Mapping of the preferred direction in the motor cortex

Abstract

Directional tuning is a basic functional property of cell activity in the motor cortex. Previous work has indicated that cells with similar preferred directions are organized in columns perpendicular to the cortical surface. Here we show that these columns are organized in an orderly fashion in the tangential dimension on the cortical surface. Based on a large number of microelectrode penetrations and systematic exploration of the proximal arm area of the motor cortex while monkeys made free reaching 3D movements, it was estimated that (i) directional minicolumns are approximately 30 mum in width, (ii) minicolumns with similar preferred directions tend to occur in doublets or triplets, and (iii) such minicolumns tend to repeat every approximately 240 mum (estimated width of a column), with intermediate preferred directions represented in a gradient. These findings provide evidence for an orderly mapping of the preferred direction in the motor cortex.

Fig. 1.

Design of the mapping experiment. (A) Schematic illustration of the projection of recording sites onto the cortical surface along anatomical columns. The procedure is described in detail in ref. . Open and slashed circles denote recording and projected sites, respectively. (B) Frequency distribution of pairwise distances on the cortical surface (≤1,200 μm) between projected recording sites (bin size, 30 μm).

Fig. 2.

Location on the flattened motor cortical surface (9) of the preferred directions of directionally tuned sites. The octant of the unit sphere to which a preferred direction belongs is color-coded depending on the sign of the directional cosines [x, y, z] as follows: blue, [x ≥ 0, y ≥ 0, z ≥ 0]; magenta, [x ≥ 0, y ≥ 0, z < 0]; green, [x ≥ 0, y < 0, z ≥ 0]; light gray, [x ≥ 0, y < 0, z < 0]; turquoise, [x < 0, y ≥ 0, z ≥ 0]; red, [x < 0, y < 0, z ≥ 0]; white, [x < 0, y ≥ 0, z < 0]; yellow, [x < 0, y < 0, z < 0]. A, anterior; L, lateral.

Fig. 3.

Circular grid used to count recording sites and sites with similar directional tuning to the center site. The fraction of similarly tuned sites over all sites (in a circular bin) was used for further analysis to remove potential influence of variations in the sampling distribution of recorded sites.

Fig. 4.

Normalized periodogram (13) of detrended observed data using a circular grid of 30-μm annulus. The period of statistically significant points is displayed next to the points. The levels of statistical significance of the labeled points were as follows. a, P = 0.032; b, P = 0.017.

Fig. 5.

Normalized periodogram of detrended original data using a circular grid of 10-μm annulus. The levels of significance of the labeled points were as follows. a, P < 0.004; b, P = 0.018; c, P = 0.027; d, P = 0.03; e, P = 0.01.

Fig. 6.

Schematic model of mapping of the preferred direction in motor cortex. The quantity ξ(c·j) denotes the angle formed between the PD vector c at the center of the circle and the PD vector j at site j.

Fig. 7.

Schematic diagram illustrating the periodic pattern of Fig. 6 in a filled patch of the map. Dimensions (and numbers of columns) are arbitrary.