Melatonin treatment of chronic sleep disorders.
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Child↗
Biomedical subjects
R D Freeman
Publications and source records attributed to R D Freeman.
Who is a visually impaired child?
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Blindness, Cortical↗
Neural mechanisms underlying binocular fusion and stereopsis: position vs. phase.
The visual system utilizes binocular disparity to discriminate the relative depth of objects in space. Since the striate cortex is the first site along the central visual pathways at which signals from the left and right eyes converge onto a single neuron, encoding of binocular disparity is thought to begin in this region. There are two possible mechanisms for encoding binocular disparity through simple cells in the striate cortex: a difference in receptive field (RF) position between the two eyes (RF position disparity) and a difference in RF profile between the two eyes (RF phase disparity). Although there have been studies supporting each of the two encoding mechanisms, both mechanisms have not been examined in a single study. Therefore, the relative roles of the two mechanisms have not been determined. To address this issue, we have mapped left and right eye RFs of simple cells in the cat's striate cortex using binary m-sequence noise, and then we have estimated RF position and phase disparities. We find that RF position disparities are generally limited to small values that are not sufficient to encode large binocular disparities. In contrast, RF phase disparities cover a wide range of binocular disparities and exhibit dependencies on orientation and spatial frequency in a manner expected for a mechanism that encodes binocular disparity. These results indicate that binocular disparity is mainly encoded through RF phase disparity. However, RF position disparity may play a significant role for cells with high spatial frequency selectivity, which are constrained to small RF phase disparities.
Animals↗
The neural coding of stereoscopic depth.
Stereopsis is a process by which the visual system gauges the relative depth of objects in three-dimensional space by measuring minute positional differences between left and right images. According to the standard notion, this information is thought to be encoded in the primary visual cortex by differences in receptive field (RF) positions for the two eyes. We have developed in alternative model by which stereoscopic information is coded and transformed through a hierarchical chain of processing in the primary visual cortex. Initially, first-order neurons of the visual cortex, simple cells, encode depth information by a scheme based on differences in internal receptive field structure between left and right eyes. Further abstraction of information is achieved by a subset of second-order neurons, complex cells, that are well suited for the detection of depth information in a manner unaffected by positional variations of objects. We review physiological evidence from studies of the cat and monkey that are relevant to the proposed scheme.
Animals↗
Attention deficit hyperactivity disorder in the presence of Tourette syndrome.
Increasing attention to comorbidity in Tourette syndrome has led to a search for factors that would help in the development of treatment strategies for attention deficit hyperactivity disorder. Despite arguments and some research supporting various viewpoints, it is still unclear to what extent the frequency of co-occurrence is high in Tourette syndrome. It also is unclear whether attention deficit hyperactivity disorder and obsessive-compulsive disorder are part of a spectrum with related genetic or environmental factors. One of the main difficulties is obtaining a truly representative community sample and agreeing upon the categorical boundaries. The consequences of this argument are discussed. Effective treatment of attention deficit hyperactivity disorder in the presence of tics and Tourette syndrome has undergone considerable change from an absolute contraindication of stimulants to cautious use. For those whom stimulants fail or produce unacceptable side effects, nonstimulant medications are available in increasing variety. Much more methodologically adequate research needs to be undertaken with them.
Attention Deficit Disorder with Hyperactivity↗
Encoding of binocular disparity by complex cells in the cat's visual cortex.
