Academic journal article Attention, Perception and Psychophysics

Opposite Haptic and Visual Induction Effects Observed with Titchener's [Perpendicular]

Academic journal article Attention, Perception and Psychophysics

Opposite Haptic and Visual Induction Effects Observed with Titchener's [Perpendicular]

Article excerpt

Nineteen psychology undergraduates were requested to "grasp" the lines of a computer image of Titchener's [perpendicular] with a thumb and index finger pincer grip immediately after the line to be grasped had been indicated and the whole figure extinguished. The hand remained visible. Lines were of three different lengths (6.5, 7, and 7.5 cm), and [perpendicular]s were presented at eight different orientations (rotated in steps of 45°). Lines also had to be judged longer, shorter, or equal, relative to one another. Haptically, subjects responded correctly to the [perpendicular]s' undivided lines, but scaled their responses to the divided ones according to the undivided ones. Categorical judgments yielded comparable psychometric functions for both kinds of lines, with a significant shift of points of subjective equality from a difference in length between lines of 9.8% to 13.5% when the divided or the undivided line was target. Haptic sensitivity, therefore, appears to be constrained by context, whereas the classical "visual illusion," associated with Titchener's [perpendicular], may be regarded a mere decisional criterion shift.

Visual illusions have long been regarded as a window onto neural processes. During the last two or three decades, much has been made of the often observed discrepancy between these illusions?f perceptual effectiveness and their relative irrelevance for motor coordination. Servos, Carnahan, and Fedwick (2000), for example, found considerable visual illusions of lengths, but fairly appropriate grasp responses, with the so-called horizontal.vertical illusion (Titchener's [perpendicular] [Titchener, 1901]; cf. Finger & Spelt, 1947), and a similar effect was described by Aglioti, DeSouza, and Goodale (1995) for the Ebbinghauscircles illusion (although Franz, Gegenfurtner, Bulthoff, & Fahle, 2000, could not replicate this effect; cf. DeLucia, Tresilian, & Meyer, 2000, p. = 65, note to Table 3, for diverging interpretations of Aglioti et al.'s study). The most striking demonstration of the contrast in question probably goes with induced motion: Although subjects vividly experienced a target jumping opposite the displacement of a frame, they still correctly pointed to the target's true position (Bridgeman, Kirch, & Sperling, 1981). These remarkable behavioral dissociations have commonly been viewed consistent with the existence of two functionally distinct systems within visual cortex, one subserving action, the other categorization (Bridgeman, Lewis, Heit, & Nagle, 1979; Goodale & Milner, 1992; Milner & Goodale, 2006; cf. Creem & Proffitt, 2001; Rizzolatti & Gallese, 2006; Trevarthen, 1968; Ungerleider & Mishkin, 1982, for related theorizing).

Results that do not fit this picture were reported by Vishton, Rea, Cutting, and Nunez (1999). Also utilizing Titchener's [perpendicular], these authors succeeded in demonstrating the possibility of both perceptual correctness and motor illusion: Subjects withstood the visual deception when, for metric and analogous judgments, they had been instructed to focus on one line of the display (Experiments 2 and 3), but the same (as well as new) subjects haptically erred with three-finger grasps (Experiment 4, which included a replication of Experiments 2 and 3). On the basis of this evidence, Vishton et al. (1999) suggested that the relevant opposition need not be perceptual judgment versus action, but absolute versus relative size perception.

Several objections can be raised to Vishton et al.'s (1999) work. One problem, mentioned by the authors themselves (p. 1670), is that a three-finger grasp is severely constrained biomechanically. The haptic illusion observed, then, may well have more to do with this inherent difficulty of the motor task than with relational perceptual processing. Furthermore, Vishton et al. used pictorial stimuli that could not really be grasped-subjects merely had to leave ink traces on paper. …

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