Pwceptrral and Motor Skills, 1975, 41, 225-226.
@ Perceptual and Motor Skills 1975
BISECTING THE PONZO ILLUSION A. LEWIS HILL Institute for Basic Resemch i n Mental Reiwdation' Summary.-A modification of the Ponzo illusion, in which the judged lines are centered i n different sized obliques, was presented to matched normal and mentally retarded males ( n s = 8 ) under conditions of equal and unequal retinal sizes. Magnitude of illusion was affected by 1Q (WISC) and by physical proximity rather than retinal contour. The results seem to imply a central rather than peripheral explanation for the illusion. Quina and Pollack (1971) reported that the over-all Ponzo illusion results from basic contour inreraction processes such that lines near the apex are overestimated while lines near the open end are underestimated. They reported that the middle line was not affected. The lack of contour effects on the middle line could be due to the physical distance between the line and the contours, the retinal distance between the line and the contours, o r presenting the line in the middle of the field. T o test these variables further, the Ponzo illusion was bisected into two separate figures, each presenting the test lines centered within oblique lines. T h e standard (Fig. 1 A ) was presented 90" to S's right while the comparison figure ( 1 B ) was presented 90" to his left. The standard consisted of a 2- X 1/8-in. vertical line centered within two 5 X 1/8-in. oblique ( 4 5 " ) lines. Variable vertical lines ranging from 1 1/2 in. to 3 3/4 in. long in 1/4-in. steps were presented, centered within two 5- x 1/8-in. oblique lines of the comparison figures. The stimuli were constructed with luminous paint and testing was conducted under dark room conditions. In one condition Ss sat equally distant ( 6 0 in.) from each figure. In a second condition S sat 4 0 in. from the standard and 8 0 in. from the comparison so that the figures were now retinally equal. Condition order was counterbalanced within groups and points of subjective equality were obtained by the method of constant stimuli.
FIG. 1.
The Ponzo illusion bisected into two separate figures
Eight normal and eight mentally retarded males (determined by the Wechsler Intelligence Scale for Children), matched for chronological age, were tested. The retarded Ss were diagnosed as cultural familials without indication of organiciry. They ranged in age from 1 5 to 18 yr. and in IQ from 5 1 to 68. Mean illusion scores ranged from 1 0 to 2 5 % for normal Ss and from 15 to 40% for the retarded Ss in the two conditions. Thus, when both the standard '1050 Forest Hill Road, Staten Island, N.Y.10314.
A. L. HILL and comparison lines are centered within obliques, an illusion is present. Retarded Ss obtained larger illusions than normals ( K . 1 , = 8.42, p .01), indicating that the Ponzo illusion, when presented sequentially, is influenced by intelligence. This finding seems to agree with Pollack (1969) that illusions which involve a sequence of stimulus events are influenced by the growth of cognitive functions. These group differences might also be interpreted as evidence that central rather than retinal mechanisms are responsible for the Ponzo illusion. N o differences were found between the two con.05), further indicating that the illusion is probably not the ditions ( K . , , = 3.9, p result of interaction of basic contour processes at the retinal level. This result extends rhe previous findings of Hill (1971) based on the Poggendorff illusion. In both the Ponzo and Poggendorff illusion, the distance from the subject to the figure, i.e., visual angle of the stimulus, does not seem to influence the magnitude of the illusion whereas the physical contour distances do.
REFERENCES HILL, A. L. Poggendorff illusion: effects of intelligence, viewing distance, and space between the vertical lines. Psychonomic Science, 1971, 25, 71-72. POLLACK,R. H . Some implications of ontogenetic changes in perception. I n D. Elkind & J. Flavell (Eds.), Essays in cognitive development: studies in honor o f Jean Piaget. New York: Oxford Univer. Press, 1969 Pp. 365-407. QUINA, K., & POLLACK,R. H. A parametric invesc~gat~on of the Ponzo illlusion under conditions of tachistoscopic exposure. Proceedrr~gsof the American Psycho~ogica~ Association, 1971, 9, 77-78. [Also reported In Quina, K., & Pollack, R. H. Effects of test line position and age on the ma nitude of the Ponzo illusion. Perception find Psychophysics, 1972, 12, 253-256.f
Accepted J d y 14, 1975.
