Influence of presentation order on force control in softness exploration

Humans can optimize haptic perception by tuning their exploratory behavior. In softness exploration humans use more force when expecting a pair of hard objects as compared to soft ones, and this force control improves softness discrimination [1], [2]. Such force tuning seems to be based on implicit prior information about the upcoming compliance category [3]. In previous studies, prior information was implicitly induced by presenting blocks of trials of the same category (hard or soft). Here, we studied force control when hard and soft stimulus pairs alternate according to a predictable pattern. Participants had to decide which one of two silicone stimuli was softer. Soft and hard trials were presented in random order, in blocks, in alternating order (short pattern), or alternating always two hard and two soft trials (long pattern). We confirmed the finding of force tuning to compliance for blocked as compared to random presentation. The predictable presentation patterns also influenced force control, but not in the expected direction. We conclude that implicit expectations from sequences can be used in force control, but they are not sufficient for successful tuning. A further sequential analysis shows that forces are not only adapted by simple reactive trial-by-trial.mechanisms.