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R D Freeman

Publications and source records attributed to R D Freeman.

At least 91 records · Page 5Linked to original sources
PubMed1985

Is reduced vernier acuity in amblyopia due to position, contrast or fixation deficits?

Poor vernier acuity, exhibited by amblyopes, may reflect anomalies related to eccentric fixation, deficient position sensitivity, or reduced contour visibility. We have examined these factors by measuring contrast and vernier sensitivities with stimuli consisting of extended sinusoidal gratings of several spatial frequencies. Vernier thresholds were measured using both a classical single step position change and also a grating that was position-modulated sinusoidally along its entire length. For both types of target amblyopes exhibited deficient displacement sensitivity although accurate fixation was not strictly required. The vernier deficits were not critically dependent upon the type of displacement used, and the magnitudes of the vernier and contrast sensitivity deficits were closely related. Both were largest at high spatial frequencies, and those amblyopes with larger contrast sensitivity deficits also had larger vernier acuity deficiencies. Typically, contrast sensitivity and vernier acuity were normal, or nearly so, at very low spatial frequencies. Also, vernier acuities for the amblyopic and non-amblyopic eyes were approximately equal if grating contrast was set at some fixed multiple of detection threshold. We did not find a close relationship between the magnitude of the vernier deficit and reported perceptual distortions.

Amblyopia↗
PubMed1985

Comparison of response properties of cells in the cat's visual cortex at high and low luminance levels.

Receptive-field organization of cells in the cat's striate cortex and lateral geniculate nucleus (LGN) was investigated by using bars of light as stimuli. The aim was to determine if differences occur between conditions of high and low luminance levels. Of 72 cortical cells studied, the receptive fields of 63 were clearly different at high compared with low luminances. Units that gave on-off responses to flashed bars, for example, typically displayed on-only responses at low luminance. By far the most frequent change was that off responses were reduced or absent at low luminance levels. Of 63 cells that showed clear changes, 54 were of this type. This altered receptive-field organization appears to remain for extended periods (we have monitored the steady-state case for up to 2 h). Additional tests allow us to rule out the possible influence of overall changes in response strength and scattered light. To see if similar changes in receptive-field organization are present at the level of the LGN, we recorded from a small number of cells in the LGN (n = 10) and from an additional five afferent fibers in the cortex. In each case, there was a change in center-surround organization between high and low luminance levels similar to that previously reported for retinal ganglion cells. The excitatory responses from the surround for both on-center and off-center cells were absent at low luminance. Taken together, the results suggest that surround responses that can be elicited from ganglion cells and LGN cells make an important contribution to the receptive-field organization of cortical neurons. Changes in receptive-field organization of cortical cells are apparently not accompanied by alterations of other basic response properties. Orientation (7 cells) and spatial frequency (53 cells) selectivity remain relatively unchanged when measured at different luminances. Although optimal spatial frequency is slightly lower at low luminance levels, the low spatial frequency attenuation remains unaltered. Since receptive-field changes between high and low luminance levels suggest that a unit's classification may also vary, we examined simple and complex cell characteristics using sinusoidal gratings (65 cells). Contrary to what we had anticipated, the degree of modulation of responses was relatively independent of luminance, indicating that cell classification does not vary with stimulus luminance.

Animals↗
PubMed1985

Contrast gain control in the cat's visual system.

