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Biomedical subjects

Donald E Mitchell

Publications and source records attributed to Donald E Mitchell.

8 recordsLinked to original sources
PubMed2006

Short periods of concordant binocular vision prevent the development of deprivation amblyopia.

Based in part on deprivation studies, it is generally agreed that the development of vision and of the central visual pathways of higher mammals such as cats and primates is experience-dependent. Past deprivation experiments employed periods of exclusively abnormal early visual input. Because of the absence of any normal visual input, such studies indicate only the extremes to which the visual system can change in response to visually driven activity (i.e. its capabilities) rather than provide insight into the role of early visual input in normal development (i.e. what it actually does). We examined the possibility that certain visual input, i.e. normal concordant binocular vision, may be more efficacious than others with respect to its effects on the developing visual system and on vision. On a daily basis, one type of visual input, i.e. normal binocular experience (BE), was pitted against abnormal (monocular exposure, ME) input in order to see if one was more effective. We show that 2 h of daily normal concordant, but not discordant, BE outweighs or protects against as much as 5 h of daily abnormal input to permit the development of normal grating acuity and alignment accuracy in the two eyes. Further, we show that splitting the period of BE into two 1-h periods straddling the period of ME was ineffective, thereby indicating the 2 h of BE each day must be continuous to protect against the development of amblyopia.

Adaptation, Physiological↗
PubMed2005

Haphazard neural connections underlie the visual deficits of cats with strabismic or deprivation amblyopia.

Identification of the neural basis of the visual deficits experienced by humans with amblyopia, particularly when associated with strabismus (strabismic amblyopia), has proved to be difficult in part because of the inability to observe directly the neural changes at various levels of the human visual pathway. Much of our knowledge has necessarily been obtained on the basis of sophisticated psychophysical studies as well as from electrophysiological explorations on the visual pathways in animal models of amblyopia. This study combines these two approaches to the problem by employing similar psychophysical probes of performance on animal models of two forms of amblyopia (deprivation and strabismic) to those employed earlier on human amblyopes (Hess & Field, 1994, Vis. Res., 34, 13397-13406). The tests explore two competing explanations for the visual deficits, namely an evenly distributed loss of neural connections (undersampling) with the amblyopic eye as opposed to disordered connections with this eye (neural disarray). Unexpectedly, the results in animal models of deprivation amblyopia were not in accord with expectations based upon an even distribution of lost connections with the amblyopic eye. However, the results were similar to those observed in a strabismic amblyopic animal and to strabismic amblyopic humans. We suggest that deprivation amblyopia may be accompanied by an uneven loss of connections that results in effective neural disarray. By contrast, amblyopia associated with strabismus might arise from neural disarray of a different origin such as an alteration of intrinsic cortical connections.

Amblyopia↗
PubMed2004

The spatial localization deficit in visually deprived kittens.

We measured the spatial localization abilities (alignment accuracy) of visually deprived kittens by use of similar spatially bandpass stimuli (Gaussian blobs) to those employed for the assessment of human amblyopes. The tests of vision were conducted on kittens reared with either strabismus or following different periods of monocular deprivation. As with amblyopic humans, the deficits in alignment accuracy were scaled in proportion to blob size and were not only considerably larger than those of grating acuity but also were not correlated with either the acuity or contrast sensitivity losses. Tests with stimuli of various contrast revealed that the deficits could not be explained in terms of the contrast sensitivity loss in this eye. The positional deficits that arise from anomalous visual development are independent of the contrast sensitivity loss and profound.

Amblyopia↗
PubMed2003

Brief daily periods of binocular vision prevent deprivation-induced acuity loss.

The role of experience in the development of the central visual pathways has been explored in the past through examination of the consequences of imposed periods of continuously abnormal or biased visual input. The massive changes in the visual cortex (area 17) induced by selected early visual experience, especially monocular deprivation (MD) or experience (ME) where patterned visual input is provided to just one eye, are accompanied by profound and long-standing visual deficits. Although the use of exclusively abnormal experience permits identification of those aspects of the visual cortex and of visual function that can be influenced by visual experience during development, this approach may provide a distorted view of the nature of the role of visual experience because of the absence of any normal visual input. In this study a different approach was used whereby animals were provided daily with separate periods of normal (i.e., binocular exposure) and abnormal (monocular exposure) visual experience. We show that 2 hr of daily normal concordant binocular experience (BE) can outweigh or protect against much longer periods of monocular deprivation (MD) and permit the development of normal visual acuities in the two eyes. This result is not what would be expected if all visual input had equal influence on visual development.

Animals↗
PubMed2002

Correlated binocular activity guides recovery from monocular deprivation.

Monocular deprivation (MD) has much more rapid and severe effects on the ocular dominance of neurons in the primary visual cortex (V1) than does binocular deprivation. This finding underlies the widely held hypothesis that the developmental plasticity of ocular dominance reflects competitive interactions for synaptic space between inputs from the two eyes. According to this view, the relative levels of evoked activity in afferents representing the two eyes determine functional changes in response to altered visual experience. However, if the deprived eye of a monocularly deprived kitten is simply reopened, there is substantial physiological and behavioural recovery, leading to the suggestion that absolute activity levels, or some other non-competitive mechanisms, determine the degree of recovery from MD. Here we provide evidence that correlated binocular input is essential for such recovery. Recovery is far less complete if the two eyes are misaligned after a period of MD. This is a powerful demonstration of the importance of cooperative, associative mechanisms in the developing visual cortex.

Animals↗
PubMed2002

The present and potential impact of research on animal models for clinical treatment of stimulus deprivation amblyopia.

OBJECTIVE: With the benefit of hindsight based on an additional 20 years of research, we review a question posed originally by Marg of whether animal models for stimulus deprivation amblyopia in children are valid or useful for clinical application. METHOD: Following a review of relevant research on animal models, the human clinical literature on treatment of stimulus deprivation amblyopia has been reviewed with respect to past and current impact of animal research on clinical treatment. In addition, we speculate on the potential future clinical impact of animal work on developmental plasticity in the visual cortex that is directed towards an understanding of its underlying molecular basis. CONCLUSIONS: Animal research that has begun to define the timing, nature and sites of critical periods in the central visual pathways with greater precision than was known 20 years ago has had a demonstrable impact on clinical practice. In turn, these changes in clinical practice have produced far better outcomes than prior to 1980, for both the acuity of the amblyopic eye and for binocular functions such as stereopsis.

Amblyopia↗