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 directio nal sensitivity of primary visual cortical (V1) neurons. An alternativ e hypothesis is that the OKN asymmetry is due to an immaturity in the ability of cortical neurons to influence the activity of subcortical s tructures that directly control OKN. We addressed this issue by studyi ng the directional sensitivity of V1 neurons in young infant monkeys. METHODS. The neuronal activity of V1 units was recorded from anestheti zed and paralyzed rhesus monkeys ranging in age from 6 days to 8 weeks using standard extracellular single-unit recording methods. For compa rison, V1 units from normal adult monkeys were also studied. Using dri fting sinusoidal gratings of the optimal spatial frequency and a moder ate 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 v ector summation method. RESULTS. NO clear signs of nasotemporal asymme tries in direction tuning were found in our cell population from infan t monkeys. However, the overall directional sensitivity and the peak m on ocular response amplitudes of these units were significantly lower, and binocular suppression was greater during the first 4 weeks of lif e than in adults. CONCLUSIONS. The OKN asymmetry in young infants may be more closely associated with the lower overall directional sensitiv ity and the subnormal responsiveness of V1 neurons rather than with an obvious asymmetry in the directional properties of V1 neurons.