The process of acquiring motor skills through the sustained performance of complex movements is associated with neural plasticity. However, it is unknown whether even simple movements, repeated over a short period of time, are effective in inducing cortical representational changes. Whether the motor cortex can retain specific kinematic aspects of a recently practiced movement is also unknown. We used focal transcranial magnetic stimulation (TMS) of the motor cortex to evoke isolated and directionally consistent thumb movements. Thumb movements then were practiced in a different direction. Subsequently, TMS came to evoke movements in or near the recently practiced direction for several minutes before returning to the original direction. To initiate a change of the TMS-evoked movement direction, 15 or 30 min of continuous training were required in most of the subjects and, on two occasions, as little as 5 or 10 min. Substantially smaller effects followed more direct stimulation of corticofugal axons with transcranial electrical stimulation, pointing to cortex as the site of plasticity. These findings suggest that the training rapidly, and transiently, established a change in the cortical network representing the thumb, which encoded kinematic details of the practiced movement. This phenomenon may be regarded as a short-term memory for movement and be the first step of skill acquisition.

The dorsal premotor cortex is a functionally distinct cortical field or group of fields in the primate frontal cortex. Anatomical studies have confirmed that most parietal input to the dorsal premotor cortex originates from the superior parietal lobule. However, these projections arise not only from the dorsal aspect of area 5, as has long been known, but also from newly defined areas of posterior parietal cortex, which are directly connected with the extrastriate visual cortex. Thus, the dorsal premotor cortex receives much more direct visual input than previously accepted. It appears that this fronto-parietal network functions as a visuomotor controller-one that makes computations based on proprioceptive, visual, gaze, attentional, and other information to produce an output that reflects the selection, preparation, and execution of movements.

The first motor (MI) cortex of the rat was identified as the region from which movements could be evoked by the lowest intensity of electrical stimulation. The location of this region was correlated with cytoarchitecture in the frontal and parietal cortex. Two frontal areas can be discerned in Nissl-stained sections: (1) the medial agranular field, marked by a pale-staining layer III and a compact layer II, and (2) the lateral agranular field, which has more homogeneous superficial layers and a broad layer V containing large, densely staining cells. Both of these regions project to the spinal cord and can therefore be included in the somatic sensorimotor cortex. MI in the rat coincides with the lateral agranular field but also overlaps with part of the adjacent granular cortex of the first somatic sensory (SI) representation. We conclude that the rat MI cortex can be identified by microstimulation techniques and by cytoarchitecture in the rat.

A paradigm is described for recording the activity of single cortical neurons from awake, behaving macaque monkeys. Its unique features include high-density microwire arrays and multichannel instrumentation. Three adult rhesus monkeys received microwire array implants, totaling 96 -704 microwires per subject, in up to five cortical areas, sometimes bilaterally. Recordings 3-4 weeks after implantation yielded 421 single neurons with a mean peak-to-peak voltage of 115 ؎ 3 V and a signal-to-noise ratio of better than 5:1. As many as 247 cortical neurons were recorded in one session, and at least 58 neurons were isolated from one subject 18 months after implantation. This method should benefit neurophysiological investigation of learning, perception, and sensorimotor integration in primates and the development of neuroprosthetic devices.

