Visual Motion And The Stabilization Of Gaze

National Eye Institute

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Abstract

There is extensive psychophysical evidence that visual motion can be sensed by low-level (energy-based) and high-level (feature-based) mechanisms. Our interest was in the motion detectors underlying the initial ocular following response that can be elicited at ultra-short latency by sudden motion of a large textured pattern. Ocular following responses were elicited in human subjects by applying horizontal motion to vertical square-wave grating patterns lacking the fundamental. In the frequency domain, a pure square-wave is composed of the odd harmonics ? 1st, 3rd, 5th, 7th etc ? such that the 3rd, 5th, 7th etc have amplitudes that are 1/3rd, 1/5th, 1/7th etc that of the 1st, and the missing fundamental stimulus lacks the 1st harmonic. Motion consisted of successive quarter-wavelength steps, so the features and 4n+1 harmonics (where n=integer) shifted forwards, whereas the 4n-1 harmonics ? including the strongest Fourier component (the 3rd harmonic) ? shifted backwards (spatial aliasing). Thus, the net Fourier energy and the non-Fourier features moved in opposite directions. Initial ocular following responses, recorded with the electromagnetic search coil technique, had minimum latencies of 60-70ms and were always in the direction of the 3rd harmonic, e.g., leftward steps resulted in rightward ocular following. Thus, the earliest ocular following responses were strongly dependent on the motion of the major Fourier component, consistent with mediation by oriented spatio-temporal visual filters as in the well-known energy model of motion detection. Introducing inter-stimulus intervals of 10-100ms (during which the screen was uniform grey) reversed the initial direction of tracking, consistent with extensive neurophysiological and psychophysical data suggesting that the visual input to low-level motion detectors has a biphasic temporal impulse response.