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PubMed2006

Effect of connectivity and bistability on the visual potentials evoked by illusory figures.

The present study aimed at testing functional hypotheses regarding two brain potentials elicited by illusory figures. Accordingly, the N1 potential indexes mechanisms connecting the separate parts of the illusory form, whereas a subsequent negative potential indexes compensatory processes triggered by perceptual difficulty. Here, perceptual difficulty was induced by bistability; that is, by equating the probability of perceiving the illusory form to that of perceiving the independent separate parts. We compared the brain potentials evoked by a strongly connected illusory square, with almost no bistability, with those evoked by a weakly connected illusory square presenting strong bistability. Consistent with our hypotheses, the latter figure evoked the smallest N1 and a larger negative component peaking at 360 ms (N360). These results strengthen the link between N1 and connection and between negativity to perceptual difficulty and perceptual difficulty.

Adult↗
PubMed1991

Visual evoked potentials elicited by subjective contour figures.

In order to investigate the relationship between the appearance of illusory figures and the wave form of visual evoked potentials (VEPs), 8 different visual pattern stimuli were presented to 8 normal subjects. Four of the stimuli (experimental stimuli) produced subjective figures and contours (squares and discs). The 4 other stimuli (reference stimuli), although equal to the experimental stimuli in the amount of physical energy, did not produce the illusion of squares or discs. Electrodes were placed on the scalp at central and occipital locations. Three prominent peaks in the occipital record were observed in all subjects. An amplitude difference of VEP N180 (N2) between the subjective figures and the reference stimuli was found in the values for each subject. Enhancement of the VEP of the illusory figure stimuli was observed for a specific component (N2), whereas the amplitude values at the central components and the occipital P120 (P2) and P280 (P3) were almost the same as the reference values. The VEP (N2 component) amplitude enhancement at the occipital area for subjective figure stimuli suggests that illusory contour formation takes place at higher levels in the visual system. This was known from experiments using dichoptic presentation.

Adult↗
PubMed1990

Parietal P3 response as an indicator of stimulus categorization: increased P3 amplitude to categorically deviant target and nontarget stimuli.

Two experiments were performed in which we compared the effects of selected non-deviant versus categorically deviant stimuli on parietal P3 under a variety of conditions in which one, both, or neither stimulus was a target of an experimental task. Subjects were repeatedly presented with series of 8 numeric stimuli and 1 alphabetic (Deviant) stimulus. P3 amplitudes to target and nontarget Deviant stimuli were consistently and significantly larger than to other, non-deviant targets and nontargets, respectively. Nontarget Deviant stimuli evoked P3 amplitudes comparable to those evoked by low-probability non-deviant targets. The observed differences indicate that P3 amplitude is a sensitive indicator of perceived category differences, and that the effect of category deviance on parietal P3 amplitude is independent of task response classification (target or nontarget) and of response probability.

Adolescent↗
PubMed1995

Patterns of cerebral activation while mental images are rotated and changed in size.

Event-related brain potentials were recorded while subjects performed either a rotation or a size scaling transformation of a mental image. Images had to be rotated 0 degrees, 60 degrees, or 120 degrees or their size had to be enlarged by factors of 1:1, 1:3, or 1:5. Both tasks were accompanied by pronounced negative slow potentials, which extended over several seconds. The relative maximum of these shifts emerged at central to occipital leads. Over the occipital cortex, the negative potential had a similar amplitude level in all conditions and both tasks. However, at parietal and central areas, the negative slow wave changed in relation to the difficulty of the task. The amplitude increased with increasing rotation demands and if the scaling operation required an exact computation of the coordinates of the image. None of these effects could be attributed to an inverse change of P300.

Adult↗
PubMed1988

Amplitude variability of the transient visual evoked response.

The amplitude repeatability of pattern reversal visual evoked response using a 5.5 and 40' arc check was investigated in twelve subjects with each subject recording ten 64 sweep runs. The variability by visual inspection or by analysis of variance suggests that a single run will provide a representative amplitude for a 40' arc check whereas the 5.5' check will require more runs merged together before the amplitude is stable. The larger spread of error encountered with the 40' arc check may point to a visual component contributing to more amplitude variability. The possibility of making some assessment of the level of noise present during recording is discussed.

Adolescent↗
PubMedDate not supplied

Pattern-reversal VEP and cortical SEP latency prolongations in epilepsy.

Twenty ambulatory outpatients with generalized tonic-clonic seizures with primary generalized discharges and photoconvulsive response on electroencephalogram (EEG) and 11 ambulatory outpatients with partial complex seizures with or without secondary generalization were studied with pattern-reversal light-emitting diode (LED) stimulator visual evoked potential (VEPs) and short-latency median nerve cortical somatosensory evoked potentials (SEPs). The patients with primary generalized epilepsy had significantly prolonged latencies of VEP components P2 and N3 and SEP component P22. The patients with partial epilepsy had significantly prolonged latency of VEP component N3. It is concluded that both functional and structural factors may cause a slowing of central impulse conduction.

