Endogenous attention modulates the amplitude and phase coherence of steady-state visual-evoked potentials (SSVEPs). In efforts to decipher the neural mechanisms of attentional modulation, we compared the time course of attentional modulation of SSVEP amplitude (thought to reflect the magnitude of neural population activity) and phase coherence (thought to reflect neural response synchronization). We presented two stimuli flickering at different frequencies in the left and right visual hemifields and asked observers to shift their attention to either stimulus. Our results demonstrated that attention increased SSVEP phase coherence earlier than it increased SSVEP amplitude, with a positive correlation between the attentional modulations of SSVEP phase coherence and amplitude. Furthermore, the behavioral dynamics of attention shifts were more closely associated with changes in phase coherence than with changes in amplitude. These results are consistent with the possibility that attention increases neural response synchronization, which in turn leads to increased neural population activity.

We investigated coordinated movements between the eyes and head (“eye-head coordination”) in relation to vision for action. Several studies have measured eye and head movements during a single gaze shift, focusing on the mechanisms of motor control during eye-head coordination. However, in everyday life, gaze shifts occur sequentially and are accompanied by movements of the head and body. Under such conditions, visual cognitive processing influences eye movements and might also influence eye-head coordination because sequential gaze shifts include cycles of visual processing (fixation) and data acquisition (gaze shifts). In the present study, we examined how the eyes and head move in coordination during visual search in a large visual field. Subjects moved their eyes, head, and body without restriction inside a 360° visual display system. We found patterns of eye-head coordination that differed those observed in single gaze-shift studies. First, we frequently observed multiple saccades during one continuous head movement, and the contribution of head movement to gaze shifts increased as the number of saccades increased. This relationship between head movements and sequential gaze shifts suggests eye-head coordination over several saccade-fixation sequences; this could be related to cognitive processing because saccade-fixation cycles are the result of visual cognitive processing. Second, distribution bias of eye position during gaze fixation was highly correlated with head orientation. The distribution peak of eye position was biased in the same direction as head orientation. This influence of head orientation suggests that eye-head coordination is involved in gaze fixation, when the visual system processes retinal information. This further supports the role of eye-head coordination in visual cognitive processing.

It is well known that compression of visual space occurs near the saccade goal when visual stimuli are briefly flashed at various locations on a visual reference just before a saccade. We investigated how presaccadic compression of visual space affected the apparent size of an object. In the first experiment, subjects were instructed to report the apparent number of multiple bars briefly presented around the time of saccade onset. The reported number of four bars began to decline at the 50 ms mark before a saccade and reached a minimum near the saccade onset. This confirms that the compression of visual space occurs just before saccades. In the second experiment, subjects judged the apparent width of a rectangle (a single element) or four bars (four elements) presented just before saccades. The apparent width of the four-bar stimulus was compressed just before saccades, but that of the rectangle stimulus was not compressed. Experiment 3 shows that the width compression of the four-bar stimulus is consistent with the width change predicted by compression of position. These findings indicate that the shape of a single object is not distorted at the saccade goal during presaccadic compression of visual space. In addition, experiment 4 indicates that the apparent width of a flashed stimulus just before saccades depends on the processing of global shape. This extends the definition of a visual object during presaccadic compression of visual space to not only a solid element but also a constellation of multiple elements. Furthermore, the results from these experiments suggest that presaccadic compression of visual space does not prevent object recognition underlying an attentional mechanism in generating saccadic eye movements.

The present study investigated the role of size and view on face discrimination, using a novel set of synthetic face stimuli. Face discrimination thresholds were measured using a 2AFC match-to-sample paradigm, where faces were discriminated from a mean face. In Experiment 1, which assessed the effect of size alone, subjects had to match faces that differed in size up to four-fold. In Experiment 2 where only viewpoint was manipulated, a target face was presented at one of four different views (0 degree front, 6.7 degrees, 13.3 degrees, and 20 degrees side) and subsequent matches appeared either at the same or different view. Experiment 3 investigated how face view interacts with size changes, and subjects matched faces differing both in size and view. The results were as follows: (1) size changes up to four-fold had no effect on face discrimination; (2) threshold for matching different face views increased with angular difference from frontal view; (3) size differences across different views had no effect on face discrimination. Additionally, the present study found a perceptual boundary between 6.7 degrees and 13.3 degrees side views, grouping 0 degrees front and 6.7 degrees side views together and 13.3 degrees and 20 degrees side views together. This suggests categorical perception of face view. The present study concludes that face view and size are processed by parallel mechanisms.