ORCID Profile
0000-0002-1592-3532
Current Organisation
University of Miami
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Publisher: Elsevier BV
Date: 05-1994
DOI: 10.1016/0167-8760(89)90045-7
Abstract: Steady-state pattern-reversal electroretinograms (PERG) were recorded from both monkeys and humans in response to tartan patterns modulated in both space and time in either luminance contrast or chromatic contrast. In both species, all types of patterns cause a strong modulation of the second-harmonic of the PERG. There was no measurable dependency of the PERG on the colour of the stimulus per se: both in humans and monkeys, stimuli with green-black, red-black or yellow-black modulation of the same mean luminance and of the same contrast, produced identical results. However, chromatic stimuli with modulation between equiluminant red and green produced a qualitatively different PERG: the litude was lower, particularly at high temporal frequencies, and there was a clear phase lag corresponding to a difference in processing time of about 20 ms.
Publisher: Elsevier BV
Date: 11-1996
DOI: 10.1016/0042-6989(96)00032-6
Abstract: 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.
Publisher: Cambridge University Press (CUP)
Date: 09-1994
DOI: 10.1017/S0952523800003837
Abstract: We have recorded steady-state PERGs from five macaque monkeys in response to red-green plaid patterns reversed sinusoidally in contrast. The patterns had either a pure luminance contrast (red-black, green-black, yellow-black), pure red-green color contrast, or a variable amount of luminance and color contrast. By varying the relative luminance of the red-to-total luminance (color ratio) of red-green patterns, a value could be obtained at which the PERG litude was either minimum or locally maximum, and the phase was most lagged. This value was very similar to that producing equiluminance in human observers, and was considered to be equiluminance for the monkey. The phase of the PERG to chromatic stimulus was systematically lagged compared with that to luminance stimuli, by an amount corresponding to about 10–20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 80 ms for color contrast compared with 63 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. As a function of temporal frequency, the chromatic PERG function was clearly low-pass with a cutoff around 15 Hz, whereas that to luminance was double-peaked and extended to higher temporal frequencies, around 30 Hz. For both luminance and chromatic stimuli, the litude of PERGs increases with increasing stimulus contrast. By summing vectorially the luminance and chromatic responses of appropriate contrasts, we were able to predict with accuracy the response as a function of color ratio. In two monkeys, the optic chiasm was sectioned sagittally causing total degeneration of ganglion cells in the nasal retina, without affecting the temporal retina (verified by histology). In these animals, there was a strong response to both luminance and chromatic patterns in the temporal retinae, but none to either type of pattern in the nasal retinae, suggesting that the PERG to both luminance and chromatic stimuli arises from the inner-retinal layers. Electrophysiological studies suggest that the PERG to chromatic stimuli is probably associated with the activity of P-cells. P-cells may also make a major contribution to the PERG of luminance stimuli, although M-cells may also participate. The above findings on normal monkeys all agree with those reported in the accompanying paper for humans (Morrone et al., 1994), so similar conclusions can probably be extended to human PERG.
Publisher: Cambridge University Press (CUP)
Date: 09-1994
DOI: 10.1017/S0952523800003825
Abstract: We have studied the steady-state PERG in human subjects in response to red-green plaid patterns modulated either in luminance or in chromaticity or both. By varying the relative luminance of the red and green components, a value could be obtained at which the PERG litude was either minimum or locally maximum. This always occurred at equiluminance, as measured by standard psychophysical techniques. PERG litude and phase were measured as a function of spatial and temporal frequency of sinusoidal contrast reversal. In both space and time, the response to chromatic patterns was low-pass, while that to luminance was band-pass, and extended to higher spatial and temporal frequencies. The phase of the PERG to chromatic stimuli was systematically lagged compared with that to luminance stimuli, by an amount corresponding to about 20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 67 ms for color contrast compared with 47 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. For both luminance and chromatic stimuli, the litude of PERGs increases with increasing stimulus contrast. By summing vectorially the responses to appropriate luminance and chromatic contrasts, we were able to predict with accuracy the response as a function of color ratio (ratio of red to total luminance). The above findings all agree with those reported in the accompanying paper for the monkey PERG (Morrone et al., 1994), and indicate that the differences in response latency and integration time of luminance and chromatic stimuli observed by psychophysical and VEP techniques may arise at least in part from the properties of retinal mechanisms.
Publisher: Elsevier BV
Date: 07-1992
DOI: 10.1016/0042-6989(92)90214-4
Abstract: 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 litude (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 litude. The effects of aging on the VEP were more dramatic and depended on the spatial and temporal properties of the stimulus. VEP litudes (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 litude. 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 litude 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.
Publisher: Elsevier BV
Date: 06-1999
DOI: 10.1016/S0042-6989(98)00288-0
Abstract: We have measured reaction time (RT) to motion onset in two groups of subjects (average ages: 70 and 29 years), for horizontal gratings of 1 c deg-1, modulated in either luminance or colour (equiluminant red-green), for various contrasts and speeds. For both old and young subjects, RTs depended on both speed and contrast, being faster at high speeds and high contrasts, and showed a stronger contrast dependency for chromatic gratings. The older subjects were systematically slower than the younger subjects. The difference between old and young RTs varied with condition, being 30-40 ms more at the slow than at the fast speed. The relative difference in RTs in different stimulus conditions shows that at least some of the increase in response time with age has a sensory origin. The results relate well to previous work on visual evoked potentials.
No related grants have been discovered for Vittorio Porciatti.