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Biomedical subjects

L Mecacci

Publications and source records attributed to L Mecacci.

At least 19 recordsLinked to original sources
PubMed1995

Pattern reversal visual evoked potentials in patients with hemineglect syndrome.

To investigate basic visual information processing in patients with hemineglect syndrome, pattern reversal visual evoked potentials (VEPs) were recorded in 21 brain-injured patients (10 with neglect symptoms) and 6 healthy subjects. The stimulus was a checkerboard which varied in check size or temporal frequency, presented to the left or right visual field. VEPs recorded in neglect patients to stimuli presented in the subjectively neglected left visual field were comparable in amplitude to those recorded to stimuli presented in the normal right visual field. For stimuli presented centrally, there was no difference in VEPs between neglect patients and brain-damaged patients without neglect. These results support neuropsychological theories that state that the neglect syndrome is more closely related to deficits at post-sensory levels than to impairments in basic visual processing. Some evidence, however, suggests that the nature of the interaction between the two visual hemi-fields may also be altered in neglect patients.

Aged↗
PubMed1993

On spatial frequencies and cerebral hemispheres: some remarks from the electrophysiological and neuropsychological points of view.

The spatial frequency hypothesis on hemispheric specialization gave rise to contradictory experimental results, commented on in Brain and Cognition by Christman (1989) and Peterzell (1991). The question is discussed through a review of the electrophysiological and neuropsychological research on hemispheric asymmetry of spatial frequency processing. The general hypothesis of the hemispheric specialization for this basic visual information appears to be supported by recent works on evoked potentials by gratings and checkerboards. However, an interaction between the cerebral hemisphere, spatial frequency, and temporal frequency was found more than a sharp dichotomy between low (right hemisphere) and high spatial frequencies (left hemisphere), as indeed it was proposed by the spatial frequency hypothesis. Other relevant physical parameters in generating the hemispheric asymmetry were found to be the contrast and the visual field size. The neuropsychological research on brain-injured patients has given some further evidence of the hemispheric asymmetry in spatial frequency processing. In conclusion, it is argued that the major merit of the spatial frequency hypothesis was in the attempt to investigate the hemispheric specialization of lower and higher levels of visual information processing from the perspective of a unified computational conception of visual perception.

Brain↗
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↗
PubMed1990

Handedness and hemispheric asymmetry of pattern reversal visual-evoked potentials.

Pattern reversal visual-evoked potentials (EP) from temporal leads in the two hemispheres of 26 right-handed (14 right-eye-dominant and 12 left-eye-dominant) and 10 left-handed (left-eye-dominant) adults were recorded. Checkerboard patterns (check sizes: 5.7 and 17 min of arc) at 1 and 8 Hz were reversed. Stimuli (a) subtended 6 degrees of visual field, (b) subtended 1 degree of visual field (foveal condition), and (c) were restricted to the annular portion of the visual field around the fovea (peripheral condition). Larger EP amplitudes in right or left hemisphere in relation to handedness, temporal frequency, and visual field condition were recorded. Eye dominance of dextrals appeared to play a role in determining the hemispheric asymmetry. Previous literature data and present results in relation to the hypothesis of different hemispheric specialization for basic visual information are discussed.

Adult↗
PubMed1990

Contrast and hemispheric asymmetry: an electrophysiological investigation.

Visual evoked potentials by 8 Hz phase-reversed sinusoidal gratings of different spatial frequency were recorded from occipital and temporal leads of left and right hemispheres. Seven adult subjects were investigated. At low contrast, the VEP amplitudes in the two hemispheres were symmetrical. At medium and high contrast, the amplitudes were larger in one hemisphere. On the contrary, the "phase advance" as a function of contrast was comparable in the two hemispheres. The results were discussed according to the literature on different neural population involved in contrast perception.

Adult↗
PubMed1990

The effects of visual field size on hemispheric asymmetry of pattern reversal visual evoked potentials.

Visual evoked potentials (VEPs) were recorded in 17 females (10 right-handers and 7 left-handers) and 17 males (10 right-handers and 7 left-handers). The stimulus was a checkerboard pattern phase-reversed at a rate of 1 Hz, binocularly viewed by the subject. Two experimental conditions were tested: field size (the stimulus was enlarged from 1 to 30 degrees of visual angle) and occlusion (the central part of the stimulus was occluded by experimental scotomata of various size--from 1 to 16 degrees). Responses recorded at occipital and temporal leads of the two hemispheres were compared. The P100-N145 amplitude was larger at occipital than at temporal leads, it augmented increasing the size of the stimulus and diminished occluding progressively larger portions of the central field. Hemispheric asymmetries emerged with large field sizes and were not abolished by the presence of experimental scotomata. The hemispheric asymmetry was related to handedness (left-handers had larger amplitudes in the right hemisphere, right-handers in the left hemisphere), but not to sex. Hemispheric asymmetries in VEPs latency were not present. In comparison to males, females showed larger amplitudes and shorter latencies. These results point out that hemispheric asymmetries are found also for the processing of elementary visual stimuli like checkerboards, and they depend both on stimulus parameters (field size) and subject variables (handedness).

Adult↗
PubMed1989

Influence of spatial frequency and handedness on hemispheric asymmetry in visually steady-state evoked potentials.

