Adriana Fiorentini

The visual cortex as a spatial frequency analyser

Abstract Unitary responses to sinusoidal gratings either moving or alternating in phase have been investigated in the optic tract, lateral geniculate body and visual cortex of the cat as a function of the spatial frequency, position of the grating with respect to the cell receptive field and grating contrast. From the retina to the simple cells of the cortex there is a progressive narrowing of the spatial frequency band at which the cells are sensitive. The response of simple cells changes systematically with the position of an alteranting grating on the cell receptive field. The response of cells of the retina, geniculate body and visual cortex increases monotonically when the contrast of the grating is increased. For the simple cortical cells and for a part of complex cells there is a linear relation between the amplitude of the response and the logarithm of grating contrast.

The unresponsive regions of visual cortical receptive fields.

Abstract We have recorded from simple and complex cells in area 17 of the cat cortex. Around the classical receptive field (regions within which a moving or flashing bar can elicit a response from a cell) there are large regions which dramatically influence the cells responsiveness. In some cells these regions are facilitatory, in others, inhibitory. Since these regions do not respond in isolation to moving or flashing bars, we called them unresponsive regions of the receptive field. Both the inhibitory and facilitatory unresponsive regions show spatial-frequency selectivity. All the facilitatory unresponsive regions show orientation selectivity, while most of the inhibitory unresponsive regions are not orientation selective. Both the facilitatory and the inhibitory unresponsive regions contribute to the size-selectivity of the cell. The facilitatory unresponsive regions show a great influence on the orientation selectivity of the cell, in that they sharpen the orientation channel of the cell. The inhibitory unresponsive regions, on the contrary, have not such an effect.

A cortical area that responds specifically to optic flow, revealed by fMRI

The continuously changing optic flow on the retina provides information about direction of heading and about the three-dimensional structure of the environment. Here we use functional magnetic resonance imaging (fMRI) to demonstrate that an area in human cortex responds selectively to components of optic flow, such as circular and radial motion. This area is within the region commonly referrred to as V5/MT complex, but is distinct from the part of this region that responds to translation. The functional properties of these two areas of the V5/MT complex are also different; the response to optic flow was obtained only with changing flow stimuli, whereas response to translation occurred during exposure to continuous motion.

Neural Correlate of Perceptual Adaptation to Gratings

Exposure of simple cells of the cat striate cortex to high-contrast drifting gratings greatly reduces the subsequent response of the cells to low-contrast gratings. This adaptation effect has an average duration of 30 seconds and shows interocular transfer and selectivity for spatial frequency and orientation. This effect is strikingly similar to the perceptual adaptation to high-contrast gratings.

Learning in grating waveform discrimination: Specificity for orientation and spatial frequency

Abstract The effects of practice in the discrimination of briefly flashed gratings were investigated by a forced-choice procedure with error correction in a number of tasks requiring discimination either of pairs of complex gratings of different waveforms or of “simple” (sinusoidal) gratings of slightly different spatial frequency. The percentage of correct responses progressively increases with repetition of trials up to 100–200 trials and then levels off, remaining rather constant thereafter even after days or weeks, in all tasks involving discrimination of complex gratings. However, when the gratings are set perpendicular to those used for the training sessions, or their spatial frequency is changed by 1 octave, the effects of previous perceptual learning are lost, while transfer of learning effects is obtained for smaller changes in orientation (±30°) or spatial frequency (±12 octave). The spatial frequency discrimination of sinusoidal gratings does not improve with a comparable number of trials.

A second neural mechanism of binocular depth discrimination

1. Rotation of an object about its horizontal axis, towards or away from the viewers eyes, usually causes the images of its contours to have slightly different orientations on the two retinae.

