Head-injured patients and control group: implicit versus explicit measures of frequency of occurrence.

This study was conducted in order to investigate the possibility that effortful processes are involved in the retrieval stage of the putative automatic task-frequency judgment. Head-injured (HI) and control groups were tested on a frequency of occurrence task under explicit-intentional retrieval (i.e., frequency estimation) and implicit-unintentional retrieval (i.e., word-stem priming) conditions. Subjects were presented with a list of nouns that appeared once, three times, and six times. Following presentation, subjects were first given a priming task, then a recall task, and finally a frequency judgment task. Although the control group performed better than the HI group on recall and frequency judgment tasks, the groups did not differ on the priming task. The results are discussed in terms of the relationship between effortful and automatic memory processes.