Refractoriness and the healthy brain: a behavioural study on semantic access.

While many behavioural studies on refractory phenomena in lexical/semantic access have focused on the mechanisms involved in the oral production of names, comprehension tasks have been almost exclusively used in neuropsychological studies on brain damaged patients. We report the results of two experiments on healthy participants conducted by means of speeded word to picture matching tasks. They assess the effects of the same variables examined in the study of refractory access dysphasic patients: semantic distance and word frequency (experiment 1) and presentation rate and serial position effects (experiment 2). Semantic access patients usually show little effect of word frequency but a large semantic distance effect. However, critical in characterising the syndrome as 'refractory', effects of presentation rate and serial position should also be present. The experiments involved the use of a deadline response procedure. The critical manipulation was the absence of a Response Stimulus Interval (RSI) in the fast presentation rate conditions; slower presentation rates involved 1 s RSI. With these manipulations the typical behavioural pattern of performance provided by semantic access dysphasic patients was reproduced. Semantic distance effects were more powerful than word frequency effects (experiment1). Presentation rate effects were found and, most important for a "refractory" account of the effects, a serial position effect was obtained (experiment 2). These results provide the first evidence of such a broad range of refractory effects at the same time in comprehension tasks in healthy subjects and support a purely semantic account for the locus of refractoriness. Moreover, error analysis showed a predominance of perseverative errors with subsequent representations of the same target, supporting a failure of cognitive control mechanisms as the cause of refractory behaviour. The findings are discussed in the light of current models of lexical and semantic processing.