the distractors could be in blue and placed where the remaining letters are in Fig. 3. The size of the
letters was about 1.1 x 0.9° of visual angle (Font: Arial, 40pt, bold) and they each had a circle with a radius of about 1.2° of visual angle surrounding them. The experiment was programmed using MATLAB (The MathWorks Inc, Natick, MA, USA) with the Psychophysics Toolbox extension (Kleiner, Brainard & Pelli, 2007).
Figure 3. Example of a stimulus array. The target represents the single green-coloured letter A and the purple letters are the distractors. The targets always randomly appeared as either an A, D, B or E on a random position on the diagonal axes (45°).
EEG Recording
The EEG system consisted of an actiCHamp amplifier (Brain Products, Gilching, Germany) with Ag/AgCl active electrodes. Twenty-six scalp electrodes extracted a continuous EEG signal and six electrodes were placed on the outer sides of the eyes (bipolar HEOG), above and below the right eye (VEOG) and on both earlobes (offline bipolar reference). The Cz electrode served as reference and the Afz as ground. The electrodes were attached to a headset which was placed on the head of the participant. Conducting gel was inserted between each electrode and the scalp of the participant so as to measure electrophysiological activity.
Impedance was maintained below 25 kΩ.
For this study, we measured the N2pc and CDA components. Offline data was analysed using Brain Vision Analyzer (Brain Products, Gilching, Germany). We re-referenced the raw EEG to the average of both earlobes and band-pass filtered it between 0.1 and 30 Hz. EEG was first segmented into two segmentations delimited by the start and end markers of the pre- surprise and post-surprise conditions. A second segmentation into 1000ms epochs was then performed on each condition which corresponded to the experimental trials. We excluded trials contaminated with blinks and vertical eye movements (VEOG channel difference above ± 60 μV), horizontal eye movements (steps in HEOG channel above ± 30 μV) during the first 300
ms, and muscular artefacts (electrodes above ± 80μV) during the whole period. On the signal of the remaining epochs, we performed an Independent Component Analysis on the data so as to exclude blink and saccade components. This was done to avoid the loss of too many trials for analysis. Finally, we computed difference waves between both hemispheres, depending on whether the target was presented in the left or right half of the screen, to compute the presence of N2pc and CDA for each condition and both conditions collapsed. The data for analysing N2pc was extracted from electrodes PO7/8, P7/8 and P3/4 between 220 ms and 260 ms after stimulus onset, and the data for CDA was extracted from electrodes PO7/8 and P7/8 between 575 ms and 1000 ms after stimulus onset. These procedures were done for both the experimental group and control group.
Dataset Exclusions
In our experimental group, one dataset was excluded because the subject took too long to finish the task and a second dataset was excluded due to the presence of artefacts in more than 35% of trials. Overall, 13 subjects were kept for final analysis. In the control group, 3 datasets were excluded due to the presence of artefacts in more than 55% of trials. Therefore, 12 subjects remained for final analysis of this group.
Procedure
At the start of the experiment, each participant was placed at approximately 60 centimetres away from the screen. The EEG headset was placed and the electrodes measuring ocular movement were attached to the face. Each trial started with a gray background and fixation cercle at its center. When ready, the participant had to press the SPACE button when focused on the fixation cercle in order to start the trial. Once the SPACE button was pressed, the trial resembled the sequence shown in Fig. 4. A fixation cross was presented in the center of the screen for a random duration between 600 and 1100 ms, followed by the stimulus array for 150 ms, followed by a second fixation cross for 1000 ms. For the experimental group, the task included two parts corresponding to both conditions of the experiment which were separated by a “surprise” trial: during the first 192 trials (the single attribute condition), the response array, which appeared after the second fixation cross, presented four letters to choose from. The participant had to report the identity of the last target, meaning the letter which was of a different colour to the others, using the arrow keys to select the correct letter in the four letter configuration on the screen (see Fig. 4). At the 193rd trial, the stimuli were followed by
a surprise question asking the participant to report the colour of the last target from memory.
The four possible colours were presented on the screen and the participant had to choose the last target’s colour using the arrow keys (see Fig. 4). A second response array appeared right afterwards asking the participant to report the identity of the last target as well, just as he was instructed earlier. Finally, during the 192 trials following the surprise trial (the double attribute condition), the participant was asked to report both the colour and the identity of the last target just as they did in the surprise trial. Along with the ERPs, we also measured accuracy of response as a dependent variable.
