Two experiments were performed to investigate conditions affecting the Müller-Lyer illusion and its decrement with practice. The first experiment was a methodological study concerned with the setting of the variables before adjustment by the subject. The results indicated that the method employed may determine whether a decrement occurs with repeated trials. The evidence suggested the most suitable method to employ in the succeeding experiment. The second experiment was performed to investigate the effects of practice with another illusion figure on the magnitude of illusion on the Müller-Lyer figure. The practice figure was the same as the Müller-Lyer illusion figure except that circles were replaced by the obliques. It was found that the magnitude of the initial illusion is a decreasing monotonic function of the amount of preliminary training. This finding is interpreted as meaning that in pretraining subjects are practiced in disregarding the context (circles) of the horizontal lines of the figure and this transfers positively to the Müller-Lyer task. / Thesis / Master of Arts (MA)

Three experiments were conducted to investigate the two visual systems hypothesis (Milner & Goodale, 1995) and the planning-control hypothesis (Glover, 2002). Experiment 1 required the participants to make rapid aiming movements to 25 cm and 35 cm tails-in, no tails, or tails-out Müller-Lyer stimuli following a 0 ms or 5000 ms no-vision delay. In Experiment 2, the participants executed their movements with full vision of the Müller-Lyer vertices that either remained the same or changed to a different configuration upon movement initiation. Vision was occluded either 350 ms or 450 ms after the onset of the movement. Experiment 3 was similar to Experiment 2, except the amount of visual feedback for on-line control was constrained to 200 ms, 400 ms, or 600 ms. The results of these experiments are problematic for both hypotheses. The participants exhibited a similar pattern of illusion-induced bias in both short and long delay conditions. In addition, the magnitude of the aiming bias increased as the movement unfolded (Experiment 1). Furthermore, even though participants were engaging in on-line control the illusion continued to exert its effects on aiming during the latter stages of the movement (Experiment 2). This effect was also observed when participants had sufficient time to process visual feedback in order to modify their movements (Experiment 3). Taken together, the results suggest that on-line control is biased by visual illusions. / Thesis / Master of Science (MSc)

No description available.

This study was designed as an attempt to extend Schneider and Shiffrin's (1977) automatic versus controlled processing distinction into the area of visual perception. Hasher and Zacks (1979) proposed a continuum of automatic processes, with processes which encode the fundamental aspects of the flow of information as the anchor of the continuum. They presented evidence that depressed people perform more poorly than nondepressed on effortful (controlled) memory tasks, but not on automatic tasks. Tachistoscopic and free inspection presentation of Piaget's (1961/1969) primary geometric illusions meet two of Hasher and Zack's criteria for automatic and effortful tasks, respectively. Consequently, multiple regression techniques were used to determine the relationship between depression (operationally defined as score on the Beck Depression Inventory) and method of presentation of a Piagetian primary illusion, the Müller-Lyer. Furthermore, correlations were determined between tachistoscopic versus free inspection of the Müller-Lyer illusion and forward versus backward digit span (operationally defined as score on the WAIS Digit Span subtest), since forward and backward digit span have been linked theoretically to automatic and effortful processing, respectively.