Brain Oscillations as a Window Into Human Cognition.

Common everyday activities---riding a bike to work, taking notes on a lecture, or choosing food at the market---involve various neural processes, including perception, movement, and memory. These varied neural processes are supported by distinct brain regions. How do different brain regions coordinate their activities? Theoretical work suggests that brain oscillations have a critical role in this process, but the precise relations between human behavior and brain oscillations are unknown. I performed five studies examining the human brain activity involved in working and spatial memory. First, I analyzed scalp-recorded brain oscillations during a working-memory task. Here, I found that theta (4-8 Hz) oscillations in distinct regions related to different aspects of working memory. This indicates that oscillations in the same frequency band simultaneously serve varied roles across the brain. Next, I examined the temporal relation between brain oscillations and single-neuron spiking activity, using intracranial recordings from neurosurgical patients performing a navigation task. This analysis revealed that single-neuron spiking was synchronized to oscillations in the theta and gamma (30-100 Hz) bands. Each of these frequency bands facilitated a different type of spike timing and thus appeared to have unique functional roles. I also analyzed how the amplitude of oscillations varies during navigation, and I found oscillatory patterns related to virtual movement and searching for unknown objects. In the gamma band, these navigation-related oscillations were lateralized to the right hemisphere. This suggests that the right hemisphere has a unique role in navigation and demonstrates that gamma oscillations identify the active brain regions in particular task. Finally, in addition to investigating neural correlates of broad cognitive processes, I examined whether neuronal oscillations reveal the neural representations of individual stimuli. Analyzing intracranial recordings during a working-memory task, I found that the amplitude of gamma oscillations encoded the identity of the currently viewed letter. These letter-specific gamma patterns coincided both with overall increases in gamma amplitude and with simultaneous changes in theta phase. Overall, my work shows that brain oscillations are a unique signal that reveals how human behaviors are linked to both large-scale brain dynamics and to the activities of individual neurons. These oscillatory patterns reveal similarities and differences between human and animal neural activity, and this may be useful for identifying the unique characteristics of the human brain.