Phonological short-term memory in conduction aphasia

Abstract

Background: Within cognitive neuropsychological models conduction aphasia has been conceptualised as a phonological buffer deficit. It may affect the output buffer, the input buffer, or both. The phonological output buffer is a short-term storage, responsible for the short-term maintenance of phonological units until their articulation, as well as for phonological and morphological composition. The phonological input buffer holds input strings until they are identified in the input lexicon. Thus the phonological buffers are closely related to phonological short-term memory (pSTM), and hence it is important to assess pSTM in conduction aphasia. Because the input and output buffers play different roles, impairment in each of them predicts different impairments in the patient's ability to understand certain sentences, to learn new words and names, and to remember and recall lists of words and numbers for short time periods.

This research was supported by a research grant from the National Institute for Psychobiology in Israel (Friedmann 2004-5-2b), by the Israel Science Foundation (grant no. 1296/06, Friedmann), and by the ARC Centre of Excellence in Cognition and its Disorders (CCD), Macquarie University.

Aims: This study explored in detail pSTM in individuals with conduction aphasia, comparing individuals with input and output deficits, recall and recognition tasks, and stimuli of various types. It also tested pSTM in six age groups of healthy individuals, assessing the effect of age on various types of stimuli. This paper presents a new battery of 10 recall and recognition span tests, designed to assess pSTM in aphasia and to measure spans and effects on spans.

Methods & Procedures: The participants were 14 Hebrew-speaking individuals with conduction aphasia, 12 with input or input-output phonological buffer deficit, and 2 with only output deficit, and 296 healthy individuals.

Outcomes & Results: The analyses of the spans and effects on pSTM in the 10 tests indicated that all the participants with conduction aphasia had limited pSTM, significantly poorer than that of the control participants, and no semantic STM impairment. They had shorter spans, smaller length and similarity effects, and larger sentential effect than the controls. The individuals with conduction aphasia who had an impairment in the phonological input buffer showed deficit in both the recall and recognition span tasks. The individuals with the output conduction aphasia showed impairment only in the recall tasks. The healthy individuals showed age effect on span tasks involving words, but no effect of age on span tasks of nonwords.

Conclusions: pSTM is impaired in conduction aphasia, and different pSTM impairments characterise different types of conduction aphasia. Output conduction aphasia causes difficulties only when verbal output is required, whereas input conduction aphasia also causes a deficit when only recognition is required. This suggests that rehearsal can take place without the phonological output buffer. Age differentially affects pSTM for words and nonwords in healthy adults: whereas the encoding of words changes, the ability to remember nonwords is unchanged.

Keywords:

• Aphasia
• Conduction aphasia
• Working memory
• Phonological buffer
• Short-term memory
• STM
• Hebrew

Notes

This research was supported by a research grant from the National Institute for Psychobiology in Israel (Friedmann 2004-5-2b), by the Israel Science Foundation (grant no. 1296/06, Friedmann), and by the ARC Centre of Excellence in Cognition and its Disorders (CCD), Macquarie University.

¹For the three individuals who were tested 2–3 months post onset, all tests were administered within a short time, and each session included retesting of span test. No change in spans was detected for either of them within the testing period.

²Because some of the aphasic participants had phonological output deficits as well, it was impossible in certain cases to decide whether an erroneous response indicated a recall failure or a phonological output deficit. We wanted to avoid a false lexical effect as a result of mistakenly accepting a response with mild phonological errors as a correct recall response in the various word spans tasks, while rejecting inaccurate responses in the nonword span. We therefore analysed the spans in three ways: in the first only accurate responses were accepted as correct recall responses, counting all types of errors as incorrect recalls. In the second analysis we were more permissive and accepted also phonological errors (“dable” or “cable” for “table”) as correct recalls, and finally we also accepted responses that indicated correct semantic encoding in the absence of phonological word form (such as pointing to the ear for the word “ear”, or giving a definition). The first two counts yielded exactly the same spans for each of the participants. On the more permissive count, only the spans of a single participant, TG, changed: his phonologically similar word span increased from 2 to 2.5; his basic word span changed from 2.5 to 3, and his long word span changed from 1 to 3.5 words. The results presented in the results section are the results of the two first counts.

³Because no differences were found between the two youngest control age groups in any of the tests, we combined their data to a single 20–40 age group and these data were compared to the performance of the aphasic patients within this age range.

⁴Because both long and short recall spans were quite small for the participants with input conduction aphasia, it would be interesting in future studies to compare words of a single syllable to words with two syllables to evaluate length effect in the least demanding stimuli.

⁵The different performance in recall and recognition tasks in the output buffer participants, whereby recall was impaired but recognition was normal, can also be seen in the increased difference between the recognition and recall tasks. An ANOVA analysing the interaction between span type (recall or recognition) and the three groups: controls, mixed input-output, and output phonological buffer, yielded F(2,58) = 4.75, p = .01, with Tukey post hoc analysis indicating that the difference is between the output buffer conduction participants and the normal controls (p < .05).

⁶Several interesting questions arise with respect to the lexicality effect when coupled with impairments in various loci in the language-processing model. R. Martin reported on individuals who had impaired semantic STM with unimpaired semantics (at the semantic lexicon and the conceptual level). Thus their deficit can be conceived as a deficit at some semantic buffer that sits in the input and output to and from the semantic lexicon. It is clear that these participants could not use the semantic buffer for remembering words, but why couldn't they use the phonological lexicons for that? It would have been conceivable that a direct route between phonological input lexicon and phonological output lexicon, for example, would support the recall of words over nonwords even when the semantic buffer is impaired. In future research with patients who show this pattern (semantic STM impairment, no lexicality effect) it would be interesting to test whether they are also impaired in the input or output phonological lexicon or the direct connection between them. If the phonological lexicons are intact, and the participants still do not show a lexicality effect, this would suggest that the semantic buffer is placed within this lexical-phonological route, between the phonological lexicons. Another interesting point is that individuals with impaired phonological output lexicon might also show reduced lexicality effect, so the lack of lexicality effect cannot be directly taken to indicate impairment in the semantic buffer.