Visual memory for vernier offsets

Human ability to perceive and remember precise spatial relationships was investigated in a vernier acuity task. An initial ("standard") vernier stimulus with a variable offset was presented for 100 msec. After a delay of 1, 4, or 8 sec, another vernier target (the "variable" stimulus) followed, also for 100 msec. Observers compared the offsets of the two stimuli with each other. For very small offsets, discrimination between smaller and larger offsets was very precise, in the hyperacuity range. Thresholds increased linearly with the spatial offset of the standard stimulus, suggesting that the precision of the mechanism solving this spatial task scales with offset size. Control experiments suggested that this effect was not due to variations in retinal eccentricity. Thresholds also increased with increasing delay between the presentations of the two stimuli. By varying the delay, we directly measured the fading of the spatial memory trace.