A 71-year-old man is brought by his family for evaluation of progressive memory loss over the past two years. Neuropsychological testing reveals deficits in short-term memory encoding, attention, and executive function. MRI shows bilateral hippocampal atrophy and cortical thinning in temporoparietal regions. The PMHNP is explaining to the family why cholinesterase inhibitors are being considered for this patient. Which of the following best describes the cholinergic deficit underlying this patient's cognitive presentation?