a key feature of the brain is its remarkable ability to form associations between coincident events , allowing organisms to learn from the past and thereby predict the future . when individual neurons are coincidently active they will promote a strengthening of synaptic connections . in 1949 , donald olding hebb ( 1949 ) proposed that if two neural signals enter the brain and simultaneously excite a post synaptic neuron , the strength of one or both of the synapses could increase such that , in the future , when only one of the signals enters the brain , that signal alone could trigger a response in the postsynaptic neuron . the later identification of associative long - term potentiation ( ltp ) has provided a cellular mechanism for hebb 's first postulate . the conceptual reduction of learning to three neurons has proved tremendously informative , and extensive evidence suggests that hebb 's first postulate is a likely mechanism of learning at the level of the individual neuron ( hebb , 1949 ) . ( a ) schematics of hebbian associative plasticity . cells that fire together wire together . from left to right : presynaptic cell ( black ) does not activate post synaptic cell when firing on own . when two presynaptic cells fire simultaneously post synaptic cell fires action potential . now when the same presynaptic cell fire on own ( red ) it now activates the post synaptic cell . ( b ) schematic of the second hebbian postulate : reverberating ensemble of neurons . from left to right : the network may reverberate when a sensory signal ( black arrow ) enters at one node in an interconnected neurons ensemble ( grey small arrows ) . this signal will remain active in the ensemble by moving from neuron to neuron within the network . the path of transmission will vary depending upon neurons history of activity , current synaptic strengths and other sensory signals entering the network . this reverberant activity can become coincident with new neural signals ( middle , black arrows ) , thereby facilitating  hebbian postulate one plasticity at some synapses in the network ( in red ) . while most work has emphasized coincidence detection at the level of individual neurons , coincidence detection likely also occurs between neurons within micro circuits . indeed , in addition to the idea that two coincidently active presynaptic neurons result in increased synaptic strength , hebb also postulated that associations are distributed across ensembles of neurons . however , in the brain of a behaving organism variable latencies of coincident external signals will result because of different sensory signals via separate afferent paths , as well as separate regional processing of sensory signals . to accommodate the variable latencies and still make associations between the externally coincident events , the brain uses such tools as oscillations and neuronal ensemble activity within microcircuits , both of which can facilitate coincidence detection ( douglas et al . , 1995 ; johnson and ledoux , 2004 ; johnson et al . , 2008 ; klausberger et al . , 2004 ; pare et al . , 2002 ; pelletier and pare , 2002 ; seidenbecher et al . hebb 's second postulate was extrapolated from lorente de no 's original idea that reverberating networks are a fundamental organizing principle of the brain ( hebb , 1949 ; sejnowski , 1999 ) . hebb argued that ensembles of neurons are a unit of temporary memory storage and coincidence facilitation . thus , information can actively reverberate within the ensemble of neurons in order to maintain its activity and to coordinate additional coincidence detection of different sensory signals ( douglas et al . , 1996 ; hebb , 1949 ; rao and sejnowski , 2001 ; sejnowski , 1999 ) . according to hebb , the ongoing neuron to neuron signaling may skip neurons and incorporate others as it cycles ( figure 1b ) . thus , according to hebbian logic , memory formation not only involves synaptic strengthening in feedforward circuits , it also involves reverberating signals in ensembles of neurons ( hebb , 1949 ) . we examined the lateral amygdala ( la ) a region known for associative memory formation for evidence of reverberating neuronal ensembles ( figure 2 ) . from there , it is sent to the lateral amygdala ( la ) either directly (  low road ) , or via a longer route , through the auditory cortices , for higher processing of the auditory signal , therefore providing the la with more detailed information (  high road ) . therefore , information processed through the high road ( blue ) reaches the la later than the direct thalamic processed information ( green ) . cells in the la are interconnected and provide a recurrent structure for possible reverberating activity in the la , facilitating coincidence detection between afferent information and intra - amygdala processing , thus enabling hebbian plasticity for storage of emotional memory traces in the la . emotional memory and especially classical fear conditioning ( also called pavlovian fear conditioning ) , is one of the most described associative memory systems in neurobiology ( fanselow and ledoux , 1999 ; ledoux , 2003 ; maren and quirk , 2004 ; sah et al . , conditioned fear has been observed in most animals from invertebrates to humans , and has been extensively studied in rodents ( fanselow and ledoux , 1999 ; ledoux , 2003 ; maren and quirk , 2004 ) . one input is an initially neutral stimulus , which becomes the conditioned stimulus ( cs ) . the other input is a biologically significant event , the unconditioned stimulus ( us ) . the pairing of these stimuli results in a conditioned response ( cr ) such that the initially neutral stimulus ( cs ) now elicits the same behavioral response as the us ( fanselow and ledoux , 1999 ; maren and quirk , 2004 ) . in auditory fear conditioning a neutral auditory tone is paired with an electric shock such that presentation of the auditory tone alone elicits fear responses . anatomically , the two auditory pathways converge in the la ( armony et al . , 1996 ; humeau et al . , 2005 ; romanski and ledoux , 1992 ) . the acoustic signal conveying the tone cs enters the la via the auditory thalamus and the auditory association cortex ( armony et al . , 1997 ; romanski et al . , both of these pathways enter the la , where they converge with somatosensory / nociceptive us signals , coming directly through the sensory thalamic complex via the spino - thalamic tract ( figure 2 ) ( ledoux , 2003 ; maren and quirk , 2004 ) . the two auditory routes are generally thought to provide different aspects of the cs to the la , with the thalamic input providing a rudimentary version of the cs (  low road , rapid ) and the cortical input (  high road , slow ) providing more details . while either the thalamo - la or cortico - la pathways is sufficient for conditioning to simple auditory stimuli , the cortical pathway appears to be required for learning about more complex stimuli ( ledoux , 2000 ; pare et al . , 2004 ) . divergent sensory information from the same event can be processed by parallel neural paths within the brain . the two auditory pathways to the la both synapse onto the same la principal neurons where they contact different types of dendritic spines ( humeau et al . , 2005 ) . the auditory input via the auditory cortex arrives some 30  ms later ( armony et al . , 1995 , 1998 ; ledoux , 2000 ; li et al . , 1996 ; quirk et al . , 1997 ) . for these two sensory signals to become rejoined in the la one has therefore to envisage a mechanism for the brain to sustain the activity triggered by the thalamic event for sufficient time to meet the cortically processed signal . one possible mechanism that would allow for the two temporally segregated sensory inputs to converge in time as well as in space is a recurrent network within the la . this network would allow for thalamo - la signals to feedback to the superior parts of the la during conditioning where they will meet incoming cortical signals ( johnson and ledoux , 2004 ; johnson et al . , 2008 ) . current associative plasticity models of conditioned fear rely on the concept of unidirectional information processing ( herry et al . , 2008 ; ledoux , 2003 ; maren and quirk , 2004 ; , 1995 , 1998 ; ledoux , 2000 ; li et al . , 1996 ; quirk et al . , 1997 moreover , current models do not generally incorporate the role of hebbian reverberation or recurrent circuits in associative plasticity during conditioned fear . in order to fully understand the contribution of the la network to conditioned fear behavior , both the structure of the la network and its functional behavior in relation to its sensory inputs needs to be known . recent work has shown the utility of field potentials in interpreting physiological data especially related to ensembles of neurons . these recent advances have included functional measures of the discrete volume of tissue recorded in the field potential ( katzner et al . , 2009 ; pesaran , 2009 ) , additionally , increasing evidence points to the potential advantages of the field potential over single unit activity in predicting both the fmri bold signal and importantly behavior ( goense and logothetis , 2008 ; mazzoni et al . field potentials have been used to study plasticity in the la but have not been used extensively to study la network activity ( figures 2 and 3 ) . field potentials ( in red ) evoked in the lateral amygdala ( la ) by stimulation of auditory afferents ( internal capsula in vitro , auditory thalamus in vivo ) showed multiple negative potentials both in vitro ( slice ) and in vivo ( awake rat ) . we developed a method for fitting and extracting peaks that represent reliable synchronized activity ( in black ) . five and nine peaks were isolated in the slice and the awake preparation , respectively . note that not all peaks are present in a given sample ( e g. here n5 is missing for the in vitro example , or n7 for the in vivo example ) . to enable comparison between the two preparations , latencies of each of the isolated peaks were normalized with respect to the latency of the first monosynaptic peak ( n1 ) . this transformation showed a high correlation between the two preparations ( r2  =  0.994 ; p  <  0.005 ) , suggesting similar intra - network rhythmicity in both preparations , and therefore similar network behavior . based on the logic that reliable activity across different animals would reflect aspects of the intrinsic la network structure we developed a method for the identification of regularity in the polysynaptic field potential peaks ( cruikshank et al . , 2002 ; johnson et al . , 2008 ; kudoh and shibuki , 1997 ; mitzdorf , 1985 ) we found that different network responses can be recorded at the same time from the lad - s and lad - i giving some measure of the localized nature of the networks recorded from . we identified both observable and hidden peaks ( figure 3 ) ( peaks summated within larger peaks , usually indicated by changes in negative slope ) from the field potential waveform using automated peak fitting . because any single stimulation may trigger events that could represent both random and reliable responses , we used a statistical method to extract temporally invariant peaks across animals . these techniques allowed us to study both the temporal structure and plasticity in localized la network ( figures 24 ) . ( a ) high - frequency stimulation ( hfs ) of thalamic afferents produces long - term potentiation ( ltp ) of the monosynaptic field potential response recorded in the superior part of the lad , a potentiation that lasted at least 90  min . at the same time , long - term depression ( ltd ) of the intra - la network was observed . this was evidenced by a depression of the monosynaptic field potential evoked in the superior part of lad ( lad - s ) in response to the stimulation of the inferior part of the lad ( lad - i ) ( b ) ; this was also observed as a general decrease in the amplitude of polysynaptic peaks ( filled red dots ) , representing ltd in the network activity ( c ) . thalamic afferents enter the lad from a dorsomedial direction and then apparently course in a ventral direction through the la ( doron and ledoux , 1999 , 2000 ; ledoux et al . this is the direction of neural activity and information flow considered in current models ( herry et al . , 2008 ; ledoux , 2003 ; maren and quirk , 2004 ; in contrast , in the network we have identified , neural activity appears to propagate in all directions throughout the lad . importantly this propagation also includes an apparent recurrent feed back from the inferior to superior parts of lad ( figure 2 ) . reverberating activity can be observed to last for up to 40  ms in vitro and in the awake rat apparent polysynaptic activity can be observed to continue beyond 240  ms . using a new method for extracting signatures of network activity from the extracellular field potential , we measured time - locked mono and polysynaptic activities as peaks in the field potential traces ( johnson et al . , 2008 ) . we named these peaks sequentially from the monosynaptic negative peak n1 to polysynaptic peaks n2 through n5 ( n1 to n9 in vivo ) ( figure 3 ) . we localized the origin of each of these peaks to either the superior or inferior lad ( lad - s , lad - i ) . upon stimulation of thalamic afferents in vitro , monosynaptic activity is detected first in the lad - s and then 3  ms later in the lad - i . after the initial monosynaptic activation of the lad , polysynaptic activity is seen throughout . in the lad - s the initial polysynaptic activity n2 is mediated by following n3 and n4 peaks appear to be local to the lad - s because trans - section of the lad - s and lad - i did not affect these peaks . finally , polysynaptic peak n5 , like n2 , is also derived from the lad - i because it is also lost after the mid la trans - section . as a confirmation , correlation of latencies from polysynaptic peaks obtained from the lad - s with and without lad - i trans - section reveals a best fit regression when peaks n2 and n5 are considered absent . the same pattern of polysynaptic excitatory events appears in vivo in the awake rat . upon stimulation of the medial geniculate nucleus of the thalamus , both mono and polysynaptic peaks the polysynaptic peaks are also negative peaks which follow the negative monosynaptic field excitatory post synaptic potential ( fepsp ) . the latency of the peaks is longer than that observed in the in vitro preparation . however , while the latencies are not identical the inter - peak relationship follows a matching pattern in the vivo and in vitro preparations . this is evident by the very close correlation between in vitro and in vivo patterns of peak latencies ( figure 3 ) . given the matching pattern of polysynaptic peaks in vivo and in vitro , at least for the first 93 and 39  ms respectively , one interpretation is that they may reflect the same or similar intrinsic network structures . the logic of this idea is that in vitro brain 400  m thick slice will contain large sections of the same network that exists in vivo . moreover , stimulating the same thalamo amygdala afferent pathways will trigger the same intrinsic network activity . since field potentials are thought to measure network activities within a 250  m radius ( katzner et al . , 2009 ; pesaran , 2009 ) , it is plausible that the network identified in 400  m thick slices in vitro is the same as the one identified in vivo . the temporal organization of the recurrent lad network is consistent with known cortico - amygdala latencies . an auditory tone , as used in conditioned fear , triggers in the la , first a fast monosynaptic response via thalamic afferents , followed by a later monosynaptic response via cortical afferents . our in vitro and in vivo data suggest that between the thalamic and cortical monosynaptic events , precisely timed recurrent activity reverberates within the lad . both the recurrent n2 and the lad - s local n3 are active within lad - s at the same time as the activity from the monosynaptic cortical input ( armony et al . this provides a mechanism for convergence across time of the thalamic and cortical sensory information . this converging information will include elements of the same original sound that is processed directly to la via the thalamus , as well as via the cortex . in addition , it is also possible that other sensory information , for example tone and light , as well as aspects of the tone and shock could also show temporal converge via the reverberating la network . thus , the reverberating and recurrent networks of the lad provide a mechanism to facilitate coincident interaction between cortical and subcortical information . first , the temporal coordination of sensory signals in reverberating ensembles facilitates convergence dependent up - regulation of synaptic strength ( seen as ltp ) . in contrast , reverberating ensembles may also facilitate the intra - network weakening of synaptic strength ( seen as ltd ) . both ltp and ltd are supposed to play a critical role in the storage of memory , by increasing contrast between strengthened and weakened synapses , synaptic weakening likely plays a critical role in networks by preventing saturation and refining the memory trace . in addition , as some synapses within the network are strengthened , the increased activity in the network could ultimately lead to run away excitation . ltp in the thalamic to la pathway leads to ltd in the lad - i to lad - s pathway ( figures 4a , b ) . if the polysynaptic network is the same recurrent network , changes in the polysynaptic peaks would also be predicted . induction of ltp in the thalamic to la pathway leads to ltd in the polysynaptic peaks . this plasticity within the reverberating la network provides further evidence the network may contain key features of hebbian reverberating ensembles . another feature of recurrent networks which has been studied more recently is that they appear to utilize nmda receptor mediated mechanisms to promote propagation of activity ( tegner et al . , 2002 ; interestingly , several key behaviors have been shown to be dependent upon nmda receptors , and therefore may be based on key aspects of recurrent reverberating network activity . for example , nmda receptors at recurrent ca3 to ca3 synapses have been shown to underlie feedback and are essential for the ca3 mediated pattern completion of spatial cues in a water maze task ( nakazawa et al . , 2002 ) . in addition , blockade of nmda receptors in the la disrupts second order fear conditioning ( gewirtz and davis , 1997 ) , a fear learning based on the association between two different css , after one cs has previously been associated with a us ( debiec et al . , 2006 ; it is conceivable that the blockade of the second order fear conditioning could be due to the disruption of la recurrent connectivity which is mediated in part by nmda receptors ( johnson and ledoux , 2004 ; johnson et al . , 2008 ) . hebb 's first postulate has been tremendously influential in the neurobiology of learning and memory . in contrast variables in the second postulate  reverberation and  recurrent network have been harder to define and measure . nonetheless the idea of memory formation being facilitated by reverberating networks may explain how temporally divergent but behaviorally relevant neural signals become linked . hebbian - like reverberating networks in the lad appear to be structured to facilitate coincident associations between temporally divergent cortical and subcortical sensory afferents . in addition , transmission of activity within the network is nmda receptor dependent and the synapses at each node in the network are plastic . these data raise important questions for future research on how network structure and behavior facilitate the encoding of conditioned fear memory traces ( johnson and ledoux , 2004 ; johnson et al . , 2008 ) . further studies are needed to specify the degree to which recurrent connectivity converges onto single neurons , and to characterize the precise rules that govern plasticity at network synapses . many structural and functional questions remain . how does this reverberant activity interact with oscillatory behavior to facilitate sensory and synaptic plasticity ? recent data suggest that the amygdala stores fear memories with much more sensory detail than once supposed ( debiec et al . , 2006 ; doyre et al . , 2007 ) which raises important questions about how detailed memory traces are encoded by la networks . for example , does the reverberatory activity participate in the refinement of the la memory trace ? does nociceptive information transmitted from the thalamus directly to the la converge in the la with auditory information from the auditory cortex ? does sensory information from the different senses also converge in the la and thereby undergo associative plasticity using the thalamic and cortical routes to the la ? while more research is needed on the functional behavior of hebbian ensembles , recent studies of la networks have opened a door into the next level of analysis of emotional memory ( johnson et al . , 2008 ) . 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 study of memory in most behavioral paradigms , including emotional memory paradigms , has focused on the feed forward components that underlie hebb 's first postulate , associative synaptic plasticity . 
 hebb 's second postulate argues that activated ensembles of neurons reverberate in order to provide temporal coordination of different neural signals , and thereby facilitate coincidence detection . 
 recent evidence from our groups has suggested that the lateral amygdala ( la ) contains recurrent microcircuits and that these may reverberate . additionally this reverberant activity is precisely timed with latencies that would facilitate coincidence detection between cortical and sub cortical afferents to the la . 
 thus , recent data at the microcircuit level in the amygdala provide some physiological evidence in support of the second hebbian postulate .