Specific long-term memory traces in primary auditory cortex

Adaptation in human visual cortex as a mechanism for rapid discrimination of aversive stimuli

The ability to react rapidly and efficiently to adverse stimuli is crucial for survival. Neuroscience and behavioral studies have converged to show that visual information associated with aversive content is processed quickly and accurately and is associated with rapid amplification of the neural responses. In particular, unpleasant visual information has repeatedly been shown to evoke increased cortical activity during early visual processing between 60 and 120 ms following the onset of a stimulus. However, the nature of these early responses is not well understood. Using neutral versus unpleasant colored pictures, the current report examines the time course of short-term changes in the human visual cortex when a subject is repeatedly exposed to simple grating stimuli in a classical conditioning paradigm. We analyzed changes in amplitude and synchrony of large-scale oscillatory activity across 2 days of testing, which included baseline measurements, 2 conditioning sessions, and a final extinction session. We found a gradual increase in amplitude and synchrony of very early cortical oscillations in the 20-35 Hz range across conditioning sessions, specifically for conditioned stimuli predicting aversive visual events. This increase for conditioned stimuli affected stimulus-locked cortical oscillations at a latency of around 60-90 ms and disappeared during extinction. Our findings suggest that reorganization of neural connectivity on the level of the visual cortex acts to optimize early perception of specific features indicative of emotional relevance.

Dual time scales for categorical decision making in auditory cortex

Category formation allows us to group perceptual objects into meaningful classes and is fundamental to cognition. Categories can be derived from similarity relationships of object features by using prototypes or multiple exemplars, or from abstract relationships of features and rules . A variety of brain areas have been implicated in categorization processes, but mechanistic insights on the single-cell and local-network level are still rare and limited to the matching of individual objects to categories . For directional categorization of tone steps, as in melody recognition , abstract relationships between sequential events (higher or lower in frequency) have to be formed. To explore the neuronal mechanisms of this categorical identification of step direction, we trained monkeys for more than two years on a contour-discrimination task with multiple tone sequences. In the auditory cortex of these highly trained monkeys, we identified two interrelated types of neuronal firing: Increased phasic responses to tones categorically represented the reward-predicting downward frequency steps and not upward steps; subsequently, slow modulations of tonic firing predicted the behavioral decisions of the monkeys, including errors. Our results on neuronal mechanisms of categorical stimulus identification and of decision making attribute a cognitive role to auditory cortex, in addition to its role in signal processing.

Neural representations during sleep: From sensory processing to memory traces

In the course of a day, the brain undergoes large-scale changes in functional modes, from attentive wakefulness to the deepest stage of sleep. The present paper evaluates how these state changes affect the neural bases of sensory and cognitive representations. Are organized neural representations still maintained during sleep? In other words, despite the absence of conscious awareness, do neuronal signals emitted during sleep contain information and have a functional relevance? Through a critical evaluation of the animal and human literature, neural representations at different levels of integration (from the most elementary sensory level to the most cognitive one) are reviewed. Recordings of neuronal activity in animals at presentation of neutral or significant stimuli show that some analysis of the external word remains possible during sleep, allowing recognition of behaviorally relevant stimuli. Event-related brain potentials in humans confirm the preservation of some sensory integration and discriminative capacity. Behavioral and neuroimaging studies in humans substantiate the notion that memory representations are reactivated and are reorganized during post-learning sleep; these reorganisations may account for the beneficial effects of sleep on behavioral performance. Electrophysiological results showing replay of neuronal sequences in animals are presented, and their relevance as neuronal correlates of memory reactivation is discussed. The reviewed literature provides converging evidence that structured neural representations can be activated during sleep. Which reorganizations unique to sleep benefit memory representations, and to what extent the operations still efficient in processing environmental information during sleep are similar to those underlying the non-conscious, automatic processing continually at work in wakefulness, are challenging questions open to investigation.

