The spatial resolving power, contrast sensitivity, and receptive field properties of retinal ganglion cells were studied in cats reared with either convergent or divergent squint in one eye. Sustained-X cells in the area centralis of the squinting eye of the cats with esotropia without alternating fixation showed significantly poorer spatial resolution, and reduced contrast sensitivity compared with cells in the area centralis of the normal eye. These amblyopic sustained-X cells in the area centralis of the squinting eye had receptive field characteristics similar to those found in immature cells of young kittens. They had a shallow sensitivity gradient within a relatively widespread centre zone and a weak and widespread inhibitory surround. In contrast, the sustained cells in the area centralis of the normal eye revealed a typical, well defined, small centre zone with its sensitivity gradient extremely steep and its inhibitory surround strong and confined. A minor degree of amblyopia was also found in transient Y-cells in the area centralis of the squinting eye of these cats. However, no loss of resolving power was found in the cells in the area centralis of the squinting eye of the cats with esotropia or exotropia which showed alternating fixation. Thus, amblyopia occurs in those eyes which have lost the use of the area centralis as the normal visual axis during early postnatal development, and its organic lesion is already apparent in the retinal ganglion cells--the third order neurone in the afferent visual system. It is suggested that the loss of the ability to fixate results in inadequate stimulation of the central retinal ganglion cells due to the habitual presence of blurred images at the area centralis which prevents their full development during the critical period.

Patients classified into the above 3 groups were then correlated with clinical findings. The first group of patients (category 1), showed either a number of neurological symptoms to suggest multiple sclerosis without any visual deficit or a sudden transient loss of vision accompanied by pain in the affected eye. They were finally diagnosed as multiple sclerosis with optic neuritis. The second group of patients (category 2), had similar case histories to those of category 1 but showed a more severe, permanent loss of vision, commonly with temporal pallor and pupillary defects. The final diagnoses of these patients were either multiple sclerosis or compressive or ischaemic optic neuropathies. The third group of patients all showed bilateral, gradual, painless loss of vision but were subdivided into 2 distinct clinical diagnostic categories: (a) toxic amblyopia (category 3), and (b) bilateral optic atrophy of unknown cause (category 4).

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SUMMARY1. Recordings of single cells were made in layers A and Al of the lateral geniculate nucleus of kittens raised with convergent squint in one eye, and morphological studies of cells representing the different parts of the visual fields in these layers were also made from histological sections.2. For the normal eye, cells receiving inputs from the nasal and temporal visual fields were evenly represented up to the periphery, whereas for the squinting eye, no cells which permitted quantitative studies of receptive field properties could be found in the periphery of the nasal field.3. The loss of nasal field, represented by the loss of functional cells in the LGN layer Al fed by the squinting eye, depended on the severity of the squint. The greater the angle of convergent squint, the greater the loss of nasal field represented by the loss of functional cells.4. The cells fed by the squinting eye's temporal visual field retained their brisk function, although minor modifications in the receptive field organisation were apparent.5. The mean perikaryal size was smaller and the cell-density higher for cells in layers fed by the squinting eye. As found for the functional loss of cells, the shrinkage of perikaryal size and the increase of celldensity was smallest in the zones fed by the temporal visual field, and greatest in the zones fed by the peripheral nasal visual field.6. The functional and morphological changes in the cells in the LGN, which receive inputs from the nasal field of the squinting eye, are attributed to part of the temporal retina being hidden behind the bridge of the nose. It is proposed that this is a consequence of disuse atrophy, due to lack of stimulation during the sensitive period of development.