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McCollough effects, a group of visual contingent aftereffects involving color and contour, have been the subject of a recent body of literature. While a number of investigations have simply reported variations of the original phenomenon, others have closely examined characteristics of these effects and the conditions under which they can be optimally produced. Indications that their spatial specificity is similar to that of single cortical cells investigated electrophysiologically in animals have inspired several models; however, the extraordinary persistence of the effects is difficult to explain at a single-unit level. -Findings which suggest that McCollough effects have characteristics like those of learned responses may help to resolve this paradox.

The time course of development of visual acuity for square wave gratings was measured behaviourally in a number of cats that were reared in total darkness until they were either 4 (5 cats) or 6 (1 cat) months of age. Less extensive measurements were also made on animals reared in a similar manner until they were either 1 1/2 or 10 months old. Initially, all the animals appeared to be blind, but signs of vision became evident after periods of time in an illuminated environment that ranged from a few days, in the case of animals dark-reared for only 1 1/2 to 4 months, to 1 to 2 months for those animals that were deprived for 6 months or more. Thereafter, visual acuity as measured on a jumping stand progressively improved, reaching, in the case of animals deprived for 4 months, values that were comparable to those of normal animals (6.9 cycles/deg) after 4 months of exposure to light. The animal deprived for 6 months remained apparently blind for a month and eventually attained an acuity (5.7 cycles/deg) that was somewhat less than that of normal animals. The fact that such high acuities can be achieved after periods of binocular deprivation that extend throughout the classically defined "critical period" suggest that the effect of dark-rearing is somehow to impede the natural decline with age in the degree to which cortical neurones are susceptible to environmental modification.

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Development of the haptic system was evaluated by examining object exploration and recognition in sighted children between the ages of 3 and 8 years. To determine the importance of visual experience for these abilities, the performance of seven congenitally blind children was compared with that of sighted peers matched for age and gender. Performance was evaluated in terms of the speed and correctness of object identification, thoroughness of exploration of object parts, representation of the global form versus local parts of objects, and the possible role of critical parts in object identification. Four types of common objects were presented: normal-sized, miniaturized small, miniaturized large, and oversized objects. All subjects were required to manipulate and identify these objects haptically, without the aid of vision. Results revealed the emergence of a developmental pattern in all performance measures for sighted children. Older sighted children were not only able to recognize more objects and to do so more quickly, but also were more thorough in their exploration patterns. With increasing age, children appear to change their representation of objects from one based predominantly on global shape to one that incorporates a balance of global shape and specific local parts. In agreement with this, critical parts also played a role in object identification, particularly in older children. Blind and sighted children did not differ in any performance measures, which suggests that previous visual experiences do not determine tactile exploration strategies and are not essential for haptic object recognition.