Figure 4. Experimental paradigm used for our experimental group. All trials started with a fixation cross, continued with the stimulus array and ended with a last fixation cross alone. Then, participants were asked to report the identity of the target defined by the differentiating colour during the first 192 pre-surprise trials (as in 1.). In this experiment, the single attribute condition consisted of these trials. In the 193rd surprise trial, participants were asked by surprise to report the colour of the last target and then, again, its identity (as in 2.). In the 192 post-surprise trials, participants had to continue reporting both the colour and identity of the target, but without it being a surprise (it was expected; as in 3.). In this experiment, the double attribute condition consisted of these trials. In all trials, participants were instructed to focus on the fixation cross as long as it was present (so even when the stimuli appeared). In the experiment performed on the control group, participants were asked to report the identity and colour of the target defined by the differentiating colour during the first 192 pre-surprise trials. In this experiment, the double attribute condition consisted of these trials. In the 193rd quasi-surprise trial, participants were asked by surprise to report only the identity of the last target (no longer the colour). In the 192 post-surprise trials, participants had to continue reporting only the identity. In this experiment, the single attribute condition consisted of these trials.
The procedure was the same for the control group except for one difference. Conditions of the pre-surprise and post-surprise trials were reversed whereby both attributes (identity and colour) were to be reported during the pre-surprise trials and only one attribute (identity) was to be reported during the post-surprise trials. Therefore, in this experiment, the double attribute condition refers to the pre-surprise trials and the single attribute condition refers to the post- surprise trials. The surprise trial presented a “quasi-surprise” memory test asking to report only the identity of the target from then onwards instead of both attributes.
Behavioural Results
Table 1 shows the behavioural results for the experimental group. As we can see, the percentage of correct responses is 96% for identity during the (pre-surprise) single attribute condition and is 97% for identity and 99% for colour during the (post-surprise) double attribute condition. Paired-sample t-tests were performed to test the difference in performance between identity reporting in both conditions and between colour and identity within the double attribute condition. There was no significant difference between identity reporting in the single attribute condition and double attribute condition (-1.2%) t(12) = 1.70, p = .115. However, there was a significant difference between colour reporting and identity reporting in the double attribute condition, (-2.4%) t(12) = 4.72, p <.001, showing that performance for colour was again better than for identity. This difference, although small and close to ceiling, suggests that colour was more easily detected than identity when participants were expected to report both attributes. These percentages also suggest that the stimulus was very salient and easily detectable. To test for AA, we compared performance in the 193rd “surprise” trial with performance in the 194th trial (that is, the first post-surprise trial). In the 193rd trial, we found 61% correct responses for colour (8 out of 13 participants). In the 194th trial, we found 92%
correct responses for colour (12 out of 13 participants). A Pearson’s Chi Squared test was conducted on correct responses for colour in the 193rd (surprise) and 194th trial to test whether AA occurred. Results showed an insignificant difference between both trials, X2(1, N = 13) = 2.44, p = .118 suggesting that AA did not occur in this experiment, as there were not significantly fewer subjects with a correct response on the surprise trial compared to the 194th trial (see Annexe B for outputs).
Pre-surprise trials Single Attribute
condition
Surprise 194th Trial Post-surprise trials Double Attribute
condition
Identity 96% N/A N/A 97%
Colour N/A 61% 92% 99%
Table 1. Behavioural results of the experimental group. Statistical analyses show a significant difference between pre-surprise identity report and post-surprise colour report and between post-surprise identity report and post-surprise colour report. There is no significant difference between colour report in the 193rd and 194th trial confirming that AA did not occur.
Table 2 shows the behavioural results for the control group. As we can see, the percent of correct responses is at 95% for identity and 99% for colour during the (pre-surprise) double attribute condition and is 97% for identity during the (post-surprise) single attribute condition.
Paired sample t-tests were performed to test the difference in performance between identity reporting in both conditions and between colour and identity within the double attribute condition. There was no significant difference between identity reporting in the single attribute condition and double attribute condition, (-2%) t(11) = 1.64, p = .130. However, there was a significant difference between colour reporting and identity reporting in the double attribute condition, (-3%) t(11) = 2.68, p = .021. Once again, this shows that performance for colour was slightly better than for identity. Both performances suggest that the target was very salient.
