Old and recent approaches to the problem of non-verbal conceptual disorders in aphasic patients

From the first research on aphasia, it has been shown that, in addition to verbal communication disorders, aphasic patients often have difficulty on non-verbal cognitive tasks, which can actually be solved without the use of language. In this survey, I will discuss in a historical perspective the different interpretations provided by classical and contemporary authors to explain this puzzling observation. First, I will take into account the different positions of classical authorities on this topic, starting from the first debates (mainly based on anatomo-clinical observations) on the organisation of language in the brain. Then, I will attempt to summarize the work of authors who have tackled this complex issue more recently, in systematic investigations using methods drawn from experimental psychology, to clarify the meaning of non-verbal cognitive disorders in aphasia. Finally, in the last part of the survey, I will discuss the interpretation of proponents of the ‘semantic hub’ hypothesis who have tried to analyse and explain the differences between the non-verbal semantic defects of patients with semantic dementia and aphasic stroke patients. The hypothesis which assumes that most non-verbal cognitive disorders observed in aphasic patients are due to a preverbal conceptual disorder, which cannot be attributed to a loss of semantic representations but rather to a defect in their controlled retrieval, seems substantially confirmed. Nevertheless, two main issues must still be clarified. The first is that some of the non-verbal cognitive defects of aphasic patients seem due to the negative influence of language disturbances on abstract non-verbal cognitive activities, rather than to a preverbal conceptual disorder. The second issue concerns the exact nature and the neuroanatomical correlates of the defective controlled retrieval of unimpaired conceptual representations, which should subsume most of the non-verbal cognitive disorders of aphasic patients.