Interactions of crowding, overlap masking and surround suppression

Crowding, the difficulty in recognizing an object in clutter, is distinct from overlap masking and surround suppression. Little is known about whether, and how, these three spatial interaction phenomena interact with one another. We sought to examine how the magnitude of crowding varies with different flanking elements, some of which also elicit effects of overlap masking and/or surround suppression. Contrast thresholds for identifying the orientation of a target E stimulus (0.5°) presented at 5° right of fixation were measured for six observers. The E-target was presented alone (UNFLANKED), or according to one of these conditions: [1] FLANKED by four randomly oriented Es of the same size at a Weber contrast of 0.2; [2] overlaid by a FULL-FIELD of letter noise of RMS contrast of 0.2 (spatial-frequency content matching that of the E-target); [3] DONUT: similar to [2] but the noise field spared the target location; [4] DONUT-HOLE: letter noise covered only the target location. Several target-"flanker" separations were tested. These conditions were tested in a random order in separate blocks of trials in which the contrast of the E-target varied according to a staircase procedure. In general, threshold elevation (TE: threshold for a condition/unflanked threshold) was the lowest for DONUT, and increased for FLANKED and DONUT-HOLE (even higher than FULL-FIELD). At the smallest target-"flanker" separation, TEs averaged 5.4, 12.5 and 19.6 for DONUT, FLANKED and DONUT-HOLE, respectively (FULL-FIELD: TE=10.4). Similarly, the extent of spatial interaction (the target-"flanker" separation at which threshold was no longer elevated) was the smallest for DONUT, and was 1.5× and 2× larger for FLANKED and DONUT-HOLE, respectively. At any given separation, TEs of DONUT and DONUT-HOLE did not summate linearly to yield the TE of FULL-FIELD. Our results suggest that crowding, overlap masking and surround suppression interact with one another but the combinations are not additive.