We compared performance in the 193rd “surprise” trial with performance in the 194th trial in order to test whether there would be a difference due to the “quasi-surprise” question. During the 193rd trial, we found 75% correct responses for identity (9 out of 12 participants) and 100%
(12 out of 12 participants) during the 194th trials. These results suggest that participants’
performance in reporting identity was slightly impacted by the quasi-surprise memory test, that is, it reduced slightly. However, Pearson’s Chi-Squared test found that the difference was non- significant X2(1, N = 13) = 0.68, p = .411; see Annexe B for outputs).
Pre-surprise trials Double Attribute
condition
Surprise 194th Post-surprise trials Single Attribute
condition
Identity 95% 75% 100% 97%
Colour 99% N/A N/A N/A
Table 2. Behavioural results of the control group. Statistical analyses show a significant difference between pre-surprise identity report and pre-surprise colour report and between post-surprise identity report and pre-surprise colour report.
Electrophysiological Results
We will begin by reporting the electrophysiological results for the appearance of N2pc in the experimental group. Fig. 5 a) shows the topographic distribution during a 40 ms time frame centred on the peak of N2pc (starting at 220 ms from stimulus onset), as can be seen in the computed difference waves (contralateral-ipsilateral) of all conditions collapsed in Fig.5 b). The scalp distribution shows that the differential activity between contralateral and ipsilateral electrodes during this time frame is maximal on posterior-occipital electrodes, namely electrode sites PO7/8 and P7/8, as expected of N2pc. This activity seems to spread on electrode sites P3/4 as well, which is the reason we included them in our analysis. We conducted a one-sample t-test against 0 on these electrode sites. This was done for each experimental condition. For the single attribute condition, results for all electrode sites of interest showed a significant difference from 0 confirming the presence of N2pc, PO7/8: (- 1.37μV), t(12) = 5.34, p <.001; P7/8: (-1.01μV), t(12) = 4.08, p <.001; P3/4: (-1.09μV), t(12)
= 8.68, p <.001. For the double attribute condition, results for all electrode sites of interest showed a significant difference from 0 confirming the presence of N2pc as well, PO7/8 : (- 1.10μV), t(12) = 5.61, p <.001; P7/8: (-0.94μV), t(12) = 6.70, p <.001; P3/4: (-0.79μV), t(12)
= 6.51, p <.001. A one-sample t-test was also done on the electrode sites measuring vertical and horizontal ocular movements EOGL/EOGR for each condition, so as to eliminate the possibility that the observed activity would be due to this kind of movement. Results for these analyses showed a non-significant difference against 0 for the single attribute condition, (- 0.16μV), t(12) = 0.97, p = .349, and a non-significant difference against 0 for the double attribute condition, (-0.18μV), t(12) = 1.34, p = .206, suggesting that the observed activity was not due to these movements (see Annexe C for outputs).
Figure 5. a) Topographic distribution of N2pc of the experimental group (both conditions). It represents the differential activity during a 40 ms time frame starting at 220ms after stimulus onset centred on the peak of N2pc. As expected, the activity is maximal on electrodes PO7/8, P7/8 and spreads on electrode sites P3/4. b) Computed difference waves (contralateral-ipsilateral) of all conditions collapsed for all 3 pairs of electrode sites. The blue window represents the chosen time-frame. The black line represents the activity from electrodes PO7/8, the red line represents the activity from electrodes P7/8 and the blue line represents activity from electrodes P3/4.
Fig. 6 a) shows the computed difference waves (contralateral-ipsilateral) for electrode sites of interest in each condition. A repeated-measures 2(condition: single attribute, double attribute) x 3 (pair of electrodes: PO7/8, P7/8, P3/4) ANOVA was conducted on the mean amplitude differences (i.e. the magnitude of N2pc). Neither a main effect nor an interaction effect was observed in our results for N2pc for our experimental group. No main effect of condition suggests that the amplitude of N2pc remained stable across conditions, supporting our hypothesis regarding this component (see Annexe C for outputs).
Figure 6. Computed difference waves of both conditions for each pair of electrode sites for the experimental group. Each graph represents the activity in one of the three pairs of electrode sites of interest. The first graph shows the activity for electrodes PO7/8, the second shows the activity for electrodes P7/8 and the third shows the activity for electrodes P3/4. In each graph, the black line represents the single attribute condition and the red line represents the double attribute condition. a) represents the chosen time frame for the analysis of N2pc. We can see that the amplitude is at the same level in both conditions for electrode sites P7/8, but the amplitude of N2pc seems slightly lower in the double attribute condition for electrode sites PO7/8 and P3/4.
b) represents the chosen time frame for analysis of CDA. Overall, CDA seems to be similar in amplitude in both conditions.
