Demonstrating the qualitative differences between semantic aphasia
and semantic dementia: A novel exploration of nonverbal semantic
processing
Issue title: Primary Progressive Aphasia and Post-Stroke
Aphasia: Some Complementary Insights into Brain-Behavior
Relationships/Hemispatial Neglect and Related Disorders
Affiliations: School of Psychological Sciences, University of
Manchester, Manchester, UK | Department of Psychology, University of York, York,
UK | Stroke and Dementia Research Centre, St George's
University of London, London, UK
Note: [] Address for correspondence: Prof. M.A. Lambon Ralph,
Neuroscience and Aphasia Research Unit (NARU), Zochonis Building, School of
Psychological Sciences, University of Manchester, Oxford Road, Manchester, M13
9PL, UK. Tel.: +44 161 275 2551; Fax: +44 161 275 2873; E-mail:
matt.lambon-ralph@manchester.ac.uk. Web:
www.psych-sci.manchester.ac.uk/naru
Abstract: Semantic dementia (SD) implicates the anterior temporal lobes (ATL)
as a critical substrate for semantic memory. Multi-modal semantic impairment
can also be a feature of post-stroke aphasia (referred to here as "semantic
aphasia" or SA) where patients show impaired regulatory control accompanied by
lesions to the frontal and/or temporo-parietal cortices, and thus the two
patient groups demonstrate qualitatively different patterns of semantic
impairment [1]. Previous comparisons of these two patient groups have tended to
focus on verbal receptive tasks. Accordingly, this study investigated nonverbal
receptive abilities via a comparison of reality decision judgements in SD and
SA. Pictures of objects were presented alongside non-real distracters whose
features were altered to make them more/less plausible for the semantic
category. The results highlighted a number of critical differences between the
two groups. Compared to SD patients, SA patients: (1) were relatively
unimpaired on the two alternative forced choice (2AFC) decisions despite
showing a comparable degree of semantic impairment on other assessments; (2)
showed minimal effects of the plausibility manipulation; (3) were strongly
influenced by variations in the regulatory requirements of tasks; and (4)
exhibited a reversed effect of familiarity – i.e., better performance on
less commonly encountered items. These results support a distinction between
semantic impairments which arise from impaired regulatory processes (e.g., SA)
versus those where degraded semantic knowledge is the causal factor (e.g., SD).
SA patients performed relatively well because the task structure reduced the
requirement for internally generated control. In contrast, SD patients
performed poorly because their degraded knowledge did not allow the
fine-grained distinctions required to complete the task.