Auditory short-term memory behaves like visual short-term memory

Are the information processing steps that support short-term sensory memory common to all the senses? Systematic, psychophysical comparison requires identical experimental paradigms and comparable stimuli, which can be challenging to obtain across modalities. Participants performed a recognition memory task with auditory and visual stimuli that were comparable in complexity and in their neural representations at early stages of cortical processing. The visual stimuli were static and moving Gaussian-windowed, oriented, sinusoidal gratings (Gabor patches); the auditory stimuli were broadband sounds whose frequency content varied sinusoidally over time (moving ripples). Parallel effects on recognition memory were seen for number of items to be remembered, retention interval, and serial position. Further, regardless of modality, predicting an item's recognizability requires taking account of (1) the probe's similarity to the remembered list items (summed similarity), and (2) the similarity between the items in memory (inter-item homogeneity). A model incorporating both these factors gives a good fit to recognition memory data for auditory as well as visual stimuli. In addition, we present the first demonstration of the orthogonality of summed similarity and inter-item homogeneity effects. These data imply that auditory and visual representations undergo very similar transformations while they are encoded and retrieved from memory.

Unbalanced synaptic inhibition can create intensity-tuned auditory cortex neurons

Intensity-tuned auditory cortex neurons have spike rates that are nonmonotonic functions of sound intensity: their spike rate initially increases and peaks as sound intensity is increased, then decreases as sound intensity is further increased. They are either "unbalanced," receiving disproportionally large synaptic inhibition at high sound intensities; or "balanced," receiving intensity-tuned synaptic excitation and identically tuned synaptic inhibition which neither creates enhances nor creates intensity-tuning. It has remained unknown if the synaptic inhibition received by unbalanced neurons enhances intensity-tuning already present in the synaptic excitation, or if it creates intensity-tuning that is not present in the synaptic excitation. Here we show, using in vivo whole cell recordings in pentobarbital-anesthetized rats, that in some unbalanced intensity-tuned auditory cortex neurons synaptic inhibition enhances the intensity-tuning; while in others it actually creates the intensity-tuning. The lack of balance between synaptic excitation and inhibition was not always apparent in their peak amplitudes, but could sometimes be revealed only by considering their relative timing. Since synaptic inhibition is essentially cortical in origin, the unbalanced neurons in which inhibition creates intensity-tuning provide examples of auditory feature-selectivity arising de novo at the auditory cortex.

Developmental hearing loss eliminates long-term potentiation in the auditory cortex

Severe hearing loss during early development is associated with deficits in speech and language acquisition. Although functional studies have shown a deafness-induced alteration of synaptic strength, it is not known whether long-term synaptic plasticity depends on auditory experience. In this study, sensorineural hearing loss (SNHL) was induced surgically in developing gerbils at postnatal day 10, and excitatory synaptic plasticity was examined subsequently in a brain slice preparation that preserves the thalamorecipient auditory cortex. Extracellular stimuli were applied at layer 6 (L6), whereas evoked excitatory synaptic potentials (EPSPs) were recorded from L5 neurons by using a whole-cell current clamp configuration. In control neurons, the conditioning stimulation of L6 significantly altered EPSP amplitude for at least 1 h. Approximately half of neurons displayed long-term potentiation (LTP), whereas the other half displayed long-term depression (LTD). In contrast, SNHL neurons displayed only LTD after the conditioning stimulation of L6. Finally, the vast majority of neurons recorded from control prehearing animals (postnatal days 9-11) displayed LTD after L6 stimulation. Thus, normal auditory experience may be essential for the maturation of synaptic plasticity mechanisms.

The cognitive auditory cortex: task-specificity of stimulus representations

Auditory cortex (AC), like subcortical auditory nuclei, represents properties of auditory stimuli by spatiotemporal activation patterns across neurons. A tacit assumption of AC research has been that the multiplicity of functional maps in primary and secondary areas serves a refined continuation of subcortical stimulus processing, i.e. a parallel orderly analysis of distinct properties of a complex sound. This view, which was mainly derived from exposure to parametric sound variation, may not fully capture the essence of cortical processing. Neocortex, in spite of its parcellation into diverse sensory, motor, associative, and cognitive areas, exhibits a rather stereotyped local architecture. The columnar arrangement of the neocortex and the quantitatively dominant connectivity with numerous other cortical areas are two of its key features. This suggests that cortex has a rather common function which lies beyond those usually leading to the distinction of functional areas. We propose that task-relatedness of the way, how any information can be represented in cortex, is one general consequence of the architecture and corticocortical connectivity. Specifically, this hypothesis predicts different spatiotemporal representations of auditory stimuli when concepts and strategies how these stimuli are analysed do change. We will describe, in an exemplary fashion, cortical patterns of local field potentials in gerbil, of unit spiking activity in monkey, and of fMRI signals in human AC during the execution of different tasks mainly in the realm of category formation of sounds. We demonstrate that the representations reflect context- and memory-related, conceptual and executional aspects of a task and that they can predict the behavioural outcome.

Persistent responsiveness of long-latency auditory cortical activities in response to repeated stimuli of musical timbre and vowel sounds

Long-latency auditory-evoked magnetic field and potential show strong attenuation of N1m/N1 responses when an identical stimulus is presented repeatedly due to adaptation of auditory cortical neurons. This adaptation is weak in subsequently occurring P2m/P2 responses, being weaker for piano chords than single piano notes. The adaptation of P2m is more suppressed in musicians having long-term musical training than in nonmusicians, whereas the amplitude of P2 is enhanced preferentially in musicians as the spectral complexity of musical tones increases. To address the key issues of whether such high responsiveness of P2m/P2 responses to complex sounds is intrinsic and common to nonmusical sounds, we conducted a magnetoencephalographic study on participants who had no experience of musical training, using consecutive trains of piano and vowel sounds. The dipole moment of the P2m sources located in the auditory cortex indicated significantly suppressed adaptation in the right hemisphere both to piano and vowel sounds. Thus, the persistent responsiveness of the P2m activity may be inherent, not induced by intensive training, and common to spectrally complex sounds. The right hemisphere dominance of the responsiveness to musical and speech sounds suggests analysis of acoustic features of object sounds to be a significant function of P2m activity.

Auditory associative memory and representational plasticity in the primary auditory cortex

Historically, the primary auditory cortex has been largely ignored as a substrate of auditory memory, perhaps because studies of associative learning could not reveal the plasticity of receptive fields (RFs). The use of a unified experimental design, in which RFs are obtained before and after standard training (e.g., classical and instrumental conditioning) revealed associative representational plasticity, characterized by facilitation of responses to tonal conditioned stimuli (CSs) at the expense of other frequencies, producing CS-specific tuning shifts. Associative representational plasticity (ARP) possesses the major attributes of associative memory: it is highly specific, discriminative, rapidly acquired, consolidates over hours and days and can be retained indefinitely. The nucleus basalis cholinergic system is sufficient both for the induction of ARP and for the induction of specific auditory memory, including control of the amount of remembered acoustic details. Extant controversies regarding the form, function and neural substrates of ARP appear largely to reflect different assumptions, which are explicitly discussed. The view that the forms of plasticity are task dependent is supported by ongoing studies in which auditory learning involves CS-specific decreases in threshold or bandwidth without affecting frequency tuning. Future research needs to focus on the factors that determine ARP and their functions in hearing and in auditory memory.

Associative representational plasticity in the auditory cortex: a synthesis of two disciplines

Historically, sensory systems have been largely ignored as potential loci of information storage in the neurobiology of learning and memory. They continued to be relegated to the role of "sensory analyzers" despite consistent findings of associatively induced enhancement of responses in primary sensory cortices to behaviorally important signal stimuli, such as conditioned stimuli (CS), during classical conditioning. This disregard may have been promoted by the fact that the brain was interrogated using only one or two stimuli, e.g., a CS(+) sometimes with a CS(-), providing little insight into the specificity of neural plasticity. This review describes a novel approach that synthesizes the basic experimental designs of the experimental psychology of learning with that of sensory neurophysiology. By probing the brain with a large stimulus set before and after learning, this unified method has revealed that associative processes produce highly specific changes in the receptive fields of cells in the primary auditory cortex (A1). This associative representational plasticity (ARP) selectively facilitates responses to tonal CSs at the expense of other frequencies, producing tuning shifts toward and to the CS and expanded representation of CS frequencies in the tonotopic map of A1. ARPs have the major characteristics of associative memory: They are highly specific, discriminative, rapidly acquired, exhibit consolidation over hours and days, and can be retained indefinitely. Evidence to date suggests that ARPs encode the level of acquired behavioral importance of stimuli. The nucleus basalis cholinergic system is sufficient both for the induction of ARPs and the induction of specific auditory memory. Investigation of ARPs has attracted workers with diverse backgrounds, often resulting in behavioral approaches that yield data that are difficult to interpret. The advantages of studying associative representational plasticity are emphasized, as is the need for greater behavioral sophistication.

Enhanced anterior-temporal processing for complex tones in musicians

OBJECTIVE

To examine how auditory brain responses change with increased spectral complexity of sounds in musicians and non-musicians.

