Sensitivity of lgn neurons in infant macaque monkeys

Purpose. To understand the neuronal factors limiting visual sensitivity in infant primates. Methods. We studied the responses of neurons recorded in parts of the LGN representing the central visual fields in paralyzed, opiate-anesthetized 1- and 4-wk old macaque monkeys; comparison data were taken from animals older than 6 mo. We tested each neuron with achromatic sinusoidal gratings varied in spatial and temporal frequency and contrast, and we also studied the effects of added spatiotemporal white noise. Results. In agreement with earlier reports, we found that neurons in the infant monkeys had relatively poor spatial resolution; sensitivity to high temporal frequencies was also lower than in adults. When tested with gratings of near-optimal spatio-temporal frequency, however, most LGN neurons in the infant monkeys gave brisk and reliable visual responses that were qualitatively similar to those seen in older animals. Spontaneous and evoked response rates and contrast gain were modestly lower in the infants, but response variability was also lower and therefore statistical measures of sensitivity and susceptibility to masking noise showed little difference between infant and adult neurons. Especially in the 1-wk-old animals, a substantial fraction of neurons lacked spontaneous activity. This resulted in hard contrast thresholds not seen in adult animals; in consequence, masking noise often paradoxically enhanced visual responses by subthreshold summation, a result not seen in adults. Conclusions. The maturation of visual responses in macaque LGN consists largely of changes in spatial and temporal scale, accompanied by modest changes in responsiveness and little or no change in sensitivity.