We have examined the idea that the adaptation of cortical neurons to local contrast levels in a visual stimulus is functionally advantageous. Specifically, cortical cells may have large differential contrast sensitivity as a result of adjustments that center a limited response range around a mean level of contrast. To evaluate this notion, we measured contrast-response functions of cells in striate cortex while systematically adapting them to different contrast levels of stimulus gratings. For the majority of cortical neurons tested, the results of this basic experiment show that contrast-response functions shift laterally along a log-contrast axis so that response functions match mean contrast levels in the stimulus. This implies a contrast-dependent change in the gain of the cell's contrast-response relationship. We define this process as contrast gain control. The degree to which this contrast adjustment occurs varies considerably from cell to cell. There are no obvious differences regarding cell type (simple vs. complex) or laminar distribution. Contrast gain control is almost certainly a cortical function, since lateral geniculate cells and fibers exhibit only minimal effects. Tests presented in the accompanying paper (37) provide additional evidence on the cortical origin of the process. In another series of experiments, the effect of contrast adaptation on physiological estimates of contrast sensitivity was evaluated. Sustained adaptation to contrast levels as low as 3% was capable of nearly doubling the thresholds of most of the cells tested. Adaptation may therefore be an important factor in determinations of the contrast sensitivity of cortical neurons. We tested the spatial extent of the mechanisms responsible for these gain-control effects by attempting to adapt cells using both a large grating and a grating patch limited to that portion of a cell's receptive field from which excitatory discharges could be elicited directly (the central discharge region). Adaptation was found to be an exclusive property of the central region. This held even in the case of hypercomplex cells, which received strong influences from surrounding regions of the visual field. Finally, we measured the time course of contrast adaptation. We found the process to be rather slow, with a mean time constant of approximately 6 s. Once again, there was considerable variability in this value from cell to cell.

Acclimatization↗
PubMed1985

Contrast gain control in the kitten's visual system.

We have studied the effects of contrast adaptation on cortical cells from 4- and 6-wk-old kittens (49 and 47 cells, respectively) using sine-wave grating stimuli. We wished to know if the effects of adaptation to different contrast levels are more extensive than those in adult animals. Our experiments involved adapting cells to different contrasts (3.1, 12.5, and 50%) while concurrently measuring their contrast-response functions at each of these different levels. We found qualitatively that the effects of adaptation in the kitten are similar to those we have previously documented in adult animals (19). Contrast-response functions are laterally shifted along the log-contrast axis, effectively matching the response range of the cells to prevailing contrast levels. The degree to which this occurred varied from cell to cell. The average degree to which cells showed these effects, as assessed both qualitatively and quantitatively, was greater for kittens than for adult cats, and greater for 4-wk-old kittens than for those aged 6 wk. This suggests that susceptibility to adaptation varies as a function of age. Additional studies were undertaken with the intent of localizing these adaptive effects. First, lateral geniculate cells and fibers (n = 23) were studied with our standard protocol, and second, we investigated the degree to which the effects of adaptation of cortical cells transferred interocularly.(ABSTRACT TRUNCATED AT 250 WORDS)

Acclimatization↗
PubMed1985

Pattern evoked potentials from the cat's retina.

We have studied electroretinograms (ERG) in the cat using phase-reversed sinusoidal gratings as a stimulus. Our purpose was to characterize response properties of this type of ERG. One basic question we addressed was whether the response to a grating stimulus is actually pattern specific. For the purpose of comparison, we used the same stimulus to investigate mass potentials from the lateral geniculate nucleus (LGN) and the visual cortex. The pattern ERG consists mainly of a vitreous negative after potential peaking shortly (120-200 ms) after reversal of the pattern. There is a notable absence in the pattern ERG of a b-wave that, however, can be elicited by a step increase of luminance over a uniform field. Pattern ERG amplitudes decrease monotonicaly with increasing spatial frequency and show no low-frequency attenuation when the pattern is phase reversed in square-wave fashion. This is markedly different than evoked potentials from the LGN and visual cortex that show band-pass characteristics. On the other hand, sinusoidal phase reversal reveals a clear attenuation of the pattern ERG amplitude at low spatial frequencies, whereas this type of stimulation produces very poor responses from LGN and visual cortex. The low spatial-frequency attenuation in the pattern ERG shows that the generating mechanism involves lateral interactions. There is thus a clear pattern-specific component in the pattern ERG. The pattern ERG has a surprisingly high contrast threshold relative to those estimated from cortical and LGN evoked potentials. Above threshold, pattern ERG response amplitude increases rapidly with contrast, but it often shows saturation at high contrast levels. These saturation points are generally high when contrast thresholds are high so that the rising portion of the contrast-response functions have fairly uniform slopes. Contrast-response curves from the LGN and cortical potentials are quite different from those for the retina in that amplitudes increase approximately linearly with log contrast over a 2-log-unit range (1 to 100%).

