The effect of congruency on visual exploration and episodic memory performance in Alzheimer's disease and semantic dementia.

Mounting evidence points to the role of semantic knowledge in modulating how we perceive, and subsequently remember, experiences. In healthy aging, prior knowledge becomes increasingly important to guide visual exploration during episodic encoding and retrieval and can hinder performance when incongruous with to-be-learned information. It remains unclear, however, how the dynamic integration of visual information and prior knowledge is altered in neurodegenerative disorders, and whether this impacts oculomotor behaviour. Here, we explored how degradation of the conceptual knowledge base in semantic dementia (SD) impacts the acquisition and retrieval of new information, and how such changes relate to oculomotor behaviour. Ten well-characterised cases of SD were compared to 12 disease-matched cases of Alzheimer's disease (AD) and 24 healthy older Controls. Participants completed a visual search task followed by a surprise memory task where target objects were displayed in either semantically congruent or semantically incongruent locations. Oculomotor performance was evaluated by measuring the time participants spent exploring target congruent areas in each condition. Relative to Controls, visual search and memory performance was significantly compromised in AD, as indexed by slower response times, reduced task accuracy, and more extensive visual exploration directed towards target congruent areas. In contrast, SD patients scored in line with Controls for all behavioural and oculomotor measures on the visual search task when target objects were displayed in semantically incongruent locations. Overall, our findings suggest that degradation of the semantic and episodic memory systems in dementia differentially impacts visual exploration and memory retrieval depending on stimulus congruency.