Inferring the depth of 3-D objects from tactile spatial information

Four psychophysical experiments were conducted to examine the relation between tactile spatial information and the estimated depth of partially touched 3-D objects. Human participants touched unseen, tactile grating patterns with their hand while keeping the hand shape flat. Experiment 1, by means of a production task, showed that the estimated depth of the concave part below the touched grating was well correlated with the separation between the elements of the grating, but not with the overall size of the grating, nor with the local structure of the touched parts. Experiments 2 and 3, by means of a haptic working memory task, showed that the remembered depth of a target surface was biased toward the estimated bottom position of a tactile grating distractor. Experiment 4, by means of a discrimination task, revealed that tactile grating patterns influenced speeded judgments about visual 3-D shapes. These results suggest that the haptic system uses heuristics based on spatial information to infer the depth of an untouched part of a 3-D object.