An Investigation of Haptic Object Discrimination and Cue Combination

Abstract

Perception relies on the integration of numerous noisy inputs (cues). Cue combination has been relatively understudied in somatosensation, compared to vision and audition. Here, we investigated whether haptic cutaneous and hand configuration cues are combined optimally to discriminate between coin-sized discs of different sizes. When the hand is open such that the thumb and index fingers span the diameter of a disc to contact its perimeter, cutaneous cues occur from the indentation of the skin caused by the curvature of the disc (smaller discs cause greater indentation). Simultaneously, the hand configuration cue (relating to the perceived distance between fingers), provides an additional cue to size. These cues may vary in their reliability. In three experiments involving 34 participants, we measured these cues and considered three hypotheses for how humans may use them: humans rely solely on the least noisy cue (Winner-Take-All Model), humans combine cues based on a simple average (Average-Measurement Model), or humans combine cues via an optimal weighted average (Optimally-Weighted Model). Each experiment tested participants using a two-interval forced-choice (2IFC) paradigm with 3D printed disc stimuli. On each trial, under occluded vision, participants felt two discs sequentially and responded which felt larger. Participants were tested with each finger’s cutaneous cue alone, the hand configuration cue alone, and all cues together. In two experiments, the presented discs were both circular. In a third experiment, unknown to participants, some of the presented discs were oval-like cue conflict stimuli. Participant performance was compared to predictions of the cue combination models. We conclude that humans may combine haptic cutaneous and hand configuration cues optimally to judge the size of held objects.