Discrimination of differences in speed and flicker rate depends on directionally selective mechanisms

The present study compared discriminations of differences in speed to differences in temporal frequency and examined the role of directionally selective mechanisms in such discriminations. In measuring the contrast dependence of speed and temporal frequency discriminations two different techniques were used to reduce the role of directionally selective mechanisms. The first was the virtual elimination of directional selectivity in the visual cortex of cats by stroboscopic rearing. The second was the reduction of directional sensitivity in normal humans and cats by testing with gratings of high spatial and low temporal frequency. Discrimination of the temporal frequency of sinusoidal gratings flickered in counterphase was worse than discrimination of speeds of moving gratings. Under conditions that maximize the sensitivity of directional mechanisms (low spatial, moderate temporal frequency) Weber fractions for speed and flicker in all normal observers (cats and humans) were constant at higher contrast and increased only as contrast began to approach threshold. In strobe-reared cats sensitivity for direction was 10 times lower than sensitivity for detection. They were able to discriminate speeds and temporal frequencies only at contrasts that exceeded contrast threshold for direction. This was also true for a normal cat whose sensitivity for direction was reduced by increasing the spatial frequency of the grating. In all cases Weber fractions for flicker as a function of contrast were greater than but paralleled those for speed.