The effects of ageing on the pattern electroretinogram and visual evoked potential in humans

We have recorded patterns electroretinograms (PERGs) and visual evoked potentials (VEPs) from 14 elderly subjects (mean age 72 yr) and 12 young subjects (mean age 21 yr) in response to stimulation by high contrast sinusoidal grating patterns of variable spatial frequency (at 9 Hz) and temporal frequency (at 1.7 c/deg). The major effect of aging on the PERG was an aspecific reduction in amplitude (of about 40%) at most spatial and temporal frequencies, together with a small but systematic phase lag. Control measurements suggest that senile miosis may be responsible for the phase lag, but not for the reduction in amplitude. The effects of aging on the VEP were more dramatic and depended on the spatial and temporal properties of the stimulus. VEP amplitudes (at 1.7 c/deg) were significantly lower for the aged at low temporal frequencies (below about 6 Hz), but were similar at high temporal frequencies. At 9 Hz, there was no effect of spatial frequency on VEP amplitude. At high temporal frequencies (above 10 Hz), the latencies of VEPs (estimated from the rate at which phase varied with temporal frequency) were similar for old and young (94 and 99 msec respectively). Below 10 Hz, however, the latencies of the old observers was much greater (153 compared with 108 msec). The second-harmonic phase of VEPs of the old but not the young decreased considerably with spatial frequency, by about 1.9 pi radians (52 msec) over the range from 0.5 to 11 c/deg. The selective reduction in amplitude at low temporal frequencies, the longer latencies at low temporal frequencies and the phase lag at high spatial frequencies are consistent with the hypothesis that mechanisms sensitive to high spatial and low temporal frequencies are selectively degraded by aging.

Lack of cortical contrast gain control in human photosensitive epilepsy

Television and video games may be powerful triggers for visually induced epileptic seizures. To better understand the triggering elements of visual stimuli and cortical mechanisms of hyperexcitability, we examined eleven patients with idiopathic photosensitive epilepsy by recording visually evoked potentials (VEPs) in response to temporally modulated patterns of different contrast. For stimuli of low–medium, but not high, temporal frequency, the contrast dependence of VEP amplitude and latency is remarkably abnormal for luminance contrast (black–white), but not so for chromatic contrast (equiluminant red–green) stimuli. We conclude that cortical mechanisms of contrast gain control for pattern stimuli of relatively low temporal frequency and high luminance contrast are lacking or severely impaired in photosensitive subjects.

Spatial frequency rows in the striate visual cortex

Abstract We have recorded the unitary activity of the neurons of area 17 of the cat either in penetrations perpendicular to the surface of the cortex or parallel to it. We have examined the functional architecture of this cortical area with respect to the spatial frequency of the visual stimulus (sinusoidal gratings) at which each neuron is most sensitive. In penetrations perpendicular to the surface of the cortex (columns) the cells of the various layers or even of the same layer have the same preferred orientation but different preferred spatial frequencies. In penetrations parallel to the surface of the cortex the cells of the same layer or sublayer present a variety of preferred orientations, but maintain the same preferred spatial frequency and visual acuity over long microelectrode track distances.

Visual ageing: unspecific decline of the responses to luminance and colour.

We have investigated whether ageing affects selectively the responses to equiluminant patterns of pure colour contrast. In two groups of subjects (mean ages 29 and 72 yr) contrast thresholds were measured psychophysically for the detection and for the discrimination of the direction of motion of drifting gratings. The gratings were modulated either in pure luminance contrast (and uniform colour), or pure chromatic contrast (red-green equiluminant gratings). In subjects of the same age groups, visual evoked potentials (VEP) were recorded in response to gratings with either pure luminance contrast or pure colour contrast sinusoidally reversed in contrast at various temporal frequencies. It was shown that psychophysical contrast sensitivity for equiluminant patterns deteriorates significantly with age, and VEP latency increases. However, these effects of ageing on the responses to patterns of pure colour contrast are substantially the same as those observed in the same subjects for stimuli with pure luminance contrast. The results suggest that ageing causes a small and unspecific decline of the response of the visual system to luminance and colour contrast.