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PubMed1990

Event-related potentials to visual and language stimuli in normal and dyslexic children.

Event-related potentials (ERPs) were recorded in 31 normal and 21 dyslexic children, the latter with visual processing impairment, in three visual reading-related tasks: a nonalphabetic, an alphabetic, and a lexical decision task (two levels of difficulty in each). ERPs were recorded from fifteen active electrodes for 1.5 s following stimulus presentation; analyses were done on difference waves. N2 and P3 components were usually longer in latency and P3 was lower in amplitude in the dyslexic than in the normal children. The P3 had a flatter, broader distribution in the dyslexics than in the controls, which was very similar to the effect of difficulty in the two easier tasks. The lack of Group X Age interactions is inconsistent with either a maturational lag between the groups of children, or with the dyslexic children catching up to normals in terms of their speed of processing. Thus, the dyslexic children required more time to process the visual information across tasks (with only their letter classification being equal to that of the normal controls) and approved to have a different developmental pattern in their ERPs from that of the normal children.

Arousal↗
PubMed1992

Taking attention to task: P300, task response probability, and within-category deviation detection.

Previously, we reported enhanced P3 amplitudes to between-category deviations among high and low probability events. Here, we tested the effects of a within-category difference. Subjects performed a go/no-go button press task as they viewed repeated, randomly-ordered presentations of nine double-digit numbers. Eight numbers fell within a prescribed range (42-49, standards); prior to testing, subjects selected one standard number for later recall. A ninth, out-of-range (91, deviant) number was also included. Subjects were tested under two conditions, in which they responded either to one (low response probability) or to seven (high response probability) standard nonselected numbers, designated as targets. Relatively larger P3s were consistently apparent only when the deviant nontarget was associated with a low probability response to a nontarget. There was a selective effect of nontarget response probability on P3 amplitude to the deviant nontarget. Our results indicate that within-category deviation detection is facilitated by "controlled" attention to the structure of the stimulus field.

Adolescent↗
PubMed1992

Analysis of pattern-reversal visual evoked-potential topography: replicability in normal subjects and comparisons with Alzheimer's disease.

This paper reports two studies using multivariate analysis to aid the interpretation of cross-coherence of multiple electrode sites in evoked potential responses. In the first study, the replicability of principal components and multidimensional scaling was evaluated by applying both methods to pattern-reversal visual evoked potentials recorded from 28 scalp sites in two subgroups of sex- and age-matched healthy subjects, and then comparing the results. Four dimensions replicated in the two initial multidimensional scaling solutions and appeared to reflect differences in: 1) anterior versus posterior scalp areas, 2) laterality, 3) separation of frontal and occipital sites from other scalp regions, and 4) proximal versus distal placements. The initial principal components did not match well in the two groups, but rotation to congruence improved their replicability and ultimately yielded axes similar to those of the multidimensional scaling dimensions. In the second study, Alzheimer's disease and sex- and age-matched control subjects were evaluated. The four axes identified were the same as those described above, but after the solutions were rotated to align them, the group differences appeared negligible. Examination of the components and dimensions from both studies showed some consistent departures from being merely reflectors of site location, and the apparently visual dimension appeared clearly in all four groups. Judged on the basis of initial interpretability and replicability of the solutions, the results suggest that multidimensional scaling, with appropriate transformation, may provide an effective tool for analyzing pattern-reversal visual evoked-potential topography.

Aged↗
PubMed1993

Modulation of event-related potentials by word repetition: the role of visual selective attention.

Event-related potentials (ERPs) were recorded while subjects viewed visually presented words, some of which occurred twice. Each trial consisted of two colored letter strings, the requirement being to attend to and make a word/nonword discrimination for one of the strings. Attention was manipulated by color in Experiment 1, and color and a precue were used in Experiment 2. As in previous ERP studies of word repetition, a positive offset to repeated words developed when both first and second presentations were the focus of attention. In Experiment 2, ERPs showed evidence of positive-going repetition effects in all conditions in which at least one of the two presentations of the repeated word was attended. In the visual modality, the positive-going ERP repetition effect occurs only when at least one of the two presentations of a repeated item is the object of attention, which suggests that one or more of the processes reflected by the effect is capacity limited.

Adult↗
PubMed1994

Processing of auditory stimuli during auditory and visual attention as revealed by event-related potentials.

