extensive evidence from studies employing a variety of mammalian species indicates that memory is organized in multiple brain systems that differ in terms of the type of memory they mediate ( for reviews see squire et al . , 1993 ; packard and knowlton , 2002 ; white and mcdonald , 2002 ; squire , 2004 ) . neurobiological evidence that the brain contains multiple memory systems has led to the proposal of several  dual - memory  theories designed to define the psychological operating principles that distinguish different forms of memory ( for review see kesner , 1998 ) . each of these theories was derived primarily from a comparison of the pattern of spared and impaired learning and memory functions observed following damage to the hippocampal system . in addition , the operating principles described in many dual - memory theories , particularly those derived from the animal literature , are heavily influenced by the classical debate between  cognitive for example , the memory functions of the mammalian hippocampal system have been described as essentially neo - tolmanian in nature , while the types of learning spared following hippocampal system damage are often readily interpreted by hullian - like s - r or  habit  learning theories ( e.g. , hirsh , 1974 ; o'keefe and nadel , 1978 ; mishkin and petri , 1984 ) . in rats , evidence for the multiple memory systems hypothesis is found in experiments comparing the effects of manipulations of the hippocampal system and caudate - putamen ( i.e. , dorsal striatum ) . in studies using pairs of tasks with similar motivational , sensory , and motoric processes , lesions of the rat hippocampal system and dorsal striatum result in a double dissociation of task acquisition ( packard et al . , 1989 ; packard and mcgaugh , 1992 ; mcdonald and white , 1993 ; kesner et al . , 1993 ) . for example , lesions of the fimbria - fornix impair acquisition of  win - shift  behavior in the radial maze , a learning task that requires rats to remember which maze arms have been visited within a daily training session ( packard et al . , 1989 ) . however , fornix lesions facilitate acquisition of a simultaneous visual discrimination win - stay  radial maze task , in which food baited maze arms are signaled by a light cue ( packard et al . , 1989 ) . in contrast , lesions of the dorsal striatum impair acquisition of the win - stay radial maze task , but have no effect on acquisition of win - shift behavior ( packard et al . , 1989 ; another early experiment in rats demonstrating a double dissociation between the mnemonic functions of the hippocampal system and dorsal striatum used two versions of a two - platform water maze task . in this task , two rubber balls protruding above the water serve as cues . one ball ( correct ) is on a rectangular platform that can be mounted to escape the water , and the other ball ( incorrect ) is mounted on a thin rod and thus does not provide escape . the two balls also differ in visual appearance ( i.e. , vertical versus horizontal black / white stripes ) . in a cognitive version of the task , the correct platform is located in the same location on every trial , but the visual appearance of the ball varies . thus , this version of the task requires rats to learn to approach the correct ball on the basis of spatial location , and not visual pattern . in the s - r habit version of the task , the correct platform is located in different spatial locations across trials , but the visual pattern is consistent . thus , this task can be acquired by learning an approach response to the visual cue ( i.e. , pattern discrimination ) . lesions of the fornix , but not the dorsal striatum , impair acquisition of the cognitive task ; whereas lesions of the dorsal striatum , and not hippocampus , impair acquisition of the habit task ( packard and mcgaugh , 1992 ) . a further example of the differential roles of the hippocampus and dorsal striatum in memory involves the use of a plus - maze task that served as a  battleground  for the debate between cognitive and s - r learning theorists during the late 1940's  early 1950 's ( for review see restle , 1957 ) . the plus - maze apparatus allows an animal to approach a goal box ( e.g. , east or west ) from one of two start boxes ( e.g. , north or south ) . in a  dual - solution  version of the task , rats are trained to start from the same start box ( e.g. , south ) and obtain food in a consistently baited goal box ( e.g. , west ) . in describing the manner in which rats acquire this task , cognitive learning theorists argued that rats learn the spatial location of the food reward ( i.e. ,  place  learning ) . however , stimulus - response learning theorists argued that rats instead learn to make a specific body turn at the choice point ( i.e. ,  response  learning ) . the use of these two possible learning mechanisms can be assessed in a probe trial administered after training , in which rats are started from the opposite start box ( e.g. , north ) . on the probe trial , rats that approach the spatial location that contained food during training are designated  place learners , whereas rats that make the same body turn that was reinforced during training are designated  response  learners . although intact rats can use both types of learning in performing the task ( depending in part on intra- and extra - maze environmental conditions ; for review see restle , 1957 ) , the multiple memory systems hypothesis raises the possibility that these two types of learning may have distinct neural substrates . this hypothesis was addressed in a plus - maze study designed to differentiate the role of the hippocampus and the dorsal striatum in memory ( packard and mcgaugh , 1996 ) . rats were trained in a daily session from the same start box to obtain food from a consistently baited goal box . following a week of daily training the animals prior to the probe trial , rats received intra - dorsal striatal or intra - hippocampal injections of either vehicle saline or the local anesthetic lidocaine , a compound that produces a localized and reversible neural inactivation via a blockade of voltage - gated sodium channels . rats receiving vehicle injections into the hippocampus or dorsal striatum were predominantly place learners on the probe trial . intra - hippocampal , but not intra - striatal injections of lidocaine blocked expression of place learning . thus , the functional integrity of the hippocampus , but not dorsal striatum is necessary for expression of place learning . in order to assess whether the dorsal striatum might be selectively involved in response learning , we took advantage of previous findings indicating that with extended training in the plus - maze intact rats  switch  from the use of place learning to response learning ( for review restle , 1957 ) . accordingly , we trained the rats for an additional week and then administered a second probe trial . rats receiving vehicle injections into either the hippocampus or dorsal striatum were predominantly response learners on the second probe trial , confirming previous reports of a shift from the use of place information to response learning with extended training . intra - hippocampal injections of lidocaine had no effect on the expression of response learning . in contrast , rats receiving intra - striatal lidocaine displayed place learning ( i.e. , a blockade of the use of response learning observed in control rats ) . these findings provide further evidence for the differential roles of the hippocampus and the dorsal striatum in memory . they also suggest that , in this dual - solution task , place learning is acquired earlier than response learning and that with extended training the control of learned behavior  shifts  from the hippocampus to the dorsal striatum . finally , in addition to lesion studies , double dissociations between the roles of the hippocampus and the dorsal striatum in memory have also been observed following post - training intracerebral drug injections ( e.g. , packard and white , 1991 ; packard et al . , 1994 ; packard and teather , 1997 , 1998 ) . for example , post - training intra - hippocampal injections of dopaminergic agonists selectively enhance memory in a win - shift radial maze task , while similar injections into the dorsal striatum selectively enhance memory in a win - stay radial maze task ( packard and white , 1991 ) . moreover , post - training intra - hippocampal injections of the glutamatergic nmda receptor antagonist ap5 selectively impair memory in a hidden platform water maze task , while similar injections into the dorsal striatum selectively impair memory in a visible platform water maze task ( packard and teather , 1997 ) . in both the hippocampus and the dorsal striatum , the effects of the post - training treatments are time - dependent ; injections delayed 2 h post - training have no effect on memory . the time - dependent nature of these post - training injections strongly implicates these brain regions in the modulation of memory processes ( mcgaugh , 1966 ) , and the task - dependent nature of the treatments indicate selective roles for the hippocampus and dorsal striatum in different types of memory . finally , when taken together , double dissociations between the mnemonic functions of the hippocampus and the dorsal striatum in cognitive and habit memory have also been demonstrated in other mammalian species , including monkeys ( e.g. , teng et al . , 2000 ; fernandez - ruiz et al . , 2001 ) and humans ( e.g. , martone et al . , 1984 ; heindel et al . , 1988 ; butters et al . , 1994 ; knowlton et al . , in view of the numerous studies supporting a multiple systems view of memory organization in the mammalian brain , there has been an increasing interest in examining various factors that might influence the relative use of these different systems . in this context we have conducted several experiments assessing the effects of  emotional arousal  produced by acute stress or drug - induced anxiety on the use of cognitive and habit memory systems . for example , in one study ( kim et al . , 2001 ) , rats were exposed to a pre - training stress regimen ( restraint stress and intermittent tail - shock ) and trained in a dual - solution water maze task in which a visible escape platform is always located in the same spatial location . training in this task appears to involve a parallel activation ( mcdonald and white , 1994 ) of hippocampus - dependent cognitive memory ( swim to the same spatial location ) and dorsal striatal - dependent habit memory ( swim to the visible cue ) . on a probe trial given after task acquisition , the use of these two learning strategies is assessed by moving the visibly cued platform to a new spatial location and observing whether rats continue to swim to the old location or , alternatively , continue to approach the visible cue . rats that were administered the pre - training stress - regimen acquired the task at a normal rate . however , on a probe trial 24 h after training , the previously stressed rats displayed a predominant use of habit learning , approaching the cued platform in its new location and showing significantly fewer visits to the old spatial location ( kim et al . these findings suggest that , in a task in which both hippocampus - dependent and dorsal striatal - dependent learning is adequate for acquisition , acute stress may bias rats toward the use of striatal - dependent habit memory . we have expanded on these findings by examining the effects of drug - induced anxiety on the relative use of multiple memory systems . accordingly , we selected doses of the -2 adrenoreceptor antagonists yohimbine and rs 79948197 that have been shown to possess anxiogenic properties in rats ( e.g. , handley and mithani , 1984 ; guy and gardner , 1985 ; white and birkle , 2001 ) . using a water maze version of the dual - solution plus - maze task described earlier , we observed that pre - training peripheral injections of either drug produced a robust use of response learning relative to place learning ( packard and wingard , 2004 ) . a similar bias toward the use of response learning in the dual - solution water maze task is also observed if the drugs are injected prior to memory retrieval ( i.e. , prior to the probe trial ) , and importantly the effects of pre - training or pre - retrieval injections of rs 79948197 are not due to state - dependency ( elliot and packard , 2008 ) . in addition , pre - training exposure to an ecologically valid stressor ( trimethlythiazoline , a component of fox feces odor ) also biases rats toward the use of dorsal - striatal response learning in the dual - solution water plus - maze task ( packard and carlin , unpublished data ) . taken together , experiments involving the administration of acute stress or anxiogenic drug injections in lower animals suggest that , at least in some learning situations , robust levels of emotional arousal may bias the brain toward the use of a habit memory system . interestingly , lower levels of trait anxiety have been recently shown to correlate positively with place recognition memory and with a preference for the use of hippocampus - dependent place learning in rats ( hawley et al . , 2011 ) . the studies reviewed above suggest that acute stress or peripheral anxiogenic drug injections may influence the relative use of multiple memory systems . however , they do not directly identify the neuroanatomical structure(s ) that confer the ability of emotional arousal to favor habit learning and memory . in this context there are at least two lines of evidence from animal studies that support the hypothesis that the bla may mediate a modulatory influence of emotional arousal on different memory systems . first , this brain structure has been historically linked to emotional behavior in mammals ( e.g. , kluver and bucy , 1939 ) , and in rats intra - bla injection of various drugs induces an anxiogenic behavioral and physiological profile ( e.g. , nagy et al . , 1979 ; scheel - kruger and petersen , 1982 ; sanders and shekhar , 1991 ) . second , decades of research has implicated the bla as a critical brain site for the memory modulatory effects of drugs that influence several transmitter systems , including those activated by emotional arousal ( for review see mcgaugh , 2004 ) . according to the hypothesis that the bla functions as a general  memory modulatory system , efferent projections of the bla influence the consolidation of memory in other brain structures . consistent with this idea , extensive evidence indicates that the bla modulates memory processes occurring in both the hippocampus and the dorsal striatum ( packard et al . , 1994 , 1996 ; packard and teather , 1998 ; roozendaal and mcgaugh , 1996 , 1997 ) . in order to examine whether the bias toward habit memory produced by peripheral anxiogenic drug administration may involve the bla , rats trained in the dual - solution plus - maze tasks received injections of rs 79948197 directly into this brain structure . on the later drug - free probe trial , these rats showed a significant use of response learning relative to control rats ( packard and wingard , 2004 ) . this finding indicates that intra - bla injections of rs 79948197 mimic the effects of peripheral administration of the drug . moreover , we subsequently demonstrated that the dose of rs 79948197 that produces this memory modulatory influence is also anxiogenic when injected into the bla ( wingard and packard , 2008 ) . one question raised by these findings in the dual - solution plus - maze task concerns whether intra - bla injections bias rats toward the use of habit learning by directly facilitating striatal - dependent response learning , or in a perhaps more indirect manner by impairing hippocampus - dependent place learning . in order to address this question , we trained rats in single - solution  versions of the water plus - maze task that required rats to use either response or place learning . in the single - solution plus - maze tasks , the start points used varied between north and south . in the response task , the spatial location of the escape platform varied equally across trials ( east or west ) and the same body turn response ( e.g. , left ) was consistently reinforced . in the place task , the escape platform was always located in the same spatial location ( e.g. , west ) and the body turn responses ( left and right ) were equally reinforced . separate groups of rats trained in these tasks and receiving post - training intra - bla injections of an anxiogenic dose of rs 79948197 displayed enhanced acquisition of the response learning task and impaired acquisition of the place learning task ( wingard and packard , 2008 ) . this behavioral profile is consistent with the hypothesis that the facilitation of response learning produced by the drug results from an impairing effect on hippocampus - dependent place learning . indeed , we have previously observed that post - training neural inactivation of the dorsal hippocampus also enhances response learning and impairs place learning . an interfering competitive action of the hippocampus during training in the single - solution response task presumably occurs because the spatial location of the escape platform varies across trials ( schroeder et al . , 2002 ; chang and gold , 2003 ; for a review on competition between multiple memory systems see poldrack and packard , 2003 ) . in a final recent set of experiments , we examined whether the bla is critical for the ability of rs 79948197 to both enhance response learning and impair place learning when the drug is administered peripherally . separate groups of rats trained in either the single - solution response or place tasks received post - training peripheral injection of an anxiogenic dose of rs 79948197 and concurrent intra - bla injections of the local anesthetic bupivacaine . in this study , both the enhancing and the impairing effect of peripheral administration of rs 79948197 on response and place learning , respectively , were blocked by neural inactivation of the bla ( packard and gabriele , 2009 ) . these findings provide compelling evidence for a critical role for the bla in mediating the influence of emotional arousal on different types of memory . in summary , lower animal studies indicate that peripheral or intra - bla administration of an anxiogenic drug(s ) can bias rats toward the use of dorsal - striatal habit memory and that the influence of the bla on the relative use of multiple memory systems appears to modulate the degree of interference between cognitive and habit memory . in the remaining discussion , we consider the extent to which stress may affect hippocampal and dorsal striatal morphology , whether emotional arousal may also influence the relative use of multiple memory systems in humans , and briefly describe possible implications of this hypothesis for understanding the role of learning and memory processes in various psychopathologies . when an organism senses a threat , stress hormones are released via the hpa axis and bear a significant and unequal impact upon various brain structures related to learning and memory ( for review see mcgaugh , 2002 ) . among these brain structures , the hippocampus and dorsal striatum appear to be differentially affected by stress . in rats , chronic stress causes atrophy of hippocampal neurons of the ca3 region ( watanabe et al . , 1992 ; magarios and mcewen , 1995 ) and hypertrophy in the dorsal striatum ( dias - ferreira et al . , 2009 ) . also , in a longitudinal mri study , rats subjected to 21 days of chronic restraint stress displayed a 3% reduction in hippocampal gray matter volume , an effect that was not observed in non - stressed controls ( lee et al . numerous studies have shown that people who have been exposed to trauma and developed post - traumatic stress disorder ( ptsd ) generally have smaller hippocampi than those who did not develop ptsd ( gilbertson et al . , 2002 ; bremner et al . , 2003 ; lindauer et al . , 2004 ; in addition , there is evidence that people with ptsd or obsessive - compulsive disorder ( ocd ) have enlarged caudate nuclei ( looi et al . , 2009 ; radua et al . , it is unclear whether chronic stress affects the relative size of these brain structures or if smaller hippocampi and larger caudate nuclei precede the development of these anxiety disorders . indeed , the nature of the relationship between hippocampal gray matter volume and ptsd has attracted considerable debate ( bremner , 2001 ; pitman , 2001 ) . in favor of a causal relationship , there is evidence that stressful life events can negatively affect hippocampal morphology in humans . using a longitudinal mri paradigm , researchers measured hippocampal gray matter volume at two time points , separated by a three month interval . the number of stressful life events experienced during the three - month interval was positively correlated with a reduction in gray matter volume of the right hippocampus ( papagni et al . , 2011 ) , suggesting that the relationship between ptsd and hippocampal volume may be causal . interestingly , a single stressful experience may be sufficient to affect the morphology of the hippocampal formation . several studies have shown that acute stress suppresses neurogenesis of progenitor cells in the dentate gyrus of rodents ( galea et al . , 1996 ; gould et al . , 1997 ; tanaput et al . , 2001 ) and non - human primates ( gould et al . , 1998 ) . moreover , in humans hydrocortisone administration decreased activation of the hippocampus in the retrieval phase of a declarative memory task ( oei et al . , 2007 ) . it is less clear whether acute or chronic stress affects the function of the dorsal striatum . however , evidence from human neuroimaging studies suggests that the dorsal striatum may play a role in the processing of negative stimuli . in healthy subjects , the dorsal striatum responds intensely when viewing unpleasant pictures , relative to positive or neutral pictures ( carretie et al . , in addition , in anxious subjects , dorsal striatal activation increases when reading negative words as opposed to positive or neutral ones ( roiser et al . , 2008 ) . lastly , subjects with huntington 's disease ( a disease associated with deterioration of the dorsal striatum ) are impaired in their ability to recognize negative facial expressions ( gray et al . , 1997 ; johnson et al . , 2007 ) as described earlier , numerous studies in rats suggest a dynamic impact of emotional arousal on hippocampus - dependent and dorsal striatal - dependent memory systems . specifically , acute stress and/or anxiety appears to bias rats to solve dual solution tasks by employing an s - r habit learning strategy , at the expense of the competing cognitive memory system ( packard and wingard , 2004 ; wingard and packard , 2008 ) . to the extent that the influence of emotional arousal on the hippocampus and dorsal striatum is similar in rodents and humans , it is possible that the effect of stress on the relative use of memory systems observed in rodents may also be observed in humans . indeed , recent studies building on the earlier research in lower animals indicate that both acute and chronic stress may also bias human subjects to implement an s - r habit learning strategy to solve a dual solution task . for example , schwabe et al . ( 2007 ) trained subjects to locate a over 12 trials , subjects could locate the win - card by using a spatial strategy ( i.e. , the card is always in the same spatial location ) or a stimulus - response strategy ( i.e. , the card is always next to the plant ) . on the 13th trial , the plant was moved to a different location which allowed the experimenters to assess the type of learning strategy used . prior to training , subjects were exposed to an acute stressor consisting of giving a speech and performing mental math in front of an audience . subjects in this stressed condition implemented an s - r strategy to locate the win - card significantly more often than controls . interestingly , high salivary cortisol at the time of learning predicted habit behavior in stressed and non - stressed conditions . in a subsequent study , subjects with higher scores on a chronic stress questionnaire implemented an s - r strategy in a 2-d dual solution task significantly more often than subjects with lower scores ( schwabe et al . , 2008a ) . therefore , similar to the effects of acute stress , chronic stress also appears to favor habit learning , at the expense of spatial learning , in humans . consistent with these findings , another study assessed the effect of emotional arousal on retention of the striatal - dependent weather prediction task ( steidl et al . , 2011 ) . for each trial in this task , subjects are asked to predict the weather based on a random set of cards depicting different abstract shapes . each possible combination of cards has a predetermined probability of signifying rain or sunshine , and after each prediction subjects are given feedback as to whether their prediction was correct or incorrect . in this particular study , in order to manipulate the level of emotional arousal , subjects were presented with either  arousing  or  neutral pictures during acquisition of the task . in a retention test given 1.5 months after training , subjects who had viewed the arousing pictures exhibited sustained memory for the task , whereas subjects who had viewed the neutral pictures exhibited considerable memory decay . previous studies using fmri or enlisting subjects with selective damage to the basal ganglia have indicated that the striatum has a central role in the initial acquisition of the weather prediction task ( knowlton et al . , therefore , it could be interpreted that heightened emotional arousal may have further enhanced the role of the striatum during training , thus strengthening the memory and improving performance in the retention test . however , it should be noted that the hippocampus is implicated in later performance of the weather prediction task ( knowlton et al . , 1994 , 1996 ) , suggesting that a hippocampus - dependent learning system may have also had a role in the enhanced retention ( steidl et al . , interestingly , some research has shown that stress promotes habit behavior in instrumental learning tasks as well . in one study ( schwabe and wolf , 2009 ) , human subjects were exposed to the socially - evaluated cold pressor test or a non - stressed control condition and then trained on two instrumental tasks , each associated with a distinct food outcome . after training , one of the food outcomes was devalued by feeding the subject with the food until satiation . in a subsequent extinction test , subjects exposed to pre - training stress continued performing the same instrumental response despite it being associated with the devalued food outcome ( i.e. , pre - training stress prompted habitual behavior ) . subjects unexposed to stress decreased the instrumental behavior associated with the devalued food outcome , suggesting the use of a more cognitive , goal - directed learning system . in a separate fmri study , it was observed that habit behavior in the food devaluation paradigm was associated with increased activation of the dorsal striatum , while goal - directed behavior was associated with increased activation of the ventromedial prefrontal cortex ( tricomi et al . , 2009 ) . therefore , stress - induced habit behavior in this task may also represent a shift to a dorsal striatal - dominant activation pattern . while behavioral stressors appear to bias humans toward the use of a habit memory system at the expense of the competing cognitive system , studies investigating the role of the human stress hormone cortisol seem to yield opposite results . for example , one study observed that low basal cortisol levels were associated with the use of an s - r habit strategy , whereas higher levels were associated with the use of a spatial strategy in a dual solution virtual maze task ( bohbot et al . however , subjects ' cortisol levels did not correlate with their scores on the perceived stress questionnaire , suggesting that sample cortisol levels in this study may not have reflected actual feelings of stress . the effect of stress on the hippocampus is typically described as following an inverted u - shape , with high and low levels of stress leading to impairments and a  middle  level being optimal for hippocampus function . considering that no subjects reported high stress , the authors suggest that the higher cortisol readings in this study may actually represent the middle of this inverted - u , thus favoring the use of a hippocampus - dependent memory system . another study investigating the effect of cortisol showed that orally administered hydrocortisone biased women to solve a dual solution task using a spatial strategy , at the expense of the habit system ( schwabe et al . interestingly , exogenous cortisol treatment was also associated with poorer performance in both spatial and response learners . to explain these results in the context of earlier work , the authors hypothesize that under low cortisol levels the hippocampus controls behavioral output . under moderate levels , hippocampus function declines and the dorsal striatum seizes control , and under high levels hippocampus and dorsal striatum function decline , but the balance between systems is restored and the hippocampus regains control . it is interesting to note a contrast between this explanation and the one proposed by bohbot et al . ( 2008b ) suggest that the hippocampus gains control at low and high cortisol levels , bohbot et al . the different methods used in these studies ( e.g. , behavioral tasks ; monitoring endogenous cortisol vs. manipulating exogenous cortisol ) may in part account for this discrepancy . in this context , it is worth noting that human fmri studies have established a negative correlation between cortisol levels and hippocampus activity ( oei et al . , 2007 ) . aside from a potential u - effect , there may be another explanation for the seemingly opposite effects of cortisol . using the instrumental food - devaluation paradigm discussed earlier , researchers found that only concurrent administration of the 2-noradrenergic receptor antagonist yohimbine and hydrocortisone promoted habit behavior in humans , whereas administration of hydrocortisone alone resulted in goal - directed , cognitive behavior ( schwabe et al . , 2010 ) . therefore , it appears that increases in both cortisol and norepinephrine may be required to induce a habit bias in dual solution tasks . this finding may explain why a behavioral stressor like the socially - evaluated cold pressor test , which increases plasma norepinephrine ( blandini et al . , 1992 ) and salivary cortisol ( schwabe et al . , 2008 ) , can induce a habit bias ( schwabe and wolf , 2009 ) , whereas a pharmacological increase in cortisol alone does not ( schwabe et al . in summary , both chronic and acute stress appears to bias humans to solve dual solution tasks with a habit memory system , consistent with the research in lower animals . however , when manipulating and monitoring cortisol levels , a more complex picture emerges , and the effect of stress on the relative use of memory systems may follow a u - shaped curve or depend on an interaction between both cortisol and norepinephrine . extensive research indicates that in some learning situations the hippocampus and dorsal striatum vie for control of behavioral output ( for review see poldrack and packard , 2003 ) . for example , in rats , lesions , or neural inactivation of the hippocampus can lead to enhanced acquisition of striatal - dependent habit tasks ( e.g. , packard et al . , 1989 ; packard and mcgaugh , 1992 ; schroeder et al . , 2002 ) . in view of evidence that high levels of stress can impair hippocampus - dependent learning in rats , it has been suggested that the stress or anxiety - induced shift to habit learning may result from the hippocampus relinquishing control and releasing the habit memory system from competition ( wingard and packard , 2008 ) . consistent with this idea , numerous human studies have shown that high levels of stress at the time of encoding or retrieval impair performance in hippocampus - dependent memory tasks ( schwabe et al . , 2009 ; merz et al . , 2010 ; schwabe and wolf , 2010 ; thomas et al . , 2010 ) . therefore , in dual solution tasks , it is possible that stressed individuals are more proficient in solving the task with their unimpaired habit system and thus , in order to preserve performance levels , implement an s - r strategy as opposed to a cognitive strategy . consistent with this interpretation , subjects with lower scores in episodic memory tasks were more likely to be response learners in a dual solution virtual maze task ( bohbot et al . , 2011 ) . it is important to note that stressed human subjects may be generally unaware of alternative strategy options ( e.g. , spatial ) for solving a given dual solution task ( schwabe et al . , 2007 ; schwabe and wolf , 2009 ) . therefore , it is unlikely that stressed individuals make a conscious decision to abstain from cognitive solutions and opt for a habit learning strategy . rather , stressed human subjects may rely on their habit system , simply because the available cognitive solutions go unnoticed . as mentioned earlier , stressful life events are associated with reduced gray matter volume of the right hippocampus in humans ( papagni et al . , 2011 ) . this finding may be relevant for understanding the effect of stress on memory systems , particularly as the relative size of the hippocampus and dorsal striatum may predict the learning strategy used . in one study ( bohbot et al . , 2007 ) , researchers utilized a virtual eight - arm radial maze that could be solved by associating distal cues ( e.g. , mountains , trees , etc . ) with the location of the correct maze arms ( i.e. , a spatial strategy ) or by memorizing the sequence of left and right arrow presses on the keyboard that lead to the correct maze arms ( i.e. , an s - r strategy ) . to determine which strategy a subject used , mri scans revealed that greater density in the hippocampus was positively correlated with the number of errors in the final probe trial ( suggesting the use of a spatial strategy ) and that greater density in the dorsal striatum was negatively correlated with the number of errors in the probe trial ( suggesting the use of an s - r strategy ) . chronic stress may potentially exert its influence on multiple memory systems by affecting the relative volume of the hippocampus and dorsal striatum . whether the more modest morphological changes induced by acute stress could underlie the habit bias remains to be determined . as previously described , several studies in lower animals indicate that the effects of emotional arousal on memory depend on the integrity of the bla ( for review see mcgaugh , 2004 ) and also implicate this brain region in orchestrating the use of multiple memory systems during periods of high emotional arousal ( e.g. , packard and wingard , 2004 ; wingard and packard , 2008 ; packard and gabriele , 2009 ) . interestingly , human fmri studies reveal that the degree of amygdala activation during encoding positively correlates with the recall of emotion - laden memories ( hamann et al . , 1999 ; however , to our knowledge , no studies have investigated the relationship between amygdala activation and the relativeuse of memory systems in humans . finally , human case studies reveal that acute or chronic anxiety may underlie the development and persistence of several psychopathologies with  habit - like  behavioral features , including for example ocd , post - traumatic stress disorder , and drug addiction . for instance , in ocd , the exaggerated fear of germs or infection may cause a person to  solve the problem  habitually , thus leading to excessive hand washing ( jones and menzies , 1998 ) . in post - traumatic stress , a significantly traumatic experience can lead to the development of non - context - specific cued recall of the memory . in this way , some aspects of ptsd may be analogous to the previously described studies showing a stress - induced facilitation of s - r habit ( or , cued ) learning and concomitant disregard for the spatial context of the learning environment ( schwabe et al . in addition , several studies have evidenced a relationship between acute stressors and relapse into habit - like drug seeking behavior in lower animals ( shaham and stewart , 1995 ; shepard et al . , 2004 ; buffalari and see , 2009 ) and humans ( kosten et al . , moreover , there is increasing evidence that the dorsal striatum plays an important role in the expression of drug - seeking behaviors in animals ( ito et al . , 2002 ; fuchs et al . , 2006 ; see et al . , 2007 ) and humans ( garavan et al . , thus , one might speculate that anxiety and/or stress may influence relapse in drug addiction by favoring the use of habit memory to guide the expression of maladaptive behaviors . further research examining the relationship between the neural bases of emotional arousal and the relative use of multiple memory systems may prove useful for understanding various human psychopathologies . the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest . the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .