**1. Introduction**

74 Haptics Rendering and Applications

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The two main purposes of this chapter are to review past evidence for a systematic spatial bias in the perception of surface orientation (geographical slant), and to report two new experiments documenting this bias in the manual haptic system. Orientation is a fundamental perceptual property of surfaces that is relevant both for planning and implementing actions. Geographical slant refers to the orientation (inclination or pitch along its main axis) of a surface relative to the gravitationally-defined horizontal. It has long been known that hills appear visually steeper than they are (e.g., Ross, 1974). Only recently has it been documented that (1) there is also bias in the haptic perception of surface orientation (Hajnal et al., 2011), and that (2) similar visual and haptic biases even exist for small surfaces within reach (Durgin, Li & Hajnal, 2010).

To provide a context for understanding the present experiments, we will first provide an overview of the prior experimental evidence concerning bias in the perception of geographical slant. First we will discuss findings from both vision and haptic perception that have documented perceptual bias for surfaces in reach. We will then review the literature on the visual and haptic biases in the perception of the greographical slant of locomotor surfaces such as hills and ramps. At the intersection of these two literatures is the historical use of haptic measures of perceived geographical slant, and we will therefore review these measures with particular attention to understanding some pitfalls in the use of haptic measures of perception. We next contrast these haptic measures with proprioceptive measures of perceived orientation and discuss the problem in interpreting calibrated actions as measures of perception.

Having laid out these various past findings we will then report two novel experiments that demonstrate spatial biases in the haptic experience of real surfaces. The experiments include both verbal and non-verbal methods modelled on similar findings we have reported in the visual domain. Following the presentation of the experimental results we will discuss issues of measurement in perception – especially pertaining to the interpretation of verbal reports, and conclude with a discussion of functional theories of perceptual bias in the perception of surface orientation.
