Capturing the spatial percepts evoked by moving tactile stimuli: a novel approach

Abstract

Forced-choice procedures are conventionally used to study the percepts evoked by stimuli that move across the skin and enable an unbiased estimation of subjects’ sensory capacities. These procedures, however, require subjects to assign complicated percepts to one of a small number of experimenter-defined response categories, none of which may satisfactorily describe the perceptual experience. To address this limitation, we developed a psychophysical approach, which graphically captures spatial information about a moving stimulus in a holistic manner. Briefly summarized, the stimulus object controlled for location, velocity, direction and distance is moved across the skin of a blind-folded subject, after which the subject draws its path on a life-size, two-dimensional photograph of the body region stimulated. Using this approach, we demonstrated that the drawings contain perceptually relevant information, estimates of direction discrimination and subjective traverse length derived from the drawings closely parallel data obtained with forced-choice and magnitude estimation methods, respectively, and generate comparable psychophysical functions of stimulus velocity. In addition, information is represented in the complex shapes of the curves and in the locations at which they are drawn. Analyses of these latter features support the hypothesis that non-sensory factors (individual subject biases) also affect the drawings.

Introduction

The complexity of the percepts evoked by simple, point stimuli moved across the skin was demonstrated over 100 years ago by Hall and Donaldson [10]. To minimize response bias and other sources of among- and within-subject variability in measurements of sensory performance, psychophysical procedures have largely ignored the multidimensional and time-varying nature of the percepts. For example, forced-choice procedures are typically employed in studies of direction discrimination [4], [5], [9] and of velocity discrimination [8]. Forced-choice procedures require that the subject assign the complex percept to one of a small number of categories, none of which may accurately describe the perceptual experience. For example, to assess direction discrimination stimuli are usually moved across the test site in two opposing directions, described to the subject prior to testing. After each stimulus, the subject is required to select one of these two options even though the stimulus path perceived might be curved substantially and its average direction different from the stimulus directions by as much as 90°. To maximize performance, the subject selects as his response the stimulus direction for which the difference is <90°.

As a second example, magnitude estimation procedures are typically employed for studies in which subjects judge the length of skin traversed (and the duration or velocity of stimulus motion [6], [7], [8], [13]. Perceived deviations from a linear path are common for a stimulus moving even in a straight line, and it is unclear how this affects the subject's unidimensional response regarding the subjective distance of movement. Few attempts to capture holistic information about the percepts evoked by space- and time-varying stimuli have been reported. Noteworthy in this regard is the work of Langford et al. [11], who asked subjects to draw the path taken by a stylus moved across a linear chord, 12 cm in length, on the volar forearm. These investigators found that the curves were drawn over shorter lengths of skin as the velocity of motion was experimentally varied from 2 to 18 cm/s, and with less irregularity in shape. To our knowledge, the investigators did not pursue this method further to understand tactile perception. Our aim has been to develop computer-assisted approaches for the collection of subjects’ graphical responses (this report) and sophisticated quantitative analyses of the curve data obtained [12]. Based on the ecological and intuitive nature of this approach, we hypothesized that it would be useful in characterizing the altered tactile perception of patients with peripheral or central neural disorders [1]. In order to compare the results with available data obtained with conventional psychophysical procedures, we studied the facial region.