Where do semantic errors come from?

We report the performance of two brain-damaged subjects (RGB and HW) whose frequent errors in spoken production are nearly always semantically related to the target word. Both subjects show similar, high rates of these "semantic" errors in oral naming and oral reading; yet neither subject makes semantic errors in comparable written tasks. Further, results of a variety of lexical tasks with the same stimuli demonstrate unimpaired comprehension of printed or spoken words, including those that are orally produced as semantic errors. These patterns of performance are interpreted as resulting from damage to the phonological output lexicon. The postulated deficit is contrasted to the hypothesis of impairment to the lexical-semantic component, required to explain performance by brain-damaged subjects described elsewhere who make seemingly identical types of oral production errors to those of RGB and HW, but, in addition, make comparable errors in writing and comprehension tasks.