Donald E. Mitchell

Meridional amblyopia: Evidence for modification of the human visual system by early visual experience

Abstract Subjects with ocular astigmatism habitually experience contours of one orientation as clearer than all others. The largest difference in clarity is between lines oriented parallel to to the two principal meridians, which in the vast majority of cases lie close to vertical and horizontal. In contrast to normal subjects, many astigmats after full correction of their refractive error show large differences in their acuity for gratings of these two orientations. The acuity is substantially reduced for the orientation which is habitually the most defocused by the astigmatic optics. We have shown (a) that the reduced acuity is of neural origin, (b) that it does not fully recover subsequent to optical correction of the astigmatism, and (c) that the amount by which acuity is reduced is correlated with the degree of astigmatism. It is argued that the abnormal early visual experience of an astigmat induces matching neural alterations to occur within the visual system, which in turn result in the substantial differences in the ability to resolve contours of different orientations.

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.

Visual resolution in young kittens

Abstract A longitudinal study of the development of the visual acuity of young kittens was made using a behavioural method similar in principle to that employed by Lashley many years ago for his studies of pattern vision in the rat. The visual acuity for gratings rose gradually from about 1.2 c/deg at 35 days of age to reach adult values of between 5 and 6 c/deg when the animals were from 3 to 4 months old.

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.

A Neural Effect of Partial Visual Deprivation in Humans

Certain human subjects have considerable sensitivity differences in the visual resolution of vertical gratings as compared to horizontal gratings. Although only subjects with pronounced ocular astigmatism exhibit this effect, the differences are of neural, rather than optical, origin. It is argued that the resolution anisotropies result from early abnormal visual input caused by astigmatism. This abnormal input permanently modifies the brain.

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

SummaryThe 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.

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. Examples 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.

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 animals 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.

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.

Visual Resolution and Experience: Acuity Deficits in Cats Following Early Selective Visual Deprivation

Cats reared during the first 5 months of life in environments that contain contours of a single orientation show a diminished ability to resolve gratings of the orthogonal orientation in later life. It is argued that these perceptual deficits result from changes in the organization of the visual cortex induced by the selected early visual input.