We are currently investigating in more detail horizontal and vertical asymmetries for face processing using only gazes. As mentioned before, gaze processing is crucial in face perception and it is not clear to what extent it contributed to our previous findings. A potential limit in previous experiments was that eyes were not displayed equidistant from the center (but images of faces were). One could argue that they should be because eyes are the most important feature in face processing and our task required above all shifts of attention between the eye regions of faces while the rest of the face was completely irrelevant. Thus, in a series of experiments we wanted to know if we could replicate the reaction time advantage in the upper left quadrant with gaze presented in total isolation and equidistant from the central fixation cross. We jointly studied horizontal and vertical asymmetries by presenting gaze stimuli in each quadrant of the visual field. In the first experiment (Figure 16, A), only eye regions were displayed; we used rectangular cuts from face stimuli used previously. In the second experiment (Figure 16, B), we presented solely eyes, nose and eyebrows were concealed from the eye region. The upper visual field advantage was expected since object recognition is better in the upper visual field, reflecting stronger projections into the ventral stream. Concerning horizontal asymmetry, the left advantage for global processing was expected in the first experiment but not in the second experiment, when the eyes were displayed in complete isolation, because global processing in not needed anymore (Conty et al., 2006). Our preliminary findings suggest that, in both experiments, we were able to replicate our previous finding of an upper left visual field advantage. This result is surprising in particular we were not expecting left visual field advantage for eyes displayed alone. One possible explanation could be that the left hemifield advantage that we observed was caused by elevation, the upper left condition being significantly different from the other three. Instead of comparing quadrants, a control experiment (Figure 16, C) comparing only horizontal (left vs. right) and vertical position (upper vs. lower) may give us more information about the previous observed effects.
Figure 16. In panel A, four rectangular eye region faces are displayed, one in each quadrant of the subject’s visual field, the singleton is on the upper right quadrant. In Panel B, same stimuli as previously but nose and eyebrows are concealed. In panel C, eyes alone are presented on horizontal and vertical axes. NB. For the original stimuli (protected database), the contrast between iris and sclera is less salient.
We are expecting to replicate the upper visual hemifield advantage but as in Conty et al.
(2006), to lose the laterality effect. If so it will give us additional evidence suggesting that looking at a quadrant is more than just adding horizontal plus vertical expected simple effects. Again our findings suggest that the display of meaningful stimuli such as faces or gazes in the upper left position lead to a behavioral advantage, presumably because of the activation of the ventral stream of the right hemisphere where specialized modules of processing are located.
In Chapter 1, we investigated horizontal and vertical perceptual asymmetries. For vertical asymmetry, we found a lower visual field advantage for geometrical shapes and an upper visual field advantage for face processing. However, the difference between sensory processing (lower visual field) and object recognition (upper visual field) is not clear in literature.
It would be interesting to investigate this difference in greater detail. One way to do so would be to use schematic faces. Schematic faces would be made up of different features; dots for eyes, a vertical line for the nose and horizontal lines for the mouth and eyebrows (Figure 17, A).
The participant’s task would be to determine the nose orientation of the singleton face which could be slightly tilted (45°) to the left or the right. As schematic faces should normally elicit the same processing as real faces (Maratos, Garner, Hogan, & Karl, 2015), we could predict an upper visual field advantage for object recognition/face perception even if these basic features are similar to the ones used in Experiments 1-6. This experiment would be a good way to test whether vertical asymmetry is material-specific or, as postulated by the literature, relies on object recognition regardless of the type of stimuli used. Furthermore, given that schematic faces should also be treated as global stimuli, a left advantage would also be postulated similarly to Experiments 7-9. Besides the proposed experiment, a control manipulation should present the same features in a scrambled way, in order to confirm that the potential effects observed do indeed rely on one prototypical configuration of features perceived as a face (Figure 17, B).
In this experiment, similarly to our experiments on geometric shapes, we would anticipate a lower field advantage (visual discrimination) and a right hemifield advantage because local processing would be required. Finally, a third possible manipulation will be to present schematic faces upside down (Figure 17, C). As in Experiment 8, we would predict that the upper hemifield advantage for object recognition would remain but the left advantage for holistic processing should disappear.
Figure 17. In panel A, four schematic faces are displayed, one in each quadrant of the subject’s visual field, the singleton is on the lower left quadrant. In Panel B, the same features as previously but displayed in a scramble way.
In panel C, schematic faces are presented upside down.
Our findings did not allow us to conclude about eventual asymmetries in the distribution of attention within our visual field depending on stimuli. One experiment that we are currently preparing will aim to investigate if one type of stimulus (face or geometrical form) causes a stronger attentional capture when displayed in one visual quadrant compared to the other three. Spatial attention capture will be measured by the evoked potential N2pc using electroencephalography (EGG). This electrophysiological component is measured based on the difference of activation between two posterior electrodes (PO7–PO8), one ipsilateral and one contralateral to the target (Eimer, 1996). Consistent with our behavioral results, we could anticipate a stronger N2pc in the upper left quadrant for faces and a stronger N2pc in the lower
right quadrant for visual forms. For sensory processing, previous findings using array items have found a larger N2pc in the lower versus the upper visual field (Luck, Girelli, McDermott, & Ford, 1997) but they only investigated horizontal asymmetries. To our knowledge, no studies have investigated attentional capture for face stimuli using N2pc. Interestingly, Quek and Finkbeiner (2015) manipulating endogenously oriented spatial attention suggested that spatial attention plays a role but is not sufficient to explain the specialization of upper visual field for face processing.