Hugh T. Blair

Optical activation of lateral amygdala pyramidal cells instructs associative fear learning

Humans and animals can learn that specific sensory cues in the environment predict aversive events through a form of associative learning termed fear conditioning. This learning occurs when the sensory cues are paired with an aversive event occuring in close temporal proximity. Activation of lateral amygdala (LA) pyramidal neurons by aversive stimuli is thought to drive the formation of these associative fear memories; yet, there have been no direct tests of this hypothesis. Here we demonstrate that viral-targeted, tissue-specific expression of the light-activated channelrhodopsin (ChR2) in LA pyramidal cells permitted optical control of LA neuronal activity. Using this approach we then paired an auditory sensory cue with optical stimulation of LA pyramidal neurons instead of an aversive stimulus. Subsequently presentation of the tone alone produced behavioral fear responses. These results demonstrate in vivo optogenetic control of LA neurons and provide compelling support for the idea that fear learning is instructed by aversive stimulus-induced activation of LA pyramidal cells.

Hippocampal place cells acquire location-specific responses to the conditioned stimulus during auditory fear conditioning.

We recorded neurons from the hippocampus of freely behaving rats during an auditory fear conditioning task. Rats received either paired or unpaired presentations of an auditory conditioned stimulus (CS) and an electric shock unconditioned stimulus (US). Hippocampal neurons (place and theta cells) acquired responses to the auditory CS in the paired but not in the unpaired group. After CS-US pairing, rhythmic firing of theta cells became synchronized to the onset of the CS. Conditioned responses of place cells were gated by their location-specific firing, so that after CS-US pairing, place cells responded to the CS only when the rat was within the cells place field. These findings may help to elucidate how the hippocampus contributes to context-specific memory formation during associative learning.

Putting Fear in Its Place: Remapping of Hippocampal Place Cells during Fear Conditioning

We recorded hippocampal place cells in two spatial environments: a training environment in which rats underwent fear conditioning and a neutral control environment. Fear conditioning caused many place cells to alter (or remap) their preferred firing locations in the training environment, whereas most cells remained stable in the control environment. This finding indicates that aversive reinforcement can induce place cell remapping even when the environment itself remains unchanged. Furthermore, contextual fear conditioning caused significantly more remapping of place cells than auditory fear conditioning, suggesting that place cell remapping was related to the rats learned fear of the environment. These results suggest that one possible function of place cell remapping may be to generate new spatial representations of a single environment, which could help the animal to discriminate among different motivational contexts within that environment.

Neural substrates for expectation-modulated fear learning in the amygdala and periaqueductal gray

A form of aversively motivated learning called fear conditioning occurs when a neutral conditioned stimulus is paired with an aversive unconditioned stimulus (UCS). UCS-evoked depolarization of amygdala neurons may instruct Hebbian plasticity that stores memories of the conditioned stimulus–unconditioned stimulus association, but the origin of UCS inputs to the amygdala is unknown. Theory and evidence suggest that instructive UCS inputs to the amygdala will be inhibited when the UCS is expected, but this has not been found during fear conditioning. We investigated neural pathways that relay information about the UCS to the amygdala by recording neurons in the amygdala and periaqueductal gray (PAG) of rats during fear conditioning. UCS-evoked responses in both amygdala and PAG were inhibited by expectation. Pharmacological inactivation of the PAG attenuated UCS-evoked responses in the amygdala and impaired acquisition of fear conditioning, indicating that PAG may be an important part of the pathway that relays instructive signals to the amygdala.

Placing prediction into the fear circuit

Pavlovian fear conditioning depends on synaptic plasticity at amygdala neurons. Here, we review recent electrophysiological, molecular and behavioral evidence suggesting the existence of a distributed neural circuitry regulating amygdala synaptic plasticity during fear learning. This circuitry, which involves projections from the midbrain periaqueductal gray region, can be linked to prediction error and expectation modulation of fear learning, as described by associative and computational learning models. It controls whether, and how much, fear learning occurs by signaling aversive events when they are unexpected. Functional neuroimaging and clinical studies indicate that this prediction circuit is recruited in humans during fear learning and contributes to exposure-based treatments for clinical anxiety. This aversive prediction error circuit might represent a conserved mechanism for regulating fear learning in mammals.

Conversion of a Phase- to a Rate-Coded Position Signal by a Three-Stage Model of Theta Cells, Grid Cells, and Place Cells

As a rat navigates through a familiar environment, its position in space is encoded by firing rates of place cells and grid cells. Oscillatory interference models propose that this positional firing rate code is derived from a phase code, which stores the rats position as a pattern of phase angles between velocity‐modulated theta oscillations. Here we describe a three‐stage network model, which formalizes the computational steps that are necessary for converting phase‐coded position signals (represented by theta oscillations) into rate‐coded position signals (represented by grid cells and place cells). The first stage of the model proposes that the phase‐coded position signal is stored and updated by a bank of ring attractors, like those that have previously been hypothesized to perform angular path integration in the head‐direction cell system. We show analytically how ring attractors can serve as central pattern generators for producing velocity‐modulated theta oscillations, and we propose that such ring attractors may reside in subcortical areas where hippocampal theta rhythm is known to originate. In the second stage of the model, grid fields are formed by oscillatory interference between theta cells residing in different (but not the same) ring attractors. The models third stage assumes that hippocampal neurons generate Gaussian place fields by computing weighted sums of inputs from a basis set of many grid fields. Here we show that under this assumption, the spatial frequency spectrum of the Gaussian place field defines the vertex spacings of grid cells that must provide input to the place cell. This analysis generates a testable prediction that grid cells with large vertex spacings should send projections to the entire hippocampus, whereas grid cells with smaller vertex spacings may project more selectively to the dorsal hippocampus, where place fields are smallest.

Scale-Invariant Memory Representations Emerge from Moiré Interference between Grid Fields That Produce Theta Oscillations: A Computational Model

The dorsomedial entorhinal cortex (dMEC) of the rat brain contains a remarkable population of spatially tuned neurons called grid cells (Hafting et al., 2005). Each grid cell fires selectively at multiple spatial locations, which are geometrically arranged to form a hexagonal lattice that tiles the surface of the rats environment. Here, we show that grid fields can combine with one another to form moiré interference patterns, referred to as “moiré grids,” that replicate the hexagonal lattice over an infinite range of spatial scales. We propose that dMEC grids are actually moiré grids formed by interference between much smaller “theta grids,” which are hypothesized to be the primary source of movement-related theta rhythm in the rat brain. The formation of moiré grids from theta grids obeys two scaling laws, referred to as the length and rotational scaling rules. The length scaling rule appears to account for firing properties of grid cells in layer II of dMEC, whereas the rotational scaling rule can better explain properties of layer III grid cells. Moiré grids built from theta grids can be combined to form yet larger grids and can also be used as basis functions to construct memory representations of spatial locations (place cells) or visual images. Memory representations built from moiré grids are automatically endowed with size invariance by the scaling properties of the moiré grids. We therefore propose that moiré interference between grid fields may constitute an important principle of neural computation underlying the construction of scale-invariant memory representations.

Electrical Synapses Control Hippocampal Contributions to Fear Learning and Memory

Neuronal gap-junction channels containing connexin 36 proteins participate in consolidation of fear memories. The role of electrical synapses in synchronizing neuronal assemblies in the adult mammalian brain is well documented. However, their role in learning and memory processes remains unclear. By combining Pavlovian fear conditioning, activity-dependent immediate early gene expression, and in vivo electrophysiology, we discovered that blocking neuronal gap junctions within the dorsal hippocampus impaired context-dependent fear learning, memory, and extinction. Theta rhythms in freely moving rats were also disrupted. Our results show that gap junction–mediated neuronal transmission is a prominent feature underlying emotional memories.

The lateral amygdala processes the value of conditioned and unconditioned aversive stimuli

The amygdala is critical for acquiring and expressing conditioned fear responses elicited by sensory stimuli that predict future punishment, but there is conflicting evidence about whether the amygdala is necessary for perceiving the aversive qualities of painful or noxious stimuli that inflict primary punishment. To investigate this question, rats were fear conditioned by pairing a sequence of auditory pips (the conditioned stimulus, or CS) with a brief train of shocks to one eyelid (the unconditioned stimulus, or US). Conditioned responding to the CS was assessed by measuring freezing responses during a test session conducted 24 h after training, and unconditioned responding to the US was assessed by measuring head movements evoked by the eyelid shocks during training. We found that pre-training electrolytic lesions of the amygdalas lateral (LA) nucleus blocked acquisition of conditioned freezing to the CS, and also significantly attenuated unconditioned head movements evoked by the US. Similarly, bilateral inactivation of the amygdala with the GABA-A agonist muscimol impaired acquisition of CS-evoked freezing, and also attenuated US-evoked responses during training. However, when amygdala synaptic plasticity was blocked by infusion of the NR2B receptor antagonist ifenprodil, acquisition of conditioned freezing was impaired but shock reactivity was unaffected. These findings indicate that neural activity within the amygdala is important for both predicting and perceiving the aversive qualities of noxious stimuli, and that synaptic plasticity within LA is the mechanism by which the CS becomes associated with the US during fear conditioning.

Stress-Induced Alterations in Hippocampal Plasticity, Place Cells, and Spatial Memory

Acute, inescapable, and unpredictable stress can profoundly modify brain and cognition in humans and animals. The present study investigated the ensuing effects of 2-h variable “audiogenic” stress on three related levels of hippocampal functions in rats: long-term potentiation, place cell activity, and spatial memory. In agreement with prior findings, we observed that stress reduced the magnitude of Schaffer collateral/commissural–Cornu Ammonis field 1 long-term potentiation in vitro, and selectively impaired spatial memory on a hidden platform version of the Morris water maze task. We also observed that stress impaired the stability of firing rates (but not firing locations) of place cells recorded from dorsal Cornu Ammonis field 1 in rats foraging freely on a novel open-field platform located in a familiar surrounding room. These findings suggest that stress-induced modifications in synaptic plasticity may prevent the storage of stable “rate maps” by hippocampal place cells, which in turn may contribute to spatial memory impairments associated with stress.