REACTION-TIME TO MOTION ONSET OF LUMINANCE AND CHROMATIC GRATINGS IS DETERMINED BY PERCEIVED SPEED

We measured reaction times for detecting motion onset for sinusoidal g ratings whose contrast was modulated in either luminance or chromatici ty, for various drift rates and contrasts. In general, reaction rimes to chromatic gratings were slower than to luminance gratings of matche d cone contrast, but the difference in response depended critically on both contrast and speed. At high image speeds there was virtually no difference, whereas at low speeds, the difference was pronounced, espe cially at low contrasts. Al high image speeds there was little depende nce of reaction times on contrast (for either luminance or colour), wh ereas at low speeds the dependence was greater, particularly for chrom atic stimuli. This pattern of results is reminiscent of those found fo r apparent speed of drifting luminance and chromatic gratings. We veri fied the effects of contrast on perceived speed, and went on to show t hat the effects of contrast on reaction times are totally predictable by the perceived speed of the stimuli, as if it were perceived rather than physical speed that determined reaction times. Our results suppor t the idea of separate systems for fast and slow motion (with separate channels for luminance and colour at slower speeds), and further sugg est that apparent speed and reaction times may be determined at a simi lar stage of motion analysis. (C) 1998 Elsevier Science Ltd. All right s reserved.