The threshold for the detection of the dark interval between two flashes of light (DIT) was investigated for 240 school children aged [6][7][8][9][10][11][12][13][14][15][16][17] Pollack (1969) has put forth a two-process theory to explain age changes in visual perception. One of these processes, nondevelopmental in nature. involves a progressive loss in receptor sensitivity due to physiological aging. Within the framework of this theory those phenomena which show nondevelopmental age changes (Type I) are not correlated with intellectual ability, but appear to be related directly to receptor efficiency and such stimulus variables as intensity, duration, contrast, and orientation. The operation of all these variables depends on the sensitivity and efficiency of the visual receptor system. Weale (1963) has shown that lenticular density and pigmentation, as well as retinal pigmentation, increase with age, while pupil size decreases. The pinpointing of the precise mechanisms involved, however, requires further physiological investigation. Some behavioral research exists that tends to confmn the above notions. Polla<:k's investigation of contour detectability thresholds in childhood (I963c) supports the above. Both sensitivity to contour as produced by brightness contrast and the magnitude of the Mueller-Lyer illusion were found to decline between ages 8 and 12, and intelligence played no significant role. Two experiments by Pollack (1965a. b) on backward figural masking showed that figure-grounu contrast of the target and contour orientation of the target and mask were significant variables in producing the masking effect with adults. Those conditions that produced the maximal amount of backward masking in adults were presented to children ranging in age from 7 to 10 years. The interstimulus interval permitting masking declined steadily as a function of increasing chronological age. Adult Ss exhibited masking with even shorter intervals than the l()..year-olds. There was no correlation with IQ. Again, stimulus variables interacting with the declining sensitivity of the visual receptor system were believed to be the factors responsible for the decline in masking with age.The other process put forth by Pollack ( 1969) is "developmental" in that age changes seem to be related to central cognitive functions and abilities that are hierarchically organized. Those phenomena showing developmental age changes (Type II) are correlated with intellectual ability and do not depend as much upon classical stimulus variation. Presentation of such phenomena usually involves a sequence of discrete stimulus events, registered by the S as traces, which are then centrally compared and contrasted. Pollack (1964) demonstrated this by converting a Type I illusion (MuellerLyer) into a Type II illusion, by altering the mode of presentation from simultaneous to successive. This was done by presenting the oblique inducing lines before presenting the main line. The classical illusion reversed its usual direction; the new iUusion increased in ma...

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32 rats were used to test the deprivation-trace hypothesis of O'Kelly and Heyer (1951). In a 2 × 2 × 2 factorial design the following variables were manipulated: delay between saline injection and bar-press testing, 30 min. vs 240 min., presence or absence of food during the delay period, and schedule of reinforcement, VI-¼ min. vs VI-1 min. The only significant differences were the main effects of schedule and delay. The prediction was supported, but alternative interpretations not requiring the hypothesis were offered.

Changes in subjective median plane (SMP)