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要旨:
Acetylcholine (ACh) is a neurotransmitter implicated in several cognitive functions and is believed
to especially affect the acquisition of new information. Learning adapts behavior to
new situations and to new categories of stimuli.
We have examined the effects of scopolamine, an antagonist of muscarinic ACh receptors,
on object categorization. Extensive behavioral pharmacological studies were carried out in
two macaques. Performance was disrupted following injections of scopolamine. When the
stimuli presented were novel, ie. when they had not been seen before the experiment,
scopolamine significantly impaired performance in the categorization task. The monkeys
were less impaired in categorizing a set of familiar stimuli, ie. stimuli that they had categorized
successfully in previous sessions. Performance also deteriorated as the stimulus became
less salient by an increase in the level of visual noise.
One of the questions of the role of cholinergic neurons in networks involved in the learning
of new stimuli and in the performance of the categorization task is the type of information
that they convey. Our recordings are aimed towards making the study of information in
these networks feasible through simultaneous recordings of several neurons during the performance
of a complex behavioral task requiring responses to a series of discrete stimuli at
parametrically varied salience levels, belonging to two categories and two familiarity levels.
In addition to the behavioral experiments, neuronal activity was recorded in one monkey
during a fixation task in which the monkey was presented with stimuli belonging to two categories.
Preliminary recordings of neuronal ensembles have been made using tetrodes or
polytrodes in the putamen, globus pallidus and nucleus basalis. All these regions have
cholinergic neurons. Whereas cholinergic neurons in the basal ganglia are interneurons involved
in local networks, their role is likely to be important as the basal ganglia are a crucial
component in a circuit that underpins learning of conditional tasks. The nucleus basalis and
other regions of the basal forebrain, on the other hand, have cholinergic neurons that
project to the amygdala, hippocampus and cerebral cortex and are considered essential
components in a variety of cognitive behaviors, including the formation of new memories.
Neurons in the putamen had no visual responses but they anticipated the reward at the end
of each trial. Neurons in the globus pallidus and the nucleus basalis were also reward predicting
but could have in addition visual responses. These responses are related to the task
performed by the monkey and may be important in the performance of the categorization
task, in which case the prediction would be that the responses would be affected when categorization
is impaired by scopolamine.
