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At least 505 records · Page 28Linked to original sources
PubMed1990

Electrophysiological evidence for parallel and serial processing during visual search.

Event-related potentials were recorded from young adults during a visual search task in order to evaluate parallel and serial models of visual processing in the context of Treisman's feature integration theory. Parallel and serial search strategies were produced by the use of feature-present and feature-absent targets, respectively. In the feature-absent condition, the slopes of the functions relating reaction time and latency of the P3 component to set size were essentially identical, indicating that the longer reaction times observed for larger set sizes can be accounted for solely by changes in stimulus identification and classification time, rather than changes in post-perceptual processing stages. In addition, the amplitude of the P3 wave on target-present trials in this condition increased with set size and was greater when the preceding trial contained a target, whereas P3 activity was minimal on target-absent trials. These effects are consistent with the serial self-terminating search model and appear to contradict parallel processing accounts of attention-demanding visual search performance, at least for a subset of search paradigms. Differences in ERP scalp distributions further suggested that different physiological processes are utilized for the detection of feature presence and absence.

Adolescent↗
PubMed1999

Visual event-related potentials evoked by using a virtual reality display.

This study aimed at investigating whether a virtual reality display (VRD) is an appropriate tool for evoking visual event-related potentials (VEPs). VEPs evoked by VRD stimuli were highly similar in form to VEPs evoked by using a computer monitor, both having two dominant peaks, labeled P100 and N200. Monitor and VRD N200 latencies and amplitudes were highly correlated. However, peak latencies were longer and the peaks were broader when stimuli were presented on the VRD. Besides, VRD P100 amplitude was smaller, and an N75 peak could be seen usually only on monitor VEPs.

Adult↗
PubMedDate not supplied

Visual evoked potential abnormalities in dyslexic children.

Developmental reading disability (dyslexia) has traditionally been attributed to impaired linguistic skills. Recent psychophysical data suggest that dyslexia may be related to a visual perceptual deficit. A few visual evoked potential (VEP) studies have addressed this hypothesis, but their results are far from consistent. We submitted 9 dyslexic subjects and 9 age- and sex-matched normal controls to checkerboard pattern reversal VEPs. The main experimental variables were: large (0.5 cycles per degree; cpd) and small (2 cpd) checks and two reversal frequencies (2.1 Hz and 8 Hz); mean luminance and contrast (60 cd/m2 and 50%, respectively) were kept constant in all four conditions. Transient VEP (2.1 Hz) parameters did not differ between controls and dyslexics at 2 cpd. At 0.5 cpd, N70 amplitude was significantly smaller and N70 latency significantly shorter in dyslexics. Amplitudes for the fundamental frequency (8 Hz), as well as for the second and third harmonics of the steady-state VEPs were smaller in dyslexics for both stimulus sizes. A discriminant analysis correctly classified each subject. Our data confirm the hypothesis of a perceptual deficit in dyslexic subjects. The abnormalities are related to spatial and temporal stimulus frequencies: they appear when large stimuli are presented, or when the stimulation frequency is high. These data support the hypothesis of selective magnocellular dysfunction in dyslexia.

Adolescent↗
PubMed2002

[A study on examination of color vision with pattern-visual evoked potential].

OBJECTIVE: To understand the specific nature of the color and brightness components of the human visual evoked potential and explore the possibility of an objective method for examining the color vision. METHOD: The transient VEP was measured in normal subjects and persons with deficient color vision by multichannel electro-physiological apparatus. In the experiment, four kinds of reversal checkerboard patterns are simulated by the computer software technology as stimuli. The patterns included black-white (95% contrast), isoluminance chromatic checkerboards red-white, green-white, red-green (100% saturation), which were 25 ' in visual angle and alternated at a rate of 0.8 Hz. The main component, P (1) wave, was analyzed. RESULTS: (1) The normal subject group: The P (1) latencies in four conditions were obtained in terms of the length of latencies, the ordinal of P (1) latencies (from short to long) was as follows: red-white < black-white < red-green < green-white. There were very significant differences among the four latencies (P < 0.01). The P (1) amplitudes elicited with the black-white were higher than those with chromatic ones. But among the P(1) amplitudes elicited with chromatic checkerboards there were no difference. (2) The group with deficient color vision: There were no significant differences in the P(1) latencies and amplitudes elicited with the four stimuli. The length of latencies with black- white stimulus was the shortest. Red-green blindness as well as deuteranopia had no response to green stimulus but did to red or red-green stimulus. (3) Between the two groups, there were no differences in the P (1) latencies, but the P (1) amplitudes of persons with deficient color vision elicited with the chromatic stimuli were obviously lower. CONCLUSION: The results show that it is feasible to use chromatic VEP for examining congenital color vision deficiency, especially for red-green blindness and deuteranopia. At present, there is lack of quantitative and qualitative results of color VEP examination.

