Integrated Calendar

In this seminar, I will consider the possibility that the totality of dark matter consists of atomic-size black holes of primordial origin. I will review the basics of this proposal, and I will discuss some key questions yet unsolved.

One of the most remarkable functions of the human brain is the ability to recall a personal experience from the past and reenact it vividly in our mind, in a way that allows us to reflect upon the memory and derive from it relevant information that can guide our future behavior. My doctoral research explored the neuronal mechanisms that enable this core cognitive function in the human brain. Using rare electrophysiological recordings obtained from neurosurgical patients for clinical purposes I investigated and characterized the complex bidirectional interactions that occur between the hippocampus and the cerebral cortex during retrieval of conscious, reportable memories.
My results are twofold. I first show that 1-2 seconds before the onset of individual recollections the hippocampus elicits transient electrical oscillations known as Sharp Wave Ripples (SWRs). Such oscillatory events have been extensively studied in animal models in recent years and were shown to reflect massive synchronization events during which millions of pyramidal neurons on the hippocampus output pathway fire simultaneously. My results demonstrate that the SWR events are selective to memory contents and play a major role in coordinating the re-activation of hippocampal-neocortical memory representations during retrieval. I show a tight coupling between SWR events and visual cortex activation, and reveal a massive peri-ripple activation of the default mode network. Second, I show that the cortex uses a flexible, goal-directed, "baseline shift" mechanism that allows the imposition of predefined boundaries on spontaneous recollections. Specifically, the results demonstrate that when free recall is limited to a particular category, the average neuronal activity level in cortical sites that represent the targeted category is steadily and significantly enhanced throughout the free recall period. Such steady-state excitatory enhancement is likely to introduce a category-specific bias in the cortical input arriving at the hippocampus, which may facilitate the reactivation of memory traces belonging to the targeted category and not others.
Altogether, the results place hippocampal SWRs firmly as a central mechanism in the retrieval of human declarative memory. They demonstrate a central role for SWRs in coordinating the hippocampus-cortical dialogue during recollection and point to a flexible "baseline shift" mechanism that can account for the remarkable ease and precision by which we can constrain this dialogue to support retrieval goals.

Zoom link to join: https://weizmann.zoom.us/j/92146113977?pwd=VmhuMEhBcTRYZDNWMVJ4bGJrR0lIdz09


Meeting ID: 92146113977
Password: 803220

Zoom: https://weizmann.zoom.us/j/96278790117?pwd=T1ZjaHlxQjlEQkFIbE12UDJCazNwZz09

Two-dimensional layered (van-der-Waals) heterostructures, made by stacking different monolayers of semiconducting transition-metal dichalcogenides, have been drawing much attention as versatile platforms for studying fundamental solid-state phenomena and for designing opto-electronic devices. Interlayer excitons, electron-hole pairs that bind to each other across the interlayer spacing in these heterostructures, hold promise as key tools for probing the interlayer interface structure, and for exploring many-body interactions(1). With long lifetimes, spin polarization, and electric tunability, interlayer excitons are also promising as flexible information carriers(2, 3). However, they were mostly studied through the scope of their visible light emission, missing essential properties such as their momentum-space image or their absorption strength, necessary for rigorous study of their many-body interactions and potential applications.
In this talk I will present our recent studies aimed at measuring such unknown interlayer exciton properties and their dependence on the heterostructure. I will show a new interlayer exciton in WSe2/MoS2 heterostructures which we discovered based on its light emission in infra-red wavelengths, rather than in the visible range(4). I will demonstrate its properties as inferred from its optical interrogation. Then, I will present the quantitative measurement of the elusive optical absorption spectrum of interlayer excitons using electric-field modulation spectroscopy, essential for coherent coupling of light to those excitons(5). Finally, I will reveal how time- and angle-resolved photoemission spectroscopy is used to image the interlayer exciton in momentum-space, yielding its size and binding energy, so far inaccessible through optics(5).