Publications

The visual system constructs object representations hierarchically. Features are detected in early visual areas, and then progressively integrated in the ventral visual pathway, with neurons at higher levels of processing showing selectivity to increasingly complex stimuli. Interactions between areas in this recurrent pathway should mediate feature integration. We tested this hypothesis by measuring inter-area interactions with and without disruptions of feature integration.

We used fMRI to measure the effect of letter crowding (which disrupts feature integration while preserving feature detection) on inter-area correlations in the ventral visual pathway. Observers viewed closely-spaced letters (8º eccentricity, presented at 1 Hz for blocks of 15-21 s, separated by 15-21 s blocks of no stimulation). Adjacent letters were displayed in alternation in the uncrowded condition and simultaneously in the crowded condition. During fMRI, observers performed a demanding contrast discrimination task at fixation to ensure that attention was diverted from the letter stimuli. In a separate psychophysics experiment, we confirmed that letter identification was impaired by crowding under these stimulus conditions. Retinotopic visual areas (including V1, V2, V3, V4) were defined in a separate session using standard procedures, and the visual word-form area (VWFA) in inferotemporal cortex was defined by measuring responses in alternation to English and Chinese character strings. Sub-regions of each retinotopic area were identified corresponding to the letter locations in the main experiment. To quantify interactions between cortical areas, we first removed the mean stimulus-driven response, separately for each cortical area and each condition (crowded, uncrowded), and then computed pairwise correlations between the residuals for each pair of cortical areas and both conditions. The residual correlations specifically reflected interactions between areas rather than common input from the stimulus.

Crowding reduced residual correlations between several pairs of visual areas, particularly between early visual areas (V1-V4) and VWFA. Differences in residual correlations occurred despite little or no differences between conditions (crowded, uncrowded) in either mean response or residual variance. Differences in residual correlations were eliminated when letters were replaced with Gabor stimuli, which are elementary features not requiring integration.

We conclude that interactions between visual areas, especially between early areas and higher inferotemporal areas, reflect feature integration during object processing.