Columnar organization in the motor cortex.

Despite many attempts to find spatial structure in the functional properties of neurons within the primary motor cortex (MI), there is still no compelling evidence for such structure despite the anatomical similarities between motor cortex and other neocortical areas. This is a longstanding puzzle in motor cortical physiology because topographic structure of function is a hallmark of primary sensory cortices including the visual, somatosensory, and auditory cortices. In particular, experimental evidence has supported the idea of vertical columns perpendicular to the cortical surface which contain neurons that share similar sensory tuning properties. Moreover, horizontal spatial structure has been observed in sensory cortices, most elegantly manifested by the pinwheel structure of orientation tuning across V1 (Bonhoeffer and Grinvald, 1991). Although early work by Asanuma provided some evidence for somatotopic, columnar organization in MI using intracortical microstimulation (Asanuma, 1975), further research by many others did not support this perspective but rather suggested distributed and overlapping representations of body parts. Namely, nearby sites in motor cortex could represent or evoke very different muscles and joints, and multiple, spatially-distributed sites could represent very similar body parts (Donoghue, Leibovic et al., 1992; Schieber and Hibbard, 1993; Sanes, Donoghue et al., 1995). By focusing on movement parameters instead of body parts, recent studies have provided evidence that a topographic organization of directional tuning does exist within the motor cortex (Amirikian and Georgopoulos, 2003; Georgopoulos, Merchant et al., 2007). However, it still remains controversial as to which movement parameters, if any, are truly encoded in single MI neurons.