Palm boards are not action measures: An alternative to the two-systems theory of geographical slant perception Frank H. Durgin a, * , Alen Hajnal a,b , Zhi Li a , Natasha Tonge a , Anthony Stigliani a a Swarthmore College, United States b University of Southern Mississippi, United States article info Article history: Received 11 December 2009 Received in revised form 25 January 2010 Accepted 27 January 2010 Available online xxxx PsycINFO classification: 2320 2323 2330 Keywords: Slant perception Haptic perception Proprioception Geographical slant abstract Whereas most reports of the perception of outdoor hills demonstrate dramatic overestimation, estimates made by adjusting a palm board are much closer to the true hill orientation. We test the dominant hypothesis that palm board accuracy is related to the need for motor action to be accurately guided and conclude instead that the perceptual experience of palm-board orientation is biased and variable due to poorly calibrated proprioception of wrist flexion. Experiments 1 and 3 show that wrist-flexion palm boards grossly underestimate the orientations of near, reachable surfaces whereas gesturing with a free hand is fairly accurate. Experiment 2 shows that palm board estimates are much lower than free hand estimates for an outdoor hill as well. Experiments 4 shows that wrist flexion is biased and noisy compared to elbow flexion, while Experiment 5 shows that small changes in palm board height produce large changes in palm board estimates. Together, these studies suggest that palm boards are biased and insensitive measures. The existing literature arguing that there are two systems in the perception of geo- graphical slant is re-evaluated, and a new theoretical framework is proposed in which a single exagger- ated representation of ground-surface orientation guides both action and perception. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Does accurate action require accurate (unbiased) perceptual representations? If by accurate action, one means effective action, then the answer is no. Action can be guided by completely biased perceptual experience, so long as that experience is predictable and stable (Durgin, 2009). Consider the actions of a watchmaker looking through a magnifying lens. The sizes of the parts of the watch are clearly distorted by the lens, but perceived actions are distorted in the same way. The watchmaker can become quite deft in the use of the magnifier. For a more ballistic action, consider the spear fisherman who must cope with refractive distortions of posi- tion at the water–air boundary. The conscious perception of hills overestimates their slopes (geographical slant) substantially (Gibson & Cornsweet, 1952; Kammann, 1967; Proffitt, Bhalla, Gossweiler, & Midgett, 1995; Ross, 1974). Proffitt et al. considered whether successful action with respect to hills depended merely on calibrated action (like the watchmaker; see also Philbeck, Loomis, and Beall (1997)), or on a separate perceptual representation that supported action (Bridgeman, Lewis, Heit, & Nagle, 1979; Milner & Goodale, 1995). They argued that experimentally induced dissociations between haptic measures and verbal reports, such as in response to fatigue, supported the idea that there were two separate perceptual repre- sentations for geographical slant, only one of which was available to conscious inspection. This claim was further developed in two studies that argued for the existence of a separate accurate motor representation of hills (Bhalla & Proffitt, 1999; Creem & Proffitt, 1998) used to guide motor actions (Creem & Proffitt, 2001; Proffitt, 2006, 2009). In this paper we will argue that the principal evidence that has been taken to support this view has been misinterpreted. We suggest, instead, that hill misperception may be a kind of mag- nification for action. There are many means of asking research participants to evalu- ate a surface’s geographical slope (orientation relative to the hori- zontal planes defined by the normal vector of gravity). One particular method has played a central role in the theoretical development of a two-systems approach to geographical slope per- ception. In a section heading entitled ‘‘Visually guided actions show little or no evidence of the phenomenal overestimation of geographical slant’’, Proffitt et al. (1995, p. 425) wrote ‘‘Our haptic measure of pitch showed very little evidence of slant overestima- tion.’’ By ‘‘haptic measure’’, they refer to adjusting an unseen palm board to match the orientation of the hill. Generically, a palm board is a flat surface that can be rotated by hand about a horizontal axis 0001-6918/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.actpsy.2010.01.009 * Corresponding author. Address: Department of Psychology, Swarthmore Col- lege, 500 College Ave., Swarthmore, PA 19081, United States. Tel.: +1 610 328 8678; fax: +1 610 328 7814. E-mail address: fdurgin1@swarthmore.edu (F.H. Durgin). Acta Psychologica xxx (2010) xxx–xxx Contents lists available at ScienceDirect Acta Psychologica journal homepage: www.elsevier.com/locate/actpsy ARTICLE IN PRESS Please cite this article in press as: Durgin, F. H., et al. Palm boards are not action measures: An alternative to the two-systems theory of geographical slant perception. Acta Psychologica (2010), doi:10.1016/j.actpsy.2010.01.009