METHODS

Event-related potentials (ERPs) and fields (ERFs) to binaural piano tones were measured in musicians and non-musicians. The stimuli were C4 piano tones and a pure sine tone of the C4 fundamental frequency (f0). The first piano tone contained f0 and the first eight harmonics, the second piano tone consisted of f0 and the first two harmonics and the third piano tone consisted of f0.

RESULTS

Subtraction of ERPs of the piano tone with only the fundamental from ERPs of the harmonically rich piano tones yielded positive difference waves peaking at 130 ms (DP130) and 300 ms (DP300). The DP130 was larger in musicians than non-musicians and both waves were maximally recorded over the right anterior scalp. ERP source analysis indicated anterior temporal sources with greater strength in the right hemisphere for both waves. Arbitrarily using these anterior sources to analyze the MEG signals showed a DP130m in musicians but not in non-musicians.

CONCLUSIONS

Auditory responses in the anterior temporal cortex to complex musical tones are larger in musicians than non-musicians.

SIGNIFICANCE

Neural networks in the anterior temporal cortex are activated during the processing of complex sounds. Their greater activation in musicians may index either underlying cortical differences related to musical aptitude or cortical modification by acoustical training.

Perceptual-learning evidence for separate processing of asynchrony and order tasks

Normal perception depends, in part, on accurate judgments of the temporal relationships between sensory events. Two such relative-timing skills are the ability to detect stimulus asynchrony and to discriminate stimulus order. Here we investigated the neural processes contributing to the performance of auditory asynchrony and order tasks in humans, using a perceptual-learning paradigm. In each of two parallel experiments, we tested listeners on a pretest and a posttest consisting of auditory relative-timing conditions. Between these two tests, we trained a subset of listeners approximately 1 h/d for 6-8 d on a single relative-timing condition. The trained listeners practiced asynchrony detection in one experiment and order discrimination in the other. Both groups were trained at sound onset with tones at 0.25 and 4.0 kHz. The remaining listeners in each experiment, who served as controls, did not receive multihour training during the 8-10 d between the pretest and posttest. These controls improved even without intervening training, adding to evidence that a single session of exposure to perceptual tasks can yield learning. Most importantly, each of the two groups of trained listeners learned more on their respective trained conditions than controls, but this learning occurred only on the two trained conditions. Neither group of trained listeners generalized their learning to the other task (order or asynchrony), an untrained temporal position (sound offset), or untrained frequency pairs. Thus, it appears that multihour training on relative-timing skills affects task-specific neural circuits that are tuned to a given temporal position and combination of stimulus components.

NMDA-dependent facilitation of corticostriatal plasticity by the amygdala

Emotions generally improve memory, and the basolateral amygdala (BLA) is believed to mediate this effect. After emotional arousal, BLA neurons increase their firing rate, facilitating memory consolidation in BLA targets. The enhancing effects of BLA activity extend to various types of memories, including motor learning, which is thought to involve activity-dependent plasticity at corticostriatal synapses. However, the underlying mechanisms are unknown. Here we show that the NMDA-to-AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) ratio is nearly twice as high at BLA as compared with cortical synapses onto principal striatal neurons and that activation of BLA inputs greatly facilitates long-term potentiation induction at corticostriatal synapses. This facilitation was NMDA-dependent, but it occurred even when BLA and cortical stimuli were 0.5 s apart during long-term potentiation induction. Overall, these results suggest that BLA activity opens long time windows during which the induction of corticostriatal plasticity is facilitated.

Effects of nucleus basalis magnocellularis stimulation on a socially transmitted food preference and c-Fos expression

Experiment 1 examined the effects of electrical stimulation of nucleus basalis magnocellularis (NBM) on a relational odor-association task--the social transmission of food preference (STFP). Rats were stimulated unilaterally in the NBM for 20 min (100 microA, 1 Hz) immediately before the social training. They were tested on their ability to remember preference for the trained food either immediately or following a 24-h delay. Stimulation of NBM improved retention regardless of delay, and additional behavioral measures (social interaction, motor activity, or exploration) did not account for such effects. Experiment 2 investigated brain regions activated after NBM electrical stimulation by examining the induction of c-Fos. This treatment led to bilateral increased c-Fos expression in prefrontal regions, such as orbitofrontal, prelimbic, and infralimbic cortices, and some hippocampal subregions (dorsal CA and ventral dentate gyrus). In contrast, no differences between groups in c-Fos expression were found in basolateral amygdala, dorsal dentate gyrus, ventral CA, or ventral subiculum. Present findings indicate that pretraining NBM electrical stimulation facilitates the acquisition of STFP, supporting a role of NBM in the early stages of memory formation, and suggest that the treatment might cause such effects by inducing neural changes, related to transcription factors such as c-Fos, in the prefrontal cortex or the hippocampal formation.

