Academic journal article Attention, Perception and Psychophysics

Titchener's [Perpendicular] Dissected

Academic journal article Attention, Perception and Psychophysics

Titchener's [Perpendicular] Dissected

Article excerpt

Published online: 18 April 2015

© The Psychonomic Society, Inc. 2015

Abstract In three experiments, two independent samples of 12 observers each visually inspected modified versions of Titchener's [perpendicular] from which the T-junctions had been deleted. For Experiment 1, the [perpendicular]'s two lines had been replaced by dashed lines not meeting in a common point; for Experiment 2, the [perpendicular] had been reduced to five dots, representing the original lines' end- and midpoints; and for Experiment 3 (in which the second sample of observers served), the [perpendicular] had been dissected into two separate lines, differently spaced from each other. Observers haptically indicated the lengths of the two orthogonal lines of the modified [perpendicular]s and verbally judged their relative lengths or the distances between the corresponding dots. The common perceptual illusions persisted in Experiments 1 and 2, but were markedly weakened in Experiment 3. Implications for a neurophysiological account of the illusions in terms of bottom-up, long-range interactions between orientation-sensitive mechanisms versus top-down activation of a figural schema are spelled out.

Keywords Visual illusion . Haptic signaling . T-junctions . Figural completion . Retinal eccentricity

As has apparently been known for centuries (Pastore, 1971), the Latin letter T gives rise to a strong and stable visual illusion: If both of its constitutive lines are equally long, the upstroke appears to be longer than the cross-stroke. In psychology, the illusion is known as Titchener's [perpendicular], since Titchener (1901) introduced an inverted Tas an illustration of the socalled horizontal-vertical illusion, which typically consists in a relative overestimation of vertical extents relative to horizontal ones-as has often been demonstrated with an L figure (e.g., Avery & Day, 1969; but see Chapanis & Mankin, 1967, for exceptions). Finger and Spelt (1947)wereamongthefirst to note that Titchener's [perpendicular] incorporates at least two illusioninducing factors: the orientation of the lines and the bisection of the horizontal line by the vertical one. Later, Künnapas (1955), by gradually transforming the [perpendicular] into an L and by presenting the figures in upright and lateral orientations, attempted to quantify the contributions of these two factors, and found bisection to be the stronger one. However, Verrillo and Irvin (1979), using the method of absolute magnitude estimation, had observers judge the lengths of individual, luminous lines in complete darkness and found no differences for horizontal versus vertical lines. They therefore concluded that "when care is taken to eliminate [contextual] cues . . . , the Horizontal-Vertical Illusion does not exist" (p. 270).

In several experiments (Landwehr, 2009, 2014, 2015a, 2015b), I have shown that the [perpendicular] illusion, albeit in a weaker form, also occurs in the haptic domain when observers visually inspect the stimulus and then haptically signal line lengths by spreading the thumb and index finger appropriately (cf. Gibson, 1966). Attempts to influence the illusion(s), by rotating the [perpendicular] into lateral or oblique positions (Hamburger & Hansen, 2010; Künnapas, 1955;Landwehr,2009, 2014)or putting it into different contexts (Landwehr, 2015a), met with limited success. The illusions even persisted in triangles and catenary-derived forms (Landwehr, 2015a, Exp. 3), suggesting that the T-junction as such does not even have to be optically specified for the illusions to occur. The present series of experiments continued with this idea and tested modified [perpendicular]s with deleted or only implicitly specified T-junctions. The embarrassing insight was that there does not appear to be any possibility to completely destroy the visual [perpendicular] illusion (the haptic illusion did vanish with branching patterns; Landwehr, 2015a,Exp.2). Implications for a neurophysiological account in terms of interactions between orientationsensitive mechanisms (Caelli, 1977) versus response determination by figural schemas (Wertheimer, 1922, 1923)willbe considered in the General Discussion. …

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