Abstract
Purpose: :
New born and young human infants have poor sensitivity to temporally modulating stimuli (e.g., Rasengane et al, 1997; Swanson and Birch, 1991; Dopkins et al, 1999; but see Regal 1981). The cortical mechanisms that may limit infant's visual performance in temporal domain have not been systematically explored. In this study, therefore, we investigated the relative maturation of temporal frequency tuning characteristics and parallel changes in visual latency of V1 and V2 neurons.
Methods: :
At 2–, 4– and 8–weeks of age, microelectrode recording experiments were conducted in both V1 and V2. Drifting sine wave gratings (contrast = 40–80%) were used as stimuli. After optimizing stimulus orientation, size, and spatial frequency for each unit, we measured neuron's responsiveness as a function of stimulus temporal frequencies. Response latency was measured by determining the time between stimulus onset and the time at which unit's response significantly exceeded the noise level over 3 consecutive bins.
Results: :
Optimal temporal frequencies and temporal resolutions of both V1 and V2 units in infant monkeys, except for V1 of 8–week–old infants, were significantly lower than those in adults. Response latencies of both V1 and V2 neurons in infant monkeys, except for V1 of 8–week–old infants, were significantly longer than those in adults. Unlike in adults, however, the response latencies of individual neurons in infant monkeys were not correlated with their temporal resolutions. In infant monkeys, both V1 and V2 neurons exhibited stronger response adaptation during the sustained responses compared to that in adults.
Conclusions: :
The present results are consistent with the psychophysical observations that temporal contrast sensitivity of human infants is immature as late as 8 months of age (roughly equivalent to 8 weeks of age in monkeys). Also the present findings add new evidence for the hypothesis that the functional maturation of the primate visual brain proceeds in a hierarchical manner (Movshon and Kiorpes, 2003; Zhang et al, 2005).
Keywords: visual cortex • development • receptive fields