Perception of motion smear in normal observers and in persons with congenital nystagmus.

PURPOSE Despite incessant motion of the retinal image, persons with congenital nystagmus (CN) usually do not report that targets are smeared. The authors investigated whether the brief stationary glimpses of a target that occur during foveation periods in the CN waveform contribute to the alleviation of perceived smear. METHODS Retinal image motion simulating that in jerk nystagmus was produced in normal observers (N = 10) who monocularly viewed either a 5-minute or a 1 degree luminous disk reflected from a horizontally oscillating mirror. Contrast sensitivities to detect each target and to perceive the presence of motion smear were determined for two simulated CN waveforms; observers also estimated the length and brightness of perceived smear for several suprathreshold target luminances. One waveform was a 7 degrees, 4-Hz ramp that included 120 msec zero-velocity intervals, simulating the foveation periods in the CN waveform. The second waveform lacked the zero-velocity simulated foveation periods. For comparison, estimates of perceived smear for physically stationary targets were obtained from three observers with CN. RESULTS Normal observers' contrast sensitivities for perceiving smear were nearly identical for the simulated CN waveforms with and without a 120-msec foveation period. Estimated length and brightness of perceived smear for suprathreshold targets increased similarly with luminance for both waveforms. Observers with CN reported substantially less smear than did normal observers. CONCLUSIONS Glimpses of a stationary retinal image during simulated foveation periods do not attenuate the perception of motion-induced smear in normal observers. In persons with CN, the perception of smear may be reduced by the extraretinal signals that accompany their eye movements.