Abstract
We recorded human initial ocular-following responses (OFRs) when vertical sine-wave gratings were subject to horizontal motion in the form of successive 1/4-wavelength steps. The gratings could occupy the full screen (45° wide, 30° high) or a number of horizontal strips, each 1° high and extending the full width of the display. These strips were equally spaced vertically. In Experiment 1, the gratings had a contrast of 32%. Increasing the number of strips could reduce the response latency by up to 20 ms, so the magnitude of the initial OFRs was estimated from the change in eye position over the initial open-loop period measured with respect to response onset. A single (centered) strip (covering 3.3% of the screen) elicited robust OFRs, and 3 strips (10% coverage) were sufficient to elicit the maximum OFR. Increasing the number of strips to 15 (50% coverage) had little impact, i.e., responses had asymptoted, and further increasing the coverage to 100% (full screen image) actually decreased the OFR so that it was now less than that elicited with only 1 strip. In Experiment 2, the contrast of the gratings could be fixed at one of four levels ranging from 8% to 64% and the OFR showed essentially the same pattern of results except that the lower the contrast, the lower the level at which the responses asymptoted. This indicated that the asymptote was not due simply to some upper limit on the magnitude of the eye movement or the underlying motion signals. We postulate that this asymptote is the result of normalization due to global divisive inhibition. We further suggest that the decrease in the OFR when the image filled the screen was due to the increased continuity of the gratings which we postulate would favor the local inhibitory surround mechanisms over the central excitatory ones.
This research was supported by the Intramural Research Program of the NIH, the National Eye Institute.