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Biomedical subjects

D E Mitchell

Publications and source records attributed to D E Mitchell.

At least 73 records · Page 4Linked to original sources
PubMed1977

A behavioural technique for the rapid assessment of the visual capabilities of kittens.

A behavioural method is described for the measurement of various visual spatial acuities in kittens as young as thirty days of age. Expamples are given of applications of the technique to measurement of the visual acuity for gratings in normal kittens as well as to studies of the time course of behavioural recovery from the effects of early monocular visual deprivation.

Age Factors↗
PubMed1977

Diffuse brain damage of immediate impact type. Its relationship to 'primary brain-stem damage' in head injury.

In a neuropathological analysis of 151 fatal non-missile head injuries, there were 19 cases with focal lesions in the dorsolateral quadrant of the brain-stem in the corpus callosum, and histological evidence of diffuse damage to white matter. Eight of these cases had not experienced a high intracranial pressure during life. All 19 cases had been rendered unconscious at the moment of impact and had remained so or in the persistent vegetative state until death. It is therefore concluded that diffuse damage to white matter may occur as a primary event at the moment of impact, that is, it is one type of immediate impact damage to the brain. It is also concluded that this type of damage is the pathological basis of 'primary brain-stem injury' since in no patient thought clinically to have sustained 'primary brain-stem injury' were abnormalities confined to the brain-stem. Since no patient with this type of brain damage recovered consciousness after injury, it is probable that diffuse damage to white matter is the most important single factor governing the outcome in a patient who sustains a non-missile head injury.

Adolescent↗
PubMed1977

A physiological and behavioural study in cats of the effect of early visual experience with contours of a single orientation.

1. Three kittens were reared in visual environments that consisted of stripes at one of three orientations - horizontal, right oblique, or left oblique. Two additional cats were reared as controls. One of these matured viewing right and left oblique stripes on alternate days. The other experienced a normal visual environment. 2. Following the completion of rearing, and after several weeks of normal visual experience, behavioural testing of the stripe-reared animals demonstrated a deficit in visual acuity for orientations which were not present in the early visual environment. No comparable deficit emerged for either of the control cats. 3. Following 1-3 years of further, normal, visual experience, each of the cats was shipped separately to California where single units were recorded from area 17 of the visual cortex and an effort made to guess the early visual history of each animal which was unknown to the experimenters. Cell samples from each experimental cat and the normal control cat allowed the physiologist to guess their early visual experience correctly. The control cat which matured viewing orthogonal sets of oblique stripes on alternate days demonstrated a bias for horizontal contours in his cell sample. In contrast to units recorded from normal cats, about 80% of which are binocular, only about 30% of the cells recorded from the stripe-reared animals could be influenced by both eyes.

Animals↗
PubMed1976

Behavioural compensation of cats after early rotation of one eye.

Two cats, each with one eye surgically rotated by about 90, were trained (by two different behavioural methods) to discriminate horizontal from vertical stripes using the normal eye alone. When they had mastered the discrimination, the normal eye was occluded and the rotated eye was tested for transfer with neither stimulus reinforced. Both cats responded immediately and consistently to the same stimulus that they had learned to choose with the normal eye. These animals seem to have adapted to eye-rotation not only in terms of spatial localization but also in terms of the recognition of the orientation of contours. These behavioural compensations cannot be accounted for by simple changes in the organization of the visual cortex.

Adaptation, Physiological↗
PubMed1975

Behavioral deficits in cats following early selected visual exposure to contours of a single orientation.

The ability of adult cats, whose early visual experience was confined to contours of a single orientation (either vertical or horizontal), to resolve gratings of different orientations was studied by operant methods. Following selective visual exposure during part or all of the first 4 months of life, the cats were trained on a simultaneous discrimination between gratings of various orientations and blank fields of the same mean luminance. The spatial frequency of the gratings was systematically altered in order to obtain an estimate of acuity based upon extrapolation to chance levels of performance. Selectively deprived cats performed as well as normally reared cats on gratings having the same orientation as that of the stripes they saw as kittens, but their performance on gratings orthogonal to these was poorer. The deficits in acuity for gratings perpendicular to the experienced orientation varied between 0.26 and 0.87 of an octave. On the other hand, control cats whose early visual experience alternated between vertical and horizontal stripes, or who were reared in an environment containing randomly oriented contours, failed to show any difference in their acuity for vertical and horizontal gratings. The acuity deficits shown by the selectively deprived animals are long-standing since they remain unchanged even after 30 months of normal visual exposure. It is argued that these perceptual deficits are a consequence of the changes in cortical physiology that other investigators have described in cats who had undergone similar early visual deprivation. Taken together, these findings provide a basis for explaining a number of human perceptual disorders.

Animals↗
PubMed1975

Interocular transfer of the motion after-effect in normal and stereoblind observers.

The extent of interocular transfer of the motion after-effect was measured in 4 stereoblind subjects and in 19 subjects having varying degrees of stereopsis. Stereoblind individuals failed completely to show any interocular transfer of this after-effect, while subjects with good stereopsis exhibited between 55 and 82 percent transfer (mean 73 percent). Furthermore, normal subjects who manifested a clear eye dominance tended to show greater transfer from the dominant to the nondominant eye than vice versa. Individuals who either had a history of a strabismus or possessed some other early impediment to clear binocular vision tended to show less transfer. Overall there was a significant positive correlation of 0.75 between the extent of interocular transfer and the subject's stereoacuity. It is argued that the extent of interocular transfer of this after-effect provides a measure of the proportion of the total number of visual cortical neurons that are binocular. Thus stereoblind humans, who show no transfer whatsoever may, like cats and monkeys deprived of concordant binocular visual input early in life, suffer from a lack of binocular neurons.

Depth Perception↗
PubMed1974

Interocular transfer of a visual after-effect in normal and stereoblind humans.

1. Following inspection of a high contrast grating a test grating of a slightly different orientation will briefly appear rotated from its true orientation in a direction opposite to that of the adapting grating.2. The extent of interocular transfer of this phenomenon (the tilt after-effect) was measured in a number of normal subjects and in four subjects (three of whom had a strabismus) who lacked stereopsis.3. In contrast to the normal subjects, none of the four stereoblind subjects showed any interocular transfer of the tilt after-effect. Amongst the normal subjects the extent of transfer of this after-effect was positively correlated with the subject's stereoacuity. Maximum transfer (70%) was found in the subject with the best stereoacuity. In many subjects transfer was greater from the dominant eye to the non-dominant eye than vice versa.4. By analogy with experiments on cats deprived of congruent visual input to the two eyes early in life it is argued that the stereoblind subjects lack any binocularly driven cortical neurones.

Adaptation, Ocular↗
PubMed1974

The effect of early astigmatism on the visual resolution of gratings.

1. Orientational differences in visual resolution were measured at a number of different luminance levels on two subjects with high astigmatism that had remained optically uncorrected until the age of 10. Because of their astigmatism both of these subjects see vertical contours more clearly than horizontal contours with the unaided eye.2. The measurements were made using sinusoidal gratings generated on the face of an oscilloscope with the refractive error carefully corrected with lenses and with the gratings viewed through 3 mm artificial pupils.3. Visual resolution was found to be much better for vertical than for horizontal gratings for both these subjects under these conditions. The difference between the contrast sensitivities for vertical and horizontal gratings was even evident with gratings having spatial frequencies as low as 1 c/deg, but became progressively more pronounced at higher spatial frequencies. In one of the subjects the visual acuity (the cut-off spatial frequency) for horizontal gratings was more than 3/4 of an octave lower than that for vertical gratings.4. This is very different from the results obtained from normal subjects who typically show only a slight reduction in contrast sensitivity for oblique gratings but resolve vertical and horizontal gratings equally well.5. The quantitative differences between the contrast sensitivities for vertical and horizontal gratings of both high and low spatial frequencies cannot be accounted for by either errors of focus in one meridian or by the presence of meridional aniseikonia.6. In order to completely eliminate any optical explanations for these findings measurements of contrast sensitivity were made using sinusoidal interference fringes formed directly on the retina, thereby bypassing the eye's optics. Since the orientational differences in resolution persisted with this method it must be concluded that they are of neural origin.7. By analogy with the effects on cortical physiology that follow early selective visual deprivation in cats and monkeys, it is argued that these orientational differences in resolution are a consequence of changes induced in the neural organization of the astigmat's visual system by the distorted visual input provided by the uncorrected astigmatism early in life. It is furthermore argued that the smaller orientational differences in resolution observed in normal eyes might similarly be induced by certain asymmetries in the early visual input.

Adult↗