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Biomedical subjects

D Spinelli

Publications and source records attributed to D Spinelli.

At least 55 records · Page 3Linked to original sources
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

Evidence for edge and bar detectors in human vision.

The structure of receptive fields of human visual detectors was investigated by studying their phase response. Observers were required to discriminate between pairs of periodic stimuli that differed in phase by 180 degrees (reversed in contrast). The stimuli comprised 256 harmonics, smoothly filtered in amplitude, and congruent in phase at the origin. Reversal discrimination thresholds were measured as a function of the phase of the harmonics. Thresholds were slightly higher for phases around 45 degrees, consistent with the idea that all discriminations were mediated by independent detectors with 0 or 90 degrees phase response (assuming probability summation between them). Discrimination thresholds were also measured with a pedestal stimulus, of phase complementary to that of the test gratings. For discriminations between 0 and 180 degrees (cosine phase), or 90 and 270 degrees (sine phase), the complementary pedestal had little effect, implying independence of detectors in sine and cosine phase. However, for discrimination between 45 and 225 degrees (stimuli containing both sine and cosine components) the complementary pedestal, which also contained both sine and cosine components, facilitated greatly discrimination thresholds. The results suggest that there exist two classes of detectors, one with a Fourier phase spectrum of 0, the other with a Fourier phase spectrum of 90 degrees. This implies that the receptive fields are symmetric, one class having even-symmetry (line-detectors), the other odd-symmetry (edge-detectors).

Differential Threshold↗
PubMed1989

Discrimination of spatial phase in central and peripheral vision.

Sensitivity to relative phase was measured for central and peripheral vision using stimuli comprising 256 harmonics, smoothly filtered in amplitude. With these stimuli, peripheral phase sensitivity was much higher than that previously reported with two-harmonic stimuli. Sensitivity did not depend on the average phase of the stimuli, nor on their second-order statistics, irrespective of the spatial frequency of the stimulus or the position in the visual field. After scaling for size, peripheral sensitivity was as high as central sensitivity. The scaling factor required to equate phase sensitivity was the same as that required to equate contrast sensitivity and grating acuity. These results suggest that phase sensitivity decreases with eccentricity at a similar rate as contrast sensitivity and grating acuity, much more slowly than the positional acuities. This is consistent with the suggestion that phase discrimination is mediated by discriminating the amplitude of the response of quasi-linear filters, and does not require mechanisms that evaluate position. It is suggested that previous measurements on peripheral phase sensitivity may reflect positional uncertainty in the periphery, rather than a deficit in phase sensitivity per se.

Differential Threshold↗
PubMed1988

Left-right visual field asymmetry in bistable motion perception.

Twelve observers viewed two alternating frames, each consisting of three rectangular bars which were displaced laterally by one cycle in one frame with respect to the other. At long interframe intervals (IFIs) observers perceived a group of three elements moving as a whole (group movement), whereas with IFIs shorter than 40-60 ms the overlapping elements in each frame appeared stationary while the third element appeared to move from one end of the display to the other (end-to-end movement). The percentage of group movement responses in central viewing was compared to those obtained for stimulus presentation in the left and right visual fields (4 deg eccentricity), for opposite horizontal directions of motion. All ten right-handed subjects showed a left-field advantage in sensitivity to group movement. The two left-handed subjects showed a similar advantage in sensitivity with right-field presentation. The effects of monocular vision, hand used in the task, spatial frequency, and contrast on visual field asymmetry were all investigated in two right-handed subjects. None of these factors affected the left-right asymmetry.

Cerebral Cortex↗
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

Complement activation in uveitis.

To determine whether complement is activated in uveitis we have measured plasma levels of C3d, a sensitive indicator of complement activation. Increased levels of C3d were found in 11 of 15 patients with idiopathic uveitis, 13 of whom had circulating immune complexes containing complement components. Since during complement activation potent mediators of inflammation are generated, it is suggested that the activation of complement, possibly triggered by uveal deposition of immune complexes, has an important role in the pathogenesis of uveitis.

