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Hatchling chicks reared with defocusing spectacle lenses compensate for the applied defocus, both refractively and through changes in eye growth, in about 1 week. In this experiment, we show that pharmacological inhibition of the retinal ON or OFF responses to light stimulation with isomers of α-aminoadipic acid results in a sign-dependent pattern of interference with the refractive compensation mechanism. An intravitreal injection of 2.5 μM l-α-aminoadipic acid inhibited the ERG ON response and inhibited refractive compensation to negative lens defocus, but not to positive or zero power lens defocus. d-α-aminoadipic acid in the same dose reduced the retinal OFF response and inhibited refractive compensation to positive lens defocus, but not to negative or zero power lenses. Thus the pharmacological manipulation of induced refractive change suggests that the retinal ON and OFF subsystems play independent roles in the emmetropization process.

The refractive state of hatchling chicks rapidly compensates to applied optical defocus through alteration in eye growth. The mechanism is capable of sensing whether the plane of focus lies in front of or behind the photoreceptors, however, its nature and site of action within the retina are unknown. We attempted to create an imbalance in the adaptation of the retinal ON and OFF mechanisms previously implicated in refractive control through pharmacological interventions, by rearing chicks from 4 to 9 days of age with a monocular 10 D, 0 D or −10 D lens, in an environment illuminated by a moving or stationary plaid of luminance gradients. When the plaid moved in one direction a local Fast-ON sawtooth luminance modulation was produced, while plaid motion in the other direction resulted in a Fast-OFF sawtooth modulation. Significantly reduced refractive compensation accompanied 10 D lensFast-OFF and −10 D lensFast-ON rearing, but not for the other conditions. Thus the refractive compensation mechanism depends on the nature of the temporal contrast of the environment, suggesting a relationship between the sign of defocus and the state of adaptation of the retinal ON and OFF subsystems.

The acuities of cells in the primary visual cortex of five tenotomized strabismic cats were measured. Previous behavioural studies have shown such animals to possess a severe amblyopia of approximately 1.5 octaves of spatial frequency, yet the acuities of both retinal ganglion and lateral geniculate X-cells are normal. The receptive fields of the cortical cells sampled were within 5° of the area centralis projection. On average, the acuities of cortical cells driven by the amblyopic eye were nearly 1 octave less than those for the non-deviating eye. However, the best cell acuities for each eye were nearly the same. The relationship between ocular dominance and cell acuity was found to be different for the two eyes despite a symmetrical ocular dominance distribution. The acuity deficit for cells driven through the amblyopic eye was present at all depths along the electrode tracks. We conclude that in this model amblyopia, the initial spatial processing deficit lies in the visual cortex, and most probably in the cells of layer IV. Further-more, the presence of a few cells driven by the amblyopic eye which can perform nearly as well as those from the fellow eye in processing high spatial frequencies gives new insight into the way in which strabismic and deprivation amblyopias differ.

Summary: Two preparations were used to study the developmental effects of prolonged blurring of retinal images on the acuities of retinal ganglion cells. Five kittens were raised from three weeks to six months of age with daily administration of atropine to one eye. Another two kittens were raised from three weeks to 16 weeks with a contact lens of high refractive power fitted to one eye. Behavioural estimates of the visual acuity were made for two animals from each group. Animals of both groups demonstrated an amblyopia in the experimental eye: visual acuity varied from 1.8 to 2.5 cycles per degree compared with 6.0 to 7.5 cycles per degree when using the normal eye. The spatial resolving properties were measured for retinal ganglion cells within the amblyopic eyes of two lens-reared cats and three atropinized cats. Brisk-sustained (X) cells were recorded from along the naso-temporal division. The acuities of ganglion cells from the lens-reared cats were indistinguishable from those from normal cats at comparable eccentricities. However, for the cats raised with atropine administration, sub-normal acuities were determined for retinal ganglion cells from all regions that were studied in the experimental eye. We conclude that blur of retinal images produced by external means has no effect on the resolving power of retinal ganglion cells. The lowered ganglion cell acuities encountered with the atropinised cats must be attributable to a secondary effect of the atropine administration. Organic changes in the retinal blood vessel pattern support this contention.

