Mohammed R. Milad

Neurons in medial prefrontal cortex signal memory for fear extinction

Conditioned fear responses to a tone previously paired with a shock diminish if the tone is repeatedly presented without the shock, a process known as extinction. Since Pavlov it has been hypothesized that extinction does not erase conditioning, but forms a new memory. Destruction of the ventral medial prefrontal cortex, which consists of infralimbic and prelimbic cortices, blocks recall of fear extinction, indicating that medial prefrontal cortex might store long-term extinction memory. Here we show that infralimbic neurons recorded during fear conditioning and extinction fire to the tone only when rats are recalling extinction on the following day. Rats that froze the least showed the greatest increase in infralimbic tone responses. We also show that conditioned tones paired with brief electrical stimulation of infralimbic cortex elicit low freezing in rats that had not been extinguished. Thus, stimulation resembling extinction-induced infralimbic tone responses is able to simulate extinction memory. We suggest that consolidation of extinction learning potentiates infralimbic activity, which inhibits fear during subsequent encounters with fear stimuli.

Recall of Fear Extinction in Humans Activates the Ventromedial Prefrontal Cortex and Hippocampus in Concert

BACKGROUND Extinction of conditioned fear is thought to form a new safety memory that is expressed in the context in which the extinction learning took place. Rodent studies implicate the ventromedial prefrontal cortex (vmPFC) and hippocampus in extinction recall and its modulation by context, respectively. The aim of the present study is to investigate the mediating anatomy of extinction recall in healthy humans. METHODS We used event-related functional magnetic resonance imaging (fMRI) and a 2-day fear conditioning and extinction protocol with skin conductance response as the index of conditioned responses. RESULTS During extinction recall, we found significant activations in vmPFC and hippocampus in response to the extinguished versus an unextinguished stimulus. Activation in these brain regions was positively correlated with the magnitude of extinction memory. Functional connectivity analysis revealed significant positive correlation between vmPFC and hippocampal activation during extinction recall. CONCLUSIONS These results support the involvement of the human hippocampus as well as vmPFC in the recall of extinction memory. Furthermore, this provides a paradigm for future investigations of fronto-temporal function during extinction recall in psychiatric disorders such as posttraumatic stress disorder.

Neurobiological Basis of Failure to Recall Extinction Memory in Posttraumatic Stress Disorder

BACKGROUND A clinical characteristic of posttraumatic stress disorder (PTSD) is persistently elevated fear responses to stimuli associated with the traumatic event. The objective herein is to determine whether extinction of fear responses is impaired in PTSD and whether such impairment is related to dysfunctional activation of brain regions known to be involved in fear extinction, viz., amygdala, hippocampus, ventromedial prefrontal cortex (vmPFC), and dorsal anterior cingulate cortex (dACC). METHODS Sixteen individuals diagnosed with PTSD and 15 trauma-exposed non-PTSD control subjects underwent a 2-day fear conditioning and extinction protocol in a 3-T functional magnetic resonance imaging scanner. Conditioning and extinction training were conducted on day 1. Extinction recall (or extinction memory) test was conducted on day 2 (extinguished conditioned stimuli presented in the absence of shock). Skin conductance response (SCR) was scored throughout the experiment as an index of the conditioned response. RESULTS The SCR data revealed no significant differences between groups during acquisition and extinction of conditioned fear on day 1. On day 2, however, PTSD subjects showed impaired recall of extinction memory. Analysis of functional magnetic resonance imaging data showed greater amygdala activation in the PTSD group during day 1 extinction learning. During extinction recall, lesser activation in hippocampus and vmPFC and greater activation in dACC were observed in the PTSD group. The magnitude of extinction memory across all subjects was correlated with activation of hippocampus and vmPFC during extinction recall testing. CONCLUSIONS These findings support the hypothesis that fear extinction is impaired in PTSD. They further suggest that dysfunctional activation in brain structures that mediate fear extinction learning, and especially its recall, underlie this impairment.

Fear Extinction as a Model for Translational Neuroscience: Ten Years of Progress

The psychology of extinction has been studied for decades. Approximately 10 years ago, however, there began a concerted effort to understand the neural circuits of extinction of fear conditioning, in both animals and humans. Progress during this period has been facilitated by a high degree of coordination between rodent and human researchers examining fear extinction. Here we review the major advances and highlight new approaches to understanding and exploiting fear extinction. Research in fear extinction could serve as a model for translational research in other areas of behavioral neuroscience.

Biological studies of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is the only major mental disorder for which a cause is considered to be known: that is, an event that involves threat to the physical integrity of oneself or others and induces a response of intense fear, helplessness or horror. Although PTSD is still largely regarded as a psychological phenomenon, over the past three decades the growth of the biological PTSD literature has been explosive, and thousands of references now exist. Ultimately, the impact of an environmental event, such as a psychological trauma, must be understood at organic, cellular and molecular levels. This Review attempts to present the current state of this understanding on the basis of psychophysiological, structural and functional neuroimaging, and endocrinological, genetic and molecular biological studies in humans and in animal models.

