44 research outputs found
Crystallization of strongly interacting photons in a nonlinear optical fiber
Understanding strongly correlated quantum systems is a central problem in
many areas of physics. The collective behavior of interacting particles gives
rise to diverse fundamental phenomena such as confinement in quantum
chromodynamics, phase transitions, and electron fractionalization in the
quantum Hall regime. While such systems typically involve massive particles,
optical photons can also interact with each other in a nonlinear medium. In
practice, however, such interactions are often very weak. Here we describe a
novel technique that allows the creation of a strongly correlated quantum gas
of photons using one-dimensional optical systems with tight field confinement
and coherent photon trapping techniques. The confinement enables the generation
of large, tunable optical nonlinearities via the interaction of photons with a
nearby cold atomic gas. In its extreme, we show that a quantum light field can
undergo fermionization in such one-dimensional media, which can be probed via
standard photon correlation measurements
Spatial Learning and Action Planning in a Prefrontal Cortical Network Model
The interplay between hippocampus and prefrontal cortex (PFC) is fundamental to
spatial cognition. Complementing hippocampal place coding, prefrontal
representations provide more abstract and hierarchically organized memories
suitable for decision making. We model a prefrontal network mediating
distributed information processing for spatial learning and action planning.
Specific connectivity and synaptic adaptation principles shape the recurrent
dynamics of the network arranged in cortical minicolumns. We show how the PFC
columnar organization is suitable for learning sparse topological-metrical
representations from redundant hippocampal inputs. The recurrent nature of the
network supports multilevel spatial processing, allowing structural features of
the environment to be encoded. An activation diffusion mechanism spreads the
neural activity through the column population leading to trajectory planning.
The model provides a functional framework for interpreting the activity of PFC
neurons recorded during navigation tasks. We illustrate the link from single
unit activity to behavioral responses. The results suggest plausible neural
mechanisms subserving the cognitive “insight” capability originally
attributed to rodents by Tolman & Honzik. Our time course analysis of neural
responses shows how the interaction between hippocampus and PFC can yield the
encoding of manifold information pertinent to spatial planning, including
prospective coding and distance-to-goal correlates
Finding the engram.
Many attempts have been made to localize the physical trace of a memory, or engram, in the brain. However, until recently, engrams have remained largely elusive. In this Review, we develop four defining criteria that enable us to critically assess the recent progress that has been made towards finding the engram. Recent \u27capture\u27 studies use novel approaches to tag populations of neurons that are active during memory encoding, thereby allowing these engram-associated neurons to be manipulated at later times. We propose that findings from these capture studies represent considerable progress in allowing us to observe, erase and express the engram