Understanding semantic and phonological processing deficits in adults with aphasia: Effects of category and typicality

Abstract

Background: Semantic and phonological processing deficits are often present in aphasia. The degree of interdependence between the deficits has been widely studied with variable findings. Semantic variables such as category and typicality have been found to influence semantic processing in healthy individuals and persons with aphasia but their influence on phonological processing is unknown.

Aims: This study examined the nature of semantic and phonological access in aphasia by comparing adults with aphasia to healthy control participants. Semantic and phonological tasks were used to assess the difference in processing requirements between and within each group as well as examine the effects of category and typicality on different stages of semantic and phonological processing.

Methods & procedures: Thirty-two persons with aphasia and ten neurologically healthy adults were administered nine tasks: Category Superordinate, Category Coordinate, Semantic Feature, Rhyme Judgment (No-Name), Syllable Judgment (No-Name), Phoneme Verification (No-Name), Rhyme Judgment (Name-Provided), Syllable Judgment (Name-Provided), and Phoneme Verification (Name-Provided). Accuracy and reaction time data were collected for each of these tasks and between-group and within-group differences were analyzed via MANOVA/MANCOVA and hierarchical clustering analyses.

Outcomes & results: Persons with aphasia performed with significantly lower accuracy than controls on phonological tasks but performed comparably on semantic tasks. Participants with aphasia were significantly slower than controls on all semantic and phonological tasks. Clustering of the nine tasks by accuracy revealed different processing requirements in the participants with aphasia compared to the control group while clustering by reaction time revealed similar trends in both groups in that phonological (no-name) items required the most processing time. Significant effects of category and typicality were noted in the semantic tasks but not in any of the phonological tasks.

Conclusions: Individuals with aphasia demonstrated overall impaired phonological processing with relatively preserved semantic processing as compared to controls. Per accuracy and reaction time measures, distinct trends in processing load for semantic tasks versus phonological tasks were seen in the individuals with aphasia whereas only speed of processing and not accuracy was impacted by phonological processing load in the control group. The results align most closely with discrete serial processing models of lexical processing as category and typicality effects were robust in the semantic tasks but not in any of the phonological tasks. Alternative explanations for these results also are discussed.

Keywords: aphasia; category; impairment; phonological; semantic; typicality.

Figures

Figure 1

Schematic representation of the tasks implemented in the current study and the stage of lexical processing each task primarily taxed, as indicated by matching outlines within stages of the model in (A) and the task outlines (B–D). A. Schematic of Lexical Processing, B. Semantic Tasks, C. Phonological No-Name tasks, D. Phonological Name-Provided tasks. See text for details.

Figure 2

Between-group difference in overall accuracy (A) and mean RT (B) in the nine tasks. Task types are separated by a vertical line with SEM task scores listed first, followed by PhN-N task scores and then PhN-P scores.

Figure 3

Dendograms of the hierarchical cluster analyses of accuracy within each group. Tasks are clustered along the vertical axis. Standardized linkage distance is plotted along the horizontal axis with the cutoff linkage distance of 75 denoted by a vertical line.

Figure 4

Dendograms of the hierarchical cluster analyses of zRT within each group. Tasks are clustered along the vertical axis. Standardized linkage distance is plotted along the horizontal axis with the cutoff linkage distance of 75 denoted by a vertical line.

Figure 5a

Mean PWA and control accuracy by category (bird, clothing, fruit, furniture, vegetable, and transportation)

Figure 5b

Mean PWA and control zRT by category (bird, clothing, fruit, furniture, vegetable, and transportation) across the nine tasks.

Figure 6a

Mean PWA and control accuracy by typicality across the nine tasks.

Figure 6b

Mean PWA and control zRT by typicality across the nine tasks.