Right anterior prefrontal cortex activation during semantic monitoring and working memory.

Areas of the adult human brain used for semantic monitoring were identified using positron emission tomography. For a series of tasks, subjects viewed a list of familiar English nouns and monitored the words for names of dangerous animals. The monitoring task used here also contained an instruction to keep track of the number or percentage of targets for report after the scan. Surface characteristics of the tasks such as stimulus rate, number of targets, and whether subjects were asked to count or estimate the number of targets were varied across multiple conditions within and between subjects. A passive word viewing condition was used as the control in all subjects. Reliable activations were identified in anterior and dorsal right prefrontal cortex [Brodmann areas (BA) 9 and 10] and left extrastriate cortex. The right prefrontal cortical locations are similar to areas that have been activated during many episodic memory tasks. This surprising finding led to a thorough review of the literature for examples of other activations within 16-mm vector distance of this right prefrontal area. Activations in the vicinity of right BA10 due to episodic memory retrieval, to various forms of working memory, and to miscellaneous tasks were found. The right prefrontal activations in the current experiment and the additional working memory and miscellaneous tasks demonstrate that, although right BA10 is routinely activated by episodic retrieval tasks, it is not uniquely activated by episodic retrieval tasks.