Evidence for stronger visuo-motor than visuo-proprioceptive conflict during mirror drawing performed by a deafferented subject and control subjects

It has been proposed that mirror drawing is difficult because of the conflict between visual and proprioceptive signals from the arm. However, even without proprioception, there should be difficulties in planning movements to visual targets observed in a mirror, as the mirror-reversed spatial information must be translated into appropriate hand actions. Mirror drawing tasks suggest these planning conflicts are likely to be most obvious at corners, when encountering sharp changes in direction. We have therefore tested the speed of mirror drawing in a chronically deafferented man and in a control group of normal subjects, and hypothesized that increases in template complexity (number of corners) would result in reduced drawing speeds in all subjects. Indeed, all subjects, including the deafferented man, showed movement durations that increased linearly as the complexity of the drawings increased. However, the deafferented man was significantly faster than the control subjects at tracing curved templates. We suggest that the major difficulty in mirror tracking is in the visuo-motor planning of actions based on mirror-reversed visual information, and is not a conflict between visual and proprioceptive signals about arm motion.