Biomedical subjects

M C Morrone

Publications and source records attributed to M C Morrone.

54 records · Page 3Linked to original sources

PubMed1987

Cross-orientation inhibition in cat is GABA mediated.

Visual evoked potentials (VEPs) were recorded from cat cortex (area 17) before, during and after application of the GABA blocker bicuculline (iontophoretic or topical). The stimuli comprised a test sinusoidal grating, and a mask grating oriented either parallel or orthogonal to the test. Both test and mask alternated in contrast at different temporal frequencies. VEPs were averaged in synchrony with the test contrast reversal, so the mask did not contribute directly to the averaged VEP response. Before application of bicuculline, both parallel and orthogonal masks attenuated the amplitude of VEPs and changed the phase response, but in different ways. Orthogonal masks lowered the slope of the contrast response curve without affecting extrapolated threshold, while parallel masks caused the curve to shift to the right. Orthogonal masks increased the phase advance, while parallel masks eliminated it. During application of bicuculline, neither parallel nor the orthogonal masks attenuated VEP amplitudes. The results suggest that although the mechanisms for the action of parallel and orthogonal masks are clearly distinct, both are mediated by the GABA-ergic inhibitory system. Given this evidence, measurement of VEP contrast response curves may provide a simple non-invasive technique for monitoring visual inhibition in humans.

Administration, Topical↗

PubMed1987

Electrophysiological correlates of positive and negative afterimages.

We report here measurements of visual evoked potential (VEP) activity after induction of afterimages in human observers. Drifting gratings normally produce no measurable phase synchronized VEP. However, after an afterimage of a high contrast grating of the same orientation and spatial frequency of that of the drifting grating has been flashed on the retina. VEPs in synchrony with the drift frequency are produced. For up to about two minutes after the flash, the VEPs were all clustered in one phase, then declined for a minute or so, to reappear 180 degrees out of phase from the first VEPs. The first group of VEPs coincides with the percept of a positive afterimage, and the second with that of a negative afterimage. Possible explanations for the existence of positive and negative afterimages and the associated VEPs are considered.

Afterimage↗

PubMed1987

Inhibitory interactions in the human vision system revealed in pattern-evoked potentials.

1. Visual evoked potentials (v.e.p.s) were recorded from human adults to investigate orientation-specific neural interactions. The stimuli were the sum of two gratings, sinusoidally modulated in space and time at different frequencies. Recordings were made for one grating (test) alone, and with another superimposed grating (mask), oriented parallel or orthogonal to the test. The amplitude and phase of the v.e.p.s at twice the test modulation frequency (second harmonic) was measured as a function of test contrast to produce contrast-response curves. 2. Orthogonal masks attenuated considerably the amplitude of v.e.p.s. The attenuation at any given contrast was approximately proportional, or multiplicative, lowering the slope of the contrast-response curve, without affecting significantly the extrapolated threshold. Parallel masks also attenuated v.e.p. amplitudes but in a different way, leaving the slope of the contrast-response curves unchanged, while elevating threshold. 3. The attenuation by orthogonal masks occurred over a wide range of test spatial frequencies, from 0.8 to 8 cycles/deg. For any given test spatial frequency, the most effective masks were those of spatial frequency similar to or lower than the test. Masks of spatial frequency 1.5 octaves higher than the test did not attenuate v.e.p. amplitudes. 4. The mask temporal frequency for maximal attenuation of v.e.p. amplitude was around 12 Hz, with stationary masks having little effect. 5. Under most conditions, the phase of the second harmonic of the v.e.p., increased with increasing contrast (phase advance). Superimposition of a parallel mask abolished phase advance, while orthogonal masks increased it. 6. Comparisons with single cortical unit and evoked potential recordings in cats suggest that the attenuation by orthogonal masks reflects intracortical inhibitory interactions between cell populations of different orientation preference.

Adult↗

PubMed1986

Seeing objects in motion.

