Contents lists available at ScienceDirect Neurobiology of Learning and Memory journal homepage: www.elsevier.com/locate/ynlme
From Pavlov to PTSD: The extinction of conditioned fear in rodents, humans, and anxiety disorders
Michael B. VanElzakker a,b,⇑, M. Kathryn Dahlgren a,b, F. Caroline Davis b, Stacey Dubois a, Lisa M. Shin a,b a Tufts University Psychology, 490 Boston Avenue, Medford, MA 02155, USA
b Massachusetts General Hospital Psychiatry, 149 Thirteenth Street, Charlestown, MA 02129, USA
In his classical conditioning and extinction experiments, Ivan Pavlov rang a bell (the conditioned stimulus; CS), immediately before giving his dogs food (specifically meat powder, the uncondi- tioned stimulus; US; Pavlov, 1927). On its own, the meat powder made the dogs salivate (the unconditioned response; UR). After repeating this predictive pairing several times, Pavlov’s dogs began salivating to the mere sound of the bell—even when no meat pow- der was presented—making salivation the conditioned response
Abbreviations: 5-HTTLPR, serotonin transporter gene linked polymorphic region; BDNF, brain-derived neurotropic factor; BNST, bed nucleus of the stria terminalis; COMT, catechol-O-methyltransferase; CR, conditioned response; CS+ or CS, conditioned stimulus; CS , never-conditioned stimulus; dACC, dorsal anterior cingulate cortex; DAT1, dopamine active transporter gene; DCS, D-cycloserine; DRD2, dopamine receptor D2 gene; EMG, electromyography; fMRI, functional magnetic resonance imaging; GABA, gamma-aminobutyric acid; HRR, heart rate response; IED, improvised explosive device; NMDA, N-methyl-D-aspartate; NPSR1, neuropeptide S receptor; OCD, obsessive–compulsive disorder; PET, positron emission tomography; PTSD, posttraumatic stress disorder; rCMRglu, regional cerebral metabolic rate for glucose; SCR, skin conductance response; SRI, serotonin reuptake inhibitor; TMS, transcranial magnetic stimulation; TPH2, tryptophan hydroxylase-2; US, unconditioned stimulus; UR, unconditioned response; vmPFC, ventromedial prefrontal cortex.
⇑ Corresponding author at: Tufts University Psychology, 490 Boston Avenue, Medford, MA 02155, USA.
E-mail address: michael.vanelzakker@gmail.com (M.B. VanElzakker).
1074-7427/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.nlm.2013.11.014
abstract
Nearly 100 years ago, Ivan Pavlov demonstrated that dogs could learn to use a neutral cue to predict a biologically relevant event: after repeated predictive pairings, Pavlov’s dogs were conditioned to antici- pate food at the sound of a bell, which caused them to salivate. Like sustenance, danger is biologically relevant, and neutral cues can take on great salience when they predict a threat to survival. In anxiety disorders such as posttraumatic stress disorder (PTSD), this type of conditioned fear fails to extinguish, and reminders of traumatic events can cause pathological conditioned fear responses for decades after danger has passed. In this review, we use fear conditioning and extinction studies to draw a direct line from Pavlov to PTSD and other anxiety disorders. We explain how rodent studies have informed neuro- imaging studies of healthy humans and humans with PTSD. We describe several genes that have been linked to both PTSD and fear conditioning and extinction and explain how abnormalities in fear condi- tioning or extinction may reflect a general biomarker of anxiety disorders. Finally, we explore drug and neuromodulation treatments that may enhance therapeutic extinction in anxiety disorders.
Ó 2013 Elsevier Inc. All rights reserved.
(CR). The sound of the bell predicted something agreeable and bio- logically valuable: food. However, not all of Pavlov’s USs were pleasant, and not all CRs conveyed his dogs’ anticipation of some- thing enjoyable. In addition to learning about nourishment sources, it is important for an organism to be able to predict threats to health and safety. For example, when Pavlov repeatedly paired the sound of a metronome (CS) with subsequent application of a small amount of sour-tasting diluted acid (US) onto a dog’s tongue, the dog eventually learned the association. Henceforth, upon pre- sentation of the CS alone, the dog exhibited what Pavlov called a ‘‘defensive reflex’’: it shook its head, salivated profusely, and moved its tongue as if to expel a toxic substance, even though no acid was there. A similar process was demonstrated with an 11-month-old child in Watson and Rayner’s famous ‘‘Little Albert’’ experiments of 1920. Watson and Rayner paired Albert’s touching of a white rat (CS) with a sudden fear-arousing noise (US) made by striking a steel bar behind him (Watson & Rayner, 2000). Upon subsequent presentations of the rat, Albert no longer exhibited his natural curiosity, but rather withdrew his hand. This learned response seemed to generalize to cotton balls, a Santa Claus mask, a brown bunny, and a black fur coat. The Little Albert experiment is an early