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PubMed1991

A programmable digital video pattern generator controlled by microprocessor for basic research and clinical applications.

A programmable digital video pattern generator controlled by a microprocessor is described as a part of any commercial or specific visual evoked potential analysis system. The generator handles all kinds of patterns (uniform, orthogonal and radial) with all known presentation modes (onset, offset and reversal) and resolution 640 x 200 square pixels. The frame rate of the generator is adjusted to be compatible to either low cost 50 Hz video monitor or expensive ones of higher frame rate. The operation parameters defined by software are: pattern selection, trigger mode, field selection and stimulation rate. The development of software applications for special research demands is desirable because of the control of the generator by a microprocessor.

Computer Graphics↗
PubMed1992

Texture segregation is processed by primary visual cortex in man and monkey. Evidence from VEP experiments.

We investigated whether the process of texture segregation can be allocated to a specific visual cortical area. We designed a stimulus to reveal the presence of a mechanism, which is specifically sensitive to a checkerboard, that is solely defined by textures segregating due to orientation differences of the constituting line segments. We recorded evoked potentials to this stimulus in man and awake monkey. A difference component, signalling texture segregation sensitivity, could be recorded from both types of subjects. Its presence depended on the spatial extent of the textures, in a manner correlating with the perceptibility of the checkerboard. This difference response could be localized in primary visual cortex by means of equivalent dipole estimations.

Animals↗
PubMed1992

Orientation selectivity of 3-month-old infants.

A modification of the visual evoked potential (VEP) technique, first employed by Braddick, Wattam-Bell and Atkinson [(1986) Nature, London, 320, 617-619] was used to estimate the orientation selectivity of 3-month-old infants. The orientation-selective VEP was recorded in response to various changes in the orientation of a square-wave grating. The magnitude (the square root of the power of the FFT) at the frequency of orientation change was assumed to represent the response to the change in grating orientation. Orientation sensitivity was then estimated by plotting the magnitude of the FFT at the frequency of orientation change as a function of the log of the orientation change in degrees. For each individual, the data were fit by nonlinear regression and threshold was defined as the largest orientation angle for which the magnitude of the FFT was zero. The results suggest that the orientation selectivity of 3-month-old infants (1.33 deg) is similar to that of adults (1.13 deg) tested with the same stimulus parameters (1 c/deg, 9 Hz). However, when adults are tested with stimulus parameters selected to optimize their VEP response (4 c/deg, 18 Hz) instead of those which optimize the infant's response, the orientation discrimination of adults improves by a factor of 2 (0.53 deg). The results obtained from adults under optimum stimulus conditions (4 c/deg, 18 Hz) approach the estimates reported in the literature for static stimuli.

Adult↗
PubMed1992

Cortical neurons: isolation of contrast gain control.

The selectivity of cortical neurons remains invariant with contrast, even though the contrast-response function saturates. Both the invariance and the saturation might be due to a contrast-gain control mechanism. To test this hypothesis, a drifting grafting was used to measure the contrast-response function, while a counterphase grating was simultaneously presented at the null position of the receptive field (where it evokes no response at any contrast). When the contrast of the counterphase grating increased, the contrast-response function shifted primarily to the right. This result is consistent with the hypothesis that there is a fast-acting gain-control mechanism which effectively scales the input contrast by the average local contrast.

Adaptation, Ocular↗
PubMed1992

Electroretinograms (ERGs) and visual-evoked potentials (VEPs) elicited by pattern displacement.

The relation between the amplitude of visual responses to a checkerboard stimulus and the degree of lateral displacement of the checks was examined across different check sizes with simultaneously recorded electroretinograms (ERGs) and visual-evoked potentials (VEPs). The amplitudes of both the b-wave and the after-potential of the ERG increase linearly with pattern displacement. However, the major components of the VEP (N70 and P100) were smaller than expected from linearity for both small checks with small displacements (thresholding) and for large checks with large displacements (saturation). These results suggest that the ERG is proportional to the number of receptors stimulated, but the VEP reflects neural processes influenced by the spatial structure of the stimulus.

Adult↗
PubMed1992

Cortical potentials evoked by short wavelength patterned light.

Pattern-evoked cortical potentials (PECPs) were recorded in response to the onset of blue and black, square wave gratings superimposed on a bright orange background. Several lines of evidence, including spectral sensitivity and the absence of response in a tritanopic subject, indicate that the measured cortical potentials reflect input from S cones. Spatial and temporal tuning of this response are comparable to psychophysical measures of the S-pathway. In agreement with previous studies of achromatic and chromatic spatial processing, a comparison of the response to patterns of different complexity indicates that spatial processing in the S-pathway can be described by linear systems analysis. An oblique effect for the S-pathway is demonstrated. Additional findings which bear on the nature of postreceptoral processing in the S-pathway are considered. Included are measurements from a patient with diabetic retinopathy. This study underscores the potential importance of the S-pathway for spatial information processing.

