Cross-modality matches of finger span and line length

Abstract

Perceived finger span-the perceived spatial separation between the tip ofthe thumb and the tip of the index finger-was measured by using cross-modal matching to line length. In the first experiment, subjects adjusted finger span to match the length of line segments presented on a video monitor, and conversely, with both hands. Subjects also made estimates of finger span in physical units ("dead reckoning"). Finger spans were measured by using infrared LEDs mounted on the tip of the thumb and the finger tip, so the hand made no contact with any object during the experiment. Unlike in previous studies, the results suggest that perceived finger span is proportional to line length and slightly shorter than the actual span, provided that corrections are made for regression bias. The effect of finger contact was assessed in a second experiment by matching line length both to free span and to spans constrained by the pinching of blocks in the same session. The matching function when subjects were pinching blocks was accelerating, consistent with previous reports. In contrast, matched line length was a decelerating function of free span. The exponent of the free span matching function in the second experiment was slightly smaller than in the first experiment, probably due to uncorrected matching biases in the second experiment.A person usually has no difficulty reaching for a glass of water and picking it up. It can be done even with the eyes closed after a couple of tries. There is, obviously, exquisite motor control based on visual and somatosensory information. But do the different sensory modalities tell us the same thing? For example, does the glass look as big as it feels? The question does not seem that interesting at first-surely the perceived size of the glass does not depend on whether it is looked at or is held in the hand. Experimentally, though, the answer depends on how the equivalence of size is measured. Magnitude estimates of span are nearly linear with respect to the actual distance between the tips of the index fingers of two hands held apart (Gogel, Wist, & Harker, 1963;Stanley, 1966;M. Teghtsoonian & R. Teghtsoonian, 1965). But the bulk of the experimental evidence suggests in contrast that the span perceived between the tips of the thumb and index finger of the same hand is a nonlinear function of the actual distance, is not veridical, and is not commensurate with perceived visual size.Jastrow (1886) provided the first such evidence by having subjects adjust finger span by sliding a movable carriage held between the thumb and index finger to match the length ofa viewed line (0.5-12 ern); or, to seThe author would like to thank Robert Teghtsoonian and Calvin Garbin for reviewing the manuscript and making many helpful comments and suggestions; Purwanto Suwondo, Lisa Menia, Jennifer Evans, and Michael Haines for technical assistance and data collection; and Robert Montz for making the blocks used in the second experiment. This work was funded by Grant NS27958 from the National Institutes of Health. Co...

Objects, Raised Lines, and the Haptic Horizontal-Vertical Illusion

“…The second experiment was also designed to overcome a further potential limitation of earlier research using the pincers response method for making length judgements. While that method is ecologically valid and reasonable (see Van Doren, 1995), there are some difficulties in using the pincers response measure. Some subjects in earlier studies (e.g., Heller et al, 1997) complained that it was difficult to know how wide apart their fingers were without vision.…”

The tactual horizontal-vertical illusion depends on radial motion of the entire arm

“…The subjects reported their size estimates in the order in which they felt them, but could always examine a line again if they wished. The subjects reported judgments with the opposite hand, using a pincers posture of the thumb and index finger, like the "C" posture of Van Doren (1995). The experimenter measured this response with a ruler, as if the participant were holding the line between the tips of the fingerprint skin of the two fingers (see Figure I in .…”

Haptic Discrimination of Distance

“…For the finger-span method, which can be used to perceive distances of hand-held objects, the relation between physical length and perceived length is a power function with an exponent ranging between 1.1 and 1.3 [16], [17], [18], [19]. For the movement method, a power function with an exponent of about 0.89–1 is reported [20], [21].…”