This paper reports estimates of the conjoint spatiotemporal tuning functions of the neural mechanisms of the human vision system which detect image motion. The functions were derived from measurements of the minimum contrast necessary to detect the direction of drift of a sinusoidal grating, in the presence of phase-reversed masking gratings of various spatial and temporal frequencies. A mask of similar spatial and temporal frequencies to the test grating reduces sensitivity considerably, whereas one differing greatly in spatial or temporal frequency has little or no effect. The results show that for test gratings drifting at 8 Hz, the tuning function is bandpass in both space and time, peaked at the temporal and spatial frequency (SF) of the test (SFs were 0.1, 1 or 5 c deg-1; c represents cycles throughout). For a grating of 5 c deg-1 drifting at 0.3 Hz, the function is bandpass in space but lowpass in time. Fourier transform of the frequency results yields a function in space-time which we term the 'spatiotemporal receptive field'. For movement detectors (bandpass in space and time) the fields comprise alternating ridges of opposing polarity, elongated in space-time along the preferred velocity axis of the detector. We suggest that this organization explains how detectors analyse form and motion concurrently and accounts, at least in part, for a variety of perceptual phenomena, including summation, reduction of motion smear, metacontrast, stroboscopic motion and spatiotemporal interpolation.

Differential Threshold↗

PubMed1986

Smooth and sampled motion.

Stroboscopic or sampled motion is indistinguishable from smooth motion if the frequency of sampling is sufficiently high. We report measurements of the minimum sample frequency for smooth motion of drifting sinusoidal gratings (extended and truncated) which varied in spatial frequency from 0.06 to 24 c/deg, in temporal frequency from 1.5 to 24 Hz and in contrast from 3 to 100 times detection threshold. Threshold sampling frequency for smoothness increased with temporal frequency and contrast, and inversely with spatial frequency. The threshold step size associated with the sampling frequency ranged from 20" arc (for gratings of 24 c/deg) to 6 deg (for gratings of 0.06 c/deg). Calculations of the spurious frequencies introduced by sampling lead us to conclude that motion appears smooth provided that sampling is above the Nyquist limit and that the amplitude of the spurious components is below their independent threshold.

Humans↗

PubMed1986

Local and global visual processing.

The fundamental sinusoidal components of a chequerboard pattern are oriented at 45 degrees to the orientation of the chequerboard edges. Removal of one of the fundamental sinusoids (at +45 degrees) creates a useful pattern for studying the mechanisms of visual analysis. Close up, the pattern appears to be oriented +45 degrees, although there is no global energy at that orientation, implying local analysis. At a distance, the perceived diagonality switches to -45 degrees implying access to global information. Measurements show that contrast thresholds for seeing diagonality at +45 degrees follow closely those for detecting the 5th harmonic component of the pattern, over a wide range of spatial frequencies and luminances. Low pass filtering also causes the pattern to be perceived according to its global energy, provided that the cutoff frequency is set to remove the fifth harmonic. We conclude that, at least for this particular stimulus, the visual system performs a local analysis if the fifth harmonic is visible and a global analysis if not.

Form Perception↗

PubMed1986

A spatial illusion from motion rivalry.

A new dynamic visual illusion is reported: contrast reversal of a horizontal and vertical plaid pattern (produced by adding two orthogonal sinusoidal gratings) causes the pattern to appear as an array of lustrous diamonds, cut by sharp lines into a diagonal lattice structure. On the basis of computer simulations it is suggested that the illusion results from rivalrous interaction of motion detectors tuned to opposing directions of motion.

Humans↗

PubMed1986

Recognition of positive and negative bandpass-filtered images.

A study is reported in which the significance for vision of low- and high-spatial-frequency components of photographic positive and negative images was investigated by measuring recognition of bandpass-filtered photographs of faces. The results show that a 1.5 octave bandpass-filtered image contains sufficient visual information for good recognition performance, provided the filter is centred close to 20 cycles facewidth-1. At low spatial frequencies negatives are more difficult to recognize than positives, but at high spatial frequencies there is no difference in recognition, implying that it is the low-frequency components of negatives which present difficulties for the visual system.

Color Perception↗

PubMed1986

Spatial and temporal properties of neurons of the lateral suprasylvian cortex of the cat.

Neurons in the posteromedial lateral suprasylvian cortex (PMLS) of cats were recorded extracellularly to investigate their response to stimulation by bars and by sinusoidal gratings. Two general types of cells were identified: those that modulated in synchrony with the passage of drifting bars and gratings and those that responded with an unmodulated increase in discharge. Both types responded to contrast reversed gratings with a modulation of activity: the cells that modulated to drifting gratings modulated to the first harmonic of contrast reversed gratings (at appropriate spatial phase and frequency), whereas those that did not modulate to drifting gratings always modulated to the second harmonic of contrast reversed gratings. No cell had a clear null point. Nearly all cells were selective for spatial frequency. The preferred frequency ranged from 0.1 to 1 cycles per degree (cpd), and selectivity bandwidths (full width at half height) were around two octaves. Preferred spatial frequency was not correlated with receptive field size, but bandwidth and receptive field size were positively correlated. Preferred spatial frequency decreased with eccentricity, at about 0.05 octaves/deg. The response of all cells increased as a function of grating contrast up to a saturation level. The contrast threshold for response to a grating of optimal parameters was approximately 1% for most cells and the saturation contrast approximately 10%. The contrast gain was approximately 25 spikes/s per log unit of contrast. All cells were tuned for temporal frequency, preferring frequencies from approximately 3 to 10 Hz, with a selectivity bandwidth approximately 2 octaves. For some cells, the spatial selectivity did not depend on the temporal frequency and vice versa. Others were spatiotemporally coupled, with the preferred temporal frequency being lower at high than at low spatial frequencies, and the preferred spatial frequency lower at high than at low temporal frequencies. Previous results showing broad velocity tuning to a bar were replicated and found to be predictable from the combined spatial and temporal tuning of PMLS cells and the Fourier spectrum of a bar. Preferred temporal frequency steadily decreased with eccentricity, at 0.025 octaves/deg. The results for PMLS cells are compared with those of other visual areas. Acuity and spatial preference and selectivity bandwidth is comparable to all areas except area 17, where they are a factor of about two higher. Temporal selectivity in PMLS is as fine as observed in other areas. The possibility that PMLS cells may be involved with motion detection and detection of motion in depth is discussed.

Animals↗

PubMed1985

Visual acuity of neurones in the cat lateral suprasylvian cortex.

The spatial acuity was measured for cells of the posteromedial lateral suprasylvian area (PMLS) of the cat. Acuities were found to be 2 cycles/degree (15 mins arc) at best, and 1 cycle/degree (30 mins arc) on average. Both best acuity and average acuity remained constant with receptive field eccentricity within 20 degrees of the area centralis, and then fell gradually with eccentricity. Acuity was good, given receptive field size, and was not correlated with receptive field size. Comparisons are drawn with other visual structures.

Animals↗

PubMed1985

Local regulation of luminance gain.

Contrast sensitivity was measured for sinusoidal gratings sampled by compressing luminance into a variable number of sample bars. This procedure does not affect the amplitude or mean luminance of the grating, but does increase the local luminance of the sample bars: the fewer the bars, the more luminous they are. It was found that sensitivity increased with bars per cycle, particularly at low spatial frequencies. Further experiments, in which the local luminance of the sampling bars (but not the average luminance of the grating) was varied by addition of veiling glare showed that contrast sensitivity varied inversely with local bar luminance (a Weber type relationship). We interpret the results as evidence of local gain control under conditions where average luminance, and hence mean photon flux, does not vary. Calculations based on variation of sensitivity with spatial frequency suggest that gain control can be very localized, with receptive fields of Gaussian space constant of 0.5' arc. The relevance of these results to modern psychophysical concepts, including the definition of contrast is discussed.

Form Perception↗

PubMed1984

The role of gamma-aminobutyric acid mediated inhibition in the response properties of cat lateral geniculate nucleus neurones.

We studied the effect of local ionophoretic application of bicuculline on the response of cat lateral geniculate nucleus (laminae A) cells to stimulation by sinusoidal gratings and spots of light. Application of bicuculline produced an increase both of spontaneous and visually driven discharge of both X and Y cells. On stimulation by drifting sinusoidal gratings, the average discharge of both X and Y cells remained constant with increasing contrast under normal conditions. Application of bicuculline caused the average discharge to increase with contrast, indicating that the constancy of the average discharge was maintained by gamma-aminobutyric acid mediated inhibition. Under normal conditions, the amplitude of response modulation of both X and Y cells to sinusoidal grating stimulation increased monotonically with stimulus contrast. During bicuculline application, the slope of the contrast-response curve for X cells but not for Y cells increased, indicating that the inhibition which dampened the modulation of X cells (but not Y cells) was contrast dependent. Application of acetylcholine also increased the average discharge and the amplitude of modulation of the cell responses, but this increase did not depend on stimulus contrast. Under normal conditions, X but not Y cells showed an attenuation of response and an increase in contrast threshold to low spatial frequencies. This attenuation vanished during bicuculline application. The shape of Y-cell response curves was unaffected by bicuculline. Bicuculline had the same effect on the non-linear component of Y-cell response as on the linear component. Although bicuculline had a different effect on the response of X and Y cells to stimulation by gratings, it reduced the antagonistic surround of both X and Y cells to a similar extent (revealed by plotting the cell receptive fields with flashed spots of light).

Acetylcholine↗

PubMed1984

Development of gamma-aminobutyric acid mediated inhibition of X cells of the cat lateral geniculate nucleus.

We studied the development of gamma-aminobutyric (GABA) mediated inhibitory processes of the lateral geniculate nucleus cells in kittens of various age groups, by measuring the effect of ionophoretic application of GABA and bicuculline on cell response to sinusoidal gratings. In young kittens (less than 30 days) we found very few Y cells only X cells and weakly responsive cells which fitted neither X nor Y classifications ('immature' cells). As with adult cells, GABA inhibits the visual response of young kitten cells. Simultaneous application of bicuculline restored responsiveness. The mean GABA current to silence cell response in young kittens was significantly higher than that obtained in adult cats. Application of bicuculline alone had little effect on young kitten X-cell responsiveness, either on the average discharge or on the amplitude of modulation to stimulation by sinusoidal gratings. For older kittens (40-45 days), bicuculline increased X-cell responsiveness, and the increase in responsiveness was dependent on stimulus contrast and spatial frequency. However, the increased responsiveness was less than that for adults. At 100 days the changes of slope of X-cell contrast-response curves during bicuculline application were similar to those observed for adult X cells. We conclude that, although GABA receptors may be present at 30 days, the GABA mediated inhibitory system does not begin to function until about 45 days and does not mature fully until about 100 days.

Aging↗

PubMed1982

Functional implications of cross-orientation inhibition of cortical visual cells. I. Neurophysiological evidence.

Simple and complex cells of striate cortex of anaesthetized and paralysed cats were stimulated with two superimposed one-dimensional grating stimuli of different orientations to investigate inhibitory effects of non-optimally oriented stimuli. We confirmed that a stimulus of orientation orthogonal to a cell's long axis significantly reduces the cell's discharge rate. Further experiments revealed the following. (i) The inhibition was typically stronger for simple than for complex cells. (ii) It is very broadly tuned for orientation, all orientations outside the cell's tuning band having a comparable inhibitory effect. (iii) Similarly, it is broadly tuned for spatial frequency. These last two results suggest that the inhibition arises not from a single cell but from a pool of cells. (iv) The pattern of the discharge of the inhibition in response to stimulation by phase-reversed sinusoidal gratings is consistent with the notion that the inhibition arises from complex cells. A second series of recordings of stimulation by visual noise patterns demonstrated how 'cross-orientation inhibition' prevents simple cells from responding to two-dimensional visual noise while allowing them to respond to comparable one-dimensional noise patterns. We suggest that this mechanism may serve to render simple cells selectively sensitive to one-dimensional stimuli, such as the contours or borders of visual objects.

Action Potentials↗

PubMedDate not supplied

Spatial neglect is associated with increased latencies of visual evoked potentials.

We have recorded steady-state visual evoked potentials (VEPs) from patients with vascular damage to their right brain hemispheres, some suffering from unilateral spatial neglect (n = 9), and some not (n = 7). VEPs were recorded in response to sinusoidal gratings of 0.56 cycle/deg contrast-reversed sinusoidally at temporal frequencies from 4-11 Hz. Stimuli were presented either to the left or to the right visual field, or to both. Confirming previous reports, reliable VEPs were recorded from stimuli in the left contralesional hemifield, of comparable amplitude to those of the ipsilesional hemifield and to those of both hemifields of brain damaged patients without neglect. However, analysis of apparent latency derived from phase data showed that the VEPs from the contralesional hemifield were systematically delayed by 30-40 ms compared with those of the ipsilesional hemifield, and compared with both hemifields of the nonneglect groups. This result suggests changes in neural processing in neglect patients.

Adult↗

PubMedDate not supplied

Spatial structure of chromatically opponent receptive fields in the human visual system.

This study investigates the receptive-field structure of mechanisms operating in human color vision, by recording visual evoked potentials (VEPs) to multiharmonic gratings modulated either in luminance or color (red-green). Varying the Fourier phase of the harmonics from 0 deg to 90 deg produced a family of stimulus profiles that varied from lines to edges. The stimuli were contrast reversed to elicit steady-state VEPS, and also randomly jittered (at a higher temporal frequency than the contrast reversal) to ensure that the evoked response resulted from the polarity reversal, rather than from local variation of luminance or color. Reliable VEPs were recorded from both luminance and chromatic stimuli at all phases, suggesting that the mechanisms sensitive to chromatic contrast and those sensitive to luminance contrast have both symmetric and asymmetric receptive fields. Contrast thresholds estimated by extrapolation of the contrast response curves were very similar to psychophysical thresholds for phase discrimination, suggesting that the VEP response is generated by mechanisms mediating phase discrimination. The results support the idea that human color mechanisms have receptive fields with a variety of spatial symmetries (including odd- and even-symmetric fields) and that these mechanisms may contribute to phase discrimination of chromatic stimuli in a similar way to what has been suggested for luminance vision.

Color Perception↗

PubMedDate not supplied

Added noise restores recognizability of coarse quantized images.

When a portrait is coarsely quantized into blocks, the block structure hides the face, although lower spatial frequencies of the original image sufficient by themselves for recognition are preserved. Recognition can be recovered by blurring the image, or otherwise attenuating the spurious higher spatial frequency components. Harmon and Julesz claim that high spatial frequencies introduced by quantized blocking mask the lower spatial frequencies which convey information about the face, preventing recognition. Here we show that recognition can be enhanced, without decreasing the amplitude of these spurious higher frequencies, by adding further high-frequency noise to the quantized image. This result is clearly at odds with a theory of high-frequency (or critical band) masking. We suggest that the added noise mutes mechanisms which would otherwise impose a block structure on the image, allowing the alternative perceptual organization of the hidden face to reemerge.

Form Perception↗

PubMedDate not supplied

Evidence for the existence and development of visual inhibition in humans.

Neural inhibition forms a major mechanism by which the nervous system refines and elaborates its input. Several recent experiments have demonstrated the existence of inhibition between orientation-selective cells of the primary visual cortex of the cat and although the precise function of this inhibition is uncertain, there is evidence that it enhances orientation tuning and that it is involved in pattern recognition. Here we report a series of experiments which, on the basis of evoked potential responses to oriented stimuli, suggest that similar processes may exist in humans. Recordings from young infants further suggest that the machinery which mediates orientation-specific interactions may not be functional at birth, but develops only after 6-8 months.

Adult↗