Adult↗
PubMed1992

Visual evoked electrical and magnetic response to half-field stimulation using pattern reversal stimulation.

The visual evoked magnetic response to half-field stimulation using pattern reversal was studied using a d.c. SQUID coupled to a second order gradiometer. The main component of the magnetic response consisted of a positive wave at around 100 ms (P100M). At the time this component was present the response to half-field stimulation consisted of an outgoing magnetic field contralateral and extending to the midline. When the left half field was stimulated the outgoing field was over the posterior right visual cortex and when the right half field was stimulated it was over the left anterior visual cortex. These findings would correctly identify a source located in the contralateral visual cortex. The orientation of the dipoles was not that previously assumed to explain the paradoxical lateralization of the visual evoked potential. The results are discussed in terms of both electrical and magnetic models of the calcarine fissure.

Adult↗
PubMed1992

Colour, contrast and the visual evoked potential.

Visual evoked potentials exhibit interesting morphological changes when they are elicited by checkerboards of different spatial and chromatic contrast, counterphasing in the foveal and lower macula field. The characteristic, positive wave of the phase-reversal visual evoked potential, for example, is preceded by an increasingly prominent negative peak as luminance contrast progressively increases above 10% and, at isoluminance, the response to red and green checkerboards becomes a predominantly monophasic negative wave. To study the nature of the morphological change we synthesized these waveforms with a computer simulation consisting of Gaussian components. The amplitudes of positive and negative components were altered until the synthesized response was closely similar to the recorded data. These Gaussian components have response characteristics which are identified with those of magnocellular and parvocellular neurones.

Color Perception↗
PubMedDate not supplied

VEP acuity, FPL acuity, and visual behavior of visually impaired children.

Forced-choice preferential-looking (FPL) and pattern-visual-evoked potential (VEP) acuity tests were conducted with 42 children with bilateral moderate to severe visual impairment (age, 4 months to 9 years). Within this group of children, FPL acuity testing was more successful than VEP acuity testing (98% versus 64%). Mean FPL acuity was better than mean VEP acuity (20/155 vs 20/290, P less than .002). For the children who completed both FPL and VEP acuity tests, an acuity difference score (FPL minus VEP) was derived, and compared to difference scores based on normative data. Significant differences were found between visually impaired (VI) and control difference scores (P less than .001). Difference scores of VI were larger than and opposite in sign to the control scores. Rating scores of visual behaviors supported the concurrent validity of the FPL acuities (P = .01), but not the VEP acuities.

Child↗
PubMed1975

Stimulus complexity, EEG abundance gradients, and detection efficiency in a visual recognition task.

Occipital EEG was monitored while subjects inspected 27 projected patterns. The number (N) and variety (V) of elements in the patterns were varied systematically. There were three levels of N (6, 12 or 24 elements) and three levels of V (circles, squares or hexagons occupying all, one half or one third of the element locations for all levels of N). Subjects were instructed that they would be required in a post-test to recognize the patterns, among patterns which had not appeared; they were also informed that the patterns had been constructed according to a set of simple rules, but the nature of these rules was not made fully explicit. The EEG was quantified by means of low-frequency analysis, yielding measures of abundance (theta, alpha and beta) and mean dominant frequency. For the recognition task, nine stimulus items were embedded among 45 items. Recognition efficiency was measured by means of the signal detection theory discrimination index (d'). The results were as follows: (i) Both N and V were inversely related to alpha abundance (P less than 0-01); (ii) the strongest relationship between stimulus parameters and the EEG held for N and EEG beta activity (13-5-19-5 Hz;P less than 0-001), where again the EEG and N were inversely related; (iii) there was a significant (P less than 0-05) direct relationship between N and theta activity; (iv) contrary to prediction, mean dominant alpha frequency decreased as N increased; (v) d' correlated significantly with a number of effects for N, i.e. subjects who exhibited greatest EEG discriminability of items during exposure of the patterns, subsequently obtained the higher detection scores in the recognition task. The work described therefore demonstrates that only only do stimulus parameters have systematic effects upon brain activity as measured by the EEG, but that such effects have functional value and reflect aspects of efficiency. The results are fully compatible with arousal theory constructs relating physiological reactivity and performance.

Adolescent↗
PubMed1979

Recognition of changes in the dimensions and categories of visual objects.

The aim of the present work is a comparison between recognition with changes in the dimensions of the objects and recognition with changes in the category of the objects. After preliminary training under tachistoscopic ocnditions, to a control set of different contour drawings are added: (1) the same drawings increased or desreased several times; (2) different new drawings with the same size. The percentage of recognized drawings is determined for such exposure time which is needed for the recognition of 60--80 per cent of the drawings in the control set. Recognition is found to be deteriorated in the case of changes both in the dimensions and in the category of the objects, the deterioration being double for the objects with changed category. This fact serves as a basis for rejecting the hypothesis that considerable changes in the dimensions could create new objects for the visual system. It was also found that recognition of pre-trained drawings is either not influenced or comparatively least influenced by the changes in the dimensions and categories. This result is probably due to the specificity of recognition of long trained objects. The results obtained do not contradict the assumption of the participation of spatial consecutive scanning of the iconic memory in the recognition of the spatial properties of visual objects.

Form Perception↗
PubMed1990

The human motion VEP as a function of size and eccentricity of the stimulation field.

A 'motion onset VEP' was elicited by the onset of a pattern drift. The amplitude of the most distinct wave (AN200) was determined on the following stimulation conditions: eccentricity, 0 to 23 deg; velocity. 1.5 to 16 deg/s; spatial frequency, 0.19 to 2.1 c/deg; and stimulation field size, 0.2 to 160 deg2, AN200 remained constant at any degree of eccentricity if stimulation field size, velocity, and spatial frequency were M-scaled according to Rovamo-Virsu's M-equations. AN200 decreased as a function of eccentricity if field size and velocity were kept constant (spatial frequency had minimal effect). The size of the cortical representation field (Sc) in this case varied with change in eccentricity (stimulation field size constant). In another experiment, it varied by change in stimulation field size (eccentricity constant). For both conditions, AN200 was proportional to log Sc.

Electrodes↗
PubMed1992

Detection duration thresholds and evoked potential measures of stereosensitivity.

Visual evoked potentials have been proposed by some researchers to be more useful than behavioral techniques to evaluate stereo performance in children and certain clinical populations. Stimulus duration detection thresholds, visual evoked potentials, and scalp electrical potential distribution maps to dynamic random dot stereograms were studied. A high degree of correspondence was found between visual evoked potential amplitudes and behaviorally determined detection thresholds. Upper field stimuli had higher detection thresholds and generated lower-amplitude visual evoked potential responses than did centrally presented stimuli. For the most eccentrically presented stimuli, lower detection thresholds were found for stimuli presented in the right visual field than the left visual field. This finding was consistent with the pattern of VEP responses to be lateralized, with higher-amplitude responses recorded over left-hemisphere sites. The study examined a proposal that the major negative component of the stereoscopic visual evoked potential originates in cortical area V1. The results failed to support the proposal and were consistent with the main negative component of the VEP being generated in V2, rather than V1.

Adult↗
PubMed1992

The effect of spatial frequency and contrast on the latency in the visual evoked potential.

The latency in the visual evoked potential was measured at spatial frequencies of 2-12 c/deg in 10 subjects. The contrast levels of the sinuosoidal grating patterns were set at 1.5, 1.75, 2.0, 2.25, 2.5, 2.75 and 3.0 log units above each subject's contrast sensitivity threshold. Two factors were shown to influence the latency: suprathreshold contrast and, to a lesser extent, spatial frequency. The visual evoked potential latencies at contrast sensitivity threshold were extrapolated. These threshold latencies showed considerable variation with spatial frequency and between subjects. Therefore, the visual evoked potential latency cannot be considered a useful tool for estimating the contrast sensitivity function.

Adult↗
PubMed1992

Visual evoked cortical potentials from transient dark and bright stimuli. Selective 'on' and 'off-pathway' testing?

The superior hemifields of five normal left eyes were stimulated by novel equal and opposite contrast pattern onset stimuli which were generated on a cathode ray tube. Patterns consisted of 144 discs, each subtending 60 min arc at the viewing distance of 40 cm and were separated by a distance equal to their diameter. Equal and opposite light changes were created by presenting the disc patterns with different luminance values on a uniform constant background (20 cd/m2). Transient visual evoked cortical potentials to the appearance and disappearance of the patterns were recorded separately and analysed. Significant amplitude differences between the responses to bright and dark stimuli were observed with light increment responses being 36-53% larger than the light decrement responses for pattern on-set and 54-80% larger for pattern off-set respectively. This finding is attributed to the difference in the input to the ON and OFF Parallel Pathways which are known to carry light increment and decrement information respectively, as well as differences in the metabolic and discharge rates of these pathways.

Adult↗
PubMed1991

Saccade onset and offset lambda waves: relation to pattern movement visually evoked potentials.

The lambda (lambda) wave is an occipital EEG potential which occurs when saccadic eye movements are made against an illuminated contrast background. There is some disagreement concerning the presence of sub-components to the lambda-wave, and its relationship to visually evoked potentials. In the present study, lambda-waves were recorded with saccades of different durations (30-110 ms) and compared to VEPs associated with pattern movements of similar durations and velocity. It was found that the lambda-wave consisted of a saccade onset component with positive sub-components at 59 and 100 ms after saccade onset, and a saccade offset component with a positive potential at 74 ms after saccade offset. With small saccades of 30 ms duration or less, these components superimposed to form a single lambda-wave. In the case of pattern movement VEPs, a movement onset component of latency 110 ms following movement onset, and a movement offset component at 89 ms after movement offset, were identified. The similar behaviour of the lambda-wave and VEP under these conditions supports the view that the lambda-wave is a visually evoked potential resulting from movement of the visual field across the retina during a saccadic eye movement.

Adult↗