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Abstract:
Rather than just passive reactions to external stimulation, much of human behavior is self-initiated, often involving free choice between alternative possibilities. However, the functional neuroanatomy underlying the formation of intentions for self-initiated behavior is still not well understood. Here, we present two studies showing that different regions of the prefrontal cortex contain predictive information about the timing and content of such an intention long before it reaches awareness.
Methods
Experiment 1
Previously, we showed that long before a conscious decision has been formed, the frontopolar cortex already encodes what motor action a subject will freely choose to make. In a novel variant of Libet’s clock task (Libet et al., 1983), subjects freely chose to make a button press with either left or right index finger immediately when they felt the spontaneous “urge” to do so. Concurrently, they monitored a letter stream, noting the letter presented when the choice first became conscious. This choice was already encoded in the spatial pattern of activation in the frontopolar cortex as early as ten seconds before the decision reached awareness. In a new decoding analysis, we tried to predict at which time a decision will be made.
Experiment 2
To verify whether the findings in Experiment 1 related to covert intentions per se, or unconscious preparation of motor output, a second experiment was conducted involving abstract intentions rather than motor intentions. Subjects monitored a letter stream, and remembered the letter presented when they first felt the spontaneous urge to perform either addition or subtraction. A number stream was presented concurrently above the letter stream, and the chosen arithmetic operation was performed on the two subsequent numbers presented after the conscious decision was made.
Results
Experiment 1
The new analysis further revealed that preSMA and SMA contained early predictive information about when the conscious decision will be formed, but not what the decision outcome will be. In contrast, the timing of the decision could not be predicted from the frontopolar cortex.
Experiment 2
Preliminary findings similarly suggest a double dissociation: early cortical activity patterns in frontopolar cortex encoded the content of the decision, addition or subtraction, while preSMA encoded the timing of the decision, but not vice versa. This suggests that our earlier findings related to covert intentions rather than motor preparation.
Taken together, our results reveal a functional dichotomy between different prefrontal regions in the pre-conscious formation of intentions for self-initiated behavior.