To examine the roles that complex cells play in stereopsis, we have recorded extracellularly from isolated single neurons in the striate cortex of anesthetized paralyzed cats. We measured binocular responses of complex cells using a comprehensive stimulus set that encompasses all possible combinations of positions over the receptive fields for the two eyes. For a given position combination, stimulus contrast could be the same for the two eyes (2 bright or 2 dark bars) or opposite (1 bright and 1 dark). These measurements provide a binocular receptive field (RF) profile that completely characterizes complex cell responses in a joint domain of left and right stimulus positions. Complex cells typically exhibit a strong selectivity for binocular disparity, but are only broadly selective for stimulus position. For most cells, selectivity for disparity is more than twice as narrow as that for position. These characteristics are highly desirable if we assume that a disparity sensor should exhibit position invariance while encoding small changes in stimulus depth. Complex cells have nearly identical binocular RFs for bright and dark stimuli as long as the sign of stimulus contrast is the same for the two eyes. When stimulus contrast is opposite, the binocular RF also is inverted such that excitatory subregions become suppressive. We have developed a disparity energy model that accounts for the behavior of disparity-sensitive complex cells. This is a hierarchical model that incorporates specific constraints on the selection of simple cells from which a complex cell receives input. Experimental data are used to examine quantitatively predictions of the model. Responses of complex cells generally agree well with predictions of the disparity energy model. However, various types of deviations from the predictions also are found, including a highly elongated excitatory region beyond that supported by a single energy mechanism. Complex cells in the visual cortex appear to provide a next level of abstraction in encoding information for stereopsis based on the activity of a group of simple-type subunits. In addition to exhibiting narrow disparity tuning and position invariance, these cells seem to provide a partial solution to the stereo correspondence problem that arises in complex natural scenes. Based on their binocular response properties, these cells provide a substantial reduction in the complexity of the correspondence problem.
Animals↗
Spatiotemporal receptive field organization in the lateral geniculate nucleus of cats and kittens.
We have studied the spatiotemporal receptive-field organization of 144 neurons recorded from the dorsal lateral geniculate nucleus (dLGN) of adult cats and kittens at 4 and 8 wk postnatal. Receptive-field profiles were obtained with the use of a reverse correlation technique, in which we compute the cross-correlation between the action potential train of a neuron and a randomized sequence of long bright and dark bar stimuli that are flashed throughout the receptive field. Spatiotemporal receptive-field profiles of LGN neurons generally exhibit a biphasic temporal response, as well as the classical center-surround spatial organization. For nonlagged cells, the first temporal phase of the response dominates, whereas for lagged neurons, the second temporal phase of the response is typically the largest. This temporal phase difference between lagged and nonlagged cells accounts for their divergent behavior in response to flashed stimuli. Most LGN cells exhibit some degree of space-time inseparability, which means that the receptive field cannot simply be viewed as the product of a spatial waveform and a temporal waveform. In these cases, the response of the surround is typically delayed relative to that of the center, and there is some blending of center and surround during the time course of the response. We demonstrate that a simple extension of the traditional difference-of-Gaussians (DOG) model, in which the surround response is delayed relative to that of the center, accounts nicely for these findings. With regard to development, our analysis shows that spatial and temporal aspects of receptive field structure mature with markedly different time courses. After 4 wk postnatal, there is little change in the spatial organization of LGN receptive fields, with the exception of a weak, but significant, trend for the surround to become smaller and stronger with age. In contrast, there are substantial changes in temporal receptive-field structure after 4 wk postnatal. From 4 to 8 wk postnatal, the shape of the temporal response profile changes, becoming more biphasic, but the latency and duration of the response remain unchanged. From 8 wk postnatal to adulthood, the shape of the temporal profile remains approximately constant, but there is a dramatic decline in both the latency and duration of the response. Comparison of our results with recent data from cortical (area 17) simple cells reveals that the temporal development of LGN cells accounts for a substantial portion of the temporal maturation of simple cells.
Analysis of Variance↗
Studies of functional connectivity in the developing and mature visual cortex.
