“Pre-semantic” cognition revisited: Critical differences between semantic aphasia and semantic dementia

Abstract

Patients with semantic dementia show a specific pattern of impairment on both verbal and non-verbal “pre-semantic” tasks: e.g., reading aloud, past tense generation, spelling to dictation, lexical decision, object decision, colour decision and delayed picture copying. All seven tasks are characterised by poorer performance for items that are atypical of the domain and “regularisation errors” (irregular/atypical items are produced as if they were domain-typical). The emergence of this pattern across diverse tasks in the same patients indicates that semantic memory plays a key role in all of these types of “pre-semantic” processing. However, this claim remains controversial because semantically-impaired patients sometimes fail to show an influence of regularity. This study demonstrates that (a) the location of brain damage and (b) the underlying nature of the semantic deficit affect the likelihood of observing the expected relationship between poor comprehension and regularity effects. We compared the effect of multimodal semantic impairment in the context of semantic dementia and stroke aphasia on the seven “pre-semantic” tasks listed above. In all of these tasks, the semantic aphasia patients were less sensitive to typicality than the semantic dementia patients, even though the two groups obtained comparable scores on semantic tests. The semantic aphasia group also made fewer regularisation errors and many more unrelated and perseverative responses. We propose that these group differences reflect the different locus for the semantic impairment in the two conditions: patients with semantic dementia have degraded semantic representations, whereas semantic aphasia patients show deregulated semantic cognition with concomitant executive deficits. These findings suggest a reinterpretation of single case studies of comprehension-impaired aphasic patients who fail to show the expected effect of regularity on “pre-semantic” tasks. Consequently, such cases do not demonstrate the independence of these tasks from semantic memory.

Semantic memory: Which side are you on?

“…3 reveals an impact of object typicality for the two patients that is indistinguishable from that for the cases with SD. Note that significant effects of familiarity and typicality are not characteristic of patients with SA Jefferies et al, 2010).…”

“…The results on DJ and GF are compared to published data obtained from patients with SD. We also refer to results from semantically impaired stroke patients, labelled semantic aphasics (SA) in the research of Jefferies et al (2008), whose semantic deficits are very different to those in SD Jefferies et al, 2010).…”

What we talk about when we talk about access deficits

“…exhibited reverse frequency effects in a delayed repetition task [64], although both had exhibited no frequency effects in synonym judgement [9] and object use [43]. In addition, these patients were part of the access deficit group 4 that showed no frequency effects in picture naming, word-picture matching, and picture and word versions of the camel and cactus test [8], but did show standard frequency effects in lexical decision, object decision and word reading [65].…”

“…As a result, it is unclear whether insensitivity to frequency should be regarded as a core phenomenon of access deficits. As will be discussed in more detail in §3c, it is possible that these inconsistent findings are a result of frequency effects being masked by other factors [66] that more strongly influence performance in access deficits, whereas the canonical storage deficit (SD) is associated with exaggerated frequency and typicality effects [2,65,67].…”

Cortical & Subcortical Dementias: A Psychoneurolinguistic Perspective

“…Comparing the performance of patients with semantic aphasia in matters concerning pre semantic tasks e.g., reading aloud, past tense generation, spelling to dictation, lexical decision, object decision, color decision and delayed picture copying, results of Jefferies et al, (2010) show that the semantic aphasia patients were less sensitive to typicality than SD patients. This study was supported by Agosta et al, (2010), notably in the part of lexical process.…”