Auditory event-related brain potentials (ERPs) were recorded during auditory and visual selective attention tasks. Auditory stimuli consisted of frequent standard tones (1000 Hz) and infrequent deviant tones (1050 Hz and 1300 Hz) delivered randomly to the left and right ears. Visual stimuli were vertical line gratings randomly presented on a video monitor at mean intervals of 6 s. During auditory attention, the subject attended to the stimuli in a designated ear and responded to the 1300-Hz deviants occurring among the attended tones. During visual attention, the subject responded to the occasional visual stimuli. ERPs for tones delivered to the attended ear were negatively displaced relative to ERPs elicited by tones delivered to the unattended ear and to ERPs elicited by auditory stimuli during visual attention. This attention effect consisted of negative difference waves with early and late components. Mismatch negativities (MMNs) were elicited by 1300-Hz and 1050-Hz deviants irrespective of whether they occurred among attended or unattended tones. MMN amplitudes were unaffected by attention, supporting the proposal that the MMN is generated by an automatic cerebral discrimination process.

Adult↗
PubMed1994

Reduced attention-related negative potentials in schizophrenic adults.

Event-related potentials were recorded from outpatient adult schizophrenics receiving maintenance doses of neuroleptics and from normal control subjects during performance of a reaction time task and a complex visual discrimination task, the Span of Apprehension. Difference potentials were computed to isolate endogenous activity associated with the processing demands of the Span task. Schizophrenics produce significantly less early endogenous negative activity than do normal subjects. This processing-related negativity reflects pattern matching activity to an attentional trace during the serial scan of the visual icon. We previously reported an identical reduction in processing-related negativity in childhood-onset schizophrenia, suggesting that this deficit is age independent. Both frontal contingent negative variation and an early frontal P3 were larger in the schizophrenics than in normal subjects, suggesting an inappropriate mobilization of nonspecific attentional resources. A later posterior P3 was significantly smaller in schizophrenics than in normal subjects.

Adult↗
PubMed1991

Visual function in Alzheimer's disease and normal aging.

We examined a wide range of visual behaviors in 59 patients with Alzheimer's disease (AD), 35 elderly control subjects, and 12 young control subjects. A subset of the patients with AD received neuro-ophthalmologic and electrophysiologic examinations in order to evaluate the integrity of the retino-calcarine pathway. Patients with AD showed significant, selective losses in visual function, including color discrimination, stereoacuity, contrast sensitivity, and backward masking, but not in critical flicker fusion. The deficits were not attributable to clinically apparent lesions of the retina or optic nerve. We therefore suggest that AD lesions in primary visual and posterior association cortices underlie the observed behavioral abnormalities.

Adolescent↗
PubMed1985

Objective evaluation of binocular function with pattern reversal VER. I. Effect of contrast.

The effect of change of contrast on the pattern reversal monocular VER, binocular VER, and binocular summation (binocular VER amplitude/monocular VER amplitude) was studied in normal subjects. The monocular VER amplitudes showed significant increase until the 0.50 contrast and saturation above that contrast. The largest binocular VER amplitude was observed at 0.20 contrast, and the amplitude tended to decrease with increasing contrast. The value of the binocular summation was highest at 0.20 contrast and decreased significantly with increasing contrast. At high contrast, there was no significant binocular summation. The difference of peak time (phase) between the binocular and the monocular VER was greatest at low contrast and decreased with increasing contrast. These results are explained by the difference of saturation level between the binocular and monocular systems. Our findings lead us to advocate strongly the use of low- to midlevel contrast for evaluating binocular function with pattern reversal VER in clinical ophthalmology.

Adult↗
PubMed1989

The pattern visual evoked cortical response in human ageing.

