Our ability to recall previous personal experiences – things that we have seen, heard, done, etc. – requires coordinated interactions among distributed brain networks that collectively encode, store, retrieve and reenact the content of our memories. Memory of this type, also known as episodic memory, enables us to transcend the 'here and now' and move mentally in time: a few seconds backwards, e.g., when we try to follow a lecture or a conversation, or a few years backwards, when we recall our last trip to Paris. Similar neurocognitive processes also enable us to envision and plan the future by intrinsic simulation of various scenarios.
In his doctoral studies at the Weizmann Institute of Science, Dr. Yitzhak (Itzik) Norman investigated the neuronal mechanisms underlying memory retrieval in the human brain. His results uncovered a bidirectional hippocampal-cortical interaction that arises 1-2 seconds prior to the onset of a conscious, reportable recollection. In this interaction, the hippocampus elicits massive neuronal activity bursts known as Sharp Wave Ripples (SWRs). Such activation events have been extensively studied in animal models (mostly rodents) in recent years and were shown to involve a synchronous discharge of millions of pyramidal neurons located on the hippocampus output pathway. By measuring local neuronal activity from both the hippocampus and the cortex of awake individuals, Dr. Norman was able to demonstrate for the first time that SWRs play a major role in recollection, coordinating re-activation of content-selective cortical representations during retrieval. Furthermore, his work characterized the potent and widespread impact of SWRs on the rest of the brain – demonstrating robust SWR-related activations in various association cortices, as well as in prefrontal regions and the default mode network, which are linked to memory functions.
Currently in the Department of Neurological Surgery at the University of California, San Francisco, Dr. Norman examines how we encode and retrieve a specific conversation in our memory. Through a combination of non-invasive brain imaging techniques (high-field fMRI) and intracranial recordings in patients (ECoG), Dr. Norman seeks to deepen our mechanistic understanding of human declarative memory and shed new light on language and memory impairments that accompany neurodegenerative diseases such as Alzheimer’s and dementia, as well as speech problems caused by epilepsy and stroke.