Tone bursts produced bands of selective 2-[14C]-deoxyglucose labelling in the inferior colliculus (IC) of the awake monkey. Low tone frequencies produced labelling in dorsal regions and high tone frequencies produced labelling in ventral regions. The position of the bands coincided with the position of a single unit with a characteristic frequency, which was the same as the frequency producing the labelling. These findings indicate that the bands of labelling represent iso-frequency contours in IC. The iso-frequency contours extended across most of the nucleus and were oriented from dorsomedially to ventro-laterally at 20–30° from the horizontal and became more vertical anteriorly. The width of the contours was as narrow as 200 µm, suggesting that the contours might represent 2 or 3 overlapping cellular laminae.

1. In order to investigate the phenomenon of acute reversibility of cortical ocular dominance, pressure block of the retinal ganglion cells and local anaesthetic block of the extraocular afferents were applied to the normal eye of six monocularly deprived cats. 2. Neither pressure blinding nor retrobulbar anaesthesia alone produced a significant increase in the proportion of single neurones recorded from area 17 which could be driven by the deprived eye. 3. Following combined pressure blinding and retrobulbar blockade, sixty‐four cells (54% of all encountered) could be stimulated through the deprived eye. 4. Of the sixty‐four cells driven by the deprived eye following both treatments a number showed direction (thirty) and orientation (thirteen) selectivity, but the majority showed non‐specific receptive field properties. 5. In four separate cases, out of the nine where the experiment could be carried out, it was possible to observe an individual neurone shift its ocular dominance. In the presence of a retrobulbar block, the four neurones could be driven first by the experienced eye alone, then by the deprived eye alone, and finally by the experienced eye again, all within a few minutes of the introduction and removal of pressure blinding. 6. These findings support previous studies which have revealed the presence of significant numbers of potentially functional cortical inputs from the deprived eye. Our new finding is that removal of tonic activity from the experienced eye, while it seems to be necessary to reveal these inputs, is not sufficient. An additional role in this phenomenon is played by extraocular afferents, possibly those involved in proprioception from the eye muscles.

1. A convergent or divergent strabismus was induced surgically in eight kittens and a cyclotropia of about 90 deg in two additional kittens. 2. Behavioural measurements were made of the visual acuity of each eye for square‐wave gratings. All eight animals that were so tested displayed a reduction of acuity in one eye relative to the other of 1.3‐2.5 octaves. 3. The activity of retinal ganglion cells was recorded within the amblyopic eye of six cats, three with a convergent strabismus, two with a divergent strabismus and one with a cyclotropia. Measurements were made of the spatial resolution with 215 on‐centre cells for horizontal and vertical gratings. 4. In contrast to other reports, we found the spatial resolution of ganglion cells in the amblyopic eye of the strabismic animals to be comparable to those of normal cats at all retinal eccentricities. In particular there was no evidence for a loss of resolution in the vicinity of the area centralis. 5. Measurement of the cross‐sectional area of cells in the lateral geniculate nucleus (l.g.n.) revealed no evidence of cell shrinkage in laminae receiving a projection from the amblyopic eye. 6. Together, these findings lead to the conclusion that the neural deficit responsible for the strabismic amblyopia in these animals did not lie in the retina but rather at more central levels of the visual pathway.

Experiments were carried out to determine the effects of different types of experimental strabismus on the acuities of retinal ganglion cells. Six kittens were raised from twenty-one days of age with an esotropia surgically induced by myectomy of the lateral rectus muscle and a large portion of the superior oblique muscle. The results are compared with those, previously reported, from five other cats also made esotropic, but by tenotomy of the lateral rectus. All animals tested behaviourally were amblyopic in the strabismic eye. For square wave gratings, the visual acuities were 1.0 to 2.5 cyc/deg through the strabismic eye compared with 6.0 to 7.5 cyc/deg through the non-deviating eye. The cut-off spatial frequencies were determined for 132 brisk sustained cells from five of the myectomized strabismic cats. There was a loss of approximately 20% in cut-off spatial frequency when compared with both normal and tenotomized cats. A correlate of the physiologically observed difference between the tenotomized cats and the myectomized cats was also found in the morphology of cells in the lateral geniculate nucleus. The tenotomized cats showed no evidence of cell shrinkage in laminae receiving a projection from the amblyopic eye whereas in the myectomized cats large differences were observed in cell cross-sectional areas between laminae receiving input from the amblyopic eye and those receiving input from the non-deviating eye. Together, these findings indicate that the presence of a neural deficit in the retina of strabismic cats is associated with the actual removal of extra-ocular muscle and probably has little to do with the optical quality of images arriving at the retina.

The relative efficacy of distributed versus massed reverse occlusion therapy in promoting recovery from the anatomical and physiological effects of monocular deprivation was studied in two experiments performed on kittens raised with one eye occluded from eye-opening until 5 weeks of age. The first experiment explored the effects of different periods (ranging from 0.5 to 4 h) of reverse occlusion imposed daily for 20 days. The second, involving a fixed period of reverse occlusion (20 h), examined recovery with respect to the distribution of that period over a varied number of brief daily sessions. Recovery was assessed in terms of changes in cortical ocular dominance and lateral geniculate cell morphology. Although recovery of both cortical ocular dominance and geniculate cell morphology showed the same overall progression with increasing periods of reverse occlusion, changes were apparent in the lateral geniculate nucleus before changes were evident in cortical ocular dominance. A given period of reverse occlusion was found to be far more effective in promoting recovery when distributed over a number of different exposure sessions than when massed together in just one or two sessions. The data suggest that there is a maximal rate of cortical recovery which is achieved with surprisingly brief daily periods of forced visual exposure of the initially deprived eye.

Regular injections of the gliotoxins D-α-aminoadipic acid and L-α-aminoadipic acid were made into the vitreous chamber of the eyes of newly-hatched chicks reared either in a normal visual environment or under conditions of monocular occlusion. Striking differences in the growth rates of the eyes from the different groups were observed. Injection of D-α-aminoadipic acid, which causes the Müller glial cells to swell and diminishes the retinal OFF-response, resulted in a marked increase in the rate of axial growth of the eye compared with normal eyes. However, injection of D-α-aminoadipic acid into occluded eyes caused a lesser growth rate than was observed in occluded control eyes. By contrast, injection of L-α-aminoadipic acid, which also causes severe glial swelling and abolishes the retinal ON-response, caused reduced eye growth in non-occluded eyes. However, injection of L-α-aminoadipic acid into occluded eyes caused eye growth in excess of that recorded in the occluded controls. We concluded that the different growth rates observed is more likely a result of the disruption of the neural ON and OFF mechanisms than of the indisposition of the Müller cells.

Corneal thickness was monitored on seven cynomolgus monkeys (Macaca fascicularis) over a 76 hour period. During this time, corneal thickness measurements were also made on six monkeys after overnight wear of a hydrogel contact lens in one eye. Mean corneal thickness was 417 ± 12um. An overall diurnal variation of 16 ± 5um (3.8 ± 1.2%) was found. During the first half hour after waking, corneal thickness changes ranged from -9um to +14um. After overnight contact lens wear, the cornea had swelled an average of 42 ± 24um (9.1 ± 4.8%). Following lens removal, the cornea returned to normal thickness within approximately 90 minutes. These results are similar to those found in humans and indicate that with respect to contact lens induced corneal thickness changes, the cynomolgus monkey is a suitable model for the physiological response to contact lens wear.

ABSTRACTThe current literature concerning experimentally‐induced strabismic, deprivation and anisometropic amblyopia is reviewed. It is demonstrated that there are at least two types of experimental strabismic amblyopia—one in which the retina is the initial locus of the amblyopia, and the other in which behaviourally measured acuity loss in cats is accompanied by normal retinal ganglion cell acuities. Classical theories are discussed in the light of new data.

The acuities of X-cells from the dorsal lateral geniculate nucleus (LGN) were measured in five cats raised with a convergent strabismus, surgically induced by tenotomy. The acuities of cells driven by the strabismic eye were not significantly different from the acuities of cells driven by the non-deviating eye over the range of eccentricities in the visual field studied (from the area centralis to over 20°). The data were also similar to X-cell acuities in the LGN of 3 normal cats. Lowered acuities of LGN X-cells driven by the deviating eye of an esotropic cat in which the strabismus was created by myectomy confirm a previous finding of a retinal locus of amblyopia associated with that preparation. In contrast, the results here implicate the visual cortex as the initial site of the deficit in spatial processing in amblyopia arising from tenotomy strabismus.