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Abstract:
Sensory processing, as well as motor planning and execution, are well-studied aspects of primate brain function. However, it is well known that cortical and subcortical structures
engage in significant activity unrelated to a sensory stimulus or motor response. Much of
this activity is not random, but rather reflects the brain’s own homeostatic mechanisms,
typically involving interactions between diverse cortical and subcortical structures. Such
activity changes are particularly pronounced during transitions of consciousness, such as
those occurring during natural sleep or anesthesia. In the present study, we examined the
dynamics of changes in sensory processing accompanying the loss of consciousness in
monkeys undergoing anesthetic induction.
We measured the auditory evoked potential (AEP) to a series of short clicks (5 Hz, 0.1
msec.) in monkeys during anesthetic induction. In all cases, the animals were brought to
the setup awake and maintained spontaneous respiration throughout the course of the
experiment. On different days, the anesthetics ketamine (KET), propofol (PRO), isoflurane
(ISO), and sevoflurane (SEVO) were administered while the animal was awake.
KET and PRO were delivered intravenously using a remote controlled syringe pump.
ISO and SEVO were provided via mask from an anesthesia machine.
Prior to anesthesia, consistent peaks appeared at 30 ms (Pa) and 50 ms (Nb) in the AEP.
The amplitude and latency of these peaks are known to be affected by anesthetic level.
We found that during induction and emergence with ISO and PRO, the changes in both
the latency and amplitude were more closely related to the animal’s visible state than to
the measured anesthetic concentration.
The results suggest that, at least for light anesthetic levels, conscious transitions draw
upon endogenous mechanisms that are impacted by, but not unambiguously determined
by, anesthetic concentration.
