Vernier discrimination with sequentially-flashed lines: Roles of eye movements, retinal offsets and short-term memory

Abstract

The two lines of a vertical vernier target were sequentially flashed (2 msec/line; 0.2–800 msec dark interval) while the subject tried to maintain an earlier position of fixation in total darkness. Vernier discrimination deteriorates with increasing dark interval: acuity threshold increases from 29″–26′; constant error generally increases. The errors are a joint result of: (1) displacements between the retinal images of the two lines that are not present in the target; these are produced by involuntary eye movements in the dark interval and (2) deterioration of memory for spatial location signalled by the first line flashed. The entire contribution of involuntary eye movements is due to its effect on retinal offset; extraretinal signals related to involuntary changes in eye position during the dark interval do not influence the discrimination at any dark interval. A systematic influence of eye-movement-produced retinal offset is measured even for the shortest dark interval of 0.2 msec (2.2 msec between centers of flashes). Memory deterioration predominates in determining both constant and variable errors with brief dark intervals; eye-movement-produced retinal offset increases in significance at longer ones. The course of memory deterioration is consistent with a random walk model. At the shortest dark interval acuity was affected by the vertical separation between the two lines of the vernier target; at longer dark intervals this influence was obscured by the eye-movement-produced increases in retinal distance and memory deterioration.