WEIGHT JUDGMENT - THE DISCRIMINATION CAPACITY OF A DEAFFERENTED SUBJECT

A weight discrimination study was undertaken to test (i) the capacity of controls and a deafferented subject (deprived of large sensory myelinated fibres from nose down), to discriminate weights with and without vision; (ii) the capacities of observers to discriminate weights while watching the deafferented and control subjects' lifting movements; (iii) the contribution of supplementary sources of sensory information (e.g. vestibular afferents) to the deafferented subject's discrimination capacity. With vision, G.L.'s liminal discrimination of weights was similar to that of the controls. In contrast, precluding vision impaired massively but not completely G.L.'s discrimination capacity so emphasizing the importance of visual kinaesthetic cues in G.L. and incidently rite importance of large myelinated sensory function in weight discrimination in controls. Kinematics recordings of G.L.'s lifting movements with vision revealed a significant correlation between weight and peak velocity of the lifting movement. This reflects a specific strategy used by G.L. to generate movements, allowing her to judge the weight of a lifted object visually. Peak velocity rather than amplitude of movement appears to be the main cue for G.L. since there was a lack of correlation between amplitude and weight lifted. For controls, none of the correlations (weight versus amplitude or weight versus velocity) was significant whether vision was available or not. When watching G.L.'s lifting performance, external observers were able to use similar cues to establish their judgments, but they were far less accurate in doing so when watching control subjects. This suggests that controls were using a strategy different from G.L.'s. Without vision, a residual discrimination capacity was observed for the heavier weights in G.L. Perception of the lightest weight disappeared after a mechanical stabilization of the head, suggesting the likely contribution of vestibular signals. Therefore, for G.L., a patient completely devoid of neck and limb proprioception our results limit the role a sense of effort or residual somatic afferents to the discrimination of the heavier weights.