Dissociation of human prefrontal cortical areas across different speech production tasks and gender groups

1. Data from a series of positron emission tomography (PET) experiments were analyzed with two goals. The first goal was to determine whether there were reliable differences in prefrontal cortex activation across two different speech production tasks. Such differences are important in determining functional subdivisions within prefrontal cortex. The second goal was to determine whether there were any gender differences across the two speech production tasks. 2. To accomplish these goals, PET subtraction images were generated for each of two speech production tasks (stem completion and verb generation). For the stem completion task, subjects viewed word stems (e.g., "GRE") and said aloud words that could complete the stems (e.g., "green"). For the verb generation task, subjects viewed nouns (e.g., "CHAIR") and said aloud words that were meaningfully related verbs (e.g., "sit"). Different groups of subjects performed the stem completion (N = 29) and verb generation (N = 32) tasks. 3. Data from each task subtraction were further divided by gender group (i.e., verb generation: male group; verb generation: female group, etc.). PET activations were separately identified in prefrontal cortex for each of the four resulting images. Activations were identified primarily in left prefrontal cortex for both tasks and both gender groups. Activations in right prefrontal cortex were small or absent. 4. Across tasks, the subtraction images showed both common activations in prefrontal cortex and one clear difference. Activations in left inferior prefrontal cortex (near Brodmann's areas 44 or 45) were observed in both male and female group images for both task subtractions. Activations in left anterior prefrontal cortex (near Brodmann's areas 10 or 46) were only observed for the verb generation subtraction images, formally demonstrating a functional dissociation between left inferior prefrontal cortex and more anterior prefrontal cortex. 5. This dissociation between prefrontal areas was highly robust and reliable across both gender groups. The left inferior prefrontal area(s) common to all of the subtraction images appears to be activated by tasks that demand high-level word retrieval and production processes. This area is distinct from the more anterior area(s), which is not always activated by such tasks. Dissociations in prefrontal areas are important because current descriptions of human functional anatomy often treat activations within large regions of cortex (e.g., dorsolateral prefrontal cortex) as single entities. 6. No qualitative differences in activation between gender groups were detected. For both subtractions, all activations identified within one gender group generalized to the other gender group. For the verb generation subtraction image, however, activations in male subjects were larger in magnitude than in female subjects.