Abstract
To cope with the vast amount of incoming visual information, we not only select regions for further processing (constructing a high-resolution representation of a small amount of input, via attention), but we summarize the visual world along multiple dimensions (constructing a low-resolution representation of a large amount of input). In studies of perceptual averaging, for example, observers are able to quickly and accurately extract the mean size of an array of shapes, or the mean orientation of an array of lines. Despite many impressive demonstrations of perceptual averaging in recent years, it is still unclear what kind of phenomenon this is. In particular, is averaging an ability (something that we can intentionally engage when asked to do so by task instructions) or is it an incidental visual process (something that occurs even without a conscious attempt to do so)? We explored perceptual averaging of orientation without an explicit averaging task, by measuring repetition suppression with EEG. Observers viewed static line segments of varying orientations arrayed in an annulus around fixation. Their only task was to respond to rare displays during which a line segment jiggled momentarily (10% of trials, later excluded from the EEG analyses). On Match displays, the mean orientation was identical to that of the previous display (though the individual line segments always had distinct orientations), whereas on No Match displays, the mean orientation differed. Across several conditions, we observed significant suppression in the EEG waveform to Match displays, compared to No Match displays. This difference occurred within approximately 250 ms of the display onset, and was primarily apparent at relatively posterior electrode sites. These and other results suggest that perceptual averaging is an incidental visual process that the mind engages in even when not explicitly tasked to do so.
Meeting abstract presented at VSS 2013