Exp Brain Res (1992) 92:139-151 Experimental BrainResearch 9 Springer-Verlag 1992 Motor cortical activity in a memorized delay task Nikolaos Smyrnis, Masato Taira, James Ashe, and Apostolos P. Georgopoulos Brain Sciences Center, Veterans Affairs Medical Center, Minneapolis, MN 55417, USA Departments of Physiologyand Neurology, Universityof Minnesota Medical School, Minneapolis, MN 55455, USA Received March 31, 1992 / Accepted July 23, 1992 Summary. Two rhesus monkeys were trained to move a handle on a two-dimensional (2D) working surface in directions specified by a light at the plane. They first captured with the handle a light on the center of the plane and then moved the handle in the direction indicated by a peripheral light (cue signal). The signal to move (go signal) was given by turning off the center light. The following tasks were used: (a) In the non-delay task the peripheral light was turned on at the same time as the center light went off. (b) In the memorized delay task the peripheral light stayed on for 300 ms and the cen- ter light was turned off 450-750 ms later. Finally, (c) in the non-memorized delay task the peripheral light stayed on continuously whereas the center light went off 750-1050 ms after the peripheral light came on. Record- ings in the arm area of the motor cortex (N= 171 cells) showed changes in single cell activity in all tasks. In both delay tasks, the neuronal population vector calculated every 20 ms after the onset of the peripheral light pointed in the direction of the upcoming movement, which was instructed by the cue light. Moreover, the strength of the population signal showed an initial peak shortly after the cue onset in both the memorized and non-memorized delay tasks but it maintained a higher level during the memorized delay period, as compared to the non- memorized task. These results indicate that the motor cortex is involved in encoding and holding in memory directional information concerning a visually cued arm movement and that these processes can be visualized using neuronal population vector analysis. Key words: Motor cortex - Visuomotor memory - Arm movement - Movement direction - Monkey Introduction The role of motor cortex in motor control is well doc- umented [see Evarts (1981) for a review]. Moreover, cell Correspondence to. A.P. Georgopoulos, Brain SciencesCenter activity in the motor cortex has also been shown to reflect more complex processes [see Georgopoulos (1991) for a review] involving, for example, spatial transformations (Georgopoulos et al. 1989; Lurito et al, 1991), spatial trajectory operations (Hocherman and Wise 1991), precuing information concerning movement direction (Riehle and Requin 1990), preparation for memorized movements (Alexander and Crutcher 1990), and preparation for movement sequences (Clark et al. 1991 ; Crammond and Kalaska 1991 ; Kettner et al. 1991 ; Mar- cario et al. 1991). Indeed, several studies have indicated that the difference between the involvement of motor cortex and premotor areas in complex visuospatial processes may be one of degree rather than of kind (Lecas et al. 1986; Alexander and Crutcher 1990; Riehle and Requin 1990; Chen et al. 1991; Clark et al. 1991; Hocherman and Wise 1991; Tanji et al. 1991), in the sense that qualitatively similar patterns of activity may be observed in these areas, although at higher propor- tions in premotor areas than in the motor cortex. In previous studies (Georgopoulos et al. 1989a, b) we documented changes in activity of motor cortical cells during an instructed delay period and showed that the neuronal population vector (Georgopoulos et al. 1983) calculated in time (Georgopoulos et al. 1984) predicted, during the delay period, the direction of the upcoming of movement in space. In the present paper, we extend these studies to the case of arm movements towards memorized targets. Changes in cell activity during delay periods preceding eye movements to memorized targets have been documented in the substantia nigra (Hikosaka and Wurtz 1983), the frontal eye field (Bruce and Gold- berg 1985), the parietal cortex (Gnadt and Andersen 1988) and the prefrontal cortex (Funahashi et al. 1989). We sought to determine whether such changes in cell activity are also observed in the motor cortex, with regard to arm movements, and whether the neuronal population vector would predict the memorized direction during the delay period. Preliminary results have been reported (Smyrnis et al. 1991).