The T-illusion in Variable Contexts K Landwehr |
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Allgemeine Experimentelle Psychologie, Universität Mainz, Germany
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| If, for the letter T, up- and cross-stroke are equally long, the upstroke will appear longer, both visually and during haptic-tactile exploration (Tedford and Tudor, 1969, Journal of Experimental Psychology, 81(1), 199-201). Recently, I discovered another, purely haptic illusion with this stimulus: When subjects had to "grasp" computer images of individual lines of the T at their respective ends with a pretended thumb and index finger pincer grip, subjects scaled their responses to the length of the upstroke when grasping the cross-stroke, but were quite correct with the upstroke as target, independently of the orientation of the T (Landwehr, 2009, Attention, Perception, & Psychophysics, 71(5) 1197-1202). With regard to the visual illusion, I found an asymmetry in illusion strength depending on which stroke served as standard. Both effects can probably be explained in terms of neural detection mechanisms that register orientation and end-points of lines (cf. Caelli, 1977, Vision Research, 17, 837-841). Since the length of lines is misestimated only in contexts (Verrillo and Irvin, Sensory Processes, 3, 261-274), future investigations of the T-illusion(s) may profit from putting the T into variable contexts. I shall report on a project that focuses on conditions to either enhance or attenuate these illusions. |
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