What do clinical populations tell us about the relationship between language and thought? It depends on which we examine.
This is the first comparative paper from our project on language and cognition in aphasia and schizophrenia. As far as I know this work is unique. OUr aim was to profile and compare the language of these two populations, for clinicial purposes as well as to approach one of the bigger questions in cognitive science: What is the relationship between language and thought, or to be a little more precise, verbal and (seemingly) non-verbal cognition?
There are the two famous and extreme positions: (1) Benjamin Lee Whorf suggested that language is the medium of thought in that it enables, but also limits higher cognition. (2) Jerry Fodor suggested that language and thought are separate cognitive “modules”, and that the former evolved to translate thoughts for communication. Then there are a number of weaker claims inbetween.
One important piece of the evidence are data from clinical populations. Aphasia is considered a primary language disorder (in most cases caught by stroke, but sometimes also the result of injury, infection, hematoma or dementia), resulting in impairment of production and comprehension. Schizophrenia is considered a general cognitive impairment, with symptoms like voice hallucinations, delusions, and disordered thought. Aphasia research has found preserved social reasoning, navigation, mathematics, and other problem solving capacities even in cases with severe language impairment due to large brain lesions across language areas. Some researchers, such as Steven Pinker, have used this evidence to support a Fodorian position. Schizophrenia, however, has been associated with language abnormalities and a decrease in linguistic capacity, which has been interpreted as evidence for language and thought being inseparable.
We tested language and other cognition in people with aphasia, people with schizophrenia and thought disorder, and people with schizophrenia without thought disorder. Our test protocol was extensive. This paper reports outcomes of standardized tests. Our verbal tests are the Boston Naming Test (BNT) as a measure of word retrieval and production, and the Test of Reception of Grammar (TROG-2), which involves matching event pictures to a given sentence. We picked the following non-verbal tests: Matrix Reasoning (visuospatial reasoning and abstract problem solving), Brixton (executive function, also visuospatial), and Pyramid and Palm Trees (non-verbal semantic memory).
(click to enlarge) Comparison between aphasia and schizophrenia (with and without thought disorder). Scores were normalized over means and standard deviations of respective control groups. Lower score = poorer performance. TROG-2: Test oof Reception of Grammar; BNT: Boston Naming Test; PPT: Pyramids and Palm Trees.
The results across groups show a clear dissociation (see Figure). People with aphasia scored much worse in verbal tests than people with schizophrenia, but overall better in non-verbal tests. Among people with schizophrenia, those with thought disorder scored worst across tests. In aphasia, correlations between verbal and non-verbal scores were weak. In schizophrenia, they were stronger.
Taken at face value, one interpretation is that the data suggest that non-verbal capacities are independent from language capacity. In problem solving and other tasks with no language demands, the group with more severe language impairment performed better than the thought impaired group with milder language difficulties. Data from our aphasic group show that primary language impairment was not a good predictor of non-verbal capacity. At the same time, data from schizophrenia showed that a person with disordered thought has disordered language - a result compatible with the position that language is a translation of thoughts.
However, standardized tests, while very useful, are blunt instruments. Both lexical and grammatical consist of many components, and newer results suggest that aphasia and schizophrenia differ not only in the severity of language impairment (e.g. lower vs. higher test scores), but that their profiles are qualitatively different. Could it be that some aspects of language processing are more strongly related to thought? As with many big topics in science, the answer to the question about the relationship between language and thought appears more complex than the extreme positions.
Our project also shows how important it is to consider evidence from different sources. In a world in which aphasia or schizophrenia did not exist, our insight into cognition would be limited in important ways.