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
defferented man, showed movement durations that
increased linearly as the complexity of the drawings
increased. However, the defferented 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.