Project Summary Episodic memory describes our ability to weave temporally contiguous elements into recollections of rich and coherent experiences. Episodic memory formation is specifically degraded by degenerative conditions such as Alzheimer’s Disease. The activity of ‘time cells’ in the mesial temporal lobe may provide a mechanism for the coding of temporal information that is necessary for the formation of these memories, and we recently published evidence of time cells in the human hippocampus using microelectrode recordings from epilepsy patients. The spike rate of these cells reliably increases at specific moments within a fixed interval, and groups (assemblies) of time cells can represent a ‘temporal space’ analogous to the manner in which hippocampal place cells are held to represent physical space, imposing temporal organization on event representations. In this proposal, we build on preliminary data to investigate the flexible participation of time cells in neuronal assemblies using established methods. We will test how time cells support serial memory by adjudicating between two models, one based on time cell activity (`time cell model’) versus a different proposed mechanism by which serial recall depends on the consistent phase offset of the spiking of hippocampal neurons relative to theta oscillations (‘phase offset model’). This experiment will test key unresolved questions about how time cells contribute to episodic memory. Finally, we will test the impact of cholinergic blockade on time cells and cell assemblies. This novel experiment builds on the preliminary data we present in our proposal showing the effects of scopolamine administration in 10 intracranial EEG subjects, which identify alterations in hippocampal theta and gamma oscillations in the setting of cholinergic blockade. Our experiments include a number of key innovations, including examining time cell activity during serial recall, assembly formation, and especially in the setting of modulation of cholinergic innervation. The data we propose to collect will fill key gaps in understanding related to time cell activity in humans and potentially establish cholinergic modulation in human intracranial EEG subjects as a method to elucidate physiological patterns during mnemonic processing and to model the effects of disorders such as Alzheimer’s Disease in this population.