The largest part of the primate prefrontal cortex has no homologue in other mammals. Accordingly, it probably confers some advantage that other mammals either lack or attain another way. Yet this advantage remains enigmatic. Not so for other parts of cortex. For example, certain visual areas encode, represent and store knowledge about objects. By analogy, perhaps the primate prefrontal cortex encodes, represents and stores knowledge about behaviors, including the consequences of doing (or not doing) something in complex and challenging situations. The long list of functions often attributed to prefrontal cortex may all contribute to knowing what to do and what will happen when rare risks arise or outstanding opportunities knock. Keywords frontal lobe; cytoarchitectonics; comparative neuroanatomy; working memory; cognitionThe earliest experiments on the frontal areas were those of the French neurologist Flourens (1824), who, on the basis of ablation studies…attributed to the frontal lobes, acting in harmony with the rest of the brain, the higher perceptual, associative, and executive functions of the mind [1, p. 447].Flourens got it right, in a way, yet his experiment and conclusions had no validity whatsoever. He studied the effect of cerebral ablations on a hen, and she did not have frontal lobes -either before or after the lesion. Research on the frontal lobes, especially the prefrontal cortex, has always been thus, and every generation seems to reach conclusions much like those that Flourens advocated in 1824 (the year of Broca's birth). The doctrine of the day seems intriguing, and authorities announce the problem solved, or nearly so: for a while, then dissatisfaction develops and the cycle begins anew. When the puzzle of prefrontal cortex is fairly solved, the account will inevitably seem familiar, even tired, if only because every possibility has probably been propounded.Flourens' misconception underscores an important principle for understanding prefrontal cortex: combining findings from different species requires a serious consideration of the relevant homologies. This opinion piece takes up that topic first, and at heart it involves a fundamental question: What is prefrontal cortex? The second topic concerns the distinction between cognitive processes and knowledge, and it involves a similarly fundamental question: The prefrontal cortex remains enigmatic, but two trends in neurosciences will help. First, comparative neuroanatomy indicates that the frontal areas shared by rodents and primates compose only part of the primate frontal lobe, which should facilitate research on these areas. Second, investigations of unshared areas have begun to focus on stored knowledge rather than cognitive processes. Conflicts of interest:The author declares no competing interests in association with this article. What does prefrontal cortex do? Grafman and his colleagues [2] have noted that most ideas about prefrontal cortex function involve cognitive processes such as working memory, retrieval of long-term memor...

The motor system includes structures distributed widely through the CNS, and in this feature article we present a scheme for how they might cooperate in the control of action. Distributed modules, which constitute the basic building blocks of our model, include recurrent loops connecting distant brain structures, as well as local circuitry that modulates loop activity. We consider interconnections among the basal ganglia, cerebellum, and cerebral cortex and the specialized properties of certain cell types within each of those structures, namely, striatal spiny neurons, cerebellar Purkinje cells, and neocortical pyramidal cells. In our model, striatal spiny neurons of the basal ganglia function in contextual pattern recognition under the training influence of reinforcement signals transmitted in dopamine fibers. Cerebellar Purkinje cells also function in pattern recognition, in their case to select and execute actions through training supervised by climbing fibers, which signal discoordination. Neocortical pyramidal cells perform collective computations learned through a local training mechanism and also function as information stores for other modular operations. We discuss how distributed modules might function in a parallel, cooperative manner to plan, modulate, and execute action.

We studied single-neuron activity in the prefrontal cortex (PF) while a monkey performed a task according to two different rules, termed conditional and spatial. The monkey viewed a video screen, and its task required a hand movement in response to the dimming of a light spot. There were four light spots on the screen: right, left, up, and down from the center. Only one of the four spots dimmed, and the degree of dimming was slight. Accordingly, the monkey needed to foveate the "correct" light spot to detect the dimming. A visual cue indicated which of the four light spots would be deemed correct and, thus, would dim on each trial. The sequence of events was as follows: a fixation spot appeared at the center of the screen; then, a cue appeared twice at one of the four potential target locations; then, the four target spots appeared; and, finally, one of them dimmed. Except for the color of an initial fixation point, the cues, their locations, and other events were identical for the conditional and spatial rules. The rules differed in one essential way. For the conditional rule, nonspatial attributes of the visual cue indicated which of the four light spots would dim, and the cue's location was irrelevant. For the spatial rule, the cue's location determined the correct target on that trial. The light spot at the location of the cue always dimmed, regardless of which cue appeared there. Our sample included 221 PF neurons showing significant task-related activity modulation, distributed among dorsal, dorsolateral, and ventral PF regions. Between one-third and one-half of the sample in each of those regions showed statistically significant activity differences that could be attributed to the rule. Selectivity for cues and/or their locations was common. However, there was no significant regional segregation of such selectivity. These data support the hypothesis that PF plays a role in the guidance of behavior according to previously learned rules.

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