1. History of Research

In 1937, at the University of Illinois, Edward Girden and Elmer Culler conducted an experiment on conditioned responses in dogs under the influence of the drug curare. In the experiment, dogs were taught a conditioned muscular response – to draw their paw away from the ground when they heard a buzzer. The buzzer was often accompanied by a small electric shock, which motivated the response. For dogs that had been under the influence of curare when they first learned the response, after the curare was no longer in their system, they were less likely to remember to draw their paw away on hearing the buzzer. Once they were given curare again, the response returned.^[1] This result indicated that the dogs' ability to recall the responses was connected to their state of consciousness. Girden and Culler's research opened the door for further investigation of the influences of state of consciousness on an organism's ability to encode memory.

Following this discovery, other researchers looked into the effect of different states of being on the ability to learn and remember responses or information. In 1964, Donald Overton conducted a study as a direct response to Girden and Culler's 1937 experiment. The study tested the effects of sodium pentobarbital on rats' abilities to learn and remember certain taught responses. These rats were randomly assigned to one of two groups – substance administered or no substance administered (the control condition) – and then placed in a simple maze and taught to escape an electrical shock. Overton found that the rats that had been administered 25 mg of sodium pentobarbital could no longer remember the proper escape response when they were later placed in the maze without the drug. However, if these rats were administered sodium pentobarbital once again and placed in the maze, they recalled the escape response they had been taught. Similarly, when Overton taught a rat the escape response under the control condition (no sodium pentobarbital administered), it could not recall that behavior when it was administered the drug and asked to perform later on. Results strongly indicated that rats performed the learned response more efficiently when in the either sodium pentobarbital or control state that they were in when they first learned it. In regard to this idea the study specifically stated "a response learned under the influence of a particular drug will subsequently reoccur (with maximum strength) only when that drug condition is reinstated".^[2]

In later years, similar studies confirmed that learning could be state-dependent. In 1971, Terry Devietti and Raymond Larson conducted a similar study in rats, seeing how memory was affected by various levels of electric shock. Their results supported the idea that the rats' ability to remember a learned response was influenced by their state.^[3] The phenomenon continued to be studied more than thirty years later. In 2004, Mohammad-Reza Zarrindast and Ameneh Rezayof studied mice, to see how memory and learning were affected by morphine. They found that when mice learned a response under the influence of morphine, they later performed it most efficiently under the influence of morphine. When mice learned the response free of morphine, they recalled it best when similarly sober. And for mice that were taught the response under the influence of morphine, once the drug wore off, they suffered amnestic effects; they could no longer remember the learned response.^[4]

The results of each of these studies points to the existence of a state-dependent memory phenomenon. Further research on the subject continues to be carried out today in order to discover further implications of state-dependent memory or other situations in which state-dependent memory might take place.

2. Biological Functions and Explanatory Mechanisms

At its most basic, state-dependent memory is the product of the strengthening of a particular synaptic pathway in the brain.^[5] A neural synapse is the space between brain cells, or neurons, that allows chemical signals to be passed from one neuron to another. Chemicals called neurotransmitters leave one cell, travel across the synapse, and are taken in by the next neuron through a neurotransmitter receptor. This creates a connection between the two neurons called a neural pathway. Memory relies on the strengthening of these neural pathways, associating one neuron with another. When we learn something, new pathways are made between neurons in the brain which then communicate through chemical signals. If these cells have a history of sending out certain signals under specific chemical conditions within the brain, they are then primed to work most effectively under similar circumstances.^[5] State-dependent memory happens when a new neural connection is made while the brain is in a specific chemical state - for instance, a child with ADHD learns their multiplication tables while on stimulant medication. Because their brain created these new connections related to multiplication tables while the brain was chemically affected by the stimulant medication, their neurons will be primed in the future to remember these facts best when the same levels of medication are present in the brain.

While there is strong evidence for the existence of state-dependent memory, it is less clear what the advantage of this circumstance might be. In 2006, researcher Lorena Pomplio and her team tackled this question as they investigated the presence of state-dependent memory in invertebrates, specifically grasshoppers. Up until this point, only vertebrates had been used to study state-dependent memory. This study found that invertebrates did indeed experience the phenomenon as well, particularly in regard to conditions of low or high nutritional intake. Pomplio and associates (2006) concluded that their results demonstrated a potential "adaptive advantage" of state-dependent learning that explains its intrinsic presence in such a wide variety of species. State-dependent memory recalls a time that the organism was in a similar condition, which then informs the decisions they make in the present. For these grasshoppers, their low nutritional state sparked cognitive connections to similar states of duress and primed the insects to make decisions they had made when faced with low nutrition in previous conditions. The paper suggests that this phenomenon allows for quick decisions to be made when an organism does not have the time or neural capability to carefully process every option.^[6]

3. Substances

Research has shown evidence for the roles that numerous substances play in state-dependent memory. For example, stimulants like Ritalin can produce state-dependent memory effects in children with hyperactive disorders.^[7] Additionally, state-dependent memory effects have been found in regard to other substances such as morphine, caffeine, and alcohol.^[4][8][9]

Substantial amounts of research have been conducted on the effects of alcohol.^[9] A very clear description of state-dependent memory is found in John Elliotson's Human Physiology (1835):

"Dr. Abel informed me," says Mr. Combe [presumably George Combe], "of an Irish porter to a warehouse, who forgot, when sober, what he had done when drunk: but, being drunk, again recollected the transactions of his former state of intoxication. On one occasion, being drunk, he had lost a parcel of some value, and in his sober moments could give no account of it. Next time he was intoxicated, he recollected that he had left the parcel at a certain house, and there being no address on it, it had remained there safely, and was got on his calling for it." This man must have had two souls, one for his sober state, and one for him when drunk.^[10]

Research shows that individuals are less likely to remember information learned while intoxicated when they are once again sober.^[9] However, information learned or memories created while intoxicated are most effectively retrieved when the individual is in a similar state of intoxication.^[9][11]

Alcoholism can enhance state-dependent memory as well. In a study comparing the state-dependent memory effects of alcohol on both subjects with alcoholism and subjects without alcoholism, researchers found that the alcoholic subjects showed greater effects for state-dependent memory on tasks of recall and free association. This is not because alcohol better produces associations, but because the person with alcoholism lives a larger portion of their life under the influence of alcohol.^[11] This produces changes in cognition and so when the person with alcoholism drinks, the intoxication primes their brain towards certain associations made in similar states. Essentially, the intoxicated and sober states of the alcoholic are in fact, different from the intoxicated and sober states of the non-alcoholic person, whose body is not as used to processing such large amounts of the substance.^[11] For this reason, we see slightly larger effects of state-dependent memory while intoxicated for chronic drinkers than for those who do not drink often.^[11]

4. Mood

State-dependent memory influenced by mood has been the subject of some controversy within the psychological field. Though research seemed to show evidence for the existence of mood-dependence in memory, this came into question later on when researchers suggested the results were actually the result of mood congruent memory, a phenomenon in which an individual recalls more information associated with their condition.^[12] For example, a person who is asked to learn a list of words while they have a cold might remember more words associated with their illness such as "tissue" or "congestion" when later asked to recall the words learned. Researchers have since been conducting experiments to unearth the truth about mood-dependent memory, though it remains difficult to completely eliminate unreliability from such studies.

Some studies have investigated the existence of mood-dependent memory, especially in individuals with bipolar disorder who generally vacillate over time between mood extremes, specifically depression and mania. In 1977, it was found that individuals with bipolar disorder performed better on a verbal association test when they were in a similar mood state to their state when the verbal associations were learned.^[13] A more recent study in 2011 similarly studied a group of individual with bipolar disorder and found evidence for mood-dependent memory on a visual task (recognition of inkblots). It was observed that subjects had better recall for these inkblots when they were in the same mood state they had been in when they first saw these inkblots. However, researchers did not find a similar effect for verbal tasks.^[14] Because the two studies do not agree on the effects of mood in regard to verbal recall tasks, further research is needed to clarify the existence of mood-dependent memory on both verbal and visual recall tasks as well as to investigate mood-dependent memory in those suffering from other mood disorders or individuals without mood disorders of any kind.

5. Pain

Evidence has been found for the existence of state-dependent learning in conditions of pain.^[15] Individuals who were subject to induced pain by placing their hands in ice water recalled a given list of words better when their hands were once again in the ice water than when their hands were placed in warm water. Participants who first learned the list of words while their hands were immersed in warm water, thus posing no threat of induced pain, recalled the words most effectively when they were in a similar warm water condition than the ice water condition. When the level of pain at the time of encoding matches the level of pain at the time of recall, individuals show an improved ability to remember a given list of words. These results show that pain states can also affect memory.^[15]

6. Recreating Internal State

Studies have shown that simply creating the same internal state that you had at the time of encoding is sufficient to serve as a retrieval cue.^[16] Therefore putting yourself in the same mindset that you were in at the time of encoding will help recall in the same way that being in the same situation helps recall. This effect called context reinstatement was demonstrated by Fisher and Craik 1977 when they matched retrieval cues with the way information was memorized.^[17]

7. Implications

State-dependent memory has widespread effects that can play roles in our everyday lives.^{[5][6][7][8][12]} For example, state-dependence can affect performance on a test or in a job interview. It can affect your memory for where you left your car keys. However, the power of state-dependent memory can also be harnessed to improve performance in school for those with learning disabilities or outcomes in therapy.^[7][8]

State-dependent memory has implications on effectiveness of psychological treatment. Evidence has also been found for the idea that an individual's state (in regard to substance) can influence the impact of psychological treatment.^[8] Patients responded better to phobia exposure therapy from one treatment session to another when they were consistent in their states of consciousness. This study found that patients who had similar levels of caffeine in their system at each session or who consistently had no caffeine in their system exhibited greater rates of improvement with fewer phobia relapses than patients who came in at various states of caffeine influence from one treatment session to another.^[8] These results show that state-dependent learning can be used to the advantage of those undergoing psychological treatment. By remaining consistent in their state of consciousness during sessions, patients can improve the likelihood of their success and decrease the possibility of relapse. Future directions for this kind of research might test substances other than caffeine for similar effects on patient performance during psychological treatment.

Evidence for state-dependent learning has been found in children with hyperactivity taking the drug methylphenidate, a drug often prescribed for treatment of ADHD symptoms, more commonly known as Ritalin or Concerta.^[7] Children with hyperactivity taking this drug during periods of learning better retained that information during subsequent periods of methylphenidate use, illustrating the effectiveness of methylphenidate in facilitating learning in children diagnosed with hyperactive disorders.^[7] However, this state-dependent learning effect of stimulant medication applies only to children diagnosed with hyperactivity. Children not diagnosed with hyperactivity show no change in retention rates due to use of stimulants such as methylphenidate or pemoline.^[7][18] These studies validate the prescription of stimulants for individuals with hyperactive disorders. The results show that the state of consciousness produced through use of these drugs improves cognitive focus in those with hyperactive disorders when taken consistently.