Bilateral representations of touch in the primary somatosensory cortex

Abstract

According to current textbook knowledge, the primary somatosensory cortex (SI) supports unilateral tactile representations, whereas structures beyond SI, in particular the secondary somatosensory cortex (SII), support bilateral tactile representations. However, dexterous and well-coordinated bimanual motor tasks require early integration of bilateral tactile information. Sequential processing, first of unilateral and subsequently of bilateral sensory information, might not be sufficient to accomplish these tasks. This view of sequential processing in the somatosensory system might therefore be questioned, at least for demanding bimanual tasks. Evidence from the last 15 years is forcing a revision of this textbook notion. Studies in animals and humans indicate that SI is more than a simple relay for unilateral sensory information and, together with SII, contributes to the integration of somatosensory inputs from both sides of the body. Here, we review a series of recent works from our own and other laboratories in favour of interactions between tactile stimuli on the two sides of the body at early stages of processing. We focus on tactile processing, although a similar logic may also apply to other aspects of somatosensation. We begin by describing the basic anatomy and physiology of interhemispheric transfer, drawing on neurophysiological studies in animals and behavioural studies in humans that showed tactile interactions between body sides, both in healthy and in brain-damaged individuals. Then we describe the neural substrates of bilateral interactions in somatosensation as revealed by neurophysiological work in animals and neuroimaging studies in humans (i.e., functional magnetic resonance imaging, magnetoencephalography, and transcranial magnetic stimulation). Finally, we conclude with considerations on the dilemma of how efficiently integrating bilateral sensory information at early processing stages can coexist with more lateralized representations of somatosensory input, in the context of motor control.

Somatosensory cortical representation of the body size

“…Conversely, in the present experiments rTMS of S1 affects size representation of both the hand contralateral and of the hand ipsilateral to the stimulated hemisphere. Recent evidence from humans also shows substantial integration of tactile information from the two hands in S1 (Tamé et al, 2016;Tamè, Pavani, Papadelis, Farnè, & Braun, 2014). Recent evidence from humans also shows substantial integration of tactile information from the two hands in S1 (Tamé et al, 2016;Tamè, Pavani, Papadelis, Farnè, & Braun, 2014).…”

“…This evidence suggests that tactile inputs contribute to the building up and updating of the internal representations of one's own body, including the visual appreciation of its size (Serino & Haggard, 2010;Tamé, Braun, Holmes, Farnè, & Pavani, 2016;Vallar & Rode, 2009); this occurs notwithstanding the absence of peripheral receptors directly coding the size and shape of body parts (Harding-Forrester & Feldman, 2018;Kaas, Qi, & Stepniewska, 2018). Hence, the amount of tactile information transmitted from the body to the cortex can directly affect MBRs (Serino & Haggard, 2010).…”

What can errors tell us about body representations?

“…Although relatively underreported, some individuals report dysynchiric phenomena in which tactile sensations are localized to the same location on the opposite side of the body. In this section, we will review evidence from individuals with different dysynchiric phenomena (including allochiria and synchiria) and discuss how these and other findings provide evidence for interhemispheric involvement in representing touch (see also Tame, Braun, Holmes, & Pavani, 2016). Next, synchiric phenomena and other aspects of tactile sensation are influenced by body position in external space (for a review, see Badde & Heed, 2016).…”

Somatosensory Misrepresentation Associated with Chronic Pain: Spatiotemporal Correlates of Sensory Perception in a Patient following a Complex Regional Pain Syndrome Spread

“…(24)] suggests an early intimate relationship between S1 across hemispheres especially for the hands that challenges the classical textbook knowledge of unilateral tactile representation. Tactile information from the body side appears to reach early ipsilateral S1 via transcallosal connections supporting bilateral integration in early stages of tactile processing.…”