Despite identical learning experiences, individuals differ in the memory formed of those experiences. Molecular mechanisms that control the neurophysiological bases of long-term memory formation might control how precisely the memory formed reflects the actually perceived experience. Memory formed with sensory specificity determines its utility for selectively cueing subsequent behavior, even in novel situations. Here, a rodent model of auditory learning capitalized on individual differences in learning-induced auditory neuroplasticity to identify and characterize neural substrates for sound-specific (vs. general) memory of the training signal's acoustic frequency. Animals that behaviorally revealed a naturally induced signal-“specific” memory exhibited long-lasting signal-specific neurophysiological plasticity in auditory cortical and subcortical evoked responses. Animals with “general” memories did not exhibit learning-induced changes in these same measures. Manipulating a histone deacetylase during memory consolidation biased animals to have more signal-specific memory. Individual differences validated this brain-behavior relationship in both natural and manipulated memory formation, such that the degree of change in sensory cortical and subcortical neurophysiological responses could be used to predict the behavioral precision of memory.

The longitudinal study aims to expand what is known about the costs and benefits of narrating stressful experiences by exploring changes in rumination within the narrative process and comparing it to changes in self-reflection. Rumination (e.g., brooding, self-criticism, and negative emotions) and self-reflection were measured in stress narratives of 56 college students. There were several goals: (1) examine changes in narrative rumination and narrative self-reflection over 3 days of writing, (2) examine the relations among the changes in narrative rumination variables and narrative self-reflection and (3) examine how changes in narrative rumination and narrative self-reflection relate to multiple measures of psychological functioning. Overall, individuals increased self-reflection over the 3-day writing task. Individuals who increased ruminative brooding across the 3 days of writing showed lower ego identity development (short term and long term) and self-esteem (short term), while increased self-criticism was positively correlated with identity distress (short term). Implications of the different aspects of narrative rumination, specifically in the context of stressful experiences, are discussed.

Generalization of fear can involve abnormal responding to cues that signal safety and is common in people diagnosed with post-traumatic stress disorder. Differential auditory fear conditioning can be used as a tool to measure changes in fear discrimination and generalization. Most prior work in this area has focused on elevated amygdala activity as a critical component underlying generalization. The amygdala receives input from auditory cortex as well as the medial geniculate nucleus (MgN) of the thalamus, and these synapses undergo plastic changes in response to fear conditioning and are major contributors to the formation of memory related to both safe and threatening cues. The requirement for MgN protein synthesis during auditory discrimination and generalization, as well as the role of MgN plasticity in amygdala encoding of discrimination or generalization, have not been directly tested. GluR1 and GluR2 containing AMPA receptors are found at synapses throughout the amygdala and their expression is persistently up-regulated after learning. Some of these receptors are postsynaptic to terminals from MgN neurons. We found that protein synthesis-dependent plasticity in MgN is necessary for elevated freezing to both aversive and safe auditory cues, and that this is accompanied by changes in the expressions of AMPA receptor and synaptic scaffolding proteins (e.g., SHANK) at amygdala synapses. This work contributes to understanding the neural mechanisms underlying increased fear to safety signals after stress.

Using a sound-reward extinction paradigm in male rats, we reveal both cortical and subcortical sensory codes for the cue-specificity of memory. In the auditory cortex, re-tuning narrowed frequency receptive field bandwidth, yielding more precise extinction behavior cued by acoustic frequency. Subcortical signals revealed in the auditory brainstem response (ABR) showed decreases in amplitude of select components of the ABR wave. Interestingly, treatment with an inhibitor of histone deacetylase 3 (HDAC3-i) facilitated both auditory cortical tuning bandwidth changes and ABR changes that were frequency-specific to the extinguished signal sound. Moreover, both changes were correlated to each other and with highly precise extinction memory at the level of behavior. Thus, we show for the first time that HDAC3 regulates the specificity of sensory features consolidated in extinction memory. Overall, the dynamics of auditory system plasticity associated with sound-specific extinction are complex. Changes in ABR amplitude induced by sound-reward learning disappeared after extinction, while changes in ABR slope that were initially induced by sound-reward learning were maintained through extinction. Moreover, plasticity of cortical re-tuning emerged only after extinction learning. HDAC3-i applied after extinction training sessions enabled sensory system plasticity to encode the extinguished sound with higher acoustic specificity (compared to vehicle controls). Both cortical and subcortical response changes to sound became unusually "tuned-in" to the acoustic frequency that had been presented under extinction conditions. Thus, HDAC3 regulates how specifically sensory features of experience are encoded into long-term memory and may exert its behavioral effects via multiple coding strategies along sensory system pathways. SIGNIFICANCE STATEMENTEpigenetic mechanisms have recently been implicated in memory and information processing. Here, we use a pharmacological inhibitor of histone deacetylase 3 (HDAC3) in a sensory model of learning to reveal, for the first time, its ability to enable unusually precise extinction memory. In so doing, we uncover neural coding strategies for memory's "specificity" for sensory cues. Extinction induced multiple forms of change at different levels of sensory processing, which highlights the complexity of extinction memory encoding. HDAC3 appears to coordinate effects across sensory levels that determine specific cue saliency for behavior. Thus, epigenetic players may gate how sensory information is stored in long-term memory and their manipulation can be leveraged to reveal neural coding mechanisms for sensory detail in memory..

In humans, females process a sound’s harmonics more robustly than males. As estrogen regulates auditory plasticity in a sex-specific manner in seasonally breeding animals, estrogen signaling is one hypothesized mechanism for this difference in humans. To investigate whether sex differences in harmonic encoding vary similarly across the reproductive cycle of mammals, we recorded frequency-following responses (FFRs) to a complex sound in male and female rats. Female FFRs were collected during both low and high levels of circulating estrogen during the estrous cycle. Overall, female rodents had larger harmonic encoding than male rodents, and greater harmonic strength was seen during periods of greater estrogen production in the females. These results argue that hormonal differences, specifically estrogen, underlie sex differences in harmonic encoding in rodents and suggest that a similar mechanism may underlie differences seen in humans.