Accueil Plasticity and signal representation in the auditory system. Partager Ajouter Me connecter M'inscrire. This is the fourth in a series of seminal meetings summarizing the state of development of auditory system neuroscience that has been organized in that great world city. Books that have resulted from these meetings represent important benchmarks for auditory neuroscience over the past 25 years. A meeting, "Neuronal Mechanisms of Hearing" hosted the most distinguished hearing researchers focusing on underlying brain processes from this era.
It resulted in a highly influential and widely subscribed and cited proceedings co-edited by professor Lindsay Aitkin. The subject of the meeting was the "Auditory Pathway - Structure and Function". It again resulted in another important update of hearing science research in a widely referenced book - edited by the late Bruce Masterton. While the original plan was to hold a meeting summarizing the state of auditory system neuroscience every 7 years, historical events connected with the disintegration of the Soviet Empire and return of freedom to Czechoslovakia resulted in an unavoidable delay of what was planned to be a meeting.
It wasn't until that we were able to meet for the third time in Prague, at that time to review "Acoustical Signal Processing in the Central Auditory System". Format Poche. Date de publication. Standard approaches are limited to measuring neural activity during conditioning trials.
Neuroscience , 1 , — Code Interne. Classical conditioning induces CS-specific receptive field plasticity in the auditory cortex of the guinea pig. Developmental plasticity of auditory cortical inhibitory synapses. If input is removed - such as when a limb is amputated - its somatotropic representation area in the sensory or motor cortex is not silenced; other sensorial receptors will stimulate central neurons that previously responded to the damaged receptors. Delete Cancel Save.
As noted previously, such studies generally found that responses to the CS are increased during pairing trials Fig. However, increased response to the CS could be due to an increased response to all stimuli in the dimension of the CS, e.
Receptive field analysis, using many stimuli along a dimension within a modality, would show increased responses across the dimension in question Fig. This would constitute general associative plasticity. On the other hand, increased responses to the CS could be due to an effect that was specific to the CS value. In this case, RF analysis would reveal maximal facilitation at the CS value with some combination of smaller increases, no changes, or decreases at other stimuli along the dimension. In toto, the result could be a shift of tuning to the value of the CS Fig.
This would constitute CS-specific associative plasticity. Receptive field analysis reveals whether learning-induced plasticity is general to the dimension of a conditioned stimulus or specific to the value of the CS. During training trials, one can determine whether or not responses to the CS changed; in this case, they increased.
However, both general and specific plasticity could produce this result. A general change revealed by receptive field analysis before and after conditioning. A CS-specific instance of associative representational plasticity, in which responses to many non-CS frequencies are reduced, producing a shift in tuning to the frequency of the CS. The importance of determining the degree of specificity of associative representational plasticity ARP is considerable. Four avenues of research will be mentioned here. First, a major advantage of receptive field analysis is that it provides for the study of specificity of neuronal plasticity.
For example, does the neural representation of a CS actually change in coding not merely in amplitude when it acquires a given association or set of associations? Given only responses to the CS before, during, and after training, it is impossible to determine the actual types of plastic changes induced by associative processes. RF or similar analysis resolves this issue, in much the same way that behavioral stimulus generalization gradients resolve the issue of the specificity of learning. Second, the nature of a change in representation can provide a means for investigating the relationships between representational plasticity and changes in behavior or behavior potential.
If both post-training receptive fields and generalization gradients were obtained within the same subjects, it would be possible to determine the relationship between the specificity of brain and behavior during learning in all sensory systems and at every level of the neuraxis brainstem, cerebellum, thalamus, cortex. Such analyses could be applied to future situations as well as immediate behavior.
For example, can the development of an ARP predict subsequent behavior in cases of behaviorally silent learning and in the future when past learning is used to solve a new problem? Third, research is currently dominated by studies of the processes necessary for the acquisition and maintenance of memory. However, all memories are about something: They have content.
The study of ARPs can be used to determine the contents of the neural substrates of memory. Associations are between and among specific elements, be they particular stimuli, responses, outcomes, contexts, or whatever. For example, are ARPs involved only in the acquisition of associations or are they as enduring as behaviorally validated memory?
Do ARPs exhibit actual consolidation after initial acquisition, become stronger over time, and less resistant to disruption? Do the forms of ARPs encode the details of associations and memories of their elements? Fourth, the study of ARPs can illuminate basic issues of associative processes. If so, to what extent are the neural substrates of effective occasion setters fundamentally the same or different from the associated elements?
