Analysis of the actions of nucleus reuniens and the entorhinal cortex on EEG and evoked population behavior of the hippocampus.

Although the hippocampal theta rhythm is thought to be linked to memory processes, its mechanism of action is unknown. Furthermore, the hippocampus forms strong connections with a functionally similar structure, the medial prefrontal cortex (mPFC). The midline thalamus appears to be an intermediate between these two structures. We recorded neurons of a midline nucleus (nucleus reuniens, RE) during theta and non-theta states. Additionally, we recorded hippocampal CA1 population responses to RE stimulation. RE cell firing patterns are classified as (i) spike rate response to stimulation (ii) determination of bursting events (iii) coherence estimation between hippocampal EEG and RE response to stimulation (within the theta frequency band of 5 - 12 Hz). The present data suggests an increase in RE spike rate due to tail pinch elicited theta activity, with no evidence of bursting activity and a weak coherence within the theta band. Furthermore, we evaluated evoked excitatory post-synaptic potentials (EPSPs) in the hippocampal CA1 to RE stimulation, as well as entorhinal cortex (EC) stimulation. We demonstrated a consistent reduction in evoked potential (EP) latency at CA1 to RE and EC stimulation during theta compared to non-theta states.

Irrepressible saccades from a tectal lesion in a Rhesus monkey.

We present a case of spontaneously occurring irrepressible saccades in an experimental Rhesus monkey. Though eye jerks are sometimes associated with cerebellar disease, central demyelination or brainstem lesions, there is little consensus on their neurological mechanisms. From neurological and anatomical investigation we report that these irrepressible saccades were caused by a discrete cerebrovascular accident that involved the rostral superior colliculus along with its commissure, and with minor invasion of periaqueductal gray and adjacent mesencephalic reticular formation. Other suspected structures, like the raphe interpositus, substantia nigra and the cerebellum, were unaffected.

Single-unit recording in the lateral geniculate nucleus of the awake behaving monkey.

In recent years, recording neuronal activity in the awake, behaving primate brain has become established as one of the major tools available to study the neuronal specificity of the initiation and control of various behaviors. Primates have traditionally been used in these studies because of their ability to perform more complex behaviors closely akin to those of humans, a desirable prerequisite since our ultimate aim is to elucidate the neuronal correlates of human behaviors. A wealth of knowledge has accumulated on the sensory and motor systems such as vision, audition, and eye movements. For more demanding behaviors where the main focus has been on attention, recordings in awake primates have begun to yield valuable data on the centers of the brain that are reactive to different attributes of this behavior. As a result, various hypotheses of the origin and distribution of attentional effects have evolved. For instance, visual attentional effects have been described not only in the higher cortical area (V4) but also in areas earlier in the visual pathway which presumably involve a feedback mechanism in the latter region. Here we outline the ways in which we have successfully used these methods to make single-cell recordings in awake macaques to show how certain behavioral paradigms affect neurons of the thalamus (with emphasis on the lateral geniculate nucleus). As we have done with established techniques these methods can be readily adapted to incorporate most behaviors needed to be tested and allow recordings to be made in virtually any part of the brain.