Abstract
SummaryElectrophysiological recording in the anterior superior temporal sulcus (STS) of monkeys has demonstrated separate cell populations responsive to direct and averted gaze 1, 2. Human functional imaging has demonstrated posterior STS activation in gaze processing, particularly in coding the intentions conveyed by gaze 3, 4, 5, 6, but to date has provided no evidence of dissociable coding of different gaze directions. Because the spatial resolution typical of group-based fMRI studies (∼6–10 mm) exceeds the size of cellular patches sensitive to different facial characteristics (1–4 mm in monkeys), a more sensitive technique may be required. We therefore used fMRI adaptation, which is considered to offer superior resolution [7], to investigate whether the human anterior STS contains representations of different gaze directions, as suggested by non-human primate research. Subjects viewed probe faces gazing left, directly ahead, or right. Adapting to leftward gaze produced a reduction in BOLD response to left relative to right (and direct) gaze probes in the anterior STS and inferior parietal cortex; rightward gaze adaptation produced a corresponding reduction to right gaze probes. Consistent with these findings, averted gaze in the adapted direction was misidentified as direct. Our study provides the first human evidence of dissociable neural systems for left and right gaze.
Highlights
Our fMRI-adaptation paradigm used a modified version of a recent behavioral experiment [8], which showed that adapting to a series of faces gazing left caused subjects to misidentify leftward gaze as direct; rightward gaze adaptation produced a corresponding pattern
FMRI adaptation is believed to have an advantage over standard fMRI paradigms because it has the potential to demonstrate separate cell populations tuned to different stimulus categories (A and B), even if they are intermixed within the same imaging voxel [9]
We predicted that if these different gaze directions are coded by separate superior temporal sulcus (STS) cell populations, adapting to one or the other gaze direction should produce reduced activation to probe faces gazing in the congruent direction relative to probes with incongruent gaze
Summary
Probes were identical to those from the Pre-adaptation phase (12 models 3 three gaze directions [10 left, 0, and 10 right] 3 two presentations; 72 stimuli in total), and subjects categorized their gaze as left, direct, or right. A contrast, which recapitulated that used for the behavioral data, compared trials on which the direction of gaze of the adaptation and probe stimuli were congruent relative to trials on which they were incongruent.
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