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Mechanisms underlying diminished novelty-seeking behavior in patients with probable Alzheimer's disease
K R Daffner1, M M Mesulam, L G Cohen
1Brigham Behavioral Neurology Group and Laboratory of Higher Cortical Functions, Division of Cognitive and Behavioral Neurology, Brigham & Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA.
Objective:
To better understand apathy and disengagement in patients with Alzheimer's disease (AD), the authors investigated possible behavioral mechanisms underlying diminished novelty-seeking activity in patients with probable AD.
Background:
Apathy and disengagement have been shown to be the most common behavioral changes associated with AD.
Method:
Patients and age-matched normal controls had their eye movements recorded while pairs of line drawings pitting an incongruous figure against a congruous figure were shown on a screen for 12 seconds. Characteristics of a subset of AD patients who were indifferent to novel visual stimuli as measured by exploratory eye movements were compared to those of a subset of AD patients who were attracted to novel stimuli to a degree similar to that of normal controls.
Results:
The indifferent patients were judged by informants, who completed a personality questionnaire, to exhibit a greater degree of apathy. The two AD groups did not differ in overall dementia severity or performance on a Saccade-to-Target Task that required shifts of attention and gaze. In a separate task, the indifferent patients were able to accurately identify the more novel stimuli in 97.5% of trials. Normal control subjects exhibited a strong bias toward processing novel stimuli, directing a higher proportion of their first fixations and dwell time to the incongruous stimuli whether the analysis was run for 3, 6, or 12 seconds of viewing. Indifferent patients did not direct their initial fixation toward novel stimuli and distributed their looking time evenly between incongruous and congruous stimuli throughout all measured intervals.
Conclusions:
The results suggest that the indifference to novelty observed in some patients with probable AD cannot simply be attributed to global cognitive decline, more elementary attentional deficits, more rapid habituation of response to novel stimuli, or an inability to discriminate upon demand between stimuli of varying degrees of novelty. It is more likely that their behavior reflects a disruption, by AD pathology, of neural systems that modulate behavioral engagement and maintain attentional bias toward novel events in the environment.