How does past stress impact fear today?

Does stress impact learning and memory?  You bet it does! Stress impacts these processes in several different ways. Today I am going to focus on the impact of stress on fear.  First, we need to define some terms.

Basic Definitions

Stress occurs when an animal experiences a challenge to the status quo. This triggers a physiological response (the stress response) that includes the release of glucocorticoids (stress hormones).

Fear conditioning is the process of acquiring a fear through classical conditioning.  One or more stimuli become associated with an aversive event.  For example, a tone is paired with a shock and the tone then causes a fear response. In this case, the shock is the unconditioned stimulus which means that it produces a fear response without any prior learning (Figure 1).

Figure 1


During the fear conditioning process, the tone is repeatedly paired with a shock (Figure 2).

Figure 2


Fear conditioning has occurred when the tone alone produces a fear response in the animal (Figure 3).

Figure 3


Once fear conditioning has been established, animals can go through the process of extinction. Fear extinction occurs when the relationship between the conditioned stimulus (e.g. tone) and the  unconditioned stimulus (e.g. the shock) is decoupled and the conditioned stimulus alone no longer produces a fear response (Figure 4). This is done by repeatedly playing the tone without following it with a shock.

Figure 4


Now that everyone is on the same page with some basic definitions, let’s take a look at how stress experienced in the recent past impacts current behavior as it relates to fear.

Stress, Fear, and Extinction

It’s easy to imagine that if an animal is currently stressed they may be more susceptible to fear.  But what if they experienced stress previously?  Within the last 10 to 15 years, several studies have looked at how a stressful experience impacts fear conditioning a few days to a week after the stressful experience. (There is also a lot of interesting research that looks at the impact of stress during development, but we will have to leave that for another time!)

Here is the typical experimental set up.  An animal is exposed to stress. This can be acute (brief) stress or it can be more chronic. Then the animal is given a “rest” period of up to seven days. After the rest period, the animal goes through a process of fear conditioning in a different context (ie location).  The stressor may be the same or different from the initial stressor. Researchers then look at how recently stressed animals behave compared to animals that were not exposed to stress at the beginning of the study (the control group).  What have they learned?

First, animals are more susceptible to fear learning if they were recently exposed to stress. This means that, compared to controls, the previously stressed animals learn fear more quickly and that the learned fear response is stronger (Figure 5).  They also show fear sensitization.  This means that they react more fearfully to their environment in general.  If they are placed in a new context and exposed to something stressful, they will show a stronger fear response than animals not previously exposed to stress. Studies show similar results regardless of whether the initial stress was acute or chronic.

Figure 5


There is also some evidence that these impacts do not necessarily emerge right away.  A study by McGuire and colleagues (2010) did not find any impact when rats were tested 16 hours after the stressful experience.  However, they did see changes after seven days. This suggests that even though an animal may appear to be “okay” after a stressful experience, they may be more impacted than we realize!

In addition, animals that have experienced previous stress showed impaired extinction to the conditioned fear. Remember, animals are stressed, then “rested” for several days to a week.  After the rest period, they go through a fear conditioning process.  The fear response that develops as a result of that fear conditioning is resistant to extinction – that means that there is little to no difference in behavior between rats than have gone through extinction and those that have not.

These studies are generally viewed as representing a rodent model for PTSD.  This is important from a research perspective because it means that the findings of rodent research may be able to be applied to the prevention and treatment of PTSD in humans. It’s also very interesting for dog owners and professionals for two reasons.

Photo credit: Viktar Masalovich

First, it provides us with useful information for preventing and treating fear-related behavior issues in dogs.  Second, it suggests that non-human animals (including dogs) may develop disorders similar to PTSD.  Why do I say “similar” and not “the same as”? Although dogs and humans (and other animals) have many similarities, we also have many important differences.  I think we can get ourselves into dangerous territory by assuming that the mental experience of dogs is exactly the same as the mental experience of humans.  On the other hand, we can also get ourselves into dangerous territory by assuming the opposite–that the experience of dogs and humans is nothing alike.  The key is finding a balance–and making sure that we stick to the facts as established by research.

In this case, the research shows us that stress experienced several days to a week ago makes rats more susceptible to reacting to and learning fear in the present.  It also makes extinction less effective. There is good reason to believe this applies to other animals as well–including dogs and humans.  What this tells us is that we need to be very mindful of preventing and treating stress in dogs. This is yet another reason to focus on positive reinforcement in training and behavior modification, rather than punishment.  It can also help owners understand why they need to work to minimize or prevent stress in their dogs–and to help them understand their dog’s behavior after they have gone through a stressful experience.

If an animal has been recently stressed, we need to be extra cautious about exposing them to potentially fearful experiences for at least a week afterward.  It also means that when treating behavior, we need to prioritize addressing the dog’s wellbeing in order to maximize success.  In some cases, this will include the use of medication to reduce the animal’s level of stress enough that behavior modification can be more effective.

Photo credit: Pedro Araújo

It also means that extinction may not be effective with certain dogs–specifically dogs that have experienced trauma.  Before you say that you don’t use extinction in behavior modification, it’s important to understand that counter conditioning involves two processes.  The first is teaching the dog to associate the conditioned stimulus with something good (e.g. men = chicken).  The second is teaching the dog that men do not predict bad things–that’s extinction! I think this research tells us that we need to develop alternatives to just using desensitization and counter conditioning as a treatment method for dogs experiencing fear.  To be clear – I am NOT saying we need to stop using desensitization and counter conditioning!  What I am saying is that we would be able to help more dogs more effectively if we expanded our tool box a bit. One major target is increasing our focus on decreasing stress in the dog’s life in general–including through increased enrichment and exercise.  Another option is trying to increase the dog’s ability to cope with stress. Sadly, we will have to leave a deeper discussion of those options for another blog post!

Remember, as always, that our knowledge is still limited.  These studies are done on rodents in a lab.  Although I do believe we can safely generalize this information to dogs to some degree, it’s important to be aware that this evidence is not as strong as it would be if we actually had clinical studies looking at these issues in pet dogs.  However, since we don’t have that, we have to go with the next best thing which is research on rodents in labs. Anytime you get new information like this, it’s important to weigh it against what you already know.  In this case, this research is very much in line with what we already know about dog behavior.  But if you find something that doesn’t match up? The best thing you can do is ask more questions and dig a little deeper. It’s critically important to continue to learn about the science of behavior as our knowledge base is ever growing and changing.

The hallmark of originality is rejecting the default and exploring whether a better option exists. – Adam Grant, The Originals


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What questions does this information raise for you?  Can you think of other implications in addition to those discussed here?  Answer in the comments section!

Note: All of the research I found looked at a maximum rest period of seven days.  However, given what we know about the long-term impacts of stress and the existence of disorders like PTSD, it seems likely that, at least in some individuals, these impacts may last much longer than seven days.


Cordero, M.I. et al. Prior exposure to single stress session facilitates subsequenct contextual fear conditioning in rats: Evidence for a role of corticosterone. Hormones and Behavior, Vol. 44, No. 4, 2003, pp. 338-345

Long, V.A. and Fanselow, M.S. Stress-enhanced fear learning in rats is resistant to the effects of immediate massed extinction. Stress, vol. 15, 2012, pp. 627-636.

McGuire et al. Enhanced fear recall and emotional arousal in rats recovering from chronic variable stress. Physiol. Behav., Vol. 101, no. 4, 2010, pp. 474-482.

Rau, V. et al. Stress-induced enhancement of fear learning: An Animal Model of Posttramatic Stress Disorder. Neuroscience and Biobehavioral Reviews, vol. 29, 2005, pp. 1207-1223.

Schmeltzer, S.N. et al. History of chronic stress modifies acute stress-evoked acoustic startle in male rats. Stress, Vol. 18, 2015, pp. 244-253.




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Kristina Spaulding

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