[Why do the aphasic patients produce semantic errors?]

One of the most striking and surprising symptoms of the aphasic patients is the production of semantic errors. This kind of errors, which consists of replacing a word by another with a different but similar meaning, may appear when speaking, as it happens when a patient say dog in reference to a cat (semantic paraphasia), when reading as it happens when a patient reads dog where is writing cat (semantic paralexia), as well as in repetition or in writing.

Some patients make these errors only in a specific linguistic modality (deep dyslexic patients make semantic errors in reading, deep dysphasic patients in repetition, etc.), but some patients make semantic errors in several modalities or even in all of them. Why these differences and what are the causes of these errors? Nowadays, models of linguistic processing built up from experiments with normal subjects provide us an answer to most of the above questions. From these models, by using appropriate tasks, it is possible to examine the reasons because of a patient makes certain errors. In fact, the results in different tasks help us to locate the deficit of each patient more than errors per se. It has been shown that sometimes the same type of error (e.g. a semantic paraphasia) can be caused by different processes in different patients. In this paper we will try to analyze which are the cognitive processes underlying semantic errors in each of the linguistic activities, when such processes are altered by injury, as well as to determine the best procedures to know what mechanisms are generating semantic errors in each aphasic patient.