Abstract
Cholinergic neurons in the basal forebrain innervate discrete regions of the cortical mantle, bestowing the cholinergic system with the potential to dynamically modulate sub-regions of the cortex according to behavioral demands. Cortical cholinergic activity has been shown to facilitate learning and modulate attention. Experiments addressing these issues have primarily focused on widespread cholinergic depletions, extending to areas involved in general cognitive processes and sleep cycle regulation, making a definitive interpretation of the behavioral role of cholinergic projections difficult. Furthermore, a review of the electrophysiological literature suggests that cholinergic modulation is particularly important in representing the fine temporal details of stimuli, an issue rarely addressed in behavioral experimentation. The goal of this work is to understand the role of cholinergic projections, specific to the sensory cortices, in learning to discriminate fine differences in the temporal structure of stimuli. A novel visual Go/No-Go task was developed to assess the ability of rats to learn to discriminate fine differences in the temporal structure of visual stimuli (lights flashing at various frequencies). The cholinergic contribution to this task was examined by selective reduction of acetylcholine projections to visual cortex (VCx) (using 192 IgG-saporin), either before or after discrimination training. We find that in the face of compromised cholinergic input to the VCx, the rats' ability to learn to perform fine discriminations is impaired, whereas their ability to perform previously learned discriminations remains unaffected. These results suggest that acetylcholine serves the role of facilitating plastic changes in the sensory cortices that are necessary for an animal to refine its sensitivity to the temporal characteristics of relevant stimuli.
Highlights
In order to negotiate the demands of the environment, we differentially allocate our resources according to the contingencies that confront us, sometimes increasing the processing capabilities of a specific modality or modulating processing of certain fields within a sensory region
The basal forebrain cholinergic system contains a continuum of projection neurons that provides the major source of cholinergic input to most neocortical and various sub-cortical regions
Each brain was designated to have a particular percent depletion based on the approximate density of stained fibers in primary visual cortex (VCx), relative to a “standard” control brain processed in the same histochemistry dish
Summary
In order to negotiate the demands of the environment, we differentially allocate our resources according to the contingencies that confront us, sometimes increasing the processing capabilities of a specific modality or modulating processing of certain fields within a sensory region. This increased capability can be manifested with regard to static characteristics of the stimuli, such as orientation or pitch, and in relation to dynamic characteristics, such as amplitude modulations. Several studies indicate that these projections are roughly organized in a topographical manner (for a detailed review see Zaborszky et al, 1999) Such an organization suggests that this system has the capacity to differentially modulate the activity of cortical regions according to behavioral demands
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