The stiffness properties of an environment are perceived during active manual manipulation primarily by processing force cues and position-based tactile, kinesthetic, and visual information. Using a two alternative forced choice (2AFC) stiffness discrimination task, we tested how the perceiver integrates stiffness-related information based on sensory feedback from one or two modalities and the origins of within-session shifts in stiffness discrimination ability. Two factors were investigated: practice and the amount of available sensory information. Subjects discriminated between the stiffness of two targets that were presented either haptically or visuohaptically in two subsequent blocks. Our results show that prior experience in a unisensory haptic stiffness discrimination block greatly improved performance when visual feedback was subsequently provided along with haptic feedback. This improvement could not be attributed to effects induced by practice or multisensory stimulus presentation. Our findings suggest that optimization integration theories of multisensory perception need to account for past sensory experience that may affect current perception of the task even within a single session.

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