Adolescent↗
PubMed2003

[Manifestations of selective visual attention in human parietal and temporal cortex according to evoked potentials].

The event-related potentials (ERPs) in visual discrimination task in parietal and temporal cortical areas were recorded in 11 young adults during passive observation (involuntary attention) and target selection (voluntary attention). The voluntary selective attention resulted in: 1) increased ERP correlation between the parietal; and temporal cortical areas; 2) increased correlation of sequential ERPs in monopolar leads (P3, P4, T3, T4, T5, T6); and 3) increased correlation of sequential ERPs in bipolar leads (P3-T3, P3-T5, P4-T4, P4-T6). The findings suggest that voluntary attention maintains a concordant activity of the parietal and temporal cortical areas in execution of visual selection tasks.

Adolescent↗
PubMedDate not supplied

[Visual evoked potentials in chessboard pattern reversion in children with optic neuritis].

Visual evoked potentials (VEP) to chess pattern reversion were studied in 17 children with optic neurites during the acute stage and in the course of convalescence 1-2 months and 1-3 years after the disease onset. Coarse VEP changes were detected in the patients during the acute stage of optic neuritis. A positive time course of VEP was followed up in late periods after the acute attack of the disease. Despite normalization of vision acuity, no complete restoration of VEP to small-cell chess patterns was seen. The findings evidence that studies of VEP to chess pattern reversion should be carried out to detect subclinical involvement of the optic nerve and to timely diagnose optic neuritis in children.

Acute Disease↗
PubMed1991

[Shape changes in contrast VEP in multiple sclerosis: relation to the type of lesion].

VEP changes in MS patients are due to the demyelinating process which causes conduction alterations as well as inhibitory mechanisms. The pattern-shift VEP can show a delay of the main positivity, a diminished amplitude or/and an altered shape and duration. Concerning to series from the literature and own observations on more than 500 MS patients 4 different types of VEP alterations can be observed: 1. NPN complex with P2 delay; 2. broadening with loss of N2 and P2 delay; 3. bifid P2, P100-P135 complex; 4. deformation and dissociation with loss of P2, N105 dominance and amplitude reduction.

Evoked Potentials, Visual↗
PubMed1991

[VEP in the determination of multiple lesions in the visual system in patients with multiple sclerosis].

18 patients with proven multiple sclerosis were examined by pattern reversal visual stimulation. Stimulations were carried out on full fields, foveal areas and horizontal half-fields of single eyes and on half-fields of both eyes. 77% of the patients demonstrated pathological alterations of the potentials in more than one kind of stimulation. In all patients pathological signs could be found, which pointed on the optic nerve. Only the results of three patients suggested a lesion within the central optic pathway. The demonstration of multiple lesions of the optic system in patients with multiple sclerosis is difficult and rarely possible.

Evoked Potentials, Visual↗
PubMed1991

Visual evoked responses to pattern reversal in children.

Pattern reversal evoked potentials (PREPs) were studied in school going children 4-15 years of age (mean 9.9 +/- 2.6 yrs), using Ag/AgCl electrodes anchored on 01-A1 and 02-A2 scalp sites. Two sets of 256 pattern reversal stimuli with check sizes 32' and alternation rate 1 Hz were applied to each eye and evoked responses thus obtained were averaged and analysed by the inbuilt computer of the evoked potential recorder. The latency of various components of visual evoked potentials along with P1 amplitudes were recorded for the right and the left eye separately in boys and girls. The normative data are being reported and these do not show eye and gender differences in children.

