My (Elizabeth’s) current project has been to map out somatosensation (i.e. our sense of position) of the human arm across a horizontal workspace. Is a somatosensory fovea (an area of the workspace where we are more sensitive to our hand’s position)? Are we systematically biased across the workspace? One hypothesis is that our sense of limb position is experience-dependent: maybe we are most sensitive in the centre of the workspace where we perform much of our tool use, or maybe our perception is biased towards this location. A second possibility is that our limb geometry influences somatosensation. For example, larger changes in joint angle may give the brain more information about the location of the limb at a new position.
We have developed a novel paradigm to test somatosensation in which subjects make judgments about the location of their hand relative to a remembered somatosensory reference position. The most robust result of this experiment was that somatosensory bias was in opposite directions for the right and left hands. Regardless of hand dominance and workspace location, subjects tested using their right hand were biased towards the left, while those tested using the left hand were biased towards the right. The results of this study were repeated in a second experiment where the memory component of the test was removed and instead subjects made judgments relative to a visual reference.
A follow-up question we’re pursuing is: How does the brain combine somatorsensory information from two limbs? Does the brain average the information and thus the bias? Is the perception of one hand dominant? Below are psychophysical functions for two subjects who each performed three somatosensory tests: one with each hand and one using both hands. These preliminary data suggests that the perceptual bias when using both hands lies somewhere between the biases of the right and left hands.
[x-axis: distance from reference position (m); y-axis: probability of reporting "right"]
Right hand bias = -5.69 mm; Both hands bias = -2.47 mm; Left hand bias = 10.34 mm
[x-axis: distance from reference position (m); y-axis: probability of reporting "right"]
Right hand bias = -1.44 mm; Both hands bias = 1.55 mm; Left hand bias = 3.67 mm
Posted by ewilso2 