@ Perceptual and Motor Skills 1975
BISECTING THE PONZO ILLUSION A. LEWIS HILL Institute for Basic Resemch i n Mental Reiwdation' Summary.-A modification of the Ponzo illusion, in which the judged lines are centered i n different sized obliques, was presented to matched normal and mentally retarded males ( n s = 8 ) under conditions of equal and unequal retinal sizes. Magnitude of illusion was affected by 1Q (WISC) and by physical proximity rather than retinal contour. The results seem to imply a central rather than peripheral explanation for the illusion. Quina and Pollack (1971) reported that the over-all Ponzo illusion results from basic contour inreraction processes such that lines near the apex are overestimated while lines near the open end are underestimated. They reported that the middle line was not affected. The lack of contour effects on the middle line could be due to the physical distance between the line and the contours, the retinal distance between the line and the contours, o r presenting the line in the middle of the field. T o test these variables further, the Ponzo illusion was bisected into two separate figures, each presenting the test lines centered within oblique lines. T h e standard (Fig. 1 A ) was presented 90" to S's right while the comparison figure ( 1 B ) was presented 90" to his left. The standard consisted of a 2- X 1/8-in. vertical line centered within two 5 X 1/8-in. oblique ( 4 5 " ) lines. Variable vertical lines ranging from 1 1/2 in. to 3 3/4 in. long in 1/4-in. steps were presented, centered within two 5- x 1/8-in. oblique lines of the comparison figures. The stimuli were constructed with luminous paint and testing was conducted under dark room conditions. In one condition Ss sat equally distant ( 6 0 in.) from each figure. In a second condition S sat 4 0 in. from the standard and 8 0 in. from the comparison so that the figures were now retinally equal. Condition order was counterbalanced within groups and points of subjective equality were obtained by the method of constant stimuli.
FIG. 1.
The Ponzo illusion bisected into two separate figures
Eight normal and eight mentally retarded males (determined by the Wechsler Intelligence Scale for Children), matched for chronological age, were tested. The retarded Ss were diagnosed as cultural familials without indication of organiciry. They ranged in age from 1 5 to 18 yr. and in IQ from 5 1 to 68. Mean illusion scores ranged from 1 0 to 2 5 % for normal Ss and from 15 to 40% for the retarded Ss in the two conditions. Thus, when both the standard '1050 Forest Hill Road, Staten Island, N.Y.10314.
A. L. HILL and comparison lines are centered within obliques, an illusion is present. Retarded Ss obtained larger illusions than normals ( K . 1 , = 8.42, p .01), indicating that the Ponzo illusion, when presented sequentially, is influenced by intelligence. This finding seems to agree with Pollack (1969) that illusions which involve a sequence of stimulus events are influenced by the growth of cognitive functions. These group differences might also be interpreted as evidence that central rather than retinal mechanisms are responsible for the Ponzo illusion. N o differences were found between the two con.05), further indicating that the illusion is probably not the ditions ( K . , , = 3.9, p result of interaction of basic contour processes at the retinal level. This result extends rhe previous findings of Hill (1971) based on the Poggendorff illusion. In both the Ponzo and Poggendorff illusion, the distance from the subject to the figure, i.e., visual angle of the stimulus, does not seem to influence the magnitude of the illusion whereas the physical contour distances do.
REFERENCES HILL, A. L. Poggendorff illusion: effects of intelligence, viewing distance, and space between the vertical lines. Psychonomic Science, 1971, 25, 71-72. POLLACK,R. H . Some implications of ontogenetic changes in perception. I n D. Elkind & J. Flavell (Eds.), Essays in cognitive development: studies in honor o f Jean Piaget. New York: Oxford Univer. Press, 1969 Pp. 365-407. QUINA, K., & POLLACK,R. H. A parametric invesc~gat~on of the Ponzo illlusion under conditions of tachistoscopic exposure. Proceedrr~gsof the American Psycho~ogica~ Association, 1971, 9, 77-78. [Also reported In Quina, K., & Pollack, R. H. Effects of test line position and age on the ma nitude of the Ponzo illusion. Perception find Psychophysics, 1972, 12, 253-256.f
Accepted J d y 14, 1975.
