The episodic and nonuniversal nature of behavioral disturbances of dementia challenges the notion that these symptoms result only from permanent changes in the brain. Few longitudinal data are available to correlate the occurrence of behavioral symptoms with the neurobiologic model of dementia, and most of these data are from clinical trials of institutionalized patients. Nevertheless, Dr. Cummings noted that even though behavioral symptoms are highly episodic, they tend to recur in the same subgroups of patients. The neurobiologic model, therefore, may help determine the vulnerability of a patient to behavioral disturbances but not the consistent expression of the behavioral symptom complex.