Alpha Stimulation of the Human Parietal Cortex Attunes Tactile Perception to External Space

An intriguing question in neuroscience concerns how somatosensory events on the skin are represented in the human brain. Since Head and Holmes' [1] neuropsychological dissociation between localizing touch on the skin and localizing body parts in external space, touch is considered to operate in a variety of spatial reference frames [2]. At least two representations of space are in competition during orienting to touch: a somatotopic one, reflecting the organization of the somatosensory cortex (S1) [3], and a more abstract, external reference frame that factors postural changes in relation to body parts and/or external space [4, 5]. Previous transcranial magnetic stimulation (TMS) studies suggest that the posterior parietal cortex (PPC) plays a key role in supporting representations as well as orienting attention in an external reference frame [4, 6]. Here, we capitalized on the TMS entrainment approach [7, 8], targeting the intraparietal sulcus (IPS). We found that frequency-specific (10 Hz) tuning of the PPC induced spatially specific enhancement of tactile detection that was expressed in an external reference frame. This finding establishes a tight causal link between a concrete form of brain activity (10 Hz oscillation) and a specific type of spatial representation, revealing a fundamental property of how the parietal cortex encodes information.