Byng (1988) has argued that some aphasic patients who show problems in sentence comprehension are unable to "map" a syntactic analysis of the sentence form onto the thematic roles specified by the verb or preposition in the sentence. In Byng's study, therapy aimed at improving the mapping process as applied to sentences containing locative prepositions led to improvements not only in the comprehension of reversible verb sentences. These problems were shown to be stable across three pre-intervention assessments. All assessments were computer-based and involved the matching of written sentences to pictures. A small vocabulary was used in assessment and therapy, which involved a "micro world" of three characters (ball, box and star), which could engage in a limited number of actions and could occupy a limited set of spatial relationships. Before therapy began, all the patients were given an assessment battery, which included a 40-item Verb Test and a 40-item Preposition Test. The patients were then divided into two groups, A and B. Group A received two 1-hr sessions of therapy per week for 3 weeks aimed at improving the comprehension of verb sentences, then a second full assessment, followed by the same amount of therapy aimed at improving the comprehension of preposition sentences, and finally a third assessment. Group B received the preposition therapy first, followed by the verb therapist. The therapy involved the patient and therapist interacting with the computer, either assembling pictures to match written sentences ("picture-building mode") or assembling pictures to match pictures (sentence-building mode"). Group A showed a classical "cross-over" treatment outcome. Performance on treated verb sentences improved during verb therapy and was retained when therapy switched to preposition sentences. Performance on treated preposition sentences was unaffected by verb therapy but improved when therapy switched to the processing of prepositions. Performance on untreated verb and preposition sentences showed a similar pattern, though the improvements observed were not as great. Improvement was also shown on a paper-based "Real World Test" which involved a wider range of more naturalistic sentences. Performance on a third aspect of sentence comprehension which the patients also had difficulty with, namely the comprehension of morphology, remained unchanged throughout, providing further evidence that the effects obtained were treatment-specific. The results of group B were less clear cut. Comprehension of both verb and preposition sentences improved during the period that prepositions were being treated then remained static during verb treatment. Comprehension of morphology remained unchanged throughout. At the level of the individual patient, the majority of patients obtained higher scores on both the Verb Test and the Preposition Test after therapy, but only three patients failed to show so much as a borderline improvement on either verbs or prepositions. Finally, seven of the patients returned for an additional assessment 5 months after completing the therapy. These patients, who had demonstrated significant improvements during the therapy, were shown to have maintained their improved comprehension skills.