Oscillation Phase Locking and Late ERP Components of Intracranial Hippocampal Recordings Correlate to Patient Performance in a Working Memory Task

In working memory tasks, stimulus presentation induces a resetting of intracranial temporal lobe oscillations in multiple frequency bands. To further understand the functional relevance of this phenomenon, we investigated whether working memory performance depends on the phase precision of ongoing oscillations in the hippocampus. We recorded intra-hippocampal local field potentials in individuals performing a working memory task. Two types of trials were administered. For high memory trials presentation of a list of four letters ( List ) was followed by a single letter memory probe ( Test ). Low memory load trials, consisting of four identical letters (AAAA) followed by a probe with the same letter (A), were interspersed. Significant phase locking of ongoing oscillations across trials, estimated by the Pairwise Phase Consistency Index (PPCI) was observed in delta (0.5-4 Hz), theta (5-7 Hz), and alpha (8-12 Hz) bands during stimulus presentation and recall but was increased in low memory load trials. Across patients however, higher delta PPCIs during recall in the left hippocampus were associated with faster reaction times. Because phase locking could also be interpreted as a consequence of a stimulus evoked potential, we performed event related potential analysis (ERP) and examined the relationship of ERP components with performance. We found that both amplitude and latency of late ERP components correlated with both reaction time and accuracy. We propose that, in the Sternberg task, phase locking of oscillations, or alternatively its ERP correlate, synchronizes networks within the hippocampus and connected structures that are involved in working memory

State-Dependent Cortical Network Dynamics

Neuropsychiatric illness represents a major health burden in the United States with a paucity of effective treatment. Many neuropsychiatric illnesses are network disorders, exhibiting aberrant organization of coordinated activity within and between brain areas. Cortical oscillations, arising from the synchronized activity of groups of neurons, are important in mediating both local and long-range communication in the brain and are particularly affected in neuropsychiatric diseases. A promising treatment approach for such network disorders entails ‘correcting’ abnormal oscillatory activity through non-invasive brain stimulation. However, we lack a clear understanding of the functional role of oscillatory activity in both health and disease. Thus, basic science and translational work is needed to elucidate the role of oscillatory activity and other network dynamics in neuronal processing and behavior. Organized activity in the brain occurs at many spatial and temporal scales, ranging from the millisecond duration of individual action potentials to the daily circadian rhythm. The studies comprising this dissertation focused on organization in cortex at the time scale of milliseconds, assessing local field potential and spiking activity, and contribute to understanding (1) the effects of non-invasive brain stimulation on behavioral responses, (2) network dynamics within and across cortical areas during different states, and (3) how oscillatory activity organizes spiking activity locally and long-range during sustained attention. Taken together, this work provides insight into the physiological organization of network dynamics and can provide the basis for future rational design of non-invasive brain stimulation treatments.Doctor of Philosoph