Tuesday, February 19, 2008

On somatosensory binding

Somatosensory experiences, like those of sight and sound, are projected on to the CBS. But unlike sight or sound, the mapping between particular somatosensory receptors of the human body to the positions in the CBS is dynamic and depends on the exact pose of the body at the particular instant. For example, consider a highly localized somatosensory stimulus like a pin prick. If someone gets pricked on a finger but he waves that hand wildly, the sensation of pain travels a trajectory in the CBS, and the exact position is determined by the motor feedback which tells the brain the position of the hand. This remapping to the CBS enables one to coordinate other senses with the pain stimulus. For example, when you are getting bitten by a mosquito on the leg, your hand automatically reaches towards the correct part of the leg to hit it, even though at that time, the actual way the leg is folded is not brought to conscious attention. And simultaneously your eyes also get drawn to the direction of the bite.

Of course, the dynamic mapping is not a hundred percent accurate. One extreme pathological case is the phenomenon of phantom limb pain suffered by amputees. In this case, the pain is projected on to a part of the CBS which doesn't correspond to any part of the person's body. But even otherwise one can assume that the mapping is not accurate to within a fraction of an inch because of the limitations of the motor feedback mechanisms. Which is in a way good, since it would allow us to perform a simple thought experiment. If we have the index fingers of both the left and right hands pricked and bring them close together so that their pain centers coincide in the CBS (and this is possible without the fingers running into each other if there is some error in the motor calibration), is the qualia state indistinguishable from just one finger pricked twice as hard? If the overall sensation of pain is mapped as points in the CBS, once this reduction takes place, it would no longer be possible for the brain to figure out which finger got pricked, without other cues like visual, etc. But the important question here is, does this kind of reductionist mapping occur in the first place? From a functional standpoint, it is fairly easy for the brain to figure out which hand got pricked. It is a no-brainer (pun unintended) to find out from which hand 's nerves the pain sensation originated.

But if we accept the premise that qualia are higher level representations which subsume lower level unconscious signalling, then we somehow need to accomodate this extra information regarding which hand the pain originated from into the nature of the pain qualia itself. This means that the simple assumption that pin pricks are just pain points in the CBS is inaccurate. With a fully developed theory of qualia (if that ever happens), we will see the exact representation of somatosensory experiences and how ambiguities of the above sort are resolved within the framework of the qualia representation itself.