Touching what is out there

The conscious sensation of touch is felt at the location of the tactile stimulus: we feel the key or the pen that we pick up at our fingertips rather than in the brain where the sensory signals end up. If we use a tool to explore our surroundings, such as a walking-stick, something even more curious happens. We feel the sensation of touch taking place out there at the tip of the stick. How is that possible; after all, there certainly are no sensory receptors located at the tip of the stick!

Yamamoto and Kitazawa (2001a,b) have recently reported two studies in Nature What does it mean to say that the sensations are “referred to” the tips of the sticks? Neuroscience that may elucidate this mystery. When two stimuli were given in rapid succession to the fingertips of uncrossed hands, the subjects were remarkably good at judging which hand was stimulated first. But if the arms were crossed, reversing the spatial locations of the hands, subjects tended to misreport the order unless there was about one second between the successive stimuli. This experiment shows that the judgement of subjective temporal order is based on a reconstruction of the spatial locations of the stimuli within an internal spatial representation. It also demonstrates that the binding together of a unified body image takes time, especially when the default mapping of locations is violated.

Even more dramatically, this effect also applies to the subjective spatial location of touch at the tip of a tool! Yamamoto & Kitazawa (2001b) showed that if stimuli are elicited at the ends of tools (drumsticks), simply crossing the tools without crossing the arms resulted in misreporting the spatial locations of the stimuli at short inter-stimulus intervals: “This strongly suggests that the somatosensory signals evoked at the hands were referred to the spatial locations of the tips of the sticks before these cutaneous signals were ordered in time” (p. 980).

But what does it mean to say that the sensations are “referred to” the tips Are sensory experiences located in a ‘virtual space’ in the brain? of the sticks? We must resist the projectivist reading of such referral: There certainly is nothing literally projected out of the brain and referred to the physical tool out there in external physical space. Already Gestalt psychologists were quite clear about it — Wolfgang Köhler wrote in 1929: “Under no circumstances has the phenomenal object anything to do with the place in physical space where the ‘corresponding’ physical object is located”. Thus, we can be absolutely sure about one thing: the sensations we feel in our consciousness cannot magically travel to the tip of the external physical stick out there in physical space. What is going on, then?

An inviting – but awesome – interpretation is that the sensory experiences we subjectively feel are really located in a “virtual space” inside the brain, a space where all conscious sensations and experiences happen. Within this phenomenal space, the sensation of touch at the tip of a tool is reconstructed as happening at the same distal spatial position relative to the body image as the visual percept of the tip of a tool is located.

Since our subjective sensations cannot literally travel out of the brain, to the tips of our tools, or even to the tips of our fingers, it appears that the brain must reconstruct an internal virtual reality of consciousness within which sensations and percepts can be located in spatial relations to one another. If that interpretation is accurate, it is much easier to understand how the brain constructs the hallucinatory experience of a phantom limb, or of an entire body image in a dream: they must be phenomenal events that take place within the confines the virtual reality of consciousness.

References

Köhler W (1929/1971) An old pseudoproblem. Reprinted in: The Selected Papers of Wolfgang Köhler, 125-141. New York: Liveright.

Yamamoto S & Kitazawa S (2001a) Reversal of subjective temporal order due to arm crossing. Nature Neuroscience 4(7): 759‑765.

Yamamoto S & Kitazawa S (2001b) Sensation at the tips of invisible tools. Nature Neuroscience 4(10): 979‑980.