II-61. Cortical preparatory activity avoids causing movement by remaining in a muscle-neutral space

Motor and premotor cortex are involved in the execution of reaching movements, but are also active during the preparation of those movements. Since the arm remains motionless during preparation, we ask: why does preparatory activity not cause movement? One possible mechanism is that activity does not escape the motor cortices because output is suppressed by local inhibition. This mechanism is known to operate in the brainstem for saccade control. We have previously searched for such a ‘gating’ mechanism in the activity of inhibitory interneurons in motor and premotor cortex of monkeys. We found that inhibition does not appear to gate movement in either premotor (Kaufman et al., 2010, J Neurophys) or primary motor (Kaufman et al., 2010, SFN) cortex. We thus propose and test an alternative model. Since there are many more neurons than muscles, there must be many possible neural states for any given pattern of muscle forces. We therefore predict that there will be an ‘iso-force space’ in neural space, where any neural state within this space will produce the same muscle output. This iso-force space could be exploited to permit motor preparation without causing movement. Here, we attempt to identify such an iso-force space using neural data from two monkeys. We tested whether some dimensions in neural space might be ‘muscle-potent,’ identifiable because activity in these dimensions correlates with muscle activity, while other dimensions might be ‘iso-force.’ We found that preparatory activity preferentially occupies the iso-force (non-muscle-like) space, while movement-epoch activity explores a greater volume of both spaces. This result held both when testing trial-averaged mean data and when examining single-trial activity. Such a mechanism might have implications beyond the motor system, conceivably allowing areas to switch their functional connectivity on and off.