The effect of forward and backward masking by flash on apparent size

Subjects observed a briefly exposed square and set a variable square to match its size. In experiment 1 the square was black or white and was preceded or followed by a flash of light. The preceding adaptation field was complete darkness or an illuminated field. Marked underestimation of size occurred when flash followed the square in all conditions, compared with trials with no flash. Preceding flash produced a small reversal of this effect, the extent of which depended on whether the square was black or white. Absence of an illuminated adaptation field reduced estimates in all conditions except those where flash preceded the square. In experiment 2, a black square was used, and the stimulus duration and the interval between stimulus and flash were varied. No significant effects were found with preceding flash. When flash followed the square, apparent size increased linearly over the first 300 ms with the logarithm of the interval between onset of the square and onset of the flash, then levelled off. Experiment 3 demonstrated that the reduction in apparent size occurred with figure and interfering flash delivered to the same eye but not with dichoptic interference. The results are explained by reference to a suggestion of Georgeson that the optimum frequency of spatially tuned receptors decreases in impoverished viewing conditions. Hence the presented figures trigger receptors which normally are responsive to smaller figures, and the presented figures are therefore categorized as being smaller than they really are.