Reversed short-latency ocular following

Abstract

Using the scleral search coil technique to monitor eye movements, we recorded short-latency ocular following responses to displacement steps of large random-dot patterns. On half of the trials, the luminance of the dots and background were reversed during the step, a procedure that is known to reverse the direction of the perceived motion ("reverse phi"). Steps without luminance reversal induced small but consistent ocular following in the direction of the steps at ultra-short latency (<80 ms). Steps with luminance reversal induced small but consistent tracking at the same latency but in the direction opposite to the actual displacement. Tuning curves describing the dependence of initial ocular following on the amplitude of the displacement had a form approximating the derivative of a Gaussian and were well fit by Gabor functions, the cosine term being phase shifted approximately 180 degrees by the luminance reversal. This result is consistent with the idea that the initial ocular following is mediated, at least in part, by first-order (luminance) motion-energy detectors.

Figure 1

Mean version velocity profiles elicited by rightward stepwise displacements of a high-density random dot pattern. Continuous lines: luminance polarity remained constant across the step (upper right cartoon). Broken lines: luminance polarity was reversed during the step (lower right cartoon). Numbers indicate the amplitude (in degrees) of the image displacement. Horizontal grey bar indicates the time-window over which response amplitude is quantified. Upward displacements represent rightward movements. Traces are each means of ~180 trials. Subject, GSM.

Figure 2

Initial ocular following responses to step displacements: dependence on the magnitude and direction of the step (four subjects: GSM, FAM, DY, JKM). Ordinate: Change in version position (mean ±SE) over the 80-160ms time window (in degrees). Abscissa: Image displacement (in degrees). Open symbols: luminance polarity remained constant across the step. Closed symbols: luminance polarity was reversed during the step. Continuous lines are best-fitting Gabor functions. Positive values represent rightward version responses and step displacements.