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Biomedical subjects

D Spinelli

Publications and source records attributed to D Spinelli.

At least 19 recordsLinked to original sources
PubMed1992

The role of frame size on vertical and horizontal observers in the rod-and-frame illusion.

The rod-and-frame illusion was used to examine a proposed distinction between the mechanism responsible for frame effects on rod-adjustment errors with large displays and the mechanism responsible for errors with small displays. It was suggested that visual-vestibular mechanisms are involved only when the rod is surrounded by a large tilted frame. Errors in the perceived vertical with small frame would instead be due to purely visual mechanisms. To examine this dual process model, we compared errors at small and large frame when the body was vertical or horizontal. There is evidence to suggest that tilting the body affects visual-vestibular interactions, but there is no reason to expect that body tilt would affect intravisual interactions. Hence, we hypothesized that body tilt would increase errors for large frame, but not for small frame. Eight subjects were tested in four different conditions, corresponding to the combination of two body orientations (vertical versus horizontal) and two frame sizes (47.5 versus 10.5 deg of visual angle). Fourier analysis of data was performed. Repeated measures ANOVA tested the hypothesis about frame size and body orientation. The hypothesis was not confirmed. More specifically, we found that tilting the body increased errors for the small frame as well as for the large frame. The interaction between frame size and body orientation was not significant. Results are discussed in relation to the proposed dual-process model.

Adult↗
PubMed1992

Perception of moving and stationary gratings in brain damaged patients with unilateral spatial neglect.

Contrast sensitivity for stationary and drifting sinusoidal gratings of different spatial frequencies was measured in 17 brain damaged patients (five left-brain damaged and 12 right-brain damaged patients, six of whom showed symptoms of spatial hemi-neglect). Twenty-nine combinations of spatio-temporal stimuli were considered in the drifting condition. No specific deficit for the group of patients with neglect symptom in the perception of moving and stationary gratings was observed. Also, no systematic deficit for perceiving the direction of motion--toward the neglect hemispace or away from it--was apparent. A small, selective impairment in the detection of stationary or very slow drifting gratings of low spatial frequency was observed in the presence of unilateral cortical damage, independently from its side (left or right) and from the presence of neglect. Some difficulty to detect the direction of motion was observed in a few individual patients.

Aged↗
PubMed1990

Contrast sensitivity and low spatial frequency discrimination in hemi-neglect patients.

Contrast sensitivity for sinusoidal gratings of different spatial frequencies and low spatial frequency discrimination of high contrast squarewave gratings were measured in 26 right brain damaged patients, 15 of whom showed symptoms of spatial hemi-neglect. No difference was present between neglect and non-neglect patients in either task. A difference in contrast sensitivity restricted to low spatial frequencies was present between controls and patients, independent of neglect. Overall, results indicated that spatial vision at threshold and suprathreshold levels is not selectively impaired in patients with hemi-neglect, a finding consistent with theories which postulate a post-perceptual locus of impairment in this syndrome.

Aged↗
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

Shape, size and relative space position perception in neglect patients.

Shape, size and relative position in space perception were tested in 17 right brain damaged (RBD) patients with neglect symptoms, 10 RBD patients without neglect and 11 controls. Simultaneous pair comparison task in free vision was used. Two arrangements of the visual display were used: side by side and top-bottom. Neglect patients' performance was comparable to RBD and controls in the shape and size test, but was below the other groups in the relative space position test. The selective failure of neglect patients in the position task did not depend on the spatial arrangement of the visual display, which may indicate a general perceptual deficit, rather than a deficit restricted to the neglect hemifield.

Aged↗
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↗
PubMed1978

Accuracy of evoked potential refractometry using bar gratings.

We recorded cortical potentials evoked by phase-alternating square-wave gratings. Significant effects of varying the refractive power of the eye are obtained only at 5 cycles/degree (cy/deg) and above. The extrapolation to zero amplitude of the evoked response as a function of spatial frequency predicts the psychophysical visual acuity even for the myopic eye. As the sensitivity of meridional refraction using sufficiently fine bar gratings is better than 0.5 D it provides a suitable means for the measurement of astigmatism.

Astigmatism↗