Medial frontal cortex lesions impair the aiming component of rat reaching.

This study examined the contribution of medial frontal cortex (the medial portion of supplementary motor cortex, or Fr1) to the performance of rats on trained, or 'skilled', reaching tasks. Unilateral medial frontal cortex lesions moderately impaired reaching success on a task that demanded accuracy but they did not impair performance on a less demanding reaching task nor did they affect limb preference. Kinematic analyses indicated that the aiming component, in which the forearm of the limb is aligned along the midline of the body by adduction of the elbow, was chronically impaired. Rather than adducting the elbow to aim, the rats used a number of limb and whole body postural adjustments to compensate for incomplete or absent aiming. That medial frontal cortex is involved in the execution of at least one component of skilled reaching implies that a larger area of frontal cortex is involved in skilled limb movement than has been suggested by previous studies. The results also suggest that the different regions of frontal cortex may each have a relatively selective involvement in the execution of only a subset of the movements comprising a reach.

Skilled reaching in rats and humans: evidence for parallel development or homology.

Forelimb reaching by the rat is used as a paradigm for the experimental study of neural control, plasticity, and recovery of function after injury, in the expectation that results are generalizable to humans. The present study was done to compare rat to human reaching movements. The movements of both species were videorecorded and subjected to frame-by-frame analysis using Cartesian (spatial and velocity) and Eshkol-Wachman Movement Notation (EWMN) systems. The component movements of reaching, their sequence and velocity profiles, and their topography were similar in the two species. Both species also displayed more supination and lengthened grasping times when reaching for small as opposed to large objects. Both rats and humans moved the limb medially using the upper arm to aim it when they were required to reach through an aperture but in a free reaching test only rats continued to aim the limb. Human movements were characterized by greater blending of movement components, more variability, and independent digit use. Arguments are presented that the similarities and differences in rat and human reaching are not trivially accounted for by limb and task similarities. The many similarities in the movements of the two species provide evidence for at least parallel development or perhaps even homology.

Dopamine and skilled limb use in the rat: more severe bilateral impairments follow substantia nigra than sensorimotor cortex 6-hydroxydopamine injection.

The experiments examined the suggestion that the dopaminergic (DA) projection to the motor cortex are involved in the motor impairments that follow complete hemitelencephalic DA depletions. The neurotoxin, 6-hydroxydopamine (6-OHDA), was injected unilaterally into the sensorimotor cortex (MCtx), the ventral tegmental area (VTA), or into the substantia nigra pars compacta (SN) of rats trained to reach for food with either forelimb. The SN injections produced large (greater than 95%) unilateral striatal dopamine (DA) depletions and severe bilateral impairments in limb use. VTA and MCtx injections did not produce impairments in limb use or severe depletions of cortical DA. An effective test of the contribution of cortical DA to skilled limb use must await a more effective technique for producing selective cortical DA depletion. Nevertheless, the results suggest that the severe impairments of skilled forelimb use that follow hemitelencephalic DA depletions may stem primarily from depletion of the nigrostriatal DA projection.

Postprandial scanning by the rat (Rattus norvegicus): the importance of eating time and an application of "warm-up" movements.

We observed the movements of rats (Rattus norvegicus) after they had eaten food pellets of various size or hardness. With rooted hindlegs, they made head scans, with vibrissae in contact with the substrate, that began over the area below where they had eaten and then expanded to include almost the entire area surrounding their body. Scanning was not contingent on the presence of dropped food. It occurred when rats ate on a screen through which any dropped crumbs could fall. It also occurred when rats were trained to find food at a location distant to where they ate. Although the duration of scanning increased in proportion to the size of food consumed, when eating time was varied, using food items of similar size but different hardness, scanning increased in proportion to eating time. Postprandial scans resemble the exploratory (warm-up) movements that bridge transitions from immobility to locomotion. We propose that a subset of the movements of warm-up are co-opted in this postprandial period. It is likely that in natural foraging situations they are useful for food searching. The results suggest that although the motor system may be conservative in the number of actions that it can produce, diversity is achieved by applying fundamental patterns to many uses.

The impairments in reaching and the movements of compensation in rats with motor cortex lesions: an endpoint, videorecording, and movement notation analysis.

Reaching for food by rats, with the limb contralateral to limb area motor cortex damage, was analyzed using end-point scores, videoanalysis, and Eshkol-Wachmann Movement Notation (EWMN). End point results from groups of rats with small, medium, and large lesions showed reaching success and amount of food grasped per reach decreased with increases in lesion size. Videoanalysis and EWMN showed that the impairments were attributable to: (1) an inability to pronate the paw over the food by abduction of the upper arm, and (2) an inability to supinate the paw at the wrist to orient the food to the mouth. There were no obvious impairments in locating food using olfaction, in positioning the body in order to initiate a reach, or in clasping the digits to grasp food. There were only mild impairments in lifting, aiming, and advancing the limb. In rats with medium and large lesions, loss of pronation and supination were compensated for by a variety of whole body movements. These findings are discussed in reference to neural and behavioral mechanisms underlying recovery of function and the contribution of the motor cortex to skilled movements in the rat and other species.