Showing papers by "David J. Tolhurst published in 1973"

Psychophysical evidence for sustained and transient detectors in human vision

Abstract: 1. The sensitivity to temporally modulated sinusoidal gratings was determined. Two thresholds could be distinguished for the modulated gratings: the contrast at which flicker could be perceived and the contrast at which the spatial structure became distinct.2. The flicker detection thresholds and pattern recognition threshold varied independently as functions of the spatial and temporal frequencies, suggesting that the two thresholds represent the activity of two independent systems of channels.3. The channels detecting flicker prefer low and medium spatial frequencies. They have a pronounced decline in sensitivity at low temporal frequencies of sinusoidal modulation. They respond twice as well to gratings whose phase is alternated repetitively as to gratings turned on and off at the same rate.4. The channels responsible for the discrimination of spatial structure are most responsive at high and medium spatial frequencies. There is no decline in sensitivity at low temporal frequencies. These channels respond equally well to alternating and on/off gratings up to about 8 Hz.5. The temporal properties as revealed with sinusoidal modulation, suggest that the flicker-detecting channels would give transient responses to prolonged presentation of stimuli: the channels responsible for analysing the spatial structure would give sustained responses. The responses of the two types of channel to alternating and on/off gratings confirm this suggestion.

Edge detectors in human vision

Abstract: 1. The spatial properties of edge detectors were measured psychophysically with the technique of subthreshold addition. Subthreshold patterns used to add to an edge were lines, sine gratings, Gaussian edges, and ramps. 2. The sensitivity profile, determined from experiments on subthreshold addition of lines to an edge was an antisymmetric function, with peak sensitivity approximately ± 1·5′ from its midpoint. Its total extent was about ± 6′. 3. The spatial frequency response of edge detectors was measured in experiments on subthreshold addition of sine gratings to an edge. The spatial frequency response was peaked at about 3 c/deg, and was broadly tuned in frequency. It was approximately equal to the Fourier transform of the sensitivity profile, implying linearity of edge detectors. 4. The visibility of Gaussian edges and ramps could be explained largely in terms of the activation of edge detector neurones. 5. The role of edge detectors in perception, in creating apparent brightness, and as an explanation of contour illusions, is discussed.

The effects of temporal modulation on the orientation channels of the human visual system.

Abstract: The orientation specificity of spatial adaptation was quantified by using an equivalent-contrast transformation. The half-width at half-amplitude of the adaptation effect was 6·5° when the sinusoidal gratings were stationary, but was increased to 11° when the gratings drifted laterally. Another type of temporal modulation, flashing the gratings on and off repetitively, also increased the half-width, but only at low spatial frequencies where these stimuli actually appeared to be flickering back and forth at threshold. At higher spatial frequencies the flashing stimuli appeared to be stationary at threshold and the half-width was as small as that for truly stationary gratings. It is suggested that there are two types of channel in the visual system: they differ in their orientation specificities, in their temporal properties, and in their roles in the analysis of a spatiotemporal stimulus.