We have investigated several aspects of cortical organization in adult cats and in young kittens. First, we determined receptive field (RF) maps of correlated discharge between pairs of cortical cells. Unique bicellular RFs appear to convey high resolution information. Second, we studied the dynamics of neural interaction between pairs of cells. Using cross-correlation analysis, we studied monosynaptic and polysynaptic interactions in both kittens and cats. A somewhat surprising finding is that there were no cases of monosynaptic excitation from simple to complex cells as would be predicted by a simple hierarchical processing theory. Third, we studied length and side tuning characteristics of cortical cells and worked out the relationships between them. Fourth we carried out an investigation of binocular processing in which we compared monocular and binocular sensitivity of cortical cells with respect to contrast. Our results are comparable to those found in psychophysical work. Fifth, we examined how stereoscopic depth information is encoded by simple cells in the visual cortex. We show that structural differences in RFs of left and right eyes may be expressed in terms of phase. Phase-based encoding appears to be a very plausible alternative to the standard position-based notion. Sixth, we attempted to induce plastic changes in connections between cell pairs by long-term activation (up to 2 h) in kittens and cats. Although connection strength between some cell pairs was increased during long-term activation, there was no consistent pattern of this effect. Seventh, we attempted to study the functional basis of reported claims of RF expansion following use of an artificial scotoma. However, we found no receptive field size change from this procedure. For some cells, there is an apparent change of gain in the form of base (spontaneous) rates and absolute response levels. Finally, we have examined RF dynamics in the central visual pathways. The standard treatment of RFs in to consider only spatial aspects. But the RF is inherently both temporal and spatial in nature and we have examined the dynamics of spatiotemporal organization of RFs in central visual pathways.
Animals↗
Encoding of binocular disparity by simple cells in the cat's visual cortex.
1. Spatiotemporal receptive fields (RFs) for left and right eyes were studied for simple cells in the cat's striate cortex to examine the idea that stereoscopic depth information is encoded via structural differences of RFs between the two eyes. Traditional models are based on neurons that possess matched RF profiles for the two eyes. We propose a model that requires a subset of simple cells with mismatched RF profiles for the two eyes in addition to those with similar RF structure. 2. A reverse correlation technique, which allows a rapid measurement of detailed RF profiles in the joint space-time domains, was used to map RFs for isolated single neurons recorded extracellularly in the anesthetized paralyzed cat. 3. Approximately 30% of our sample of cells shows substantial differences between spatial RF structure for the two eyes. Nearly all of these neurons prefer orientations between oblique and vertical, and are therefore presumed to be involved in processing horizontal disparities. On the other hand, cells that prefer orientations near horizontal have matched RF profiles for the two eyes. Considered together, these findings suggest that the visual system takes advantage of the orientation anisotropy of binocular disparities present in the retinal images. 4. For some cells, the spatial structure of the RF changes over the time course of the response (inseparable RF in the space-time domain). In these cases, the change is similar for the two eyes, and therefore the difference remains nearly constant at all times. Because the difference of the RF structure between the two eyes is the critical determinant of a cell's relative depth selectivity for the proposed model, space-time inseparability of RFs is not an obstacle for consistent representation of stereoscopic information. 5. RF parameters including amplitude, RF width, and optimal spatial frequency are generally well matched for the two eyes over the time course of the response. The preferred speed and direction of motion are also well matched for the two eyes. These results suggest that the encoding of motion in depth is not likely to be a function of simple cells in the striate cortex. 6. The results presented here are consistent with our model, in which stereoscopic depth information is encoded via differences in the spatial structure of RFs for the two eyes. This model provides a natural binocular extension of the current notion of monocular spatial form encoding by a population of simple cells. Note, however, that our findings do not exclude the possibility that positional shifts of RFs also play a role in determining the disparity selectivity of cortical neurons.
Animals↗
Receptive field structure in the visual cortex: does selective stimulation induce plasticity?