Adolescent↗
PubMed2005

Pattern-reversal visual evoked potentials in patients with newly diagnosed epilepsy.

PURPOSE: The possible occurrence of evoked potential (EP) abnormalities in patients with newly diagnosed epilepsy has been little investigated. The main purpose of the present study was to investigate possible changes in pattern-reversal visual evoked potential (P-VEP) responses in newly diagnosed epilepsy patients. METHODS: By using P-VEPs, latency values of the N75 and P100 together with amplitude values of P100 were recorded in newly diagnosed idiopathic epilepsy patients. The patients comprised two groups; nonphotosensitive (non-PS), and photosensitive (PS) patients. RESULTS: Shortened N75 and normal P100 latencies of the P-VEP with higher than normal P100 amplitudes were detected in PS patients. In non-PS patients, N75 latencies of the P-VEPs were unaffected; however, P100 latencies were prolonged, and P100 amplitudes were unchanged. CONCLUSIONS: P-VEPs are different from those of controls in previously untreated idiopathic epilepsy patients. Results also indicate different P-VEP features in patients with and without photoparoxysmal responses. The changes might be the result of a disorder of one or more neurotransmitters or subtle morphologic damage such as microdysgenesis.

Adolescent↗
PubMed1997

Pattern reversal visual evoked potentials in eyes with macular holes and their fellow eyes.

PURPOSE: To investigate whether the pattern reversal visual evoked potential can be useful in the diagnosis and management of macular hole patients. METHODS: The pattern reversal visual evoked potential was measured in 66 patients with a macular hole and in 43 healthy control subjects. Check sizes of 34', 17' and 10' were applied. RESULTS: Results showed that, for the check sizes of 34', 17' and 10', eyes with a macular hole had significantly prolonged N80 and P100 latencies and a significantly reduced P100 amplitude as compared to their fellow eyes. Furthermore, for the 10' check size, the fellow eyes appeared to have a significantly reduced P100 amplitude in comparison with the control eyes, whereas N80 and P100 latencies of the fellow eyes of the macular hole patients were not prolonged. CONCLUSION: Significant pattern reversal visual evoked potential alterations were shown in eyes with macular holes and fellow eyes for small check sizes.

Aged↗
PubMed1983

Influence of luminance gradient reversal on simple cells in feline striate cortex.

1. The sensitivity of simple cells to luminance gradient reversal in bar stimuli has been investigated in the striate cortex of lightly anaesthetized cats.2. The influence of segment(s) of one polarity of contrast, partially masking or added end-on to a bar of reversed contrast, was assessed against a stationary textured background (of intermediate average luminance), which was itself without influence on cell behaviour.3. In either configuration, short segments of reversed contrast were suppressive of bar response, to an extent varying with location along the receptive-field axis, but much greater than predictable from length-summation characteristics. Response suppression, even by very short segments, was often total.4. The effects of longer segments, added end-on to a bar of opposite contrast, depended on the extent of length summation exhibited by each cell. With progressive extension of these segments, some recovery of response occurred in a few simple cells with larger receptive fields, but not in small-field simple cells. The behaviour of end-stopped simple cells was comparable in all respects to that of their end-free counterparts, within the length-summation zone; thereafter, invasion of the inhibitory end-zones by bars of either polarity elicited a generalized decline in response.5. Where responsiveness was restored by adding longer segments of opposite polarity, the response peak shifted to the discharge centre appropriate to that polarity.6. All these results were found with either polarity of contrast, i.e. a light bar plus dark segments or the converse.7. Similar results were obtained for motion in either direction across the receptive field; and also for flash-presentation, over either the centre or flanks (whether ;on' or ;off') of the receptive field.8. The effectiveness of contrasting added segments was assessed as a function of location along the receptive-field axis for flash-presented and for moving stimuli. Suppressive effects were greatest over the centre of the receptive field, declining progressively in either direction along its axis, but over greater distances than anticipated from length summation.9. None of the results presented could have been predicted from length-summation characteristics. They are strongly suggestive of gating, rather than linear, antagonistic interactions.

Animals↗
PubMed1983

Pattern-evoked responses and luminance-evoked responses in the human electroretinogram.

Electrical potentials were recorded from the cornea of the human eye in response to the onset and offset of square-wave stripe patterns varying in spatial frequency, luminance and contrast. Under high retinal illumination (50119 photopic td) the response to pattern onset was a positive wave (pattern-onset wave) followed by a negative after-potential. As long as the pattern was presented, a steady potential of positive polarity (plateau potential) was observed. The response to pattern offset was a biphasic negative-positive wave followed by a negative after-potential. A comparison between wave forms obtained with pattern onset-offset stimuli and luminance increase-decrease stimuli suggests: (a) the pattern-onset wave is the result of an interaction between a luminance-increase and a luminance-decrease response; (b) the plateau potential is mainly an additive superposition of two receptor processes originating from retinal areas that receive increases and decreases in local luminance; (c) the negative-positive potential is an a-wave and a b-wave originating from retinal areas that receive increases in local luminance. The amplitude of the pattern-onset wave was greatest at a spatial frequency around 3-4 c/deg. This behaviour was closely correlated with contrast sensitivity determined psychophysically by previous investigators. Therefore, the pattern-onset wave seems to be a pattern-evoked response. The amplitude of the b-wave following pattern offset showed a monotonic decrease with increasing spatial frequency. It is mainly a luminance-evoked response. Under low retinal illumination (457 photopic td) the pattern-onset wave and the b-wave at pattern offset were smaller. However, the pattern-onset wave had its maximum amplitude at a lower spatial frequency. When increases or decreases in space-average luminance were combined with pattern onset or offset, transitions between pattern- and luminance-evoked responses could be observed. The results suggest that a decrease in local luminance is the essential stimulus for evoking a pattern-related response while increases in local light intensity generate luminance-evoked responses.

Cornea↗
PubMed1983

Electroretinograms evoked in man by local uniform or patterned stimulation.

1. We have recorded electroretinograms (e.r.g.s) in normal subjects. Television monitors were used as stimulators. The screens were surrounded by brightly lit white reflecting surfaces to ensure that the responses were developed by defined retinal areas.2. Various types of stimuli were employed. Either (i) a pattern of dark and bright squares was reversed, to evoke a pattern e.r.g. (p.e.r.g.), (ii) the luminance of the uniform screen was abruptly increased and decreased to evoke a focal on-off e.r.g. or (iii) a pattern was made to appear and disappear from a uniform background. In each of these cases, the sequence of changes of luminance at any one point could be made identical. The aim of the experiments was to determine whether the e.r.g. was modified by the spatial organization of the stimulus.3. In other experiments a colour monitor was used so that (i) a red-green flicker, (ii) red-green pattern reversal or (iii) the appearance of a red-green pattern from a yellow background could be used as a stimulus. The responses were caused by the changes in hue, since all the colours were equiluminant.4. With black and white patterns the p.e.r.g. peaks 5 msec later than the focal on-off e.r.g. The largest response is produced by squares of 0.5-1 degrees subtense.5. The ratio of the amplitudes of the p.e.r.g. to the focal on-off response is largest for stimuli confined to the macula and smallest for those projected onto peripheral retina.6. The amplitude of responses to chequerboard reversing patterns increases nearly linearly with contrast up to the maximum contrast available.7. When patterns appear or disappear from a uniform screen, and there is an associated change in the quantity of light entering the eye, recognizable b-waves occur when the average screen luminance increases, independently of whether pattern contrast increases (appearance) or decreases (disappearance).8. When a pattern appears or disappears with no change in luminance, e.r.g.s are evoked at both ;on' and ;off'. The disappearance of the dark parts of the pattern causes the largest logarithmic increase in local retinal illumination. For patterns of square size > 4 degrees the pattern disappearance response is larger than for pattern appearance. As the square size is reduced, the appearance response grows and the disappearance response decreases. The e.r.g.s evoked by the appropriate changes in luminance of a uniform screen are no longer the same as those caused by the appearance and disappearance of the pattern.9. The responses to change of hue are 70% as large as those produced by black and white patterns. The same ratio occurs for pattern and focal on-off e.r.g.s.10. When coloured patterns appear from and disappear to a uniform field, the e.r.g.s. evoked are very similar to those recorded when the appropriate changes of hue occur in a uniform field. This result is quite different to the findings for black and white patterns (see 8 above).11. The results suggest that it is the change in local adaptation caused by the black and white patterns which modifies the e.r.g. and not the presence of contrasting borders.

Color Perception↗
PubMed1985

Spatial-frequency characteristics of neurones of area 18 in the cat: dependence on the velocity of the visual stimulus.

The spatial and temporal response properties of neurones of areas 17 and 18 were studied in single units (165) of anaesthetized and paralysed cats. The visual stimuli were drifting or alternating gratings. We confirmed and extended the observation by Tolhurst & Movshon (1975) showing that the spatial-frequency characteristics of neurones of area 17 are largely independent of the temporal parameters of drifting or alternating gratings. The spatial-frequency tuning curves of neurones of area 18 shift along the spatial-frequency axis when the velocity or the temporal frequency of the drifting grating are changed. The effect of an increase either of velocity or temporal frequency is to shift the cell spatial-frequency tuning curve down the spatial-frequency scale, keeping relatively constant the strength and band width of the response. The spatial-frequency tuning curves of neurones of area 18 do not show this temporal-frequency-dependent phenomenon when the stimuli are gratings alternated in phase. In this case neurones of areas 17 and 18 show a similar behaviour. The response properties of neurones of area 18 are compared with recent psychophysical results obtained in similar experimental conditions. The hypothesis is advanced that both areas 17 and 18 are devoted to the processing of spatial information. Area 17 would be responsible for the processing of patterns in stationary or quasi-stationary situations while area 18 would be responsible for that of patterns moving at high velocities.

Animals↗