We present a series of experiments exploring the effect of chromaticity on reaction time (RT) for a variety of stimulus conditions, including chromatic and luminance contrast, luminance, and size. The chromaticity of these stimuli was varied along a series of vectors in color space that included the two chromatic-opponent-cone axes, a red-green (L-M) axis and a blue-yellow [S - (L + M)] axis, and intermediate noncardinal orientations, as well as the luminance axis (L + M). For Weber luminance contrasts above 10-20%, RTs tend to the same asymptote, irrespective of chromatic direction. At lower luminance contrast, the addition of chromatic information shortens the RT. RTs are strongly influenced by stimulus size when the chromatic stimulus is modulated along the [S - (L + M)] pathway and by stimulus size and adaptation luminance for the (L-M) pathway. RTs are independent of stimulus size for stimuli larger than 0.5 deg. Data are modeled with a modified version of Pieron's formula with an exponent close to 2, in which the stimulus intensity term is replaced by a factor that considers the relative effects of chromatic and achromatic information, as indexed by the RMS (square-root of the cone contrast) value at isoluminance and the Weber luminance contrast, respectively. The parameters of the model reveal how RT is linked to stimulus size, chromatic channels, and adaptation luminance and how they can be interpreted in terms of two chromatic mechanisms. This equation predicts that, for isoluminance, RTs for a stimulus lying on the S-cone pathway are higher than those for a stimulus lying on the L-M-cone pathway, for a given RMS cone contrast. The equation also predicts an asymptotic trend to the RT for an achromatic stimulus when the luminance contrast is sufficiently large.

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd.m-2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7-14 c/deg at 20 deg, 5.5-7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial.(ABSTRACT TRUNCATED AT 250 WORDS)

This paper examines the effect of colour on reaction times for variations in both luminance and chromatic contrast. Results confirm the idea that reaction times are determined by a cone-opponent mechanism: Reaction times generated in response to S-cone isolating stimuli are the longest, whereas the shortest reaction times are generated by L—M-cone isolating stimuli. In addition, an asymmetry between ON and OFF opponent channels is observed for stimuli modulated on the blue—yellow axis. Reaction time (RT) is influenced by hue and chromatic contrast at isoluminant condition. In the isoluminant condition, RT decreases as luminance contrast increases. At luminance contrasts of approximately 20%, RT approaches an asymptotic value, thus becoming independent of colour for further luminance contrast increases. This asymptotic value is achieved for lower luminance contrasts as chromatic contrast increases.

Visual performance is defined as the speed and accuracy of processing visual information. Existing models of visual performance evaluate illuminated tasks in terms of luminance contrast, retinal illuminance and visual size but do not consider the chromatic properties of the task. In consequence, there is no recommendation for the chromatic contrast needed to reach a high level of visual performance. This paper shows that chromatic information is crucial to visual performance when achromatic information is weak or missing altogether. When the luminance contrast is less than approximately 0.20, some task colours with excitation purities greater than 40% can be used to achieve a level of visual performance close to 90%. For luminance contrasts higher than 0.60, visual performance is determined only by luminance information, and tends to the value for an achromatic stimulus. For luminance contrasts in the range 0.20 to 0.60, both luminance and chromatic information are important for visual performance.