Perceptual learning is a pervasive and specific improvement in the performance of a perceptual task with training. This dissertation examines the role of the neurotransmitter acetylcholine(ACh) in perceptual learning in a series of behavioral and pharmacological studies in healthy human subjects. ACh plays a role in cognitive functions such as attention and in animal models it has been found to play a role in the facilitation of neural plasticity.
The work described here focused on the learning of a visual motion direction discrimination task. In the first study described, I provide a theoretical framework for the study of learning of this task. This part examined the "oblique effect", an advantage in performing this task when stimuli are presented in cardinal, rather than oblique directions. I present both experimental evidence and a population coding model that indicate the oblique effect in behavior may rely on the unequal representation of oblique and cardinal directions in visual areas in cortex. The model suggests that the oblique effect relies on an interplay of this representation with the decoding of the stimulus in higher cortical regions.
In the second part of this thesis, participants were administered the cholinesterase inhibitor donepezil while training on the motion direction discrimination task, performed in oblique directions. As previously described, this training abolishes the behavioral oblique effect. Moreover, donepezil increased the effects of training on performance and the specificity of these effects to the oblique direction and the visual field location in which learning took place, suggesting that ACh directs learning towards cells encoding behaviorally relevant features of the stimulus.
The third part presents a study investigating the role of ACh in the allocation of voluntary visual spatial attention (which can be allocated in a goal-oriented manner) and involuntary attention (which is automatically captured by salient events). We used an anti-predictive spatial cueing task to assess the effects of pharmacological enhancement of cholinergic transmission on behavioral measures of voluntary and involuntary attention. We found that cholinergic enhancement with donepezil augments the benefits of voluntary attention but does not affect involuntary attention, suggesting that they rely on different neurochemical mechanisms.
Taken together, the results of the second and third parts of this thesis provide converging evidence for a potential mechanism of learning: ACh mediates the allocation of voluntary attention, which in turn provides a necessary substrate for learning to occur.