Recent suggestions on the involvement of the spatial frequency of visual stimuli in the hemispheric lateralization were investigated by recording steady-state evoked potentials in two groups of subjects: five right-handers and five left-handers. Sinusoidal gratings at spatial frequency of 0.5, 1, 3, 4, 6, 8, 10, 12 or 16 cpd were phase reversed at 4 Hz or 12 Hz. Evoked potentials recorded from temporal leads over each hemisphere were submitted to a FFT analysis. Results concern the amplitude of the fundamental component. In right-handers, the temporal frequency was the deciding factor of the lateralization: the evoked activities were greatest in the RH at 4 Hz and in the LH at 12 Hz. This effect was obvious for the range of spatial frequencies from 3-12 cpd. Results, discussed in terms of global/local information, suggested the existence of two transient and sustained systems. In left-handers, both the spatial and temporal parameters were relevant to the lateralization. A spatio-temporal interaction was observed which was reversed at 6 cpd.

Adult↗
PubMed1987

Hemispheric asymmetry of pattern reversal visual evoked potentials in healthy subjects.

Pattern reversal visual evoked potentials were recorded from temporal leads of both hemispheres in 3 right-handed and 3 left-handed subjects. The stimuli subtended 6 degrees of visual field (full field condition), or 1 degree of visual field (foveal condition), or were restricted to the peripheral portion of the visual field (annular condition). Check sizes varied from 16 to 2.8 min of arc. The stimuli were phase-reversed at 1 or 8 Hz. Remarkable hemispheric asymmetries were found, depending on the portion of the visual field and temporal frequency. In right-handed subjects the amplitude of visual evoked potentials recorded over the left hemisphere was larger for stimuli phase-reversed at 8 Hz in full-field and annular conditions, whereas the amplitude of visual evoked potentials recorded over the right hemisphere was larger for stimuli phase-reversed at 1 Hz in the foveal condition. Different patterns of hemispheric asymmetries were observed in left-handed subjects.

Arousal↗
PubMed1986

Hemispheric asymmetries in the visual evoked potentials to temporal frequency: preliminary evidence.

Steady-state evoked potentials were recorded in eight adult subjects from occipital and temporal leads of both hemispheres to investigate the effect of temporal frequency on the hemispheric specialization for basic visual information. A 3 cycles deg-1 grating was phase-reversed at different temporal frequencies (from 4 to 18 Hz), and the frequency spectrum of evoked potentials was computed by means of a fast Fourier transform program. Significant results were obtained for the component at twice the temporal frequency of stimulation. Occipital evoked potentials did not show hemispheric asymmetry, whereas temporal evoked potentials showed an interaction between hemisphere and temporal frequency: right and left hemispheres were respectively prominent for low (4 and 6 Hz) and for high (8-18 Hz) temporal frequencies. The results are discussed in the context of current research on hemispheric specialization for basic spatiotemporal parameters of visual information processing.

Adult↗
PubMed1984

Looking for the social and cultural dimension of the human brain.

The theories of brain function generally stem from elementary units and mechanisms to explain behavioral and psychological processes. The concept of the reflex arc is still today the paradigm of this trend. Other views have pointed out that molar and dynamic principles of brain functioning are more suitable for the explanation of complex processes. Social and cultural factors, whilst considered fundamental in the development of psychological functions, are neglected in brain research. The relevance of these factors is illustrated for what concerns some topics of growing interest in neuropsychological research, from the hemisphere specialization in people of different cultures to the individual organization of brain functioning.

Brain↗
PubMed1984

Evoked potentials by letters in printed and script forms.

Visual evoked potentials were recorded for 6 adult male subjects in response to single vowels and consonants in printed and script forms. Analysis showed the vowels in the printed form to have evoked responses with shorter latency (component P1 at about 133 msec.) and larger amplitude (component P1-N1) than the other letter-typeface combinations. No hemispheric asymmetries were found. The results partially agree with the behavioral data on the visual information-processing of letters.

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↗
PubMed1981

Effects of age and reading ability on visual discrimination.

60 children were shown 45 pairs of cartoons with the task of searching through the first cartoon and detecting which sequence of colors, geometrical patterns, or letters had the same couple of elements drawn in the second cartoon. Older children and those who were able to read performed better. The results confirm the hypothesis that the development of some visual skills is a by-product of learning to read.

Age Factors↗
PubMed1980

Visual evoked potentials basic references.

A list is given of 60 basic references (monographs and general reviews) in the field of evoked potentials and event-related brain potentials applied to vision research in man.

Bibliographies as Topic↗
PubMed1980

Eyeblink, evoked potentials and visual attention.

Eyeblink signals and evoked potentials were recorded and averaged in humans during the tachistoscopic presentation of gratings at various spatial frequencies and levels of luminance. It was confirmed that at high spatial frequencies and low levels of luminance the latency of evoked potentials increased and their amplitude decreased. At the same time there was a longer inhibition of the eyeblink, which was more precisely timed on the stimulus. This points to the existence of a central system that regulates the eyeblink in accordance with the attention involved in visual information processing.

Adult↗
PubMed1977

Spatial frequency and orientation tuning curves of visual neurones in the cat: effects of mean luminance.

Experiments have been performed on unanaesthetized and paralysed cats. The tuning curves for spatial frequency of retinal, lateral geniculate and simple and complex cells of the cortex have been determined in response to sinusoidal gratings of various spatial frequencies at different levels of mean luminance. For all neurones, decreasing the mean luminance leads to a progressive loss of spatial resolution and contrast sensitivity. Retinal ganglion cells of type X show, for scotopic levels of luminance, a flattening of their spatial frequency tuning curves in the low spatial frequency range. For geniculate and cortical neurones, on the contrary, the spatial frequency characteristics at the various levels of luminance remain practically invariant in their bandwidth. On the average, complex cells still respond to mean luminances ten times lower than simple cells. The tuning curves for orientation of cortical cells maintain, to a first approximation, the same shape at the various levels of mean luminance. The results are discussed comparing the electrophysiological with psychophysical data.

Animals↗