Patterns of dissociation in the processing of verb meanings in brain-damaged subjects

Abstract

In recent years an increasing amount of research has focused on the ways in which knowledge associated with verbs can be impaired by brain damage. Many different methods have been used to investigate verb processing disorders, but most of these methods have only been employed in a small number of studies and with relatively small numbers of subjects. As a consequence, very little information is available on the variety of disorders that are possible. In order to explore this issue further, we administered a standardised battery of six tests to a group of 89 brain-damaged subjects. The tests differ systematically with respect to the kinds of verb processing mechanisms on which they depend. The goal of the experiment was to investigate how the patterns of associations and dissociations that emerged across the tests could shed light on the organisation of the functional architecture that underlies the meanings of verbs and the computational operations that are used to manipulate them. Of the 89 subjects, 30 were impaired on at least one of the six tests. These subjects manifested a total of 22 distinct performance profiles across the tests, and each test dissociated from all of the others. These findings suggest that each test has at least some unique processing requirements that can be independently disrupted. A statistical factor analysis indicted that several distinct factors accounted for 93% of the variance among impaired performances. These factors are interpretable in terms of the major processing similarities and differences across the tests. Some of the results from the study are consistent with previous research on verb disorders. Specifically, tests involving verb production were more difficult than tests involving verb comprehension, and tests involving linguistic stimuli were more difficult than tests involving pictorial stimuli. Other aspects of our results are entirely new. Most importantly, a dissociation was found between tests that require referential processing, i.e., mapping verbs onto actions in the world, and tests that require analytic or inferential processing, i.e., decomposing verb meanings into their component semantic features. Thus, the study has significant implications for theories of the nature of verb processing.