Orientation Discrimination in Periphery: Surround Suppression or Crowding?
33.514, Sunday, May 12, 8:30 am - 12:30 pm, Vista Ballroom
Mingliang Gong1, Lynn Olzak1; 1Department of Psychology, Miami University
Both surround suppression and crowding show impaired perception when targets are presented in periphery. They have been treated as different phenomena because they vary in many properties. For example, an inward/outward asymmetry of "masking" is found in crowding but not surround suppression. However, different stimuli and tasks (detection in surround suppression vs discrimination or identification in crowding) are employed when examining inward/outward asymmetry in the two phenomena. Surround suppression uses large surround annuli and detection tasks. Crowding experiments use small localized nearby "masks" and discrimination or identification tasks. Therefore, its unclear whether the discrepancy derives from the different stimuli or the different tasks. The current study was designed to disentangle the effects of stimuli and tasks. In the experiment, participants were required to perform an orientation discrimination task on center-surround stimuli using only the right eye. Fixation points were displaced 9 degrees to the left or right of the target center. On a given trial, one of two sharp-edged, 40-minute, 4 cpd grating stimuli appeared. The observer rated certainty of leftward vs. rightward tilt on a six point scale. Performance was measured in d'. In addition to the above control condition, two masking conditions were also run, each with a hemi-annulus mask surrounding the target on the left or on the right of the target (Petrev, Popple & McKee, 2007). Preliminary data showed a strong asymmetry effect, suggesting that task is the important variable in distinguishing between crowding and surround suppression. However, a large contrast-suppression effect was also observed, as is characteristic of surround suppression but not crowding. It may be that our particular combination of task and surround reveal a neural mechanism that differs from either surround suppression or crowding.