Fear extinction in rats: Implications for human brain imaging and anxiety disorders

Fear extinction is the decrease in conditioned fear responses that normally occurs when a conditioned stimulus (CS) is repeatedly presented in the absence of the aversive unconditioned stimulus (US). Extinction does not erase the initial CS-US association, but is thought to form a new memory. After extinction training, extinction memory competes with conditioning memory for control of fear expression. Deficits in fear extinction are thought to contribute to post-traumatic stress disorder (PTSD). Herein, we review studies performed in rats showing that the medial prefrontal cortex plays a critical role in the retention and expression of extinction memory. We also review human studies indicating that prefrontal areas homologous to those critical for extinction in rats are structurally and functionally deficient in patients with PTSD. We then discuss how findings from rat studies may allow us to: (1) develop new fear extinction paradigms in humans, (2) make specific predictions as to the location of extinction-related areas in humans, and (3) improve current extinction-based behavioral therapies for anxiety disorders.

Obsessive-compulsive disorder: beyond segregated cortico-striatal pathways

Obsessive-compulsive disorder (OCD) affects approximately 2-3% of the population and is characterized by recurrent intrusive thoughts (obsessions) and repetitive behaviors or mental acts (compulsions), typically performed in response to obsessions or related anxiety. In the past few decades, the prevailing models of OCD pathophysiology have focused on cortico-striatal circuitry. More recent neuroimaging evidence, however, points to critical involvement of the lateral and medial orbitofrontal cortices, the dorsal anterior cingulate cortex and amygdalo-cortical circuitry, in addition to cortico-striatal circuitry, in the pathophysiology of the disorder. In this review, we elaborate proposed features of OCD pathophysiology beyond the classic parallel cortico-striatal pathways and argue that this evidence suggests that fear extinction, in addition to behavioral inhibition, is impaired in OCD.

A Role for the Human Dorsal Anterior Cingulate Cortex in Fear Expression

BACKGROUND Rodent studies implicate the prelimbic (PL) region of the medial prefrontal cortex in the expression of conditioned fear. Human studies suggest that the dorsal anterior cingulate cortex (dACC) plays a role similar to PL in mediating or modulating fear responses. This study examined the role of dACC during fear conditioning in healthy humans with magnetic resonance imaging (MRI). METHODS Novel analyses were conducted on data from two cohorts that had previously undergone scanning to study fear extinction. Structural and functional brain data were acquired with MRI; the functional MRI (fMRI) component employed an event-related design. Skin conductance response (SCR) was the index of conditioned responses. RESULTS We found that: 1) cortical thickness within dACC is positively correlated with SCR during conditioning; 2) dACC is activated by a conditioned fear stimulus; and 3) this activation is positively correlated with differential SCR. Moreover, the dACC region implicated in this research corresponds to the target of anterior cingulotomy, an ablative surgical treatment for patients with mood and anxiety disorders. CONCLUSIONS Convergent structural, functional, and lesion findings from separate groups of subjects suggest that dACC mediates or modulates fear expression in humans. Collectively, these data implicate this territory as a potential target for future anti-anxiety therapies.

Presence and acquired origin of reduced recall for fear extinction in PTSD: results of a twin study.

Recall of fear extinction, which is thought to aid in recovery from a psychologically traumatic event, is hypothesized to be deficient in post-traumatic stress disorder (PTSD), but this has not yet been demonstrated in the laboratory, nor has its origin been investigated. To address these two issues, 14 pairs of monozygotic twins discordant for combat exposure, in 7 of which the combat-exposed twin had PTSD, underwent a two-day fear conditioning and extinction procedure. On Day 1, subjects viewed colored light conditioned stimuli, some of which were paired with mild electric shock, followed by extinction of the conditioned responses. On Day 2, recall of Day 1 extinction learning (i.e., extinction retention) was assessed. Skin conductance response (SCR) was the dependent measure. There were no group differences during acquisition or extinction learning. However, a significant PTSD Diagnosis (in the exposed twin) x combat Exposure interaction emerged during extinction recall, with the PTSD combat veterans having larger SCRs than their own co-twins, and than the non-PTSD combat veterans and their co-twins. These results indicate that retention of extinction of conditioned fear is deficient in PTSD. Furthermore, they support the conclusion that this deficit is acquired as a result of combat trauma leading to PTSD, rather than being a predisposing factor to developing PTSD upon the stress of combat.

The Role of the Orbitofrontal Cortex in Anxiety Disorders

Abstract: Advances in neuroimaging techniques over the past two decades have allowed scientists to investigate the neurocircuitry of anxiety disorders. Such research has implicated the orbitofrontal cortex (OFC). Characterizing the role of OFC in anxiety disorders, however, is principally complicated by two factors–differences in underlying pathophysiology across the anxiety disorders and heterogeneity in function across different OFC sub‐territories. Contemporary neurocircuitry models of anxiety disorders have primarily focused on amygdalo‐cortical interactions. The amygdala is implicated in generating fear responses, whereas cortical regions, specifically the medial OFC (mOFC) and the ventromedial prefrontal cortex (vmPFC), are implicated in fear extinction. In contrast to mOFC, anterolateral OFC (lOFC) has been associated with negative affects and obsessions and thus dysfunctional lOFC may underlie different aspects of certain anxiety disorders. Herein, we aim to review the above‐mentioned theories and provide a heuristic model for conceptualizing the respective roles of mOFC and lOFC in the pathophysiology and treatment of anxiety disorders. We will also review the role of the OFC in fear extinction and the implications of this role to the pathophysiology of anxiety disorders.