Distal attribution and distance perception in sensory substitution

In sensory substitution, the user may be directly aware of distal objects, as in everyday perception, or make explicit cognitive inferences based on an awareness of the proximal stimulation. Anecdotal evidence supports the experience of distal attribution, but so far there have been few rigorous experimental tests of the claim. In this study, blindfolded participants observed a target light using a device consisting of a finger-mounted photodiode that drives tactile vibra-tion on the back. With the blindfold off and the target removed, participants moved a reference object to match the perceived egocentric distance of the target. Participants who were instructed to attend to the distal target improved significantly during 2 h of practice, whereas those instructed to attend to proximal variables showed no improvement. Unsigned error increased with ratings of proximal attention, but decreased with ratings of target object solidity, consistent with distal attribution. Performance transferred to the non-dominant arm and to a rotated body orientation, demonstrating that learning did not depend on a joint-specific sensorimotor relationship between target distance and arm configuration. The results experimentally confirm that distal attribution can occur in sensory substitution, based on a perceptual strategy rather than an explicit cognitive strategy. Moreover, they suggest that the informational basis for distal attribution is not a joint-specific sensorimotor relation, but a more abstract spatial invariant.