Cite as:
Saarela T P, Olzak L A, Laurinen P I, 2003, "Orientation-dependent spatial asymmetry of surround suppression in contrast perception" Perception 32 ECVP Abstract Supplement

Orientation-dependent spatial asymmetry of surround suppression in contrast perception

T P Saarela, L A Olzak, P I Laurinen

We investigated the spatial distribution of surround suppression in contrast perception. This has been tested only with parallel centre and surround gratings, and has been found to be uniformly distributed around the centre target. Here we have studied the spatial distribution of surround suppression in perceived contrast using both iso-oriented and orthogonal gratings. All stimuli consisted of 4 cycles deg^-1 sinusoidal gratings. Test stimulus centre was a 1-deg circular patch. Contrast of the vertical centre grating was 0.2. Surround consisted of two sectors of an annulus presented on opposite sides of the centre, either on top and below (end-configuration) or on the left and right sides (flank-configuration) of the centre. Sector angular width was 90 deg and surround diameter 5 deg. Surround grating contrast was 0.4 and orientation either vertical or horizontal (parallel or orthogonal to the centre). Comparison stimulus was a patch of variable contrast otherwise similar to the test centre. Perceived contrast of the test centre was measured by a method of constant stimuli and a 2AFC task. Suppression was greater with parallel than with orthogonal surround grating. With parallel surround the suppression was roughly equal in the end-configuration and flank-configuration. However, when surround orientation was orthog- onal to the centre, we found that the amount of suppression depended on the position of the surround. Suppression was very weak in the end-configuration, but there was a substantial increase in suppression in the flank-configuration. This indicates that, although surround suppression is relatively uniform with parallel centre and surround gratings, there may be spatial asymmetries in surround suppression that are specific to relative orientations of the centre and surround.

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