Sensory areas of adult cerebral cortex can reorganize in response to long-term alterations in patterns of afferent signals. This long-term plasticity is thought to play a crucial role in recovery from injury and in some forms of learning. However, the degree to which sensory representations in primary cortical areas depend on short-term (i.e., minute to minute) stimulus variations remains unclear. A traditional view is that each neuron in the mature cortex has a fixed receptive field structure. An alternative view, with fundamentally different implications for understanding cortical function, is that each cell's receptive field is highly malleable, changing according to the recent history of the sensory environment. Consistent with the latter view, it has been reported that selective stimulation of regions surrounding the receptive field induces a dramatic short-term increase in receptive field size for neurons in the visual cortex [Pettet, M. W. & Gilbert, C. D. (1992) Proc. Natl. Acad. Sci. USA 89, 8366-8370]. In contrast, we report here that there is no change in either the size or the internal structure of the receptive field following several minutes of surround stimulation. However, for some cells, overall responsiveness increases. These results suggest that dynamic alterations of receptive field structure do not underlie short-term plasticity in the mature primary visual cortex. However, some degree of short-term adaptability could be mediated by changes in responsiveness.
Animals↗
A flexible PC-based physiological monitor for animal experiments.
We have developed a flexible physiological monitoring and analysis system for physiological studies in which data are obtained over extended periods. Our system uses low-cost personal computer hardware to concentrate data from existing multiple monitoring devices. All monitored parameters are displayed on a single screen and recorded in a single file. The system automates the process of physiological record keeping by providing continuous displays of vital signs. In addition, audible and visual alarms are produced when vital signs are outside of acceptable ranges, prompting the experimenter to take corrective actions. The central element of the system is a program running in a dedicated manner on an IBM PC-compatible computer. The program is written in the C language and makes use of a graphics library to display traces and analysis results in real time on any standard display for the PC. This program assigns the analog channels of an A/D board to particular physiological parameters by initially reading a configuration file, which also describes the alarm conditions and analysis routine for each parameter. All hardware specific code is isolated into well-defined modules. The program is both highly flexible with regard to different sets of parameters and highly portable for different experimental and computer environments.
Animals↗
Receptive-field dynamics in the central visual pathways.
Neurons in the central visual pathways process visual images within a localized region of space, and a restricted epoch of time. Although the receptive field (RF) of a visually responsive neuron is inherently a spatiotemporal entity, most studies have focused exclusively on spatial aspects of RF structure. Recently, however, the application of sophisticated RF-mapping techniques has enabled neurophysiologists to characterize RFs in the joint domain of space and time. Studies that use these techniques have revealed that neurons in the geniculostriate pathway exhibit striking RF dynamics. For a majority of cells, the spatial structure of the RF changes as a function of time; thus, these RFs can be characterized adequately only in the space-time domain. In this review, the spatiotemporal RF structure of neurons in the lateral geniculate nucleus and primary visual cortex is discussed.
Animals↗
Neuronal mechanisms underlying stereopsis: how do simple cells in the visual cortex encode binocular disparity?
Binocular neurons in the visual cortex are thought to form the neural substrate for stereoscopic depth perception. How are the receptive fields of these binocular neurons organized to encode the retinal position disparities that arise from binocular parallax? The conventional notion is that the two receptive fields of a binocular neuron have identical shapes, but are spatially offset from the point of retinal correspondence (zero disparity). We consider an alternative disparity-encoding scheme, in which the two receptive fields may differ in shape (or phase), but are centered at corresponding retinal locations. Using a reverse-correlation technique to obtain detailed spatiotemporal receptive-field maps, we provide support for the latter scheme. Specifically, we show that receptive-field profiles for the left and right eyes are matched for cells that are tuned to horizontal orientations of image contours. However, for neurons tuned to vertical orientations, the left and right receptive fields are predominantly dissimilar in shape. These results show that the striate cortex possesses a specialized mechanism for processing vertical contours, which carry the horizontal-disparity information needed for stereopsis. Thus, in a major modification to the traditional notion of the neural basis of stereopsis, we propose that binocular simple cells encode horizontal disparities in terms of phase at multiple spatial scales. Implications of this scheme are discussed with respect to the size-disparity correlation observed in psychophysical studies.