The N1, P1, N2 and P2 components of the pattern visual evoked response (PVER) have been recorded to the onset of presentation of a sinusoidal grating pattern of 3 and 8 cycles/deg in groups of young (15-34 years) and old (53-94 years) subjects. The negative components were taken to arise from striate cortex and the positive components from extrastriate cortex. For grating contrasts of 3-40%, the time-to-peak of the N1, P1 and N2 components, but not the P2 component, increased with age while the amplitude and rise time showed no consistent changes. Comparisons between 3 and 8 cycles/deg revealed a longer time-to-peak of N1 and P1 at 8 cycles/deg within the young group though not within the old group, i.e. an additional increment in the time-to-peak had occurred at 3 cycles/deg with ageing. This was also reflected for grating contrasts which were multiples of each individual's contrast threshold, irrespective of age. Now, the time-to-peak of N1, P1 and N2 was similar in young and old groups at 8 cycles/deg. At 3 cycles/deg, however, the time-to-peak was still longer in the old group, though the intervals between components were similar to those of the young group. This suggested the requirement for a further increment in contrast to make the time-to-peak similar, due to a selective loss of sensitivity in ageing within the low-spatial-frequency channels which are additionally sensitive to temporal modulation. Thus, it appeared that, by incrementing the stimulus contrast by the appropriate amount, the PVER of the old group could be made to resemble that of the young group. These results are consistent with the occurrence of neural changes during ageing in the retino-geniculate pathway prior to the visual cortex.

Adolescent↗
PubMed1988

Cortical contrast gain control in human spatial vision.

1. We evaluated human visual cortical contrast gain using visual evoked potential (VEP) measurements. The steady-state VEP was elicited by 7.5 Hz contrast modulation of a 6 cycles/deg sinusoidal grating. The stimulus may be regarded as the sum of a steady grating (C) and a counterphase grating of the same spatial frequency (delta C). The counterphase grating is modulated sinusoidally in time. 2. The VEP was measured to combinations of different modulation contrasts (delta C) and different mean levels of grating contrast (C) which produced stimuli with contrast modulation depths (delta C/C) ranging from 0.0625 to 1.0 ('on-off'). 3. The VEP signals were Fourier analysed and the amplitude and phase of the first (7.5 Hz) and second (15 Hz) harmonic frequency components were examined. The monocular VEP to a contrast-modulated grating contains significant first and second harmonic frequency components. 4. The amplitude and phase of the monocular VEP was plotted as a function of delta C for each mean level of contrast explored. The amplitudes of both the first and second harmonic frequency components grow with increasing delta C. However, the slope of each function depends on the mean contrast (C): with higher levels of C, the slope of the function is more shallow. Furthermore, at each level of C the amplitude of the first harmonic frequency saturates at a lower delta C than does the second harmonic frequency component. VEP amplitude is therefore not determined by the absolute contrast change (delta C) alone. The VEP phase of the first harmonic frequency shows less dependence on either modulation or on mean contrast; the phase of the second harmonic frequency component is strongly dependent on mean contrast (C) but not on delta C. 5. When the second harmonic amplitude component of the VEP response (R) is expressed as Ractual/Rmax, where Rmax is the response to C = delta C (i.e. 'on-off'), all second harmonic VEP functions can be well fitted with a power function. This is not the case for the function of the first harmonic amplitude data. 6. A dichoptic VEP was obtained by presenting the steady and counterphase gratings to opposite eyes. The dichoptic VEP, in distinction to the monocular VEP, contains only a second harmonic frequency component. The amplitude of the second harmonic frequency component grows with increasing delta C, similar to the function seen for the monocular VEP.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1988

Pattern visual evoked potentials in hyperthyroidism.

Pattern reversal visual evoked potentials (VEPs) have been elicited in 16 female hyperthyroid patients before and after treatment and compared with those from a similar group of age and sex matched control subjects. No effect on latency was seen, and although larger amplitude values were noted in the thyrotoxic group these too were not significant. We would conclude that hyperthyroidism per se has little effect on the pattern reversal VEP, and any observed effect on these potentials is probably due to other factors.

Adult↗
PubMed1983

Transient visually evoked potentials to sinusoidal gratings in optic neuritis.

Transient visually evoked potentials (VEPs) to sinusoidal gratings over a range of spatial frequencies have been recorded in cases of optic neuritis. The use of the response to pattern onset in addition to the response to pattern reversal extended the range to higher spatial frequencies by up to two octaves. There was an increase in VEP delay and a greater degree of discrimination from a control group at higher spatial frequencies. This finding is discussed in the light of previous reports of luminance and checkerboard VEPs in demyelinating optic nerve disease. An attempt is made to relate amplitude changes in various VEP components to contrast sensitivity measurements in this group of patients.

Evoked Potentials, Visual↗