Animals↗
PubMed1985

Neurophysiological evaluation of the differential response model for orientation and spatial-frequency discrimination.

Recent models have attempted to reconcile low psychophysical orientation and spatial-frequency discrimination thresholds with relatively broad orientation and spatial-frequency tuning of cortical neurons. These models have relied on the ability of the neurons to convert small stimulus changes into reliable response changes. We have examined this ability in a sample of neurons from the cat's striate cortex. We present here data from two cells that reliably signaled the smallest orientation and spatial-frequency differences. Using receiver operating characteristic analysis, we find that these cells could reliably signal orientation differences of 1.84 deg and spatial-frequency differences of 0.073 octave. We compare these single-cell results to cat and human behavioral discrimination thresholds.

Animals↗
PubMed1984

Tourette's syndrome: update.

Tourette's syndrome is a widely misunderstood chronic disorder that develops in childhood and is usually lifelong. It is characterized by waxing and waning of involuntary motor and phonic tics. The features and differential diagnosis are discussed in this paper. The estimated prevalence rate of Tourette's syndrome, 0.05%, implies that this disorder is not rare. The reasons for diagnostic confusion are outlined, and the genetic and neurotransmitter features discussed. The management of Tourette's syndrome has become more effective with the availability of at least two psychoactive drugs, haloperidol and pimozide. Although the cause of this syndrome is thought to be organic, these drugs and their adverse effects are best known to psychiatrists. Psychiatric and multidisciplinary intervention is often necessary because of the frequent association of psychosocial problems, cognitive and learning difficulties, and aggravation of the symptoms by stress. The understanding of Tourette's syndrome will probably increase significantly with the advent of the newer imaging techniques and the rapid progress of research in the neurosciences.

Adolescent↗
PubMed1984

Stimulus specificity of binocular cells in the cat's visual cortex: ocular dominance and the matching of left and right eyes.

Most cells in the striate cortex respond to visual stimulation through either eye. We have examined quantitatively the matching of response specificity for the two eyes. Our intention was to determine the degree to which this matching depends on ocular dominance. We used standard single cell recording techniques and studied responses to sinusoidal gratings of different spatial frequencies, orientations, and contrasts. For all tests, stimuli were randomly interleaved both with respect to the value of each parameter, and the eye which was stimulated. After estimating ocular dominance qualitatively and quantitatively, we measured: response modulation (to help identify whether a cell was simple or complex), orientation and spatial frequency tuning, and contrast response functions (to estimate contrast thresholds). Results show that: (1) Response modulation is well matched between the two eyes, but there is a slight tendency for the dominant eye to respond with less modulation. (2) Optimal orientation and spatial frequency and their respective tuning widths were similar for the two eyes. In general, tuning functions for the two eyes differed mainly in slope. However, in each case, there was a tendency for the dominant eye to have broader tuning widths. (3) In most cases, contrast response functions for the two eyes differed mainly in their slopes. Extrapolation to spontaneous levels suggests that estimated contrast thresholds are relatively independent of ocular dominance although, again, there ws a tendency for the dominant eye to exhibit slightly lower estimated thresholds. These findings demonstrate that response characteristics between the two eyes are generally well matched regardless of relative response strength. There are, however, small but clear differences between the two eyes for all parameters we measured which are related to and demonstrate that ocular dominance influences the degree of matching between the two eyes.

Animals↗
PubMed1983

On the relation between aniseikonia and axial anisometropia.

A dichoptic, size-matching technique was used to evaluate aniseikonia in a sample of axial anisometropes. During testing, the subjects were optically corrected to minimize differences in retinal image size. A clinically significant degree of aniseikonia was measured which varied in proportion to the magnitude of anisometropia. These results may reflect interocular differences in the spatial density of the retinal elements secondary to anisometropic ocular growth.

