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Figure 2 Left: Early in development, SC neurons (SC layer) have large and diffuse receptive fields, but the refinement of retinal ganglion cell (Ret layer) axon arbors results in smaller receptive fields by adulthood in normal animals. Top: If the caudal half of the SC is ablated at birth (PT), the retinal projection compresses on the remaining SC. The mechanisms underlying the compression include a reduction in retinal axon arbor size as well as selective collateral elimination from the SC. Note that the number of retinal inputs and the amount of visual space represented by each SC neuron is conserved, although the degree of overlap between adjacent receptive fields is reduced and multiunit receptive fields are larger. Bottom: The refinement process is dependent on NMDAR (Huang and Pallas, 2001). Chronic blockade of NMDAR with D-APV prevents the refinement process, leading to large receptive fields in adults. If the NMDAR blockade is combined with target loss (PT/D-APV), then, in addition to preventing the refinement, loss of NMDAR function prevents the compensation for target loss, leading to further receptive field enlargement. Adapted from Huang, L. and Pallas, S. L. 2001. NMDA receptor blockade in the superior colliculus prevents developmental plasticity without blocking visual transmission or map compression. J. Neurophysiol. 86, 1179-1194.

compression or by the NMDAR blockade and subsequent receptive field enlargement (Razak et al., 2003; Figure 3), suggesting that an NMDAR-inde-pendent mechanism is responsible for conservation of stimulus tuning. In order to explore this possibility, we first had to understand the circuitry responsible for velocity and size tuning in normal animals. Previous models of velocity tuning in retinal and visual cortical neurons have direction tuning as a necessary component (Barlow and Levick, 1965; Borst and Egelhaaf, 1989; Ascher and Grzywacz, 2000), but the velocity-tuned neurons in hamster SC are not intrinsically directional. We found that temporally asymmetrical lateral inhibition within the receptive field is largely responsible for size tuning, and that surround suppression is a major component of velocity tuning (Razak and Pallas, 2005).

Given that inhibition underlies stimulus tuning, and blockade of NMDARs had no effect on tuning, we hypothesized that the reason stimulus tuning is preserved in compressed or NMDAR-blocked SC

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