The body is a multisensory object par excellence.  Most multisensory studies consider how information about a single stimulus object is combined across two different sensory channels.  However, focussing on body representations allows us to identify other forms of multisensory interaction. These involve top-down multisensory modulation, rather than bottom-up multisensory convergence.  In one example, viewing one’s own body improves the sense of touch, and also reduces intensity of pain.  Recent results identify multisensory circuits in the parietal lobe as the key intermediate node in the visual-somatosensory link that underlies these interactions.  Viewing the body seems to activate a representation that regulates the cortical circuits for somatosensation.

In a second example, I focus on a recently discovered interaction between vestibular signals and somatosensory signals.  The vestibular system gives the orientation of the head with respect to the external environment.  Interestingly, there is no unisensory vestibular cortex: vestibular projects are shared with other sensory channels, in particular somatosensation. We have shown that vestibular stimulation improves tactile detection, and decreases pain sensation.  It also increases the perceived distance of visual objects far from the body, without affecting peripersonal distance perception.  Thus, the sensory balancing triggered by reorienting the body relative to the environment seems to increase the influence of proximal, self-related signals, re-emphasising the self when the exteroceptive inputs change.

I conclude that multisensory modulation is an intrinsic element in setting the appropriate relation between the self and the world.  Representations of a supramodal, continuous self may be a consequence of first implementing new balances of multisensory gains when we interact with the world, and then maintaining appropriately-tuned multisensory balance.

Dr. Adrian Alsmith (University of Copenhagen · Center for Subjectivity Research)

I would like to consider the following trilemma:

(i)         A subject’s perceptual experience is unified both within and between its senses according to a single perspective.

(ii)        The perspectival nature of perceptual experience ought to be conceived as the point of origin for an egocentric frame of reference.

(iii)       Multiple distinct egocentric frames of reference are employed both within and between the senses.

Individually, each of these claims has some plausibility. Collectively, they seem to present an inconsistency: perceptual experience cannot be unified according to a single perspective, if that perspective is conceived as the point of origin for an egocentric frame of reference, when there are multiple distinct egocentric frames of reference in operation within and between the senses. I will argue that the trilemma is genuine and discuss the motivations for each of the claims constituting it, in the hope of discerning which ought to be rejected.