Irem Yildirim¹, Hojin Jang², Frank Tong¹; ¹Vanderbilt University, ²Korea University, South Korea

Visual crowding refers to impairments in perceptual discrimination that arise when a peripheral stimulus is flanked by neighboring stimuli. fMRI studies have strongly implicated extrastriate areas V2 through V4 in the suppressive interactions associated with visual crowding, but the potential role of V1 in crowding has remained controversial. Prior studies have largely focused on the modulations in fMRI amplitude caused by flanking stimuli, but such measures are susceptible to spatial spreading of the BOLD signal from the flankers and might not reflect changes in neural processing per se. Other studies have found evidence of crowding-related modulation in V1, but in a manner that is dependent on top-down attentional selection of the crowded target. Here, we investigated whether crowding may lead to loss of target information in V1 in the absence of directed attention. Participants attended to stimuli at central fixation while white or black digit symbols were presented in the near periphery on a mean-luminance gray background. The severity of visual crowding was manipulated by presenting a target digit, with two flanking digits on either side that either matched or differed in visual polarity. We used 6 number digits as stimuli, flickering black/white rapidly for 12 seconds, while participants engaged in a color discrimination task at the central fixation. Even though target saliency did not alter the BOLD amplitude in the voxels selective for the target location in V1, V2, V3 or hV4, decoding accuracies were higher for the salient target digit in all areas. The salient target could be successfully decoded in V1 and V2 while the non-salient target was also decodable in V1. These results indicated that V1 shows loss of target information due to strong crowding even in the absence of attention, suggesting that the presence of strong flankers results in competitive interactions as early as V1.

Acknowledgements: This research was supported by NEI grants R01EY035157 to FT and P30EY008126 to the Vanderbilt Vision Research Center.