Adolescent↗
PubMed1985

The iris vessels in the exfoliation syndrome: ultrastructural changes.

The ultrastructure of iris vessels in exfoliation syndrome has been investigated. Deposits of exfoliation material have been found in the vessel walls. There is evidence of different stages of pathological alterations of some vessels, culminating in the loss of endothelial cells and obliteration of the lumen associated with exfoliative deposits. These findings correlate with fluorescein angiographic alterations of the iris and stress the importance of vascular changes in this syndrome.

Aged↗
PubMed1984

Short-term and long-term timolol maleate effects on corneal sensitivity.

The authors present data concerning the effect of timolol maleate on corneal sensitivity investigated with the Cochet-Bonnet aesthesiometer and with cotton wool wisps. Among 30 subjects with open-angle glaucoma, 15 subjects underwent a short-term trial with timolol maleate 0.50% in one eye, while 15 served in a long-term trial with timolol maleate 0.50% or 0.25% in both eyes. A light reduction in corneal sensitivity, which did not reach statistical significance, was found during timolol treatment. After discussing the possible explanations of this side effect, the authors advise monitoring this parameter during the therapy.

Adult↗
PubMed1984

Orientation sensitivity in the peripheral visual field.

Orientation sensitivity in the peripheral visual field has been tested in two tasks: (a) setting horizontal the orientation of a grating at various retinal eccentricities, and (b) matching the orientation of a peripherally viewed grating as close as possible to an oblique reference viewed foveally. Both performances fall off with increasing retinal eccentricity. Magnification of the stimulus optimizes peripheral performance. Peripheral performance, optimized by magnification, varies with retinal eccentricity. It approaches, but does not reach, the foveal value (tested by the same method) at 10 deg of eccentricity, and is much lower at 20 and 30 deg of eccentricity.

Discrimination, Psychological↗
PubMed1984

Pattern reversal evoked potentials in infantile spasms.

Visual evoked potentials in response to sinusoidal gratings of various spatial frequencies, alternating in contrast at 8 Hz were recorded from infants affected by an idiopathic form of infantile spasms (West syndrome). In two infants in whom the spasms began at 3 and 5 months of age, respectively, the VEP's were found to be depressed in amplitude at and around a spatial frequency of 3 and 5 c/deg, respectively. This midfrequency depression, which has never been reported in the literature for normal infants, suggests that infantile spasms in idiopathic patients may interfere at least temporarily with the development of visual function.

Evoked Potentials, Visual↗
PubMed1983

Visual acuity in the young infant is highest in a small retinal area.

Visual potentials (VEP) evoked by alternating gratings were recorded daily in one infant between 10 and 13 weeks of age and at 20 weeks of age. The VEP amplitude was measured for a stimulus field of increasing area at low and high spatial frequencies. At low spatial frequencies the VEP amplitude increases with increasing stimulus area, while at high spatial frequencies (near visual acuity) the VEP amplitude remains constant for stimulus field sizes beyond 2 deg. These findings parallel those obtained in the adult, and indicate that visual acuity of the infant is not uniform across the retina but is highest in a small region, as in the adult.

Adult↗
PubMed1983

Development of retinal and cortical responses to pattern reversal in infants: a selective review.

Infant contrast sensitivity and visual acuity evaluated by recording steady-state visual evoked potentials (VEP) in response to sinusoidal gratings improve considerably during the first 6 months of life. Qualitatively similar findings are obtained in kittens 3-12 weeks old. In infants, the improvement in VEP acuity with age parallels the increase in retinal acuity estimated from electroretinographic (ERG) steady-state responses to sinusoidal gratings. Spatial frequency selective mechanisms with bandwidth substantially narrower than the overall contrast sensitivity function seem to be present in the visual system of very young infants. These findings are compared with data reported in the literature about the development of the cat and monkey visual system.

Animals↗