Nasotemporal directional bias of V1 neurons in young infant monkeys.

PURPOSE Optokinetic nystagmus (OKN) in young infants typically shows a temporal-to-nasal asymmetry under monocular viewing conditions. The neural basis for this asymmetry has been a matter of debate. One idea is that the OKN asymmetry reflects a similar asymmetry in the directional sensitivity of primary visual cortical (V1) neurons. An alternative hypothesis is that the OKN asymmetry is due to an immaturity in the ability of cortical neurons to influence the activity of subcortical structures that directly control OKN. We addressed this issue by studying the directional sensitivity of V1 neurons in young infant monkeys. METHODS The neuronal activity of V1 units was recorded from anesthetized and paralyzed rhesus monkeys ranging in age from 6 days to 8 weeks using standard extracellular single-unit recording methods. For comparison, V1 units from normal adult monkeys were also studied. Using drifting sinusoidal gratings of the optimal spatial frequency and a moderate contrast, we measured the responsiveness of individual units to 24 directions of stimulus movement. The preferred stimulus direction and the magnitude of the directional response bias were determined by a vector summation method. RESULTS No clear signs of nasotemporal asymmetries in direction tuning were found in our cell population from infant monkeys. However, the overall directional sensitivity and the peak monocular response amplitudes of these units were significantly lower, and binocular suppression was greater during the first 4 weeks of life than in adults. CONCLUSIONS The OKN asymmetry in young infants may be more closely associated with the lower overall directional sensitivity and the subnormal responsiveness of V1 neurons rather than with an obvious asymmetry in the directional properties of V1 neurons.