Figure 7. All-in-one topographic scalp distribution of the experimental group divided in 20 intervals starting at 300 ms after stimulus onset. We see here the evolution of the activity during 35 ms intervals from 300 ms to 1000 ms after stimulus onset. The activity over posterior electrodes becomes stable on posterior occipital electrode sites at approximately 580 ms after stimulus onset. Therefore, we chose to analyse CDA on a time frame starting at 575 ms after stimulus onset and ending at 1000 ms.
Analyses of CDA were performed on a 425 ms time frame (starting at 575ms from stimulus onset) centred on this component (Fig. 8 b)). They were not performed at the typical 300-400 ms mark (when CDA was expected), as its activity was only prominent starting at 575ms, which can be seen in the divided topographic scalp distribution (contralateral- ipsilateral) across both conditions in Fig. 7. The topographic distribution across conditions during this time is also shown in Fig. 8 a). The scalp distribution shows that the differential activity during this time frame is maximal on posterior-occipital electrodes, namely PO7/8 and P7/8 as expected of this component. As we can see, the differential activity is not spread on electrodes P3/4, which is why we did not include these sites. We conducted a one-sample t-test against 0 on electrode sites PO7/8 and P7/8. This was done for each experimental condition.
For the single attribute condition, results for all electrode sites of interest showed a significant difference against 0 confirming the presence of CDA, PO7/8: (-0.94μV), t(12) = 4.23, p <.001;
P7/8: (-0.63μV), t(12) = 2.67, p = .019. For the double attribute condition, results for all electrode sites of interest showed a significant difference from 0 confirming the presence of CDA as well, PO7/8: (-0.61μV), t(12) = 3.89, p <.001; P7/8: (-0.51μV), t(12) = 2.57, p = .023.
A one-sample t-test was also done on the electrode sites measuring vertical and horizontal ocular movements EOGL/EOGR for each condition, so as to eliminate the possibility that the observed activity would be due to this kind of movement. Results for these analyses showed a non-significant difference against 0 for the single attribute condition, (-0.07μV), t(12) = 0.35 p = .736 and a non-significant difference against 0 for the double attribute condition, (- 0.06μV), t(12) = 0.31, p = .764 suggesting that the observed activity was not due to ocular movements (see Annexe C for outputs).
Figure 8. a) Topographic distribution of CDA of the experimental group. It represents the activity during a 425 ms time frame starting at 575ms after stimulus onset which is centred on CDA. As expected, the activity is maximal on electrodes PO7/8 and P7/8. b) Computed difference waves (contralateral-ipsilateral) for both pairs of electrode sites. The blue window represents the chosen time-frame.
Fig. 6 b) shows the computed difference waves for electrode sites of interest in each condition. A repeated-measures 2(condition: single attribute, double attribute) x 2 (pair of
electrodes: PO7/8, P7/8) ANOVA was conducted on the mean amplitude differences (i.e. the magnitude of CDA). Results reveal a marginal main effect of electrode sites suggesting that the amplitude difference in both pairs of electrodes differs marginally, F(1, 12) = 4.07, p = .067. The direction of this effect suggests that the difference in amplitude for electrode sites PO7/8 is higher than the difference for electrode sites P7/8. No main effect of condition was observed, meaning that the amplitude of CDA does not vary between conditions. This does not support our hypothesis regarding this component (see Annexe C for outputs).
We shall now describe the results of the control group. Fig. 9 a) shows the topographic distribution during a 40 ms time frame centred on the peak of N2pc (starting at 220 ms from stimulus onset), which was chosen for the same reasons as for our experimental group. The scalp distribution shows that the differential activity during this time frame is maximal on posterior occipital electrode sites, namely PO7/8 and P7/8 as expected of N2pc. This activity seems to spread on electrode sites P3/4 as well, which is the reason we included them in our analysis. Therefore, we conducted a one-sample t-test against 0 on these electrode sites. This was done for each experimental condition. For the single attribute condition, results for all electrode sites of interest showed a significant difference from 0 confirming the presence of N2pc PO7/8 : (-1.33μV), t(11) = 3.04, p = .011; P7/8: (-1.05μV), t(11) = 3.38, p <.001; P3/4:
(-1.06μV), t(11) = 3.27, p <.001. For the double attribute condition, results for all electrode sites of interest showed a significant difference from 0 confirming the presence of N2pc as well PO7/8 : (-1.08μV), t(11) = 2.81, p = .011; P7/8: (-0.79μV), t(11) = 2.64, p = .023; P3/4: (- 0.60μV), t(11) = 2.64, p = .023. A one-sample t-test was also performed on the electrode sites measuring vertical and horizontal ocular movements EOGL/EOGR for each condition, so as to eliminate the possibility that the observed activity would be due to this type of movement.
Results for these analyses showed a non-significant difference against 0 for the single attribute condition, (-0.02μV), t(11) = 0.09, p = .932 and a non-significant difference against 0 for the double attribute condition, (-0.19μV), t(11) = 1.09, p = .298 (see Annexe D for outputs).
Fig.11 shows the computed difference waves for electrode sites of interest in each condition. A repeated-measures 2(condition: single attribute, double attribute) x 3 (pair of electrodes: PO7/8, P7/8, P3/4) ANOVA was conducted on the mean amplitude differences (i.e.
the magnitude of N2pc). Neither a main effect nor an interaction effect were observed for N2pc
in this second group. The lack of a main effect of condition supports our hypothesis that N2pc should remain stable across conditions (see Annexe D for outputs).
Figure 9. a) Topographic distribution of N2pc of the control group. It represents the differential activity during a 40 ms time frame starting at 220ms after stimulus onset which is centred on the peak of N2pc. As expected, the activity is maximal on electrodes PO7/8, P7/8 and P3/4. b) Computed difference waves (contralateral-ipsilateral) for all 3 pairs of electrode sites. The blue window represents the chosen time-frame.
For our control group, analyses of CDA were performed on a 425 ms time frame (starting at 575ms from stimulus onset) for the same reasons as for our experimental group. Fig. 10 shows the topographic distribution during the 425ms time frame centred on CDA. The scalp distribution shows that the differential activity during this time frame is maximal on posterior occipital electrode sites, namely PO7/8 and P7/8, as expected of this component. Therefore, we conducted a one-sample t-test against 0 on these electrode sites. This was done for each experimental condition. For the single attribute condition, results for all electrode sites of interest showed a significant difference from 0 confirming the presence of CDA, PO7/8: (- 0.78μV), t(11) = 3.47, p <.001; P7/8: (-087μV), t(11) = 6.60, p <.001. For the double attribute condition, results for all electrode sites of interest showed a significant difference from 0 confirming the presence of CDA as well, PO7/8: (-0.96μV), t(11) = 4.96, p <.001; P7/8: (- 091μV), t(11) = 4.51, p <.001. A one-sample t-test was also done on the electrode sites measuring vertical and horizontal ocular movements EOGL/EOGR for each condition, so as to eliminate the possibility that the observed activity would be due to this type of activity.
Results for these analyses showed a non-significant difference against 0 for the single attribute condition, (-0.09μV), t(11) = 0.45, p = 0.658 and a non-significant difference against 0 for the double attribute condition, (-0.07μV), t(11) = 0.25, p = 0.807 (see Annexe D for outputs).
Figure 10. a) Topographic distribution of CDA from the control group. It represents the activity during a 425 ms time frame starting at 575ms after stimulus onset which is centred on CDA. As expected, the activity is maximal on electrodes P7/8 and PO7/8. b)Computed difference waves (contralateral-ipsilateral) for both pairs of electrode sites. The blue window represents the chosen time-frame.
Fig. 11 shows the computed difference waves for electrode sites of interest in each condition. A repeated-measures 2 (condition: single attribute, double attribute) x 2 (pair of electrodes: PO7/8, P7/8) ANOVA was conducted on the mean amplitude differences (i.e. the magnitude of CDA). Results revealed no significant effects, suggesting that the amplitude of CDA does not differ significantly across conditions or across pairs of electrode sites in this experiment (see Annexe D for outputs).
Figure 11. Computed difference waves of both conditions for each pair of electrode sites for the control group. Each graph represents the activity in one of the three pairs of electrode sites of interest. The first graph shows the activity for electrodes PO7/8, the second shows the activity for electrodes P7/8 and the third shows the activity for electrodes P3/4. This time the black line represents the double attribute condition and the red line represents the single attribute condition.
a) represents the chosen time frame for the analysis of N2pc. We can see that the amplitude is at the same level in both conditions for electrode sites P7/8, but the amplitude of N2pc seems slightly lower in the double attribute condition for electrode sites PO7/8 and P3/4. b) represents the chosen time frame for analysis of CDA. Overall, CDA seems to be similar in amplitude in both conditions.
Two final analyses were performed in order to test the differences between the amplitude of N2pc and CDA across conditions and experimental groups. For N2pc, a mixed design 2 (within condition: single attribute, double attribute) x 3 (within electrode sites: PO7/8, P7/8,
P3/4) x 2 (between groups: experimental, control) ANOVA was performed. Results revealed a main effect of electrode sites, F(2, 46) = 4.20, p = 0.021 suggesting that overall the amplitude difference varies across electrode sites PO7/8, P7/8 and P3/4. We also found a marginal interaction effect between condition and group factors, F(2, 46) = 3.97, p = 0.058 suggesting that the amplitude difference between both conditions differs across both groups. Fig. 12 illustrates this effect suggesting that the difference in amplitude in the single attribute condition is higher than in the double attribute condition in the experimental group. However, the amplitude difference in the double attribute condition seems to be higher than the single attribute condition in the control group. Considering that the single attribute condition in the experimental group and the double attribute condition in the control group refer to the pre- surprise trials in both groups, these results reveal that the amplitude of N2pc is slightly larger in the pre-surprise trials than in the post-surprise trials in both groups. This suggests that this slight change in the N2pc amplitude across conditions is related to the order of experimental conditions. For CDA, a mixed design 2 (within condition: single attribute, double attribute) x 3 (within electrode sites: PO7/8, P7/8, P3/4) x 2 (between experiment: 1,2) ANOVA was performed as well, but no significant effects were observed, suggesting that the amplitude of CDA does not differ significantly across conditions or experimental groups (see Annexe E for outputs).
Figure 12. Condition x Group interaction effect for N2pc. We can see here that for the experimental group, the amplitude of N2pc decreases from the (pre-surprise) single attribute condition to the (post-surprise) double attribute condition. For the control group, the amplitude of N2pc decreases from the (pre-surprise) double attribute condition to the (post-surprise) single attribute condition.
General Discussion
Our objective in conducting our experiment was to understand the cognitive mechanisms underlying Attribute Amnesia (AA) using electroencephalography (EEG). For our experimental group, we asked participants to report the identity of a target amongst distractors during 192 trials and, at the 193rd trial, we surprised them with a memory task asking them to report the colour of the last target. They performed this task while we recorded ERP components, N2pc and CDA. In accordance with Chen & Wyble’s hypothesis that AA is due to a lack of consolidation into working memory, we expected to observe an increase in CDA’s amplitude, but a stable N2pc between the pre-surprise single attribute condition and the post- surprise double attribute condition. The results for our experimental group showed no difference in N2pc across conditions, which supports our hypothesis that the amplitude of N2pc should be stable between the single attribute and double attribute condition. However, we observed no difference in the amplitude of CDA across conditions either, which does not support our hypothesis that the amplitude of CDA should increase between the single and double attribute conditions. We performed the same experiment on a control group with a difference: the experimental conditions were reversed and a “quasi-surprise” question was asked instead of the “surprise” question, requesting participants to unexpectedly report the identity of the target and no longer the colour. We did this in order to test whether the differences observed in our experimental group would be due to any confounding variables related to the order of the experimental conditions. In this group, the quasi-surprise question seems to have slightly impacted performance for identity. This may have been due to the fact that interrupting the subject in his automatic reporting of both attributes may have interfered in his ability to respond correctly to this question. This interference may have been worsened by the fact that the subjects always responded first to the colour before reporting the identity of the last target during the pre-surprise trials. Therefore, having to suddenly only respond to identity may have been even more surprising as they were expecting to respond to colour first.
However, this conclusion requires further investigation. Concerning the electrophysiological results, we found no significant effects for our second group for both N2pc and CDA. However, comparative analyses for both groups revealed a main effect of electrode sites and a marginally significant interaction effect between condition and experiment. This interaction effect was found to reflect a general decrease in the amplitude of N2pc from pre-surprise trials to post- surprise trials, that is, from the single attribute condition to the double attribute condition for the experimental group and from the double attribute condition to the single attribute condition
