Search PubMed⌕ Search

Biomedical subjects

D E Mitchell

Publications and source records attributed to D E Mitchell.

At least 55 records · Page 3Linked to original sources
PubMed1981

Depth perception, eye alignment and cortical ocular dominance of dark-related cats.

On first exposure to light, animals that have been reared from birth until about 4 months of age in total darkness exhibit substantial visual and visuomotor deficits, which decline in severity during the first few months following exposure to light. In order to determine whether dark-reared animals eventually acquire stereoscopic vision following exposure to light we examined the binocular status of 5 dark-reared animals two of which developed convergent eye alignment. The binocular status was assessed behaviorally by measurements of the ability of each animal to perceive depth using either one or both eyes, and physiologically by documentation of the distribution of cortical ocular dominance of a sample of visual acuity, their binocular depth perception remained very poor, comparable to the monocular performance of normal cats. In marked contrast to normal animals none of the dark-reared animals, even those with normal eye alignment, performed substantially better binocularly than monocularly, a result indicating the absence of a uniquely binocular mechanism for depth perception in these animals. Although the dark-reared animals were found to retain a substantial (but reduced) complement of binocularly influenced cortical neurons, the tuning of these cells for retinal disparity must be insufficiently precise to mediate depth perception under binocular viewing conditions that is superior to that which can be achieved monocularly.

Animals↗
PubMed1981

The efficacy of brief periods of reverse occlusion in promoting recovery from the physiological effects of mononuclear deprivation in kittens.

The relative effect of short daily periods of reverse occlusion in promoting recovery from the physiological effect of monocular deprivation in kittens were examined with a view to identifying a neurophysiological basis for the visual improvement observed with minimum occlusion therapy in amblyopia. Kittens were monocularly deprived from near birth until 5 weeks of age, at which time they were reverse-sutured and housed in total darkness. Each kitten received a short period of visual exposure through its initially deprived eye each day for either a fixed number of days or for a constant total visual exposure spread over a different number of exposure sessions. Electrophysiological recordings from single cells in the visual cortex were made the day after the last visual exposure. Kittens that received daily periods of reverse occlusion as brief as 30 min for 20 days showed a substantial degree of reversal of cortical ocular dominance. Other experiments indicated that 20 hr of reverse occlusion distributed over a number of brief daily sessions was far more effective in promoting physiological recovery than the same total period of exposure imposed in only two sessions. In general these results suggest that a given period of reverse occlusion may be more effective in promoting recovery with distributed than with massed periods of occlusion.

Amblyopia↗
PubMed1980

Visual resolution of retinal ganglion cells in monocularly-deprived cats.

The spatial resolution of 87 on-centre retinal ganglion cells was measured at different retinal eccentricities in the retina of the deprived eye of two monocularly-deprived cats. Intra-ocular recording techniques were employed in order that particular attention could be placed on ganglion cells in the region of the area centralis. The spatial cut-off frequencies of both major classes on-centre ganglion cells were comparable to those observed in normal retinas at similar retinal eccentricities. In particular there was no evidence of any loss of spatial resolution of either class of cell in the vicinity of the area centralis as has been reported in the retina of the deviating eye of strabismic cats.

Animals↗
PubMed1980

Period of susceptibility of kitten visual cortex to the effects of monocular deprivation extends beyond six months of age.

The duration of the sensitive period of the kitten visual cortex to the effects of monocular deprivation was explored by studies of the behavioral and physiological recovery from extended periods of monocular occlusion imposed from birth, and by examination of the physiological effects of a 3 month period of monocular occlusion imposed on animals at either 4, 5, 6, 7 or 8 months of age. Animals monocularly deprived until 4 months of age eventually show considerable behavioral and physiological recovery from the severe deficits observed immediately following termination of the period of deprivation. The conclusion that binocular connectivity may still be altered by the nature of the animal's visual input beyond 4 months of age was supported by the results obtained from animals that were monocularly deprived at 4 months of age or older. Animals deprived at either 4, 5 or 6 months showed a clear shift of cortical ocular dominance in favour of the non-deprived eye, but those deprived at 7 or 8 months showed approximately normal ocular dominance distributions. It is concluded that the sensitive period lasts at least twice as long as previously thought, to between 6 and 8 months of age.

Aging↗
PubMed1979

Assessment of depth perception in cats.

A behavioural method is described for the assessment of depth perception of kittens. Measurement is made of the smallest separation in depth that can be discriminated between two adjacent stimuli under both monocular and binocular viewing conditions. Normal animals can discriminate much smaller separations in depth when using two eyes than with monocular viewing, implying the presence of a cue to depth that is uniquely available with binocular viewing. The test provides a quick and reliable way of screening animals for stereopsis.

Animals↗
PubMed1978

The development of vision in cats after extended periods of dark-rearing.

The time course of development of visual acuity for square wave gratings was measured behaviourally in a number of cats that were reared in total darkness until they were either 4 (5 cats) or 6 (1 cat) months of age. Less extensive measurements were also made on animals reared in a similar manner until they were either 1 1/2 or 10 months old. Initially, all the animals appeared to be blind, but signs of vision became evident after periods of time in an illuminated environment that ranged from a few days, in the case of animals dark-reared for only 1 1/2 to 4 months, to 1 to 2 months for those animals that were deprived for 6 months or more. Thereafter, visual acuity as measured on a jumping stand progressively improved, reaching, in the case of animals deprived for 4 months, values that were comparable to those of normal animals (6.9 cycles/deg) after 4 months of exposure to light. The animal deprived for 6 months remained apparently blind for a month and eventually attained an acuity (5.7 cycles/deg) that was somewhat less than that of normal animals. The fact that such high acuities can be achieved after periods of binocular deprivation that extend throughout the classically defined "critical period" suggest that the effect of dark-rearing is somehow to impede the natural decline with age in the degree to which cortical neurones are susceptible to environmental modification.

Animals↗
PubMed1978

The rate of recovery of vision after early monocular deprivation in kittens.

1. Fifteen kittens were monocularly deprived of vision by suturing the lids of the right eye together for various periods of time at different ages. A simple behavioural technique was used to assess the immediate effects of the period of monocular deprivation on the visual acuity of the deprived eye as well as the time course of any subsequent recovery.2. The extent of the recovery of vision was measured under conditions where the animal was either forced to use its deprived eye by performing a reverse suture or where the animal had both eyes open after the initial period of monocular occlusion.3. The initial effects of monocular deprivation were graded in severity according to the age at which the deprivation was imposed, ranging from apparent blindness in animals deprived at 6 weeks of age to only a small loss of acuity in kittens deprived at 12 weeks of age.4. The effects of deprivation imposed from birth were particularly severe, leading to a temporary blindness. Nevertheless after a period of time that became progressively longer with increasing deprivation, all animals showed some recovery of pattern vision over the course of the next 2 or 3 months. The extent of this recovery became progressively less as the period of deprivation was prolonged. There was even some recovery of vision (an acuity of 2.5 cycles/deg) in animals that were deprived throughout the duration of the ;critical period' to 4 months of age.5. Direct comparison of the rate of behavioural recovery between animals that were reverse sutured with that of litter-mates that received binocular input after monocular occlusion to either 45 or 60 days of age proved to be remarkably similar, although the acuity that was eventually attained by the reverse sutured animals was always slightly higher.6. The recoveries observed after reverse suturing were reasonably well correlated with changes observed in the ocular dominance of visual cortical cells under similar circumstances.7. Although the recovery in these animals can be accounted for by the simple notion of a competitive interaction between the two eyes, the recovery observed in animals that had both eyes open after the initial period of deprivation cannot be so readily explained. Evidently there must be an additional non-competitive mechanism of recovery.

Age Factors↗