Arousal (Physiology)

The hippocampal-medial prefrontal circuit has been shown to serve a critical role
in decision making and goal directed actions. While the hippocampus (HF) exerts a direct
influence on the medial prefrontal cortex (mPFC), there are no direct return projections
from the mPFC to the HF. The nucleus reuniens (RE) of the midline thalamus is strongly
reciprocally connected with the HF and mPFC and represents the major link between
these structures.
We investigated the role of RE in functions associated with the hippocampus and
the mPFC -- or their interactions. Using two different inactivation techniques
(pharmacological and chemogenetic), we sought to further define the role of RE in spatial
working memory (SWM) and behavioral flexibility using a modified delayed non-match
to sample (DNMS) working memory task. We found that the reversible inactivation of
RE with muscimol critically impaired SWM performance, abolished well-established
spatial strategies and produced a profound inability to correct non-rewarded, incorrect choices on the T-maze (perseverative responding). We observed similar impairments in
SWM following the chemogenetic (DREADDs) inactivation of RE or selective RE
projections to the ventral HF. In addition, we showed that the inhibition of RE terminals
to the dorsal or ventral HF altered task related behaviors by increasing or decreasing the
time to initiate the task or reach the reward, respectively. Finally, we examined discharge
properties of RE cells across sleep-wake states in behaving rats. We found that the
majority of RE cells discharge at high rates of activity in waking and REM and at
significantly reduced rates in SWS, with a subpopulation firing rhythmically in bursts
during SWS. We identified five distinct subtypes of RE cells that discharged differently
across vigilant states; those firing at highest rates in waking (W1, W2), in REM sleep
(R1, R2) and SWS (S1). Given the differential patterns of activity of these cells, we
proposed they may serve distinct functions in waking – and possibly in SWS/REM sleep.
In sum, our findings indicate that RE is critically involved in mnemonic and
executive functions and the heterogeneous activity of these cells support a role for RE in
arousal/attention, spatial working memory and cognition.

A signal detection experiment tested the assumption
that the results of Hillyard, Squires, Bauer and Lindsay
(1971), who found a significant late positive component
(LFC) of the averaged evoked potential only in the hit
category, were due to the response set of their Ss and
not to any unique associations of the LPC with the hit
category as interpreted by them. Phase 1 of the present
study was basically a replication of Hillyard's study.
In Phase 2 incorrect responses were punished by an
unpleasant feedback device designed to place more equal
relevance on signal and non-signal trials. Significant
LPCs appeared in both signal and non-signal brain waves
under both phases indicating that the LPC is not contingent
on any particular response category as Hillyard reported,
but may instead reflect the relevance of the category
to the experimental task as it is perceived by the S.