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Abstract:
The nucleus basalis of Meynert (NBM) provides all non-intrinsic cholinergic input to the neocortex. It has been implicated in signal detection and in cortical plasticity. Previous studies have proposed that NBM neurons respond differentially to certain characteristics of a stimulus, such as its familiarity. However, one possible explanation is that the expected reward or other arousal value of the visual stimulus and not familiarity would account for such response selectivity. To test this hypothesis, we tested two monkeys (Macaca mulatta) with visual stimuli presented at the centre of fixation while they performed a fixation task. Single electrodes and tetrodes were implanted in the NBM and the visual responses of single neurons and neuronal ensembles were recorded while three image parameters were changed:
1) familiarity/novelty,
2) image category (monkeys or flowers) and
3) coherence (salience).
In all these cases all stimuli had an equal chance of receiving a reward. We selected neurons that had visual responses that exceeded two standard deviations from the spontaneous activity for at least some stimuli for a statistical analysis. Quantification of the response selectivity of these nucleus basalis neurons using a measure of sparseness indicated that they had little selectivity for any of the three factors employed: stimulus novelty, category or coherence. All responsive neurons responded to all types of stimuli employed, including to non-coherent stimuli consisting of white visual noise. This result favours the hypothesis that the determining factor in the responses of nucleus basalis neurons is the association of a stimulus with reward.
