Changing Reference Frames during the Encoding of Tactile Events

Abstract

The mindless act of swatting a mosquito on the hand poses a remarkable challenge for the brain. Given that the primary somatosensory cortex maps skin location independently of arm posture [1, 2], the brain must realign tactile coordinates in order to locate the origin of the stimuli in extrapersonal space. Previous studies have highlighted the behavioral relevance of such an external mapping of touch, which results from combining somatosensory input with proprioceptive and visual cues about body posture [3-7]. However, despite the widely held assumption about the existence of this remapping process from somatotopic to external space and various findings indirectly suggesting its consequences [8-11], a demonstration of its changing time course and nature was lacking. We examined the temporal course of this multisensory interaction and its implications for tactile awareness in humans using a crossmodal cueing paradigm [12, 13]. What we show is that before tactile events are referred to external locations [12-15], a fleeting, unconscious image of the tactile sensation abiding to a somatotopic frame of reference rules performance. We propose that this early somatotopic "glimpse" arises from the initial feed-forward sweep of neural activity to the primary somatosensory cortex, whereas the later externally-based, conscious experience reflects the activity of a somatosensory network involving recurrent connections from association areas.

Integration of anatomical and external response mappings explains crossing effects in tactile localization: A probabilistic modeling approach

“…Moreover, a single weight parameter for each reference frame was sufficient to account for uncrossed and for crossed performance, indicating that the integration follows the same principles in both postures. Effects of body posture on touch processing have often been attributed to the external reference frame of touch (Driver & Spence, 1998;Aglioti et al, 1999;Kennett et al, 2001;Yamamoto & Kitazawa, 2001a;Shore et al, 2002;Soto-Faraco et al, 2004;Röder et al, 2004;Eimer et al, 2004;Bolognini & Maravita, 2007;Azañón & Soto-Faraco, 2008;Heed et al, 2012;Buchholz et al, 2011;. This is because body posture determines the location of the stimulated skin region in space, but should not influence the processing of any other tactile stimulus characteristics .…”

“…Moreover, the present parameter estimates invite the speculation that bottom-up influences, such as the number of stimuli, are reflected in the anatomical weights, whereas top-down influences, such as task instructions, bear on the external weights: Indeed, the anatomical representation of touch is associated with the primary somatosensory cortices, that store all sensory characteristics of a touch. In contrast, the external representation of touch is presumably established later and in higher-order brain regions (Azañón & Soto-Faraco, 2008;Heed & Röder, 2010;Overvliet et al, 2011;Soto-Faraco & Azañón, 2013;Rigato et al, 2013). The latter are tightly connected to frontal brain areas associated with cognitive control processes.…”

Somatotopic or spatiotopic? Frame of reference for localizing thermal sensations under thermo-tactile interactions

“…Tactile inputs have been shown to be coded somatotopically at early stages of processing and, subsequently, to be processed in spatiotopic space (Azañón & Soto-Faraco, 2008). The influences from the somatotopic distance have been reported in simple temporal tasks, such as tactile simultaneity judgment (Clark & Geffen, 1990;Kuroki, Watanabe, Kawakami, Tachi, & Nishida, 2010) and tactile apparent motion detection (Kuroki et al, 2010;Sherrick, 1968), and in tactile learning (Harris, Harris, & Diamond, 2001).…”

Long-range tactile masking occurs in the postural body schema

“…Head and Holmes (Head and Holmes 1911) suggested that a superficial body schema (a representation of the body in the brain) is first created in a canonical position with posture being added later to form what they called the postural schema. Their seminal study has since obtained substantial support (Shuler et al 2001;Azañón and Soto-Faraco 2008;Schntz-Bosbach et al 2009;Azañón et al 2010a, b). The present experiment investigates whether long-range masking effects occur at the level of the superficial body schema or after postural information has been added to form a postural schema.…”