Shared mechanisms support controlled retrieval from semantic and episodic memory: Evidence from semantic aphasia

Semantic cognition is supported by at least two interactive components: semantic representations and control mechanisms that shape retrieval to suit the circumstances. Semantic and episodic memory draw on largely distinguishable stores, yet it is unclear whether controlled retrieval from these representational systems is supported by shared mechanisms. Patients with semantic aphasia (SA) show heteromodal semantic control deficits following stroke to left inferior frontal gyrus (LIFG), an area implicated in semantic processing plus the control of memory and language. However, episodic memory has not been examined in these patients and although the role of LIFG in semantics is well-established, neuroimaging cannot ascertain whether this area is directly implicated in episodic control or if its activation reflects semantic processing elicited by the stimuli. Neuropsychology can address this question, revealing whether this area is necessary for both domains. We found that: (i) SA patients showed difficulty discarding dominant yet irrelevant semantic links during semantic and episodic decisions. Similarly, recently encoded events promoted interference during retrieval from both domains. (ii) Deficits were multimodal (i.e. equivalent using words and pictures) in both domains and, in the episodic domain, memory was compromised even when semantic processing required by the stimuli was minimal. (iii) In both domains, deficits were ameliorated when cues reduced the need to internally constrain retrieval. These cues could involve semantic information, self-reference or spatial location, representations all thought to be unaffected by IFG lesions. (iv) Training focussed on promoting flexible retrieval of conceptual knowledge showed generalization to untrained semantic and episodic tasks in some individuals; in others repetition of specific associations gave rise to inflexible retrieval and overgeneralization of trained associations during episodic tasks. Although the neuroanatomical specificity of neuropsychology is limited, this thesis provides evidence that shared mechanisms support the controlled retrieval of episodic and semantic memory.