Algorithms↗
DSM-IV criteria for Tourette's.
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Adaptation, Psychological↗
Familial paroxysmal kinesigenic choreo-athetosis in a child with visual hallucinations and obsessive-compulsive behaviour.
A teenage male is described, in whom Tourette Syndrome was suspected, which was later replaced by attacks of paroxysmal kinesigenic choreo-athetosis. He also exhibited bizarre, episodic perceptual distortions of his visual environment and manifestations of an obsessive-compulsive disorder. Carbamazepine treatment not only completely eliminated the recurring attacks of his choreo-athetosis but also, contrary to expectations, the visual disturbances and even the symptoms of his obsessive-compulsive disorder. Obsessive compulsive disorder should be searched for by direct questioning in all patients with basal ganglia disorders.
Adolescent↗
Binocular summation in orientation discrimination depends on stimulus contrast and duration.
Binocular summation, an improvement in visual performance with binocular viewing compared to monocular viewing, has been studied extensively in detection tasks. Monocular detection thresholds for stationary stimuli are typically about 40% higher than binocular thresholds. Binocular summation in discrimination tasks, however, is often lower and less consistent. A possible explanation for this difference is that saturation of responses limits the extent of binocular summation in discrimination tasks. To investigate this possibility, we used an orientation discrimination task and varied stimulus contrast and exposure duration. Monocular and binocular orientation discrimination thresholds were obtained using one-dimensional difference-of-Gaussian stimuli. For briefly exposed stimuli, binocular summation is greatest at low contrasts (e.g. 66% at 8% contrast) and is reduced systematically at higher contrasts so that monocular and binocular thresholds are approximately equal at contrasts above 15%. Binocular summation for low-contrast stimuli is greatest at a brief exposure duration (50 msec), is reduced at longer durations, and is not significant at durations of 100 msec or longer. Thus, binocular summation in orientation discrimination is greatest for relatively low-energy stimuli. These results are consistent with models of binocular energy summation and the hypothesis that saturation of responses after binocular combination can limit binocular summation in discrimination tasks.
Contrast Sensitivity↗
Rapid cycling in severely multidisabled children: a form of bipolar affective disorder?
A debilitating, regularly recurring, biphasic disorder is described in 6 severely multidisabled children. It was characterized by several days of lethargy, withdrawal, loss of abilities, irritability, and hypersomnolence followed or preceded by a high-energy state for several days during which the children slept very little, at times were euphoric, had improved mental ability, and were hyperactive. These cyclic episodes had been present for years but unexpectedly disappeared in one child. The etiology is unknown, in spite of detailed neurologic, metabolic, and endocrine investigations. All patients had family histories positive for affective disorder. Melatonin treatment helped to regulate the coexisting chronic sleep disorders of 3 children but failed to eliminate the cycles. Antiepileptic drug treatment, lithium, sedatives, stimulants, tranquilizers, and light therapy were largely ineffective. The children's symptoms and signs fit the diagnostic criteria of a bipolar affective illness, as it was modified for patients with associated neurologic disability; therefore, the patients appeared to have a unique disorder that closely resembles or is a variant of rapid cycling affective disorder.
Adolescent↗
Eye-poking.
Eye-poking, -rubbing and -pressing are often incorrectly grouped together because of the assumption that they represent variations of the same self-stimulating behaviour. This prospective study of 21 children shows that eye-poking is a distinct, chronic, stereotyped, self-injurious act seen mainly among severely mentally disabled individuals, who may or may not be visually impaired. Eye-poking, which leads to intense, self-induced pain, is a harmful behaviour because it can result in permanent visual loss and even in total blindness. The cause is unclear, although it is probably multifactorial. The treatment is exceedingly difficult. Further studies are needed to develop specific management techniques to prevent eye-poking, which adds to the physical and emotional burden on the caregivers.
Adolescent↗