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To better understand the inconsistent effects across rat motor cortex, US modulation of electrically-evoked movements was tested. A stimulation array was implanted on the cortical surface and US (200 kHz, 30-60 W/cm2 peak) was applied while measuring changes in the evoked forelimb and hindlimb movements. Direct US stimulation of the hindlimb was also studied. To test peripheral effects, rat blood glucose levels were measured while applying US near the liver.
No short-term motor modulation was visible (95% confidence interval: -3.5% to +5.1% forelimb, -3.8% to +5.5% hindlimb). There was significant long-term (minutes-order) suppression (95% confidence interval: -3.7% to -10.8% forelimb, -3.8% to -11.9% hindlimb). This suppression may be due to the considerable heating (+1.8°C between US
on-US conditions); effects of heat and US were not separable in this experiment. US directly evoked hindlimb and scrotum movements in some sessions. This required a long interval, at least 3 seconds between US bursts. Movement could be evoked with much shorter pulses than used in literature (3 ms). The EMG latency (10 ms) was compatible with activation of corticospinal neurons. The glucose modulation test showed a strong increase in a few trials, but across all trials found no significant effect.
The single motor response and the long refractory period together suggest that only the beginning of the US burst had a stimulatory effect. This would explain the lack of short-term modulation, and suggests future work with shorter pulses could better explore the missing forelimb response. During the refractory period there was no change in the electrically-evoked response, which suggests the US stimulation mechanism is independent of normal brain activity. These results challenge the literature-standard protocols and provide new insights on the unknown mechanism.
For successful locomotion, animals require visual and somatosensory information. Even though a number of supraspinal centers receive both in varying degrees, processing this information at different levels of the central nervous system, especially their contribution to visuo-motor and sensory-motor integration during locomotion is poorly understood.
This dissertation investigates the patterns of neuronal activity in three areas of the forebrain in the cat performing different locomotor tasks to elucidate involvement of these areas in processing of visual and somatosensory information related to locomotion. In three studies, animals performed two contrasting locomotor tasks in each and the neuronal activities were analyzed.
In the first study, cats walked in either complete darkness or in an illuminated room while the neuronal activity of the motor cortex was recorded. This study revealed that the neuronal discharge patterns in the motor cortex were significantly different between the two illumination conditions. The mean discharge rates, modulation, and other variables were significantly different in 49% of the neurons. This suggests a contextual correlation between the motor cortical activity and being able to see.
In two other studies, the activities of neurons of either the somatosensory cortex (SI) or ventrolateral thalamus (VL) were recorded while cats walked on a flat surface (simple locomotion) or along a horizontal ladder where continuous visual and somatosensory feedback was required (complex locomotion).
We found that the activity of all but one SI cells with receptive fields on the sole peaked before the foot touched the ground: predictably. Other cells showed various patterns of modulation, which differed between simple and complex locomotion. We discuss the predictive and reflective functionality of the SI in cyclical sensory-motor events such as locomotion.
We found that neuronal discharges in the VL were modulated to the stride cycle resembling patterns observed in the cortex that receives direct inputs from the VL. The modulation was stronger during walking on the ladder revealing VL’s contribution to locomotion-related activity of the cortex during precision stepping.
Motor learning is the process of improving task execution according to some measure of performance. This can be divided into skill learning, a model-free process, and adaptation, a model-based process. Prior studies have indicated that adaptation results from two complementary learning systems with parallel organization. This report attempted to answer the question of whether a similar interaction leads to savings, a model-free process that is described as faster relearning when experiencing something familiar. This was tested in a two-week reaching task conducted on a robotic arm capable of perturbing movements. The task was designed so that the two sessions differed in their history of errors. By measuring the change in the learning rate, the savings was determined at various points. The results showed that the history of errors successfully modulated savings. Thus, this supports the notion that the two complementary systems interact to develop savings. Additionally, this report was part of a larger study that will explore the organizational structure of the complementary systems as well as the neural basis of this motor learning.
Speech motor learning is important for learning to speak during childhood and maintaining the speech system throughout adulthood. Motor and auditory cortical regions play crucial roles in speech motor learning. This experiment aimed to use transcranial alternating current stimulation, a neurostimulation technique, to influence auditory and motor cortical activity. In this study, we used an auditory-motor adaptation task as an experimental model of speech motor learning. Subjects repeated words while receiving formant shifts, which made the subjects’ speech sound different from their production. During the adaptation task, subjects received Beta (20 Hz), Alpha (10 Hz), or Sham stimulation. We applied the stimulation to the ventral motor cortex that is involved in planning speech movements. We found that the stimulation did not influence the magnitude of adaptation. We suggest that some limitations of the study may have contributed to the negative results.
Distinct neuronal adaptations between simple and complex locomotion tasks were observed for neurons with different receptive field properties and fast- and slow-conducting pyramidal tract neurons. Feedforward and feedback-driven kinematic control strategies were observed for adaptations to expected and unexpected perturbations, respectively, during complex locomotion tasks. These kinematic differences were reflected in the response characteristics of motor cortical neurons receptive to somatosensory information from different parts of the forelimb, elucidating roles for the various neuronal populations in accommodating disturbances in the environment during behaviors. The results show that anatomical and physiological characteristics of motor cortical neurons are important for determining if and how neurons are involved in precise control of locomotion during natural behaviors.