Rather, two nonprimary fields A2 and ventral ectosylvian VE were studied, because it was assumed wrongly as it turned out that A1 would be less plastic, based on dominant beliefs in auditory physiology. Cats were trained in fear conditioning and developed associative pupillary dilation conditioned responses. Frequency RFs tuning functions obtained before and after conditioning revealed CS-specific plasticity: The maximum changes in response were at the CS frequency.
Both CS-specific increases and decreases were found. This ARP was retained unless subjects underwent standard extinction training, in which case, tuning returned toward or to baseline status Weinberger et al. Importantly, the sign of plasticity that developed during training trials was not necessarily the same as that which was evident in post-training RFs. This finding indicates that context can affect the expression of associative plasticity and has been attributed to performance factors during actual training trials, such as arousal and motivational factors caused by the presence of the unconditioned stimulus US Diamond and Weinberger Although such state effects cannot be eliminated during training, they can be eliminated when pre- and post-training RFs are obtained because, as explained above, RFs can be obtained with subjects in a planned, markedly different context that ameliorates or eliminates generalization of, e.
Generalization also can be prevented altogether by training subjects in the waking state but obtaining pre- and post-training RFs or frequency maps with subjects under deep general anesthesia Weinberger a. This may have reflected the fact that little was known about these auditory fields, which do not contain the fine-grain tonotopic organization found in A1.
Inquiry was thereafter directed to unit discharges in A1. The first such study involved classical fear conditioning in the adult guinea pig Bakin and Weinberger Subjects received a single brief training session of tone paired with shock. The CS frequency was selected to not be the pre-training best frequency, to allow for the detection of possible changes in frequency tuning. Subjects developed conditioned responses to the CS following a period of habituation to the CS presented alone.
Behavioral verification of associative learning in classical conditioning. Cardiac activity changes in heart rate to a tone are shown for two groups of guinea pigs. First, both groups received a tone unpaired with shock for 10 trials Sens , which resulted in an initial decrease in heart rate during the first block of five trials; this response was no longer present during the second block, perhaps indicating habituation to the tone. Subsequently, one group Condit received tone paired with shock, while the other Sens continued to receive tone and shock unpaired.
Conditioning produced cardiac deceleration CRs as soon as the first block of pairing, which continued to develop across trials. In contrast, the sensitization group showed no such growth of the CR. Immediately after training, neuronal RFs had shifted from the pre-training BF toward or all the way to the CS frequency so that it could become the new best frequency Fig.
Shifts were caused by a simultaneous increase in response to the CS frequency, while responses to the pre-training BF and many other frequencies decreased. These tuning shifts were only toward the CS frequency, so were not due to random variation. Also, responses to the CS frequency alone could develop even when a cell appeared to be nonresponsive to tones Fig. Classical conditioning produces tuning shifts. An example of a complete shift of frequency tuning of a single cell in A1 of the guinea pig from a pre-training best frequency BF of 0. Associative processes favor responses to the frequency of the CS in a variety of circumstances.
Single-unit recordings from A1 of the guinea pig. A Double-peaked tuning, with pre-training BFs at 5. The CS was selected to be 6. After conditioning 30 trials , responses to the CS frequency increased to become the peak of tuning. A cell that exhibited minimal or no response to tones before tuning developed tuning specifically to the CS frequency after conditioning 30 trials. It also developed in avoidance learning Bakin et al.
Tuning shifts are generally assessed at stimulus levels used for training, i. Tuning shifts to the CS frequency are not limited to aversive situations, as they have been found with rewarding brain stimulation as the US Kisley and Gerstein Representation of neuronal responses in A1 A before, B immediately after, and C 1 h after two-tone discrimination training. Displayed are rates of discharge y -axis as a function of tonal frequency x -axis and level of testing stimuli y -axis, 10—70 dB. The pre-training best frequency of Strikingly, consolidation, in the form of a continued development of these changes is evident.
ARPs are highly specific, consistently exhibiting increased responses only at or near the CS frequency across subjects, with decreased responses to lower and higher frequencies Fig. CS-specific increased responses and tuning shifts are associative, as they require pairing. Random or unpaired presentation of tone and shock produce increased responses across the frequency spectrum.