The level of cholinergic nucleus basalis activation controls the specificity of auditory associative memory

Learning involves not only the establishment of memory per se, but also the specific details of its contents. In classical conditioning, the former concerns whether an association was learned while the latter discloses what was learned. The neural bases of associativity have been studied extensively while neural mechanisms of memory specificity have been neglected. Stimulation of the cholinergic nucleus basalis (NBs) paired with a preceding tone induces CS-specific associative memory. As different levels of acetylcholine may be released naturally during different learning situations, we asked whether the level of activation of the cholinergic neuromodulatory system can control the degree of detail that is encoded and retrieved. Adult male rats were tested pre- and post-training for behavioral responses (interruption of ongoing respiration) to tones of various frequencies (1-15 kHz, 70 dB, 2 s). Training consisted of 200 trials/day of tone (8.0 kHz, 70 dB, 2 s) either paired or unpaired with NBs (CS-NBs = 1.8 s) at moderate (65.7+/-9.0 microA, one day) or weak (46.7+/-12.1 microA, three training days) levels of stimulation, under conditions of controlled behavioral state (pre-trial stable respiration rate). Post-training (24 h) responses to tones revealed that moderate activation induced both associative and CS-specific behavioral memory, whereas weak activation produced associative memory lacking frequency specificity. The degree of memory specificity 24 h after training was positively correlated with the magnitude of CS-elicited increase in gamma activity within the EEG during training, but only in the moderate NBs group. Thus, a low level of acetylcholine released by the nucleus basalis during learning is sufficient to induce associativity whereas a higher level of release enables the storage of greater experiential detail. gamma waves, which are thought to reflect the coordinated activity of cortical cells, appear to index the encoding of CS detail. The findings demonstrate that the amount of detail in memory can be directly controlled by neural intervention.

Tracking short-term auditory cortical plasticity during classical conditioning using frequency-tagged stimuli

Animal studies indicate that short-term plasticity during classical conditioning is a fast process. The temporal details of this process in humans are unknown. We employed amplitude-modulated tones in order to elicit the steady-state field (SSF). Conditioned stimulus (CS+) and CS- had a common low carrier frequency, however, differed in their high-frequency component. Low and high frequencies within one tone were modulated at 29 and 45 Hz, respectively. Mean fast Fourier transformation analysis of each single trial allowed extraction of the cortical response to these modulation frequencies, allowing to track cortical responses trial by trial. Mutilation pictures were used as unconditioned stimulus. Furthermore, heart rate and contingency awareness were assessed. Our main findings are the following: 1) A rapid (within 5 trials) enhancement of the amplitude of the high frequencies in contrast to the low frequency, while the high frequencies differentiated later (toward end of acquisition). This partially replicates rapid plasticity as shown before in animals. 2) Those participants who were less aware of the stimulus contingencies showed a relative heart rate acceleration and greater SSF increase to the CS+. This could possibly imply a stronger early amygdala activation in these participants, which then mediates the development of conditioning-related reorganization in auditory cortical areas.

Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems

It is still a popular view that primary sensory cortices are unimodal, but recent physiological studies have shown that under certain behavioral conditions primary sensory cortices can also be activated by multiple other modalities. Here, we investigate the anatomical substrate, which may underlie multisensory processes at the level of the primary auditory cortex (field AI), and which may, in turn, enable AI to influence other sensory systems. We approached this issue by means of the axonal transport of the sensitive bidirectional neuronal tracer fluorescein-labeled dextran which was injected into AI of Mongolian gerbils (Meriones unguiculatus). Of the total number of retrogradely labeled cell bodies (i.e. cells of origin of direct projections to AI) found in non-auditory sensory and multisensory brain areas, approximately 40% were in cortical areas and 60% in subcortical structures. Of the cell bodies in the cortical areas about 82% were located in multisensory cortex, viz., the dorsoposterior and ventroposterior, posterior parietal cortex, the claustrum, and the endopiriform nucleus, 10% were located in the primary somatosensory cortex (hindlimb and trunk region), and 8% in secondary visual cortex. The cortical regions with retrogradely labeled cells also contained anterogradely labeled axons and their terminations, i.e. they are also target areas of direct projections from AI. In addition, the primary olfactory cortex was identified as a target area of projections from AI. The laminar pattern of corticocortical connections suggests that AI receives primarily cortical feedback-type inputs and projects in a feedforward manner to its target areas. Of the labeled cell bodies in the subcortical structures, approximately 90% were located in multisensory thalamic, 4% in visual thalamic, and 6% in multisensory lower brainstem structures. At subcortical levels, we observed a similar correspondence of retrogradely labeled cells and anterogradely labeled axons and terminals in visual (posterior limitans thalamic nucleus) and multisensory thalamic nuclei (dorsal and medial division of the medial geniculate body, suprageniculate nucleus, posterior thalamic cell group, zona incerta), and in the multisensory nucleus of the brachium of the inferior colliculus. Retrograde, but not anterograde, labeling was found in the multisensory pontine reticular formation, particularly in the reticulotegmental nucleus of the pons. Conversely, anterograde, but no retrograde, labeling was found in the visual laterodorsal and lateroposterior thalamic nuclei, in the multisensory peripeduncular, posterior intralaminar, and reticular thalamic nuclei, as well as in the multisensory superior and pericentral inferior colliculi (including cuneiform and sagulum nucleus), pontine nuclei, and periaqueductal gray. Our study supports the notion that AI is not merely involved in the analysis of auditory stimulus properties but also in processing of other sensory and multisensory information. Since AI is directly connected to other primary sensory cortices (viz. the somatosensory and olfactory ones) multisensory information is probably also processed in these cortices. This suggests more generally, that primary sensory cortices may not be unimodal.

Musical scale properties are automatically processed in the human auditory cortex

While listening to music, we immediately detect 'wrong' tones that do not match our expectations based on the prior context. This study aimed to determine whether such expectations can occur preattentively, as indexed by event-related potentials (ERPs), and whether these are modulated by attentional processes. To this end, we recorded ERPs in nonmusicians while they were presented with unfamiliar melodies, containing either a pitch deviating from the equal-tempered chromatic scale (out-of-tune) or a pitch deviating from the diatonic scale (out-of-key). ERPs were recorded in a passive experiment in which subjects were distracted from the sounds and in an active experiment in which they were judging how incongruous each melody was. In both the experiments, pitch incongruities elicited an early frontal negativity that was not modulated by attentional focus. This early negativity, closely corresponding to the mismatch negativity (MMN) of the ERPs, was mainly originated in the auditory cortex and occurred in response to both pitch violations but with larger amplitude for the more salient out-of-tune pitch than the less salient out-of-key pitch. Attentional processes leading to the conscious access of musical scale information were indexed by the late parietal positivity (resembling the P600 of the ERPs) elicited in response to both incongruous pitches in the active experiment only. Our results indicate that the relational properties of the musical scale are quickly and automatically extracted by the auditory cortex even before the intervention of focused attention.

Improved cortical entrainment to infant communication calls in mothers compared with virgin mice

There is a growing interest in the use of mice as a model system for species-specific communication. In particular, ultrasonic calls emitted by mouse pups communicate distress, and elicit a search and retrieval response from mothers. Behaviorally, mothers prefer and recognize these calls in two-alternative choice tests, in contrast to pup-naïve females that do not have experience with pups. Here, we explored whether one particular acoustic feature that defines these calls-- the repetition rate of calls within a bout-- is represented differently in the auditory cortex of these two animal groups. Multiunit recordings in anesthetized CBA/CaJ mice revealed that: (i) neural entrainment to repeated stimuli extended up to the natural pup call repetition rate (5 Hz) in mothers; but (ii) neurons in naïve females followed repeated stimuli well only at slower repetition rates; and (iii) entrained responses to repeated pup calls were less sensitive to natural pup call variability in mothers than in pup-naïve females. In the broader context, our data suggest that auditory cortical responses to communication sounds are plastic, and that communicative significance is correlated with an improved cortical representation.