Aniseikonia↗
PubMed1983

Eye-pressing by visually impaired children.

Many children with severely impaired sight exhibit stereotyped mannerisms. Visual self-stimulation, e.g. eye-pressing and light-gazing, normally is restricted to the visually impaired; prolonged eye-pressing is the most common. This behaviour depends on onset of visual impairment, age, degree and quality of residual light, type of ocular abnormality, the presence of additional handicaps, and the activities in which the child is involved. Children with bilateral optic-nerve defects never press their eyes; those with retinal disorders tend to press vigorously. A possible physiological explanation is that self-stimulation occurs when the demand of the brain for meaningful visual information is not adequately met.

Adolescent↗
PubMed1983

Nonoptical determinants of aniseikonia.

Interocular differences in apparent size (aniseikonia) are typically associated with interocular differences in refractive error (anisometropia). Aniseikonia is generally thought to reflect disparities in retinal image size that often accompany anisometropia. This assumption was examined with seven highly anisometropic subjects who were tested under conditions in which no substantial retinal image size differences were present. Using a dichoptic size matching task, consistent and large (mean = 22%) aniseikonias were found. Myopic anisometropes exhibit perceptual minification, while hyperopes demonstrate perceptual magnification when using their more ametropic eye. Both ultrasonic and fundus examinations of these subjects indicate that differential retinal growth or stretching is responsible for these findings.

Aniseikonia↗
PubMed1982

Contrast gain control in the cat visual cortex.

The eye functions effectively over an enormous range of ambient illumination, because retinal sensitivity can be adapted to prevailing light levels. Higher order neurones in the visual pathway are presumably more concerned with relative changes in illumination, that is, contrast, because a great deal of information concerning absolute light level is processed at the retinal level. It would therefore be of considerable functional value if cells in the visual cortex could adapt their response levels to a steady-state ambient contrast, in a manner analogous to the sensitivity control mechanism of the retina. We have examined here the idea that adaptation of neurones in the visual cortex to ambient contrast is similar to adaptation in the retina to ambient illumination. The experiments were performed by measuring contrast response functions (response amplitude as a function of contrast) of striate neurones, while systematically adapting them to different contrast levels. Our results show that, for the majority of cortical neurones, response-contrast curves are laterally shifted along a log-contrast axis so that the effective domains of neurones are adjusted to match prevailing contrast levels. This contrast gain control mechanism, which was not observed for lateral geniculate (LGN) fibres, must be of prime importance to visual function.

Adaptation, Ocular↗
PubMed1982

A new approach to the study of binocular interaction in visual cortex: normal and monocularly deprived cats.

Retinal disparity sensitivity was examined for cells in areas 17 and 18 of the cat's visual cortex. As a stimulus, we used drifting sinusoidal gratings which were spatially phase-shifted in one eye with respect to the other. The gratings were of high contrast (80%) and optimal values of spatial and temporal frequencies and orientations were used. In the normal cat cortex, binocular cells in areas 17 and 18 were all sensitive to phase shifts, but only minimal effects were observed for monocular cells. Recordings were also made from monocularly deprived cats, but no binocular influence was found from the silent eye.

Animals↗
PubMed1982

A quantitative study of the classification and stability of ocular dominance in the cat's visual cortex.

We recorded from single cells in the cat's visual cortex to quantitatively evaluate (1) the reliability of subjective assessments of ocular dominance (101 cells) and (2) the stability of ocular dominance over time (25 cells). We found that the correlation between subjective and objective measures of this variable was poorer than expected, and was worst for cells with low overall response strengths. This result appears to reflect variability in the subjective assessment procedure. For the second part of the study, we recorded from single cortical cells of 5-week-old kittens, and made repeated objective measurements of ocular dominance over time. Twenty-four of the twenty-five cells examined were quite stable in ocular dominance for periods so long as 8 h. One unit was encountered which showed substantial progressive shifts in ocular dominance over time.

Animals↗