Transfer characteristics of X LGN neurons in cats reared with early discordant binocular vision

1. The effects of early discordant binocular vision on the functional development of the cat lateral geniculate nucleus (LGN) were investigated by quantitatively comparing responses of individual LGN neurons with their direct retinal inputs. 2. Unilateral convergent strabismus (esotropia) was surgically induced in 11 kittens at the age of 3 wk. After the animals had reached 9 mo of age, extracellular microelectrode recordings were made from individual X LGN units in lamina A and A1 of anesthetized and paralyzed cats. Responses were measured for drifting sinusoidal gratings. Within-unit comparisons of LGN action potentials (LGN output) and S potentials (retinal input) were performed to determine the nature of signal transfer in the units driven by the deviating (N = 42) or nondeviating eyes (N = 29) of strabismic cats. The results were compared with similar data (N = 29) obtained from nine normal control cats. 3. The spatial resolution of many individual LGN units in strabismic cats was abnormally reduced relative to their retinal inputs. These differences were more pronounced in units that received inputs from the nasal retina of the contralateral eye. The resolution loss was closely associated with a dramatic decrease in the strength of the receptive field center mechanism of LGN units relative to their retinal inputs. Moreover, the efficiency of signal transfer for high-spatial-frequency stimuli, determined by the transfer ratio (response amplitude of LGN action potentials/amplitude of S potentials), was significantly lower in strabismic cats compared with normal controls. 4. In strabismic cats, contrast thresholds for the action potentials of individual LGN units were significantly higher than those determined for the S potentials. In normal cats, the input-output differences in contrast threshold were negligible. The observed contrast sensitivity loss was more pronounced for high-spatial-frequency stimuli. 5. The speed of signal transfer was significantly decreased in the LGNs of strabismic animals. The visual response latencies of many, but not all, X LGN cells in the strabismic cats were abnormally long when compared with those in normal control units, whereas SP latencies were virtually the same for strabismic and normal cats. Abnormal latencies were prevalent in units that exhibited contrast threshold deficits, and were more severe among the units receiving input from the contralateral nasal retina. 6. The deficits in strabismic cats were found in the LGN units innervated by the deviating and nondeviating eyes. However, for the majority of response measures, the units innervated by the deviating eyes showed notably larger deficits. 7. We conclude that the fidelity of signal transfer from the retina to the LGN is significantly reduced in cats reared with discordant binocular visual experience. Thus the adverse effects of early strabismus are not confined, at least in cats, to the visual cortex.