Abstract
Crowding refers to the difficulty in identifying an object when it is in close proximity with neighboring objects (the flankers). In this study, we examined the temporal properties of the flankers in relation to the target that render the flankers most effective in causing crowding. Stimuli were randomly drawn from the 26 lowercase letters. On each trial, a target letter was presented for 50 ms at 10° below fixation. The flankers, on the left and right of the target at a spatial separation (center-to-center) equivalent to 0.8×, 1.25× and 2× the x-height, were presented with a target-onset to flanker-onset asynchrony (TFOA) between −100 and 125 ms where positive values mean that the target preceded the flankers. Flankers were presented for 50 ms, 25 ms or 12.5 ms in separate blocks of trials. Observers' task was to identify the target letter. In general, the identification accuracy-vs.-TFOA plots resemble inverted U-shape functions. The dip of the function, representing the magnitude of crowding, was largest for 50-ms flanker duration and at the 0.8× target-flanker spatial separation, and became smaller for shorter flanker durations or larger spatial separations. Averaged across observers, maximal crowding occurred at a TFOA ∼20 ms at 0.8× target-flanker spatial separation, shifted to ∼50 ms at 2× separation, and did not depend on flanker duration. At any given spatial separation, the right limbs of the accuracy-vs.-TFOA plots for different flanker durations followed the same function; while the left limbs also became a single function if plotted with the stimulus offset asynchrony as the independent variable. These results constrain the temporal dynamics, particularly the integration time-constants, in the neural circuitry mediating crowding. Our results also provide evidence that maximal crowding does not require the target and flankers to be presented simultaneously; conversely, simultaneous presentation of target and flankers underestimates the crowding effect.
NIH grant R01-EY012810 (SC) and NSF grant BCS 0924636 (AS & SP).