Long-term potentiation in the amygdala: a cellular mechanism of fear learning and memory

Much of the research on long-term potentiation (LTP) is motivated by the question of whether changes in synaptic strength similar to LTP underlie learning and memory. Here we discuss findings from studies on fear conditioning, a form of associative learning whose neural circuitry is relatively well understood, that may be particularly suited for addressing this question. We first review the evidence suggesting that fear conditioning is mediated by changes in synaptic strength at sensory inputs to the lateral nucleus of the amygdala. We then discuss several outstanding questions that will be important for future research on the role of synaptic plasticity in fear learning. The results gained from these studies may shed light not only on fear conditioning, but may also help unravel more general cellular mechanisms of learning and memory.

Network architecture, receptive fields, and neuromodulation: computational and functional implications of cholinergic modulation in primary auditory cortex

Two fundamental issues in auditory cortical processing are the relative importance of thalamocortical versus intracortical circuits in shaping response properties in primary auditory cortex (ACx), and how the effects of neuromodulators on these circuits affect dynamic changes in network and receptive field properties that enhance signal processing and adaptive behavior. To investigate these issues, we developed a computational model of layers III and IV (LIII/IV) of AI, constrained by anatomical and physiological data. We focus on how the local and global cortical architecture shape receptive fields (RFs) of cortical cells and on how different well-established cholinergic effects on the cortical network reshape frequency-tuning properties of cells in ACx. We identify key thalamocortical and intracortical circuits that strongly affect tuning curves of model cortical neurons and are also sensitive to cholinergic modulation. We then study how differential cholinergic modulation of network parameters change the tuning properties of our model cells and propose two different mechanisms: one intracortical (involving muscarinic receptors) and one thalamocortical (involving nicotinic receptors), which may be involved in rapid plasticity in ACx, as recently reported in a study by Fritz and coworkers.

Neonatal nicotine exposure impairs nicotinic enhancement of central auditory processing and auditory learning in adult rats

Children of women who smoke cigarettes during pregnancy display cognitive deficits in the auditory-verbal domain. Clinical studies have implicated developmental exposure to nicotine, the main psychoactive ingredient of tobacco, as a probable cause of subsequent auditory deficits. To test for a causal link, we have developed an animal model to determine how neonatal nicotine exposure affects adult auditory function. In adult control rats, nicotine administered systemically (0.7 mg/kg, s.c.) enhanced the sensitivity to sound of neural responses recorded in primary auditory cortex. The effect was strongest in cortical layers 3 and 4, where there is a dense concentration of nicotinic acetylcholine receptors (nAChRs) that has been hypothesized to regulate thalamocortical inputs. In support of the hypothesis, microinjection into layer 4 of the nonspecific nAChR antagonist mecamylamine (10 microM) strongly reduced sound-evoked responses. In contrast to the effects of acute nicotine and mecamylamine in adult control animals, neither drug was as effective in adult animals that had been treated with 5 days of chronic nicotine exposure (CNE) shortly after birth. Neonatal CNE also impaired performance on an auditory-cued active avoidance task, while having little effect on basic auditory or motor functions. Thus, neonatal CNE impairs nicotinic regulation of cortical function, and auditory learning, in the adult. Our results provide evidence that developmental nicotine exposure is responsible for auditory-cognitive deficits in the offspring of women who smoke during pregnancy, and suggest a potential underlying mechanism, namely diminished function of cortical nAChRs.

Generalization of amygdala LTP and conditioned fear in the absence of presynaptic inhibition

Pavlovian fear conditioning, a simple form of associative learning, is thought to involve the induction of associative, NMDA receptor-dependent long-term potentiation (LTP) in the lateral amygdala. Using a combined genetic and electrophysiological approach, we show here that lack of a specific GABA(B) receptor subtype, GABA(B(1a,2)), unmasks a nonassociative, NMDA receptor-independent form of presynaptic LTP at cortico-amygdala afferents. Moreover, the level of presynaptic GABA(B(1a,2)) receptor activation, and hence the balance between associative and nonassociative forms of LTP, can be dynamically modulated by local inhibitory activity. At the behavioral level, genetic loss of GABA(B(1a)) results in a generalization of conditioned fear to nonconditioned stimuli. Our findings indicate that presynaptic inhibition through GABA(B(1a,2)) receptors serves as an activity-dependent constraint on the induction of homosynaptic plasticity, which may be important to prevent the generalization of conditioned fear.

