EEG signatures for tactile object individuation on a finger tip.

Object individuation (OI), i.e., segmenting sensory signals into individual objects based on spatial or temporal cues, has been studied predominantly in vision but remained largely unexplored in touch. Using EEG we studied the neural activity associated with OI in touch. In two experiments, participants (Experiment 1: n=28; Experiment 2: n=27) were presented with one to eight pins on their index fingertip. In Experiment 1, they enumerated the number of pins, a task relying on OI. In Experiment 2, they detected a brief temporal flicker in the tactile stimulus, a task that did not involve OI. Multivariate analyses showed that neural activity around 860 msec post-stimulus encoded the number of pins, but only during enumeration. Moreover, for Experiment 1, we observed that the amplitudes of two ERP indices, the late contralateral negativity (N1000cc) and the tactile contralateral delay activity (tCDA), increased with the number of enumerated items, peaking at three objects. Time-frequency analysis revealed greater oscillatory power in the alpha-band for small numerosities (1-3 items), corresponding to the range typically associated with tactile subitizing, relative to larger numerosities (4-6 items), suggesting an attentional control mechanism that aids in separating tactile objects from the background while reducing interference from irrelevant sensory information during capacity-limited OI processing. Importantly, these neural effects were not observed in Experiment 2. Collectively, our results provide evidence that tactile object individuation is capacity-limited and is associated with distinct N1000cc, tCDA, and alpha-band activity.