Memory and lexical processing in primary progressive aphasia

Primary Progressive Aphasia (PPA) is an aphasic dementia syndrome caused by neurodegenerative disease in which linguistic abilities are selectively disrupted in the relative absence of other cognitive deficits in early stages. The studies presented in this thesis were designed to examine the integrity of word and object processing by contrasting performance on implicit and explicit memory tests in the logopenic and agrammatic subtypes of PPA. Both studies were cross-sectional in design, in that patients with PPA were compared with cognitively intact controls. Where appropriate, comparisons were made within the PPA group. Study 1 investigated word and picture recognition memory and the impact of semantically related distracters on memory in PPA. Study 2 examined how words and pictures are processed by PPA patients when they are primed by within-format or cross-format stimuli. The central hypothesis for both studies was that picture performance would be superior to that of words, as a result of the language deficit in PPA. Beyond this central hypothesis, each study was designed to identify additional areas of vulnerability in memory and lexical processing.; The results suggest that, in the logopenic and agrammatic subtypes of PPA, successful memory performance is dependent on the format of the material (i.e., words or picture) and is susceptible to interference by semantically related stimuli. Patients were able to access semantic knowledge for both words and pictures but failed to show cross-format picture-to-word priming effects, suggesting difficulty linking a precise lexical label to an object. The degradation of the signal-to-noise ratio at the level of lexical, but not object, representations can be invoked to explain these results and may provide a general mechanism for word-finding and naming deficits in PPA. This model is consistent with the partial and selective loss of neurons within the language network.; This work suggests that PPA can provide a model for exploring the symbiotic relationship between memory and language in a system where the damage is within neurocognitive networks rather than a result of vascular lesions. Results from the study have implications clinically as and contribute to the understanding of the organization of lexical processing.