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Brian Maniscalco, Elisabeth Rounis, John C. Rothwell, Richard E. Passingham, Hakwan Lau; Theta-burst transcranial magnetic stimulation to the prefrontal cortex impairs metacognitive visual awareness. Journal of Vision 2009;9(8):764. doi: 10.1167/9.8.764.
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Current theories suggest that prefrontal cortex (PFC) plays an important role in visual awareness (Dehaene 2003). This hypothesis has been supported by a number of brain imaging studies (Rees 2002). However, critics have challenged that there has been a lack of neuropsychological demonstration of the essential role of PFC in visual awareness: PFC damage does not seem to result in cortical blindness (Pollen 1995). Here we clarify this issue by showing that bilateral transcranial magnetic stimulation (TMS) to PFC affects visual awareness and metacognition.
Methods and Results
We used a recently developed TMS protocol, theta-burst stimulation, to bilaterally depress activity in dorsolateral PFC as subjects performed a visual discrimination task. We found that this stimulation impaired subjects' ability to discriminate between correct and incorrect stimulus judgments. After TMS, subjects reported lower visibility for correctly (but not incorrectly) identified stimuli, suggesting that visual awareness of effective information processing was selectively suppressed. A signal detection theoretic analysis confirmed that these results were not merely due to a change in response bias, but rather reflected a reduction of metacognitive sensitivity. The effect was specific to metacognition; TMS did not impair performance in the stimulus discrimination task, which undermines alternative explanations such as TMS impairing visual attention.
These results suggest that activations in PFC accompanying visual awareness in brain imaging experiments are not epiphenomena, but rather may reflect a critical metacognitive process. Disruption of PFC selectively impairs visual awareness of correctly processed stimuli.
S. Dehaene, C. Sergent, and J. Changeux. (2003). A neuronal network model linking subjective reports and objective physiological data during conscious perception. Proc. Natl. Acad. Sci. U S A, 100, 8520-8525.
D. A. Pollen. (1995). Cortical areas in visual awareness. Nature, 377, 293-295.
G. Rees, G. Kreiman, and C. Koch. (2002). Neural correlates of consciousness in humans. Nat. Rev. Neurosci., 3, 261-270.
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