Premotor Circuit Topology and Sensory-motor Pattern Selection

In recent years, modeling of ocular premotor circuits in the brainstem has unmasked several advantages linked to the topology of these circuits. In particular, their bilateral symmetry can implicitly provide for rich motor behavior in response to sensory patterns such as 1) coordination of vergence and version trajectories in binocular systems, 2) simple fusion of multiple sensory sources with diverse dynamics, 3) automatic selection of appropriate motor responses and platform coordination, according to bilateral sensory patterns (e.g. ocular vergence during linear fore-aft motion vs conjugate responses during interaural translation), and 4) use of parameter (circuit) switching to enable additional modes (e.g. saccades vs slow tracking). Spinal premotor pathways also include many levels of symmetry in the circuits controlling limb muscles, for example in the stretch reflex connections of agonist/antagonist muscle groups. These therefore imply a capacity to imbed locally several motor strategies, simply released by the current context and sensory inflow patterns. Hence circuit topology should be considered a key element in the process of sensorimotor mapping and in the selection of motor strategies. This is in contrast to the classical approach where desired motor patterns are selected at a cortical level, following several computational stages.