Abstract
Crowding refers to the deleterious interaction among objects that are close together. A logical expectation is that crowding is maximal when the target and flankers are closest to one another. But is this so? Here, we examined how crowding depends on the retinal and perceptual spatial/temporal proximity between the target and flankers. We compared the crowding effect with the flash-lag effect, where flashed and moving targets are perceived to be spatially proximal when they are not retinally proximal and vice versa. Stimuli were high-contrast letter Ts (1.1°) presented randomly in one of four orientations. Target-T was presented at 10° right of fixation at the 3-o'clock position. A pair of flanking Ts, one on each side of the target-T, rotated around the fixation target at a velocity of 5 rpm. Target-T, flashed for 22 ms, appeared at different target-flanker delays (TFD) with respect to the instant at which the flankers reached the 3-o'clock position. In separate blocks of trials, observers judged the orientation of the target-T (the crowding task), or its position relative to the rotating flankers (the flash-lag task). Averaged across four observers, maximal crowding (reduction in accuracy of identifying the target's orientation) occurred for TFD of −66±33(SE) ms (target before flanker). This temporal delay for maximal crowding did not correspond to the flash-lag effect, which averaged −34±17(SE) ms. A control experiment showed that when flankers were flashed briefly at the 3 o'clock positions at different TFDs, maximal crowding occurred for TFD of −52±3(SE) ms. Further, when flankers were presented at different angular positions but simultaneously with the target, maximal crowding occurred when flankers were close to the 3-o'clock position and that the “flash-lag effect” was virtually zero. Our results suggest that highest retinal or perceptual spatial/temporal proximity between target and flankers is not a necessary requirement for maximal crowding.
NIH grant R01-EY012810 (SC) and NSF grant BCS 0924636 (AS & SP).