Rapid induction of specific associative behavioral memory by stimulation of the nucleus basalis in the rat

Hypothesized circuitry enabling behavioral memory formation can be tested by its direct activation in the absence of normal experience. Neuromodulation via the cortical release of acetylcholine by the nucleus basalis (NB) is hypothesized to be sufficient to induce specific, associative behavioral memory. Previously, we found that tone paired with stimulation of the nucleus basalis (NBs) for 3000 trials over 15 days induced such memory, supporting the hypothesis. However, as standard associative memory can be established much more rapidly, we asked whether NB-induced memory develops rapidly. Adult male Sprague-Dawley rats, trained and tested in the same calm, waking state, were divided into Paired (n=5) and control (n=4) groups, each of which received a single session of 200 trials of an 8.0 kHz conditioned stimulus (CS) either paired with NBs or with unpaired presentation of NBs. Respiration, cardiac activity, and evoked potentials in the primary auditory cortex (ACx) were recorded. Memory and its degree of specificity were assessed 24 h later by presenting tones of various frequencies (1-15 kHz) in the absence of NBs to yield behavioral frequency generalization gradients. Behavioral responses to test tones consisted of interruption of ongoing respiration and changes in heart rate. Post-training behavioral generalization gradients exhibited response peaks centered on the CS frequency for the Paired group alone. Tone evoked potentials from the ACx also developed CS-specific plasticity. The findings indicate that NB induction of specific behavioral associative memory, like normal memory, can develop rapidly and is accompanied by specific cortical plasticity, supporting the view that NB engagement during normal learning produces memory.

Effects of classical conditioning on identification and cortical processing of speech syllables

The present study was designed to examine effects of learned motivational significance on processing of speech syllables in adults using a classical conditioning paradigm. Aversive white noise (unconditioned stimulus) was paired with two exemplars of /ba/ (conditioned stimulus, CS+) occurring near the category boundary of a 10-item /ba/-da/ continuum. Two corresponding /da/ syllables served as the CS- and indicated absence of white noise. High-density electroencephalogram (EEG) was recorded while participants passively listened to the stimuli. Prior to the EEG session and after intermittent conditioning, participants were asked to identify the syllables in a categorical perception task. Analysis of time-locked electrocortical data revealed amplitude modulations of the N2 (248-312 ms) component as a function of acquired stimulus properties (CS+ versus CS-). Over right hemisphere regions, negativity was specifically enhanced for the CS+ during intermittent conditioning. During extinction, this conditioning effect was paralleled by findings in the time-frequency domain, showing greater oscillatory activity in the gamma-band (25-40 Hz) range, 80-120 ms following onset of the CS+ compared with CS-. A similar pattern emerged in a later time segment of 400-600 ms (30-45 Hz). Aversive conditioning was not reflected in superior categorical perception performance. Our data indicate that electrocortical correlates of speech syllable perception are susceptible to changes induced by contingencies. Physiological differences were not manifest in behavioral advantage for a specific stimulus, i.e., the CS+, however. We conclude that training speech categorization by merely enhancing motivational relevance is not effective for conveying behavioral improvement.

Perceptual learning directs auditory cortical map reorganization through top-down influences

The primary sensory cortex is positioned at a confluence of bottom-up dedicated sensory inputs and top-down inputs related to higher-order sensory features, attentional state, and behavioral reinforcement. We tested whether topographic map plasticity in the adult primary auditory cortex and a secondary auditory area, the suprarhinal auditory field, was controlled by the statistics of bottom-up sensory inputs or by top-down task-dependent influences. Rats were trained to attend to independent parameters, either frequency or intensity, within an identical set of auditory stimuli, allowing us to vary task demands while holding the bottom-up sensory inputs constant. We observed a clear double-dissociation in map plasticity in both cortical fields. Rats trained to attend to frequency cues exhibited an expanded representation of the target frequency range within the tonotopic map but no change in sound intensity encoding compared with controls. Rats trained to attend to intensity cues expressed an increased proportion of nonmonotonic intensity response profiles preferentially tuned to the target intensity range but no change in tonotopic map organization relative to controls. The degree of topographic map plasticity within the task-relevant stimulus dimension was correlated with the degree of perceptual learning for rats in both tasks. These data suggest that enduring receptive field plasticity in the adult auditory cortex may be shaped by task-specific top-down inputs that interact with bottom-up sensory inputs and reinforcement-based neuromodulator release. Top-down inputs might confer the selectivity necessary to modify a single feature representation without affecting other spatially organized feature representations embedded within the same neural circuitry.

Food for thought: honeybee foraging, memory, and acetylcholine

Recent research implicates acetylcholine signaling through muscarinic receptors in structural changes that take place in the honