REPRESENTATIONS OF CROSS-DOMAIN ASSOCIATIONS IN WORKING MEMORY
7.2. The role of feature encoding time replicated
Experiment 7
In this experiment, we compared an integrated feature presentation with an isolated feature presentation, making use of different presentation times. Table 7.1 shows the
ensemble of the different conditions. We replicated Experiment 3 by comparing an integrated letter in location presentation shown on screen for 1500 with an isolated feature presentation in which each feature was shown for 750 ms. We replicated Experiment 6 by comparing an integrated presentation lasting 1500 ms with an isolated presentation in which each feature was on screen for 1500 ms. We added an additional condition in which integrated letter in location presentations were shown for 750 ms. This design allowed us to compare an integrated with an isolated presentation when the feature presentation times were equalized (Integrated short versus Isolated short and Integrated long versus Isolated long) or when the total presentation time was equalized (Integrated long versus Isolated short).
If feature presentation time is the main determinant for memory performance, then we should first of all observe equal memory performance scores in the integrated and isolated presentation condition when the feature presentation times are equalized (Integrated short = Isolated short and Integrated long = Isolated long). Secondly, we should observe lower memory scores in the isolated presentation condition when the total presentation time is equalized (Isolated short = Integrated long) as this necessarily results in shorter feature presentation times in the isolated than in the integrated presentation.
Table 7.1: Presentation times of the memory items in the different experimental conditions of Experiment 7
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Method
Participants and Design
Twenty-eight students of the University of Geneva took part in this experiment. Three of them did not reach the 80 % accuracy criterion for the tone discrimination task in the training and did not perform the actual experiment. Twenty-five participants (mean age = 20.08 years, SD = 5.08, 23 female) completed both sessions of the experiment, corresponding to the integrated and the isolated presentation condition. They were paid or given course credit for their participations. Presentation duration (short or long) and cognitive load (low or high) were manipulated within each session.
Materials and Procedure
The materials and procedure were roughly the same as in Experiment 3 and 6, with however some small changes. New series of letters and locations were created, ranging once again from two to six feature pairs. Only a low and a high cognitive load condition were used to avoid a too lengthy experiment. The tone discrimination task was now to be performed on three tones instead of only two. The goal was to increase the effect of the cognitive load.
Participants had to press “leftarrow” for the low frequency tone (200 Hz), “downarrow” for the medium frequency tone (500 Hz) and “rightarrow” for the high frequency tone (1000 Hz).
Feature presentation time was manipulated within subjects and corresponded to 750 (short) or 1500 ms (long). Table 7.1 shows the exact presentation times in the different experimental conditions. The integrated long and the isolated short presentation result in an equal total presentation time. The integrated short and the isolated short presentation result in an equal feature presentation time. This is also the case for the integrated long and the isolated long presentations. As in Experiment 6, the blanks after the items to be maintained were omitted.
The recall procedure was slightly eased as participants no longer had to validate each response by pressing ENTER. In the integrated condition, participants clicked the location which then turned grey. Tapping a letter made it appear in the highlighted location. A next click with the mouse made the letter in location disappear and highlighted the new location.
This procedure continued until all features had been entered. In the isolated presentation mode, participants started by clicking a location which then turned grey. Tapping a letter turned the highlighted location back to white and made the letter appear in the middle of the screen. Clicking the next location made the letter disappear and turn the clicked location grey.
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This procedure continued until all features had been entered. Span scores were calculated for verbal and spatial features per presentation condition, presentation duration and cognitive load. The same was done for cross-domain span scores.
Results
Two participants did not reach the predetermined accuracy criterion of 75 % correct responses on the tone discrimination task. Twenty-three participants were thus included for further analysis. A 2 (Order: integrated - isolated or isolated - integrated) X 2 (Domain: verbal or spatial) X 2 (Presentation mode: integrated or isolated) X 2 (Feature presentation time:
short, 750 ms, or long, 1500 ms) X 2 (Cognitive load: low or high) repeated measure ANOVA was then performed on the verbal and spatial span scores with order as between subject factor and domain, presentation mode, feature presentation time and cognitive load as within subject factors. We observed a significant effect of domain, F(1, 21) = 200.12, p <
.001, η2 = .91, with verbal span scores (M = 3.97) being better than spatial span scores (M = 2.35). Longer feature presentation times (1500 ms) resulted in significantly better recall (M = 3.40) than shorter (750 ms) feature presentation times (M = 2.92), F(1, 21) = 71.09, p < .001, η2 = .77, and the effect of cognitive load was also significant, F(1, 21) = 52.95, p < .001, η2 = .72, with high cognitive load resulting in lower recall (M = 2.93) than a low cognitive load (M
= 3.86). There was no significant effect of presentation mode, F(1, 21) < 1. The Bayesian analysis confirmed this, pBIC (H0|D) = .80. The interaction between presentation duration and cognitive load was significant, F(1, 21) = 9.47, p = .006. The effect of cognitive load was more pronounced at shorter, F(1, 21) = 44.87, p < .001, η2 = .68, than at longer presentation times, F(1, 21) = 5.53, p = .029, η2 = .21. The interaction between presentation duration and order was also significant, F(1, 21) = 4.61, p = .044. Participants starting with the integrated presentation session benefitted more from longer presentation times, F(1, 21) = 53.62, p <
.001, η2 = .72, than participants starting with the isolated presentation session, F(1, 21) = 20.65, p < .001, η2 = .50. The other effects or interactions were not significant. Figure 7.2 displays the main results.
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Figure 7.2: Span scores as a function of domain, cognitive load, presentation mode and presentation time for Experiment 7. Error bars represent the standard error from the mean.
The same analyses were performed on the cross-domain span scores in a 2 (Order:
integrated - isolated or isolated - integrated) X 2 (Presentation mode: integrated or isolated) X 2 (Feature presentation time: short, 750 ms, or long, 1500 ms) X 2 (Cognitive load: low or high) repeated measure ANOVA. As in the analysis of the verbal and spatial span score, there were similar significant effects of presentation time (M = 1.91 and M = 2.46 for the short and long presentation respectively), F(1, 21) = 51.59, p < .001, η2 = .71, and cognitive load (M = 2.39 and M = 1.98 for low and high load respectively), F(1, 21) = 29.89, p < .001, η2 = .59, but no effect of presentation mode, F(1, 21) < 1 and pBIC (H0|D) = .80. The interaction between presentation duration and cognitive load was also significant, F(1, 21) = 10.16, p = .004, and followed the pattern of results as for the verbal and spatial span score. The
interaction between presentation duration and order was no longer significant, F(1, 21) = 2.28, p > .10 and none of the other effects or interactions were. Figure 7.2 represents the main results.
Discussion
These results replicated the results obtained in Experiment 3 and 6. Keeping the total presentation time equal (Integrated long versus Isolated short) as was done in Experiment 3, resulted in better memory performance in the integrated condition. Equalizing the feature presentation time (Integrated short versus Isolated short, or Integrated long versus Isolated long) as had been done in Experiment 6 gave rise to the same memory performance in both presentation modes. It appears thus to be the case that feature encoding time plays an
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important role in the better memory performance of integrated feature presentations that has often been documented (e.g., Luck & Vogel, 1997; Morey, 2011; Prabhakaran et al., 2000).
In Experiment 3, we had suggested that the better memory performance in the integrated presentation condition could be the result of integrated maintenance. This
suggestion would have been favored if the integrated presentation mode led to an increased memory performance, independently of the feature presentation times. This was however not the case. At this point in our research, we can state that longer encoding times lead to better memory performance. We have however not made any progress on the understanding of the underlying representations. Three explanations for the results we observed in Experiment 3, 6 and 7 remain possible in terms of the underlying representations. First of all, it is possible that the integrated presentation did not result in object representations. Features would be
maintained in isolation in the integrated and in the isolated condition. As a result of equal feature presentation times, equal memory performance would then be obtained. Secondly, it might be possible that the isolated as well as the integrated presentations resulted in integrated representations. Karlsen et al. (2010) had shown that features isolated spatially or temporally could be integrated without any additional attentional cost. In their study, participants were however obliged to integrate these features as they were tested on the accuracy of the binding.
This was not the case in the isolated presentation condition in the present study. Could
participants have engaged in an integrated maintenance as well, even without this incitation to bind them? The third option would be that the integrated presentation resulted in integrated maintenance while the isolated presentation resulted in isolated feature maintenance.
Equalizing the feature presentation time could thus boost performance in the isolated
integration condition to the same level as in the integrated presentation condition. This equal performance would however only concern feature memory. The integrated presentation condition would still present the advantage of possessing object information, which would hence not be available in the isolated presentation condition. A final experiment resolved the issue.
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