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PubMed1986

Evoked potentials elicited by brief vernier offsets: estimating vernier thresholds and properties of the neural substrate.

The characteristics of the neural generator producing an evoked potential in response to the brief presentation of a vernier offset was investigated in three experiments. In the first study, averaged evoked potentials (EPs) recorded in response to a single vernier offset stimulus consisting of a horizontal line which changed from colinearity to noncolinearity for 100 msec every 1.5 sec were compared to responses elicited by other vernier configurations consisting of: stimuli with multiple offsets; stimuli presented in different orientations; targets with different offset features; and with the simple displacement of a colinear line. The results showed that a single vernier offset elicited a robust response if the offset was located in the central zone (1 degree) of the target. Other features of the target configuration were unimportant. Displacement of a colinear line over the same range without an offset evoked little, if any, response. In the second study, EPs were recorded in response to a single offset target which varied in magnitude from 21 to 82 sec of visual angle on different trials. The latency and amplitude of the EP response varied systematically with the amplitude of the vernier offset. Plots of EP amplitude against log of the offset magnitude were linear over the range of offsets employed. Straight lines fitted to the data and extrapolated to zero amplitude provided estimates of vernier threshold. These estimates agreed closely with psychophysical measures taken with the same targets and confirm the initial observations by Levi et al. (1983). In the third experiment, irrelevant contours were added to the vernier target in various spatial and temporal configurations. The addition of stationary, contiguous contours to the vernier target reduced the amplitude of the EP response when the contours were within 4-8 min of the offset, producing progressively less EP attenuation with increasing distance from the offset. However, brief presentation of the irrelevant contour (e.g. a single line passing through the offset) with an onset asynchrony relative to the vernier offset stimulus appropriate to assure simultaneity of the line-elicited EP and the offset-elicited EP yielded an enhanced response, i.e. to two responses added algebraically. The long latency of the offset evoked response and the summation results of the EPs generated by an offset and by a briefly presented linear contour suggests independence of the neural generators producing the response to these two targets.

Adult↗

PubMed1987

Contrast sensitivity and visual acuity of the pigmented rat determined electrophysiologically.

The contrast sensitivity function of the rat was assessed by investigating the relationship between the amplitude of visually evoked cortical potentials (VECP) and the spatial frequency and contrast of grating stimuli. Pattern reversal VECPs were recorded in Area 17 in the region of representation of the central binocular visual field. Transient responses were obtained with a 1 Hz contrast reversal. The mean contralateral monocular CSF peaked around 0.1 c/deg, with a threshold sensitivity of 20-25, low frequency attenuation and a high frequency cut-off of 1.18 c/deg. The mean binocular CSF showed a cut-off of 1.20 c/deg, which matches several behavioural measurements of visual acuity. The greater binocular sensitivity in the low frequency range (0.04 c/deg) could be tentatively attributed to the greater influence of the population of large ganglion cells that reaches its maximal density in the ipsilateral projection.

Animals↗

PubMed1988

Infant grating acuity is temporally tuned.

Studies of infant visual development have shown that acuity estimated with pattern visually evoked potential (VEP) techniques is higher than acuity estimated with preferential looking (PL) techniques. A major difference is that VEP stimuli are temporally modulated while PL stimuli are typically stationary. We measured PL acuity in 2-10-month-old infants for stationary gratings and for gratings phase alternating at 2.5, 7.5, 14 and 23 reversals/sec using a computer generated staircase method. The acuity functions were temporally tuned at 7.5 or 14 rev/sec for infants 3 months and older. Acuity for 7.5 and 14 rev/sec gratings was 0.5 to 1.0 octave higher than for stationary, 2.5 and 23 rev/sec gratings. When adults' grating acuity was measured foveally and 5 deg eccentrically, tuning occurred only for the eccentric targets, suggesting that the retinal area used by the infants to detect gratings acts like the adult perifovea. In a second experiment, VEP and PL acuity were both measured from the same infants using 14 reversals/sec gratings. The VEP/PL acuity difference was less for phase alternating gratings than for stationary gratings. The magnitude of the difference was age dependent, decreasing from 2 octaves at 2 months to 0.5 octave at 12 months. Even though the use of phase alternating gratings results in improved PL acuity, temporal modulation does not completely account for the difference between VEP and PL acuity.

Adult↗

PubMed1990

Localization of visually evoked cortical activity using magnetic resonance imaging and computerized tomography.

Evoked scalp potentials, computerized tomography, and magnetic resonance imaging were used together to localize cortical activity evoked by visual stimuli in humans. The temporal resolution of evoked potential measurements is sufficient to track the flow of cortical activity which evolves in epochs of a few tens of msec. Spatial localization was enhanced by deconvolving scalp potential fields with a Laplacian operator. Markers glue to the scalp served to unify the three geometric reference frames into a single computer graphics database.

Brain Mapping↗

PubMed1991

Development of motion-specific cortical responses in infancy.

The development of visual motion mechanisms has been studied in infants with a visual evoked potential (VEP) technique which isolates responses from directionally-selective mechanisms. In adults, the amplitude of this directional VEP increased with velocity up to a maximum at 15-20 deg/sec, and then declined with further increases in velocity. In a group of infants tested longitudinally, directional responses were first found at a median age of 74 days with a stimulus velocity of 5 deg/sec, and 90 days with a velocity of 20 deg/sec; this age difference was statistically significant. Initially, VEP amplitudes were significantly greater at 5 deg/sec than at 20 deg/sec. By the end of the longitudinal study, there was no significant difference in amplitudes at the two velocities. In a second group of infants, simultaneous recording of VEPs and electrooculograms indicated that eye movements tracking the stimulus were not a significant factor in the development of the directional VEP. It is concluded that the development of directional selectivity starts at low velocities, and extends to higher velocities with age.

Adult↗

PubMed1993

Development of infant contrast sensitivity to chromatic stimuli.

We have monitored the development of contrast sensitivity to equiluminant red-green chromatic patterns by monitoring visual evoked potentials (VEPs) in 13 infants. The results confirm our previous report [Morrone, Burr and Fiorentini, Proceedings of the Royal Society B, 242 (1990a)] that, before 7-8 weeks of age, there was no response to purely chromatic stimuli, while at the same age luminance stimuli of 20% contrast produced reliable responses. At all ages (even before the onset of a chromatic response) the colour mixture to yield equiluminance was similar to that of adults, suggesting that the relative proportion and efficacy of medium- and long-wave cones is similar for infants as for adults. For both luminance and chromatic stimuli, amplitude increased roughly linearly with log-contrast, so sensitivity thresholds could be predicted by linear extrapolation to the abscissa. Detailed contrast sensitivity curves were measured for four infants at various ages. The results show that luminance and chromatic contrast sensitivity develop independently at different rates, probably reflecting differential development of postreceptoral neural mechanisms.

Age Factors↗

PubMed1994

Human brain responses to different image contrasts.

Human brain activity was evoked by a dynamic random-dot display in which a square-wave grating appeared and disappeared at regular intervals. Grating visibility was determined by one of four different contrasts: texture, stereo disparity, luminance, or color. Scalp fields measured with 31 electrodes were used to estimate epicortical potential fields. The estimation procedure required detailed anatomical data for each subject. These were obtained from magnetic resonance images. A three-dimensional digitizer and a stereotactic headgear were used to accurately merge the frame of reference of the magnetic resonance image with that of the evoked potential. Epicortical potential fields provided a better indicator of where brain activity is evoked than did scalp fields. These procedures also corrected for anatomical variations between scalp and brain from subject to subject. In two right-handed female subjects, evoked activity was observed in the left posterior parietal and the right occipital, parieto-occipital and posterior temporal cortices. Evoked activity was observed in the left parietal cortex for luminance processing, in the right parietal cortex for texture processing and in the right temporal cortex for color processing, which was selective for the particular contrast.

Brain↗

PubMed1994

Organization of contour from motion processing in primate visual cortex.

A major objective of visual processing is the segmentation of the scene into separate objects. Relative motion is one of the most salient segmentation cues. In man and monkey, we recorded visually evoked potentials to a stimulus, designed to signal the presence of relative motion processing. Relative motion specific response components were only elicited when human observers perceive contours from relative motion. Equivalent dipole source localization of the responses indicated the involvement of primary visual cortex in man. This was corroborated by intracortical recordings in awake monkey, where sources of the specific components are located within the supra- and infragranular layers of primary visual cortex. It is concluded that V1 does not merely provide an input stage to contour from motion processing, but that segmentation information, based on relative motion, is present at this early cortical level.

Animals↗

PubMed1994

Spatial-frequency-tuned channels in early infancy: VEP evidence.

Spatial frequency (SF) adaptation of 3-, 6-, and 12-week-old infants produced changes in steady-state VEP amplitude to SF sweeps that closely resemble post-adaptation changes reported previously in adults. Following SF adaptation, VEP amplitude was attenuated to SFs near the adapting SF, and enhanced at SFs removed from the adapting SF in all age groups. This is interpreted as evidence for bandpass SF-tuned channels with coinhibitory interactions. The data presented here, in combination with VEP estimates of infant acuity, imply the existence of multiple bandpass SF-tuned channels with inhibitory relationships by 3 weeks of age. We hypothesize that neonatal spatial frequency filters are qualitatively adultlike.

Adaptation, Ocular↗

PubMed1996

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.

Adult↗

PubMed1985

Neurons in the amygdala of the monkey with responses selective for faces.

To investigate the functions of the amygdala in visual information processing and in emotional and social responses, recordings were made from single neurons in the amygdala of the monkey. A population of neurons (40 of more than 1000 recorded in 4 monkeys) was investigated which responded primarily to faces. These neurons typically (1) responded to some human or monkey faces, which were presented to the monkey through a large aperture shutter so that response latencies could be measured, or were simply shown to the monkey, (2) responded to 2-dimensional representations of these faces, as well as to real 3-dimensional faces, (3) had no responses or only small (less than half maximum) responses to gratings, simple geometrical, other complex 3-D stimuli, or to arousing and aversive stimuli, (4) had response latencies of 110-200 ms, (5) were located in the basal accessory nucleus of the amygdala, (6) responded differently to different faces, as shown by measures of d', and could thus over a population of such neurons code information useful for making different responses to different individuals, (7) could in some cases (9/11 tested) respond to parts of faces, and (8) in a few cases (4/19 tested) responded more to a face which produced an emotional response. A comparison made in three monkeys of the responses of these neurons with the responses of 77 neurons with face-selective responses recorded in the cortex of the superior temporal sulcus (STS) showed that the amygdaloid neurons had longer response latencies (110-200 compared to 90-140 ms), and were in some respects more selective in their responses to different faces. It is suggested that the deficits in social and emotional behavior produced by amygdala lesions could be due in part to damage to a neuronal system specialized in utilizing information from faces so that appropriate social and emotional responses can be made to different individuals.

Amygdala↗

PubMed1994

Visual electrophysiology to achromatic and chromatic stimuli in premature and full term infants.

We have previously reported on the development of black/white pattern reversal VEPs in premature babies of more than 30 weeks post-menstrual age (PMA). Unlike the flash VEP, the pattern reversal VEP shows a similar morphology to that of the full term infant and the major positive component (P1) decreased in latency with increasing PMA. The N1 and N2 components were more likely to be present with increasing maturity. In our present study we are examining the development of the transient chromatic pattern VEP. In order to produce a purely chromatic stimulus it is necessary to remove luminance cues. Based on forced choice preferential looking we developed a method of determining the isoluminant point for infants. Preference was tested for a flickering sinusoidal red and green grating over the uniform field. As sensitivity for chromatic flicker is much poorer than for luminance flicker, sensitivity is expected to be least when the residual luminance variation in the stimulus is at a minimum. The red/green luminance ratio at which this occurs represents the isoluminant point. From this method we found the subjective isoluminant point for infants of 2-3 months of age to be very close to the objective measure of isoluminance. Using this information, pattern reversal VEPs to 20 chromatic red/green and achromatic checks were studied and it would appear that pattern reversal VEPs cannot be obtained to isoluminant stimuli before 7 weeks chronological age.

Aging↗

PubMed1991

Pattern reversal visual evoked potentials in fencers.

Visual evoked potentials were recorded from occipital and temporal leads in the two cerebral hemispheres of eight fencers and eight control subjects. The stimulus was a checkerboard subtending a small (1 degree) or large (30 degrees) visual field. Significant differences in P60, N75 and P100 latency and amplitude were found between the two subject groups, especially during the processing of the large visual field. In fencers and left-handers shorter latencies were found for the large visual field condition, whereas right-handers showed an opposite trend. The results give further evidence of special patterns of visual processing in athletes, like fencers, in agreement with the literature.

Adolescent↗

PubMed1984

Pattern-reversal visual evoked potentials in photosensitive epilepsy.

VEPs to checkerboard pattern-reversal were recorded from 18 epileptic patients who had EEG photoparoxysmal responses to stroboscopic light. Patients were grouped according to whether seizures were precipitated by environmental light stimuli, or television viewing. Longitudinal studies were conducted on 8 patients treated with valproic acid. We concluded the following: (1) Latency of the major positive peak (P2) of the pattern-reversal VEP was shorter among photosensitive patients than among normal controls. This was especially true of television-sensitive patients. (2) Valproic acid, when effective in controlling seizures, lengthened the P2 latency and decreased VEP amplitude. Studies of drug effects on VEPs may help to elucidate neurochemical mechanisms of the visual cortex. (3) Because of overlap of values with normals, VEP measurements are not at present very sensitive in the diagnosis of photosensitivity. However, longitudinal studies in individuals parallel clinical changes and may be useful as objective measures of improvement.

Adolescent↗

PubMedDate not supplied

Three-dimensional human pattern visual evoked potentials. I. Normal subjects.

Pattern visual evoked potentials (PVEPs) were obtained from 30 normal adult volunteers, recording from both a conventional horizontal occipital array and three orthogonal bipolar antipodal channels approximating the three dimensions of space. Central and eccentric fixation of 60' checks and central fixation of 30' checks under binocular and monocular viewing conditions was employed. The three antipodal wave forms were displayed as a single 3-D Lissajous trajectory which contained four apices, corresponding to P40 (apex A), N70 (apex B), P100 (apex C) and N125 (apex D). The 3-D evoked potentials depicted the dynamic nature of the human PVEP in terms of changes in the 3-D voltage-voltage-voltage plots of the recordings. The orientation of the A-B, B-C and C-D curvilinear segments reflected the stimulating condition (central fixation vs. right vs. left hemi-field stimulation) for all subjects with more accuracy than did the wave forms from the conventional array. Spherical statistical methods are described for quantifying and evaluating 3-D evoked potential recordings.

Adult↗

PubMedDate not supplied

Age-related changes in pattern visual evoked potentials: differential effects of luminance, contrast and check size.

We recorded visual evoked potentials (VEPs) to checkerboard pattern-reversal stimulation in 109 normal subjects (51 males and 59 females; aged 19-84 years) in order to study the aging effect on the multiple channels of the visual system in humans. Transient VEPs to 3 check sizes (15', 30' and 50') were obtained by monocular stimulation. Two test conditions were employed: (1) a high luminance (180 cd/m2) and a low luminance (11 cd/m2) both with a fixed contrast (90%), and (2) a high contrast (85%) and a low contrast (10%) both at a fixed luminance (57 cd/m2). The major features of our results included: (1) the presence of a curvilinear relationship between P100 latency and age for all conditions, while the P100 amplitude did not show any such aging effect, (2) the age-latency function was similar between the two luminance conditions, while it was different between the two contrast conditions, and (3) the differential age effect on the P100 latency caused by changes in contrast depended on the check size. These results suggest that age-related changes in the human visual system are not uniform, but rather are different in the specific functional subdivisions. It is thus hypothesized that aging may differentially influence the separate channels of the human visual system.

Adult↗

PubMed1983

Abnormalities of visual evoked potentials by checkerboards in children with specific reading disability.

Visual evoked potentials by checkerboards of varying check sizes were recorded in the two hemispheres of 16 specific reading disabled and 8 normal children. In most of the disabled subjects a gross hemisphere asymmetry was assessed, while in the control group the usual evoked potential symmetry was observed. In some disabled subjects the evoked potentials had a larger amplitude in the right hemisphere, while in others the amplitude was larger in the left hemisphere. In a small subgroup the evoked potentials were symmetrical, but they had a smaller amplitude than in the control subjects. The results, giving evidence of a dysfunction in basic visual processing, are discussed in the context of current literature on clinical subgroups and the interhemispheric relationship in the dyslexic syndrome.

Child↗

PubMed1987

Visual search and spatial attention: ERPs in focussed and divided attention conditions.

ERPs and performance were measured in divided and focussed attention visual search tasks. In focussed attention tasks, to-be-attended and to-be-ignored letters were presented simultaneously. We varied display load, mapping conditions and display size. RT, P3b-latency and negativity in the ERP associated with controlled search all increased with display load. Each of these measures showed selectivity of controlled search, in that they decreased with focussing of attention. An occipital N230, on the other hand, was not sensitive to focussing of attention, but was primarily affected by display load. ERPs to both attended and unattended targets in focussed attention conditions showed and N2 compared to nontargets, suggesting that both automatic and controlled letter classifications are possible. These effects were not affected by display size. Consistent mapping resulted in shorter RT and P3b-latency in divided attention conditions, compared to varied mapping conditions, but had no effect in focussed attention conditions.

Adolescent↗