Sensitivity to countermodulating gratings following spatiotemporal adaptation

Contrast sensitivity reveals an oculomotor strategy for temporally encoding space

The contrast sensitivity function (CSF), how sensitivity varies with the frequency of the stimulus, is a fundamental assessment of visual performance. The CSF is generally assumed to be determined by low-level sensory processes. However, the spatial sensitivities of neurons in the early visual pathways, as measured in experiments with immobilized eyes, diverge from psychophysical CSF measurements in primates. Under natural viewing conditions, as in typical psychophysical measurements, humans continually move their eyes even when looking at a fixed point. Here, we show that the resulting transformation of the spatial scene into temporal modulations on the retina constitutes a processing stage that reconciles human CSF and the response characteristics of retinal ganglion cells under a broad range of conditions. Our findings suggest a fundamental integration between perception and action: eye movements work synergistically with the spatio-temporal sensitivities of retinal neurons to encode spatial information.

Spatial-frequency-tuned attenuation and enhancement of the steady-state VEP by grating adaptation

Steady-state visual evoked potentials (VEPs) were recorded from adults using 10% C fast spatial frequency (SF) sweeps of horizontal gratings under two conditions: (a) after exposure to a 40% C grating of 6 or 4 c/deg, and (b) after exposure to a blank screen equalling the adapting gratings in space-averaged luminance. SF adaptation attenuated VEP amplitude near the adapting SF, but maximum attenuation was displaced from the adapting SF for 6 c/deg adaptation. Small displacements in maximum attenuation would be expected if underlying neural subunits are tuned to a small number of different center SFs. In addition, SF adaptation caused amplitude enhancement 1.0-2.0 octaves below the adapting SF, providing electrophysiological evidence in humans for coinhibitory relationships among neural mechanisms that have been postulated on the basis of analogous psychophysical findings. The results are consistent with coinhibition between SF-tuned subunits and between transient and sustained mechanisms.

Spatial and temporal frequency selectivity of neurones in visual cortical areas V1 and V2 of the macaque monkey

The spatial and temporal frequency selectivity of 148 neurones in the striate cortex, V1, and of 122 neurones in the second visual cortical area, V2, of the macaque monkey were studied using sine-wave gratings of suprathreshold contrast drifting over the receptive field at the preferred orientation and direction. Neurones in V1 and V2 were selective for different but partially overlapping ranges of the spatial frequency spectrum. At retinal eccentricities of 2-5 deg from the fovea, the spatial frequency preferences for neurones ranged from 0.5 to 8.0 cycles/deg in V1 and from 0.2 to 2.1 cycles/deg in V2 and were on average almost 2 octaves lower in V2 than in V1. Spatial frequency full band widths in the two cortical areas were in the range 0.8-3.0 octaves, with a mean value of 1.8 octaves, in the parafoveal representation of both V1 and V2, and 1.4 and 1.6 octaves respectively in the foveal representation of V1 and V2. Most neurones in V1 and some in V2 responded well at temporal frequencies up to 5.6-8.0 Hz before their responses dropped off at still higher frequencies. In V1, 68% of the neurones exhibited low-pass temporal tuning characteristics and 32% were very broadly tuned, with a mean temporal frequency full band width of 2.9 octaves. However, in V2 only 30% of the neurones showed low-pass temporal selectivity and 70% of the cells had bandpass temporal characteristics, with a mean full band width of 2.1 octaves. In V2 the minimal overlap of bandpass tuning curves across the temporal frequency spectrum suggests that there are at least two distinct bandpass temporal frequency mechanisms as well as neurones with low-pass temporal frequency tuning at each spatial frequency. A matrix of spatial and temporal frequency combinations was employed as stimuli for neurones with bandpass temporal frequency selectivity in both V1 and V2. The resultant spatio-temporal surfaces provided evidence that a neurone's preference for spatial frequency is essentially independent of the test temporal frequency; however, in V2 there was some tendency for temporal frequency peaks to shift slightly towards lower frequencies when non-optimum values of spatial frequency either above or below the preferred value were tested. Neurones with pronounced directional selectivity were encountered over a wide range of spatial frequencies, although in both cortical areas there was a tendency for an increased incidence of directional selectivity among neurones which were selective for lower spatial frequencies and higher temporal frequencies.