Adolescent↗
PubMed1990

Binocular summation in visually evoked responses and visual acuity.

Monocular and binocular transient visually evoked responses (VER) were recorded on 50 adult subjects using a pattern reversal stimulus (check size 5.5 min arc). The peak-to-trough amplitude of the VER wave was measured and compared with the subjective visual acuity (Landolt C). The binocular amplitude enhancement over the monocular amplitude was around 26% across the acuity range studied, whereas the mean binocular subjective visual acuity improvement was around 11.3%, but this varied from 1% at the high acuity end to 17% at the low acuity end of the range.

Adolescent↗
PubMed1989

[Recording of visual evoked potentials in infants: characteristics and optimization of the method].

As the capacities for concentration and fixation are limited in infants and young children it is necessary to adapt the method of VEP measurement. An appropriate stimulus presentation is the appearance, disappearance mode with sequentially presented gratings of different spatial frequencies. Variation in the conditions such as fixation, attention or electrode properties, during a recording session affects all cortical responses equally. The duration of VEP examination is shortened by using rapid sequences of stimuli. The purpose of our study was to improve the presentation of the stimuli. We used a small bright monitor (9 x 9 cm) for stimulation; the test distance was 60 cm, and the mean luminance 60 cd/m2. Stimuli were achromatic gratings. These parameters were chosen in order to compare the results of our method with psychophysical measurements, such as preferential looking (PL) with Teller acuity cards. The gratings with increasing spatial frequencies were presented in a sequence of 7 steps. In order to control the influence of artefacts and the variability of the responses summation/subtraction curve of the responses, was displayed simultaneously with the VEP on the control monitor. By increasing the temporal stimulus frequency in steps from 2.5 Hz (transient stimulus) to higher values we got excellent responses from on/off-stimuli at 5 Hz frequency. Thus the signal-to-noise-ratio was improved without alteration of the spatial transfer function which is found when using higher frequencies due to the Brücke-Bartley-effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Child, Preschool↗
PubMed1988

Patterns and contrasts in ophthalmic investigation.

The Snellen test has been the most popular clinical measurement of spatial vision for over a century, but it does not fully express the visual ability of an individual. For more analytical purposes the information capacity of the visual system may be assessed by tests of contrast sensitivity and peripheral vision. The visual system selectively reduces the spatial information content of the visual field to avoid overloading the limited capacity for perception and decision making in the brain. The ways in which this reduction occurs and the processing of spatial information is of interest in many disciplines, and theoretical knowledge has been accelerated by the study of artificial intelligence. These processes may be investigated in human subjects by psychophysical and electrophysiological techniques. This provides additional information for diagnostic purposes and will form the basis of new systems of clinical investigation.

Evoked Potentials, Visual↗
PubMed1987

[Visual evoked potentials following double-pattern stimulation with pseudo-color perception].

We examined in which form an additional structural stimulus in the white checks influenced the visual evoked potential of a checkerboard reversal induced response. A pattern of vertical bars was introduced with a short delay to the checkerboard reversal. The purpose of these experiments was to find out about the discrimination threshold of the occipital cortex for timely spaced stimuli. The double stimulus was separated from the next change minimally for 800 ms. With this arrangement we avoided "steady states", which are shown by repetitive interstimulus intervals of less than 200 ms. Nine normal volunteers between 22 to 28 years of age were measured. We found a typical waveform after the P 100 potential consisting of a positive peak followed by a negative long-lasting baseline elevation. Only separations of more than 60 ms yielded this configuration. Interstimulus times of less than 60 ms resulted in refractory effects on the visual evoked response. All the subjects reported colour or brightness changes of the additional bars. These pseudo colours were very probably due to the Prevost-Fechner-Benham-Effect which is common with repetitive white and black pattern changes. Our pseudo colours showed remarkable differences to the classical designs which has led us to the conclusion that colour perception on TV monitors is influenced so far by unpredictable flicker factors.

Adult↗