Biomedical subjects

M C Morrone

Publications and source records attributed to M C Morrone.

At least 19 recordsLinked to original sources

PubMed1992

Electro-physiological investigation of edge-selective mechanisms of human vision.

This study investigates the spatial and temporal characteristics of human visual mechanisms that respond selectively to the polarity of edges. The technique was to record steady-state visual evoked-potentials (VEPs) while visually stimulating with a sawtooth waveform (a series of edges of the same polarity) periodically reversing in contrast (and hence edge-polarity) at a suitable frequency. To ensure that phase-locked VEPs resulted from polarity reversal (rather than local luminance modulation) the stimuli were randomly jittered to a new position between each contrast reversal. The jittered stimulus elicited strong and reliable second-harmonic modulation, usually about one-fifth the amplitude of standard VEPs under similar conditions. The amplitude and extrapolated thresholds of polarity-specific VEPs (relative to standard VEPs) did not vary with eccentricity (up to 10 degrees) or with stimulus orientation. The dependency on spatial frequency was similar to that of standard VEPs, but the polarity-specific VEPs tended to peak at lower temporal frequencies. Perhaps the clearest difference in the two types of VEPs was in the estimated response latency, about 140 msec for the polarity VEPs, compared with 90 msec for standard VEPs.

Contrast Sensitivity↗

PubMed1992

The effects of aging 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.

Adult↗

PubMed1991

Development of visual inhibitory interactions in kittens.

This study was designed to monitor the development of inhibitory interactions elicited in the cat visual system by oriented visual stimuli. Steady-state visual-evoked potentials (VEPs) were recorded from the scalp of 11 behaving and alert kittens while they viewed contrast-reversed sinusoidal gratings. In adult cats, the form of VEP contrast-response curves (the amplitude of second harmonic modulation as a function of stimulus contrast) was modified by superimposing a mask grating on the test. Parallel masks displaced the curves to a higher contrast region (probably via contrast gain-control mechanisms), increasing contrast threshold without affecting the slope of the curve. Orthogonal gratings, on the other hand, decrease the slope of the curve without affecting threshold (so called cross-orientation inhibition: Morrone et al., 1981). These effects are similar to those previously reported in human VEPs (Morrone & Burr, 1986; Burr & Morrone, 1987) and single cortical cat cells (Morrone et al., 1982). For young kittens of 20 days, the orthogonal mask had no effect whatsoever on the response curves, and the effect of the parallel mask was much less than for adult cats. At about 40 days, the orthogonal mask began to attenuate responses multiplicatively, and by 50 days the amount of multiplicative attenuation had reached adult levels. The effect of the parallel mask (as indicated by the increase in threshold elevation) increased gradually from 20-50 days. The results are consistent with the existence of at least two types of inhibition in cat visual neurones that develop at different rates.

Animals↗

PubMed1991

Effects of monocular deprivation on the development of visual inhibitory interactions in kittens.

A visual-evoked-potential (VEP) masking technique was used to assess the effects of short- and long-term monocular deprivation on the development of visual inhibitory interactions in kittens. VEP contrast-response curves were recorded in response to contrast-reversed sinusoidal gratings, both with and without superimposed high-contrast masks. The contrast-response curves measured from the nondeprived eye were similar to those of normal cats: with no mask VEP amplitudes increase with contrast up to saturation at about 10% contrast; parallel masks shift the curves to the right, decreasing thresholds; and orthogonal masks decrease the slope of the contrast-response curves without affecting thresholds. After monocular deprivation (either brief or extensive), the contrast-response curves without mask did not show the typical response saturation, and neither parallel nor orthogonal mask had any effect on the contrast-response curves. The masking effects did not return after 100 days of normal vision, although contrast sensitivity and acuity recovered to about half of the normal levels during that period. The results indicate that the inhibitory intracortical circuitry that mediates the orientation-dependent masking effects are highly vulnerable to visual experience.

Animals↗

PubMed1991

Two-dimensional spatial and spatial-frequency selectivity of motion-sensitive mechanisms in human vision.

Thresholds for detecting the direction of motion of drifting (8-Hz) vertical gratings [of spatial frequencies 0.1, 1.0, and 10.0 cycles per degree (c/deg)] were measured in the presence of masks that varied in both spatial frequency and orientation. Masks with different temporal properties were used. The specificity of masking was also measured for a stationary test grating of spatial frequency 3.0 c/deg. After suitable scaling and transformation, the masking data gave an estimate of the two-dimensional spatial-frequency tuning surface of cortical detector units in human vision. With the assumption of small-signal linearity and zero phase, the tuning surfaces were inverse Fourier transformed to give an indication of the size and structure of the psychophysical receptive fields of detector units. The results obtained with drifting test gratings and jittering (random phase) mask gratings indicate that motion-detector receptive fields increase in size (in cycles) with increasing spatial frequency but, at all spatial scales, have a length-width ratio of 1. These results are in close agreement with the summation results reported in J. Opt. Soc. Am. A 8, 1330 (1991). Using the same jittering mask stimuli and stationary test gratings, we confirm reports by Daugman [Vision Res. 24, 891 (1984)] and Harvey and Doan [J. Opt. Soc. Am. A 7, 116 (1990)] that motion-independent units have elongated receptive fields with a length-width ratio near 1.8. We conclude that the receptive fields of motion-dependent and -independent mechanisms in human vision are fundamentally different. The possibility that the orientation selectivity of a motion unit is sharpened by its selectivity for direction of motion is discussed.

Contrast Sensitivity↗

PubMed1990

Development of contrast sensitivity and acuity of the infant colour system.

We have monitored the development of infant colour vision by measuring chromatic contrast sensitivity and acuity in eight young infants over a period of 6 months. Steady-state visual evoked potentials (VEPS) were recorded in response to both chromatic (red-green) and luminance (red-black or green-black) patterns that were reversed in contrast over time. For most infants, no response could be obtained to chromatic stimuli of any size or contrast before 5 weeks of age, although luminance stimuli of 20% contrast gave reliable responses at that age. When responses to chromatic stimuli first appeared, they could be obtained only with stimuli of very low spatial frequency, 20 times lower than the acuity for luminance stimuli. Both contrast sensitivity and acuity for chromatic stimuli increased steadily, more rapidly than for luminance stimuli. As the spectral selectivities of infant cones are similar to those of adults, the difference in rate of development of luminance and chromatic contrast sensitivity and acuity stimuli probably reflects neural development of the infant colour system.

Age Factors↗

PubMed1989

Evidence for edge and bar detectors in human vision.

The structure of receptive fields of human visual detectors was investigated by studying their phase response. Observers were required to discriminate between pairs of periodic stimuli that differed in phase by 180 degrees (reversed in contrast). The stimuli comprised 256 harmonics, smoothly filtered in amplitude, and congruent in phase at the origin. Reversal discrimination thresholds were measured as a function of the phase of the harmonics. Thresholds were slightly higher for phases around 45 degrees, consistent with the idea that all discriminations were mediated by independent detectors with 0 or 90 degrees phase response (assuming probability summation between them). Discrimination thresholds were also measured with a pedestal stimulus, of phase complementary to that of the test gratings. For discriminations between 0 and 180 degrees (cosine phase), or 90 and 270 degrees (sine phase), the complementary pedestal had little effect, implying independence of detectors in sine and cosine phase. However, for discrimination between 45 and 225 degrees (stimuli containing both sine and cosine components) the complementary pedestal, which also contained both sine and cosine components, facilitated greatly discrimination thresholds. The results suggest that there exist two classes of detectors, one with a Fourier phase spectrum of 0, the other with a Fourier phase spectrum of 90 degrees. This implies that the receptive fields are symmetric, one class having even-symmetry (line-detectors), the other odd-symmetry (edge-detectors).

Differential Threshold↗

PubMed1989

Discrimination of spatial phase in central and peripheral vision.

Sensitivity to relative phase was measured for central and peripheral vision using stimuli comprising 256 harmonics, smoothly filtered in amplitude. With these stimuli, peripheral phase sensitivity was much higher than that previously reported with two-harmonic stimuli. Sensitivity did not depend on the average phase of the stimuli, nor on their second-order statistics, irrespective of the spatial frequency of the stimulus or the position in the visual field. After scaling for size, peripheral sensitivity was as high as central sensitivity. The scaling factor required to equate phase sensitivity was the same as that required to equate contrast sensitivity and grating acuity. These results suggest that phase sensitivity decreases with eccentricity at a similar rate as contrast sensitivity and grating acuity, much more slowly than the positional acuities. This is consistent with the suggestion that phase discrimination is mediated by discriminating the amplitude of the response of quasi-linear filters, and does not require mechanisms that evaluate position. It is suggested that previous measurements on peripheral phase sensitivity may reflect positional uncertainty in the periphery, rather than a deficit in phase sensitivity per se.

Differential Threshold↗

PubMed1989

The conditions under which Mach bands are visible.

In this paper we challenge the classical explanation of Mach bands--that they result from lateral inhibitory mechanism operating in the visual system--and present an alternative explanation based on a recent local energy model of feature detection (Morrone & Burr, 1988). A series of experiments was conducted to establish the range of parameters under which Mach bands appear on periodic waveforms of the trapezoid family. The model predicts successfully the conditions under which Mach bands appear, and the contrast necessary to see them. Other models, including those based on lateral inhibition and band-pass filtering fail to do so.

Contrast Sensitivity↗

PubMed1988

Feature detection in human vision: a phase-dependent energy model.

This paper presents a simple and biologically plausible model of how mammalian visual systems could detect and identify features in an image. We suggest that the points in a waveform that have unique perceptual significance as 'lines' and 'edges' are the points where the Fourier components of the waveform come into phase with each other. At these points 'local energy' is maximal. Local energy is defined as the square root of the sum of the squared response of sets of matched filters, of identical amplitude spectrum but differing in phase spectrum by 90 degrees: one filter type has an even-symmetric line-spread function, the other an odd-symmetric line-spread function. For a line the main contribution to the local energy peak is in the output of the even-symmetric filters, whereas for edges it is in the output of the odd-symmetric filters. If both filter types respond at the peak of local energy, both edges and lines are seen, either simultaneously or alternating in time. The model was tested with a series of images, and shown to predict well the position of perceived features and the organization of the images.

Computer Simulation↗

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↗