The role of the corticothalamic projection in the primate motor thalamus
The work in this dissertation focuses on identifying corticothalamic neurons and characterizing the signals carried along this pathway in the primate motor system. All experiments were carried out in awake primates performing a visually-guided center-out reaching task. We used stimulation in the motor thalamus (VPLo) to antidromically and orthodromically activate neurons in the primary motor cortex. Cortical neurons that showed antidromic responses to thalamic stimulation were classified as corticothalamic neurons (CT), while those that showed orthodromic responses were considered to be recipients of information from the stimulated area of thalamus. We also recorded the neural activity of thalamic neurons near the stimulation sites.We identified two distinct populations of CT neurons. The "inactive" population had virtually no spontaneous or task-related neural activity, while the "active" population showed task related activity typical of M1 pyramidal neurons. "Inactive" neurons typically exhibited long latencies and high thresholds to stimulation and were found deep within recording tracks, while "active" neurons had shorter latencies and lower thresholds to stimulation and were located more superficially in a recording track. We presumed that the "active" neurons are CT neurons that originate in cortical layer V and that the "inactive" neurons are layer VI CT neurons.We compared the neural activity of potentially connected groups of cortical and thalamic neurons. Corticothalamic neurons and the thalamic neurons located near the stimulation site from which those CT neurons were antidromically activated were grouped into CT sets and cortical neurons that showed orthodromic activation were grouped with thalamic neurons as part of TC sets. Based on the stimulation responses and narrow dispersion of preferred directions we proposed that these sets of neurons are potentially connected. Cortical neurons in both types of sets showed more sharply tuned directional responses than thalamic neurons in the same sets. Cortical neurons in CT sets modulated their activity earlier in relation to the onset of movement when compared with the thalamic neurons in the same sets and the cortical neurons in the TC sets.