In fact, general increased responses develop in A1 whether sensitization training involves a tone with random shock or a flashing light with random shock, demonstrating that general increased responses across the frequency RF are truly arousal dependent, not due to auditory processing per se Fig. Habituation produces the opposite effect, i. Summary of the effects of A conditioning, B sensitization, and C habituation on frequency receptive fields in the primary auditory cortex of the guinea pig. Data are normalized to octave distance from the CS frequency A , the presensitization best frequency B or the repeated frequency C.
Note that conditioning produces a CS-specific increased response, whereas sensitization tone—shock or light—shock unpaired produces general increases across the spectrum. Habituation produces frequency-specific decreased response. It might be thought that arousal confounds during post-training testing are responsible for the specificity of increased responses to the CS frequency.
That might occur if presentation of this frequency, alone among many other frequencies, produced arousal due to prior conditioning. However, the acoustic context is very different. Direct measurement of the CR behavior revealed no response to the CS frequency when embedded in the post-training stimulus set e. Also, the latency of neuronal response to any tone is on the order of 10—50 msec, whereas the latency to detectable EEG activation is well over msec Weinberger and Lindsley Moreover, if the CS caused arousal, then the tone following the CS within a few seconds should also show an effect, which was not the case.
Associative processes were predicted to increase the area of representation of the A1 octave band containing the CS frequency, because the tonotopic map is comprised of preferred frequencies across cortical space, and tuning shifts should increase the number of loci at which the CS frequency is preferred Weinberger et al. This has been demonstrated in a study of the relationship between the level of CS-tonal importance and representational area.
Rats were trained to associate a 6. As maps cannot easily be obtained more than once within subjects, the pre-post trainingdesign was modified to obtain post-training maps in different groups. Controls received the same schedule of tone presentations, but were trained to bar press only in the presence of a visual stimulus.
Maps of the ACx showed an expanded representation for the frequency band centered on the CS. Further, the greater the level of behavioral importance, as indexed by the level of correct performance, the larger the percent of area tuned to the CS Figs. The control group failed to develop a change in tonotopic organization, not differing from naive subjects. Effect of learning tone-contingent bar-press for water on tonotopic map in A1. Trained rats received water reward for bar-presses in the presence of a 6.
Note that training greatly increased the area of representation for the frequency band containing the 6. Level of tone importance was controlled by the amount of water deprivation; asymptotic performance was significantly correlated with level of deprivation for details, see Rutkowski and Weinberger The area of representation of the frequency band containing the 6.
In summary, the initial line of research that combines auditory physiological determination of frequency tuning with standard conditioning training revealed that associative processes modify the representation of the conditioned stimulus. Specifically, they systematically alter A1 receptive fields such that responses to the CS are facilitated, while responses to other frequencies are generally decreased, enabling tuning shifts toward and to the CS frequency.
This CS-specific tuning plasticity has the major characteristics of associative memory and the amount of increased representation of the CS is proportional to its degree of motivational importance. Following the development of this line of inquiry, several major controversies arose. These include opposing claims regarding the form of receptive field plasticity, the interpretation of its functional significance, and its underlying neural mechanisms.
More specifically, I will suggest that the underlying problem is a failure to understand the critical importance of behavioral factors, including insufficient appreciation of the need to actually obtain behavioral evidence of learning, inadequate experimental designs, and lack of knowledge of the relevant behavioral literature. Ohl and Scheich , have claimed that associative learning in the gerbil induces decreased responses at the CS frequency, i. However, their data were obtained from a behavioral paradigm that has two shortcomings.
However, there is no prior evidence that animals can learn this highly complex discrimination, particularly in a single session. The investigators provided no behavioral evidence that their subjects had learned the discrimination. Indeed, such data may not have been attainable because they used extreme massing of intertrial intervals, which varied from 0. Second, they compared RFs obtained in a pre-training state of quiet with those from a different post-training state, one involving expectation of shock and possibly new learning. The post-training state must have been different from pre-training, because Ohl and Scheich used a design in which the same acoustic context i.
The latter would constitute some sort of extinction, i. That the pre- and post-training periods differed in expectation, probable state of arousal, or in other ways, precludes attributing post-training changes in neural responses to alleged discrimination learning. Ohl and Scheich not only argue that conditioning produces CS-specific decreased responses, but that prior observations of decreased responses have been ignored by this investigator in order to promote our claim of increased responses to the CS Ohl and Scheich , , , They are correct that decreased unit responses to the CS can be found e.
Ohl and Scheich make no distinction between plasticity that is observed during training trials from that which is observed after training trials. But, a central point of the unified design presented in this article is that it reduces or eliminates performance factors, such as state changes, that must occur during training trials when a reinforcer is introduced. It does so by use of pre- and post-training RF determinations and in a context different from the context of training trials. It is well known that performance factors severely limit the interpretation of behavioral data obtained during training trials, as opposed to after training trials Rescorla a , b.
The same strictures are no less important for determining the neural correlates of associative processes during training trials. A striking example of the Ohl and Scheich fallacy is that the sign of plasticity increase or decrease in response to the CS is often different during training from after training, as noted above Diamond and Weinberger The failure of Ohl and Scheich to appreciate the essential requirement that behavioral validation of learning is necessary to support a claim that learning has occurred does not invalidate the possibility of learning-induced decreased responses to the CS.
There well may be circumstances under which such decreases do develop. But neither should the lack of behavioral evidence be ignored, as the investigators continue to do Ohl and Scheich At this time, CS-specific increases, CS-directed tuning shifts, and CS-expansions of area in the tonotopic map remain the only behaviorally validated ARPs in the primary auditory cortex.
The Ohl and Scheich findings of CS-specific decreases in A1 do not challenge this conclusion because of the absence of behavioral verification of associative learning. If this is forthcoming, then the domain of associative representational plasticity in the primary auditory cortex will have been enlarged, and a next step would be to discover the superordinate rules that govern which types of ARPs are invoked by associative processes and their respective functional consequences. Interestingly, studies in the big brown bat Eptesicus fuscus have reported CS-specific tuning shifts during tone—shock pairing Suga and Ma However, they also lack behavioral validation of association, although, in this case, behavioral data were said to have been obtained.
The investigators presented sessions of 60 trials of tone—shock pairing and reported that limb flexion-conditioned responses developed starting at trial 50 Gao and Suga However, there is reason to suspect the claims of behavioral learning. First, these investigators have consistently failed to publish individual flexion records, learning functions, or any statistical report of the behavioral data.
Second, their previous study showed a failure of acquisition of flexion CRs with tone CSs and required many days of training to obtain consistent CRs using an effective white noise CS Riquimaroux et al. Third, the animals are under whole body restraint while being trained with shock, and access to the brain is obtained by drilling a hole in the skull while they are awake.
These procedures are likely to cause considerable stress and might increase limb movement during a session. Konorski has emphasized the problem of generalized body movement in such circumstances, which can produce apparent flexion CRs. Fourth, intertrial intervals were fixed at 30 sec. Therefore, any genuine CRs could have been caused by temporal conditioning, without any control by the tonal CS itself. Only data from the first session would be clearly interpretable, but data from all sessions and subjects were combined. There are also neurobiological reasons for caution. Suga and coworkers have relied on findings from cortical inactivations using muscimol.
However, radio-tracing studies of muscimol diffusion, using smaller doses than used in these studies, has revealed a considerably more extensive spread Edeline et al. Finally, Suga and colleagues found that repeated presentation of a tone alone produced increased response to and tuning shifts toward the repeated frequency Gao and Suga This is in direct opposition to the extensive literature on the effects of repeated auditory stimulation, which produces response decrements habituation in the auditory cortices in other taxa e.
Thus, despite the temptation to assume that associative tuning shifts develop in the big brown bat during the development of conditioned responses, the claims of Suga and colleagues must remain suspect in the absence of appropriate behavioral and neural controls. Perhaps as an echolocating animal, the principles of auditory system plasticity are specialized in the big brown bat, and thus, it is not a good model for general mammalian auditory associative plasticity. I conclude with a consideration of mechanisms that may be responsible for the development of associative representational plasticity, focusing on the cholinergic system because it has garnered most attention.
There is little controversy about the importance of acetylcholine ACh in auditory cortical and other cortical plasticity. The same cannot be said for the two extant models of ARP, which are in direct conflict on all issues except the role of acetylcholine. Neuromodulators have profound effects on neuronal functions. With respect to the auditory cortex, norepinephrine can alter frequency tuning Manunta and Edeline , , dopamine can increase the area of cortical representation of a tone with which it is paired Bao et al.
Acetylcholine has long been implicated in associative learning, e. While the direct application of cholinergic agonists and antagonists to the brain both in vitro or in vivo has provided important insights into the roles of ACh in the auditory system, plasticity, and learning, this approach is necessarily limiting with respect to associative processes in behaving subjects. A major obstacle is the difficulty of achieving precise timing when it is desirable to use a cholinergic agent as a US following presentation of an acoustic CS because of the uncontrolled time lag in diffusion of drugs applied to the brain.
Electrical stimulation of the NB does permit precise timing and such stimulation is known to release ACh in the cortex Rasmusson et al. Moreover, NB stimulation appears to be motivationally neutral Pennartz ; Miasnikov et al. In the first study of this type, a tone was paired with NB stimulation as a substitute for the US in one group of waking guinea pigs or presented unpaired in another group.
Frequency-tuning curves were obtained before and after a single training session of only 40 trials Bakin et al. Pairing induced ARP, i. A series of related studies replicated and extended CS-specific frequency tuning plasticity Kilgard and Merzenich a and also demonstrated that pairing NB stimulation with other acoustic stimuli, e.
However, all of these experiments presented hundreds of trials per day for periods of weeks, thus involving from about to 12, trials. Further, some studies lack controls for nonassociative processes Kilgard and Merzenich a , b. This training approach contrasts with the studies summarized above that mimic standard behavioral conditioning protocols, e. Therefore, the extent to which massed pairing of tone and NB stimulation sheds light on basic associative processes is unclear.
Inferences that such pairing produces specific, associative memory have to meet the dual criteria of associativity and specificity. The first criterion simply requires a nonpaired control group.
The criterion of specificity can be examined by obtaining stimulus generalization gradients, obtained when a subject is trained with one stimulus CS and later tested with many stimuli. If NB stimulation paired with a tone induces specific memory about that tone, then this CS frequency should later elicit the largest behavioral responses to all tones tested, i.
The first study involved overtraining using daily trials for 15 d trials. This is the same order of magnitude used by Merzenich and colleagues in studies of the induction of neuronal plasticity by stimulation of the NB above. However, the induction of specific associative behavioral memory by NB stimulation had never been demonstrated, and we assumed wrongly as it turned out that as NB stimulation was motivationally neutral, overtraining might be necessary to induce behavioral memory. The behavioral measures were changes in heart rate and in respiration. This appears to be the first description of the induction of specific associative memory by direct stimulation of the brain.
Overtraining proved unnecessary; a single session of trials has proven to be sufficient Miasnikov et al. The minimal amount of training has not yet been determined. The frequency generalization gradients are indistinguishable from those that develop in standard learning protocols using standard motivational reinforcers Mackintosh Thus, the behavior meets the same criteria as used to assess learning under normal circumstances. Respiration responses to post-training tone and generalization gradients, showing the induction of CS-specific memory after tone paired with stimulation of the nucleus basalis.
A Examples of individual respiration records with value of respiration change index, RCI to three frequencies 2, 6, and 12 kHz for one animal each from the paired and unpaired groups. Horizontal bar indicates tone duration. The maximal response left was at 6 kHz for the paired group, but not for the unpaired group.
The group difference function right shows a high degree of specificity of respiratory responses to 6 kHz. NB stimulation not only can induce specific, associative memory, but actually control the amount of detail in that memory. Previous studies of CS-specific memory induction above had used the moderate level. As in the past, the moderate paired group developed associative CS-specific memory; the only significant increased response was at the spectral range containing the CS frequency 6.
In contrast, the weak stimulation group developed only associative memory without any frequency specificity Fig. Additional experiments are needed to further characterize the contents or detail in such memories. The current results are consistent with the conclusion that the weak paired group learned that tone or sound per se had become important, whereas the moderate paired group had learned that the CS frequency and its neighbors had become important.
In the absence of a normal US, neither group would have learned why these stimuli were important Weinberger et al.
Level of NB stimulation controls specificity of induced memory. A Pre-training responses for subjects later trained with either paired or unpaired CS tone and NB stimulation. There were no differences between the groups. B Post-training responses for the Moderate NBs groups. Note the significant difference between the paired and unpaired groups, confined to the CS-band frequencies. This indicates that training with a moderate level of NBs produced memory that was both associative and CS specific.
C Comparisons of changes within the Moderate group post- minus pre-training responses to test tones. Note that the paired group had developed a significant increase to the CS-band frequencies only, while the unpaired group had developed a significant decrease, probably indicating frequency-specific habituation due to lack of pairing with NB stimulation. D Post-training responses for the Weak NBs groups. In contrast to the Moderate NBs group, pairing produced a significant difference in response across all test frequencies compared with its unpaired controls.
This indicates that training with weak NBs was sufficient to produce associative memory but insufficient to produce memory for frequency detail, i. E Comparisons of changes within the Weak NBs groups showed that the paired group did not develop absolute increased responses, but that the unpaired group did develop significant decreases in responses across the spectrum of test frequencies.
Thus, pairing the CS with weak NBs apparently prevented a habituatory decrement in the Weak paired group, which is evident in the Weak unpaired group. In summary, these findings support the view that the NB is normally engaged during associative S—S conditioning, promoting the establishment of specific cortical plasticity that itself may be a substrate for specific behavioral memory.
During normal motivated learning, both aspects of memory may be stored when a standard reward or punishment is present and the NB is activated. In the present studies lacking a standard US, behavioral importance can be induced in the absence of a full CS—US association. A preliminary model of CS-specific receptive field plasticity was proposed at the outset of this line of research Weinberger et al. Its goal was to outline the minimum circuitry sufficient to account for both the cortical plasticity and behavioral autonomic fear conditioning Fig.
The key ideas were 1 CS—US convergence occurs first in the nonlemniscal magnocellular medial geniculate nucleus MGm , where the resultant plasticity results in shifts of tuning that favors the CS frequency; 2 projection of this MGm plasticity to the amygdala; 3 projection of the effects of the plasticity i. This model incorporated many established findings such as the convergence of auditory and nociceptive somatosensory information in the MGm Wepsic ; Love and Scott , the necessity for an intact MGm region for auditory fear conditioning LeDoux et al.
The model of Weinberger and colleagues. This model hypothesizes the minimal circuitry that would be sufficient to account for short- and long-term associative representational plasticity and rapidly acquired conditioned autonomic responses. See text for details. This model was soon shown to be incorrect. We have not been able to formulate a reasonable revision of the model that explains both the source of the brief CS-specific plasticity in the MGv and the functional implications of this plasticity.
Regarding sources, it is certainly possible that cortico-geniculate connections could be responsible, and it is intriguing that cortical stimulation appears to facilitate MGv cells while inhibiting nonlemniscal medial geniculate neurons Yu et al. But, this in itself would not explain the specificity of CS-tuning plasticity in the MGv. Thus, even reasonable speculation needs more detailed knowledge of cortico-fugal transactions in the MGv He As for the functional implications, the transient plasticity in the MGv could be responsible in part for setting up the enduring plasticity in the cortex.
But, as for the model itself, two time scales would have to be reconciled, that of less than an hour in the MGv and that of months in the auditory cortex. Mere postulation of dual temporal mechanisms in the cortex would be little more than an expression of current ignorance. Thus, while several avenues of speculation are possible, rather than construct a tenuous and untestable modification of the model, we await further empirical developments. However, the model has been successful in several respects. For example, it predicted CS-specific tuning plasticity in the MGm Edeline and Weinberger , the ability of electrical stimulation of the MGm to substitute for the shock US in fear cardiac conditioning Cruikshank et al.
It also explains why lesions of the MGm block both associative plasticity that develops in the amygdala and elsewhere in the brain, and also impairs conditioning Poremba and Gabriel Additionally, imaging studies in humans have obtained results predicted by the model. For example, aversive two-tone discriminative conditioning produces CS-specific plasticity in the primary auditory cortex and associative changes only in the medial geniculate, amygdala, basal forebrain, and orbitofrontal cortex Morris et al.
These structures, except the orbitofrontal cortex, are the core of the model. Suga and colleagues have proposed an alternative model Suga and Ma It ignores the MGm completely but rather holds that the CS and US ascend to the auditory and somatosensory cortices, respectively, and are projected to association cortex, which is said to be the locus of CS—US convergence, and then to the amygdala. This model also includes the descending corticofugal auditory system and asserts that plasticity that develops in the association cortex is relayed via the amygdala to the inferior colliculus, which in turn promotes CS-specific plasticity in A1.
Finally, cortico-collicular projections are thought to induce CS-specific response increments in the inferior colliculus, thus forming a cortico-colliculo-cortico positive feedback loop.
The investigators suggest that this positive feedback loop is turned off by the thalamic reticular nucleus TRN , which prevents auditory information from reaching A1. Suga and colleagues do not specify how or when the TRN is engaged. Their formulation also seems to ignore the fact that the auditory cortex of conditioned animals continues to receive perfectly fine auditory information. The Suga model accepts the components of the Weinberger et al. Why Suga and colleagues ignore the MGm is baffling. It projects to Layer I and the apical dendrites of pyramidal cells in A1 and in all other auditory cortical fields Winer and Morest and does so via giant axons that provide the fastest thalamo-cortical transmission Huang and Winer In addition to findings listed above that implicate the MGm and its related posterior intralaminar nucleus [PIN] in associative learning, this structure is well known to develop associative plasticity during conditioning Gabriel et al.
Moreover, its stimulation induces in the auditory cortex heterosynaptic Weinberger et al. Another inexplicable feature of the Suga model is their claim that CS—US convergence occurs in the cerebral cortex. This is incompatible with an extensive literature which shows that neither the auditory cortex, nor indeed the cerebral cortex as a whole, are necessary for simple classical conditioning DiCara et al. In summary, the Suga model includes the descending auditory system, which is a potential advantage, as its function has been obscure. However, other novel features of this model ignore or are directly contradicted by prior neural and behavioral findings.
In particular, the assessment of sensory representations preceding and following standard associative learning tasks exemplified by classical conditioning reveals the extent to which resultant neural plasticity is general or specific to signal stimuli, i. The issue of specificity is key to the construct of representation. It is of course important to find that neural plasticity during learning is associative.
This type of conclusion has been with us since the dawn of neurophysiological studies of learning and is the starting point for further inquiry. Beyond associativity is the question of the actual nature of the plasticity, that is, of its essential details, which here are somewhat simplified to general vs.
The use of sensory neurophysiological assessment of receptive fields and functional maps is a first step in revealing the nature and extent to which neural representations undergo modification in learning and memory. In the future it should be possible to similarly interrogate areas other than primary sensory cortices as more is learned about the representations contained therein and the undoubtedly more complex stimulus sets that can probe them.
As discussed previously, many issues could be illuminated by determination of the specificity of plasticity. One not previously mentioned is to determine whether or not a neural change is actually associative. This may seem odd, as heretofore specificity has been treated as the next step beyond associativity. But examination of Figure 7 , A and B may prove revealing. In fact, prior workers who found increased auditory cortical evoked potentials both during pairing and in a variant of a sensitization protocol concluded that there was no genuine associative plasticity in A1 Hall and Mark ; Mark and Hall ; for a more detailed analysis, see Weinberger It is only by obtaining responses to other frequencies that the true picture emerges.
Pairing produces a specific increase at the CS frequency, whereas sensitization protocols produce a general increase in response across the spectrum. In short, as is universally known, but not always at the forefront of research, methods constrain possible results.
The studies on associative representational plasticity in A1 to date are consistent in reporting specific increased responses to conditioned stimuli and decreased responses to most other frequencies. These opposite changes are often sufficiently large to produce shifts of tuning toward or to the frequency of the CS.
Moreover, such favored treatment of the CS can produce an expansion of the auditory cortical area that is best tuning to the frequency band containing the CS frequency. However, it is important to note that while behaviorally verified learning to date is linked to ARPs that favor response to the CS value, ARPs ultimately may be found that have other forms. Ohl and Scheich might be proven to be correct, but such findings would not invalidate those reviewed in this study.
Despite the success of research to date, other possibilities must be entertained, lest inquiry become too narrowly focused. The findings so far have concerned measures of the magnitude of response, essentially reflecting the rate of neuronal discharge. Analyses of temporal patterns of neuronal response are less well-developed, but ultimately perhaps of equal importance.
Indeed, plasticity of temporal parameters of discharge can take place in the absence of any change in the overall rate of discharge Edeline Future studies can involve many variations of the unified experimental design emphasized in this review. The advantages and limitations of several have been presented elsewhere Weinberger a , b. Recently, Fritz and Shamma have devised an ingenious variation that permits assessment of spectrotemporal receptive fields on a trial-by-trial basis in attention, thus providing a finer temporal grain of the development of plasticity than heretofore available Fritz et al.
However, the fundamental approach remains the same: combining sensory neurophysiological assessments with learning and memory paradigms in the study of basic associative processes. There is currently general agreement that the nucleus basalis cholinergic innervation of the auditory cortex is sufficient to induce ARPs. This is a promising start. The future use of post-training pharmacological manipulations, particularly those targeted at specific times in structures of interest, should greatly clarify the sequence and chemical nature of neural processes that transform sensory experiences into enduring memories.
Expanded inquiry into the roles of modulators other than ACh is clearly necessary as well.