In searches for simple geometric shapes, we only found a right hemifield advantage for identification tasks involving feature search mode. This effect disappears with simple detection tasks and when a singleton detection mode was required. For the latter, as mentioned before, we think that this finding must be interpreted with caution. We will only be able to clarify what it really going on through further experimentation. The right hemifield advantage for identification tasks was expected since our stimuli required an individual assessment of features and could not be processed holistically. As mentioned before, analytical or part-based processing is associated with the left hemisphere (Bradshaw & Sherlock, 1982; Hillger & Koenig, 1991; Rossion et al., 2000; Sergent, 1984). For low level detection tasks the results were mixed;
some authors did not find any horizontal asymmetries (Evert et al., 2003; Kitterle et al., 1990;
Michael & Ojeda, 2005) while some reported either left (Poynter & Roberts, 2012) or right hemifield advantage (Polich, 1984). In the detection tasks that we used (Experiments 4-6) we did not find any effect. Over and above the type of task used (identification versus detection), the crucial factor seems to be task demands and selective attention. If we were to consider the work of Polich (1984), who found a right hemifield advantage for detection tasks, the error rates and response times in his study increased depending on the amount of display items (4, 9, 16).
Therefore, it seems that serial attention was needed, the target did not pop out. Additionally, in Polich’s experiments, as in most experiments on visual asymmetries, no hemispheric competition is occurring; stimuli were displayed either to the left or to the right visual hemifield.
When selective attention is important to perform a task, like in our series of experiments,
several studies claim that visual hemifield differences may occur only if the task is sufficiently difficult (Evert et al., 2003; Kitterle et al., 1990; Michael & Ojeda, 2005). We therefore believe that with a really easy identification task, the right hemifield advantage will disappear as in simple detections tasks. Finally, the non-result in Experiment 6 on horizontal asymmetries could be interpreted in several ways. Based also on Experiment 3, we could argue that the right hemifield advantage disappears when singleton mode is required. However, we have to be careful with Experiment 3. Maybe this lack of horizontal asymmetries may not be due to the processing mode but simply to the task demands. Indeed, in Experiment 6, even if participants had to perform a singleton search, this task was still significantly easier than our identification tasks.
In searches for faces, once again we found the opposite effect with a left hemifield advantage for detection of a gaze singleton (Experiment 7). Faster RTs when face targets were presented in the left compared to the right hemifield were expected given that holistic processing is better in the left hemifield (Bentin et al., 1996; Bradshaw & Sherlock, 1982; Hillger
& Koenig, 1991; Rossion et al., 2000; Sergent, 1984). In Experiment 8, we were able to suppress the left hemifield advantage by presenting faces upside down, a well-known method to prevent holistic processing of faces (Freire et al., 2000). Meanwhile, featural information is not affected by orientation, which allows the processing of upside down faces to proceed analytically (Farah et al., 1995). Conty et al. (2006) found that the left hemifield advantage was also present when only the eye region was displayed, which may initially seem to contradict the role of holistic processing. Similarly to our task, their participants had to detect if a gaze singleton was present or not. However, the laterality effect disappeared when the nose and eyebrows were concealed from the eye region, reinforcing the idea that the left hemifield advantage is specific to holistic processing.
Quadrant Asymmetries
The contribution of our experiments was to investigate the efficiency of a basic perceptual process (i.e. visual search) in all quadrants of the visual field and to not only compare hemifields. In the two series of experiments we were able to discover that one quadrant was superior to the other three. With geometrical shapes, in Experiments 1-2, we did not find any interaction between horizontal versus vertical asymmetries. The absence of interaction suggests that the lower hemifield advantage for RTs was as pronounced in the right hemifield
as it was in the left hemifield. However, further analyses based on our hypotheses had suggested that participants were faster to respond in the lower right quadrant compared to the other three quadrants.
For face processing, similar to the inhibition of face recognition performance in Kessler and Tipper (2004), the search was particularly efficient when targets were presented in the upper left quadrant of the visual field. In Experiment 7, the interaction effect was due to a more pronounced difference between the upper and lower quadrant in the left hemifield than in the right hemifield. The advantage of the upper left quadrant over the others is consistent with a recent MEG study. B. Lee, Kaneoke, Kakigi, and Sakai (2009) compared extrastriate brain activity of participants engaged in a contrast-based visual search task. Square areas were randomly displayed in quadrants of the visual field. They found different response properties in the upper compared to the lower hemifield in the left hemifield. However, we note that this study focused on low-level contrast perception and more research is needed to better understand the neural basis of the upper-left bias in visual search with complex stimuli.
Attentional Asymmetries
While we have not directly addressed the issue of eventual asymmetries in the distribution of attention, our results argue against the involvement of attention. For geometric shapes, the right hemifield advantage found in Experiments 1-2 and the absence of horizontal asymmetries for Experiments 3-6 does not support the right hemisphere advantage for attentional capacities when hemispheric competition is occurring (Asanowicz et al., 2013; Du &
Abrams, 2010; Smigasiewicz et al., 2014). We think that this absence of left visual field advantage in our results is due to the fact that we used tasks that are very different from attentional tasks like RSVP or exogenous cueing. We believe that the visual search task that we used, the additional singleton paradigm without distractor, mainly relies on perceptual asymmetries. Moreover, additional experiments (non-reported in the present manuscript) suggest that even a traditional additional singleton paradigm does not show a stronger interference with distractors displayed in the left hemifield.
For face processing (Experiments 8-9), if attention had been preferentially directed to the left or upper hemifield, we would have expected a horizontal or vertical asymmetry, even with inverted faces or in an easy, saliency-based search task. However, the asymmetries disappeared, suggesting that the nature of the stimuli determines the left and upper hemifield
advantage and not an attentional preference. A similar conclusion was reached by Quek and Finkbeiner (2014a), who demonstrated that the greater susceptibility for priming in the upper hemifield persisted even when participants directed their attention to the lower hemifield because the target was more likely to appear below than above fixation.