Processes important for emmetropization, whereby the optical power of the eye comes to match its size, were examined in developing chicks. The eyes of chicks raised in a low-ceiling environment were significantly more myopic in the upper field than the eyes of control animals. Most of this effect could be accounted for by selective local increases in the depth of the posterior chamber. This is consistent with the notion that vision plays an active role in sculpting the chick's eye to achieve appropriately focussed retinal images in the different parts of the visual field. The maintenance of stable retinal images was studied in chicks by examining the visual mechanisms responsible for stabilizing the head. The head movements induced by translation or rotation of the surroundings revealed powerful stabilizing reflexes that seen to be mediated by separate mechanisms, e.g., responses to translational disturbances showed none of the naso-temporal asymmetries characteristic of the ocular stabilization mechanisms in birds that deal with rotations of the surroundings. Further, rotational oscillations of the surroundings at high frequencies evoked lateral translations of the head rather than rotations, suggesting that only the translational mechanisms respond over this part of the range. Image stabilization was also studied in monkeys by examining the visual mechanisms underlying their ocular pursuit of small moving targets. The early suppression of ocular pursuit by featured backgrounds, described by Keller & Khan (1986), was shown not to be due simply to the reduced physical salience of the track target: suppression was still seen, albeit reduced, if the path of the target was devoid of features and consisted of a dark band. In ?act, suppression was still evident even when the band was 30 degree wide. Suppression also showed interocular transfer, whereby texture seen only by one eye could suppress pursuit initiated by target motion seen only by the other eye. This indicates that suppression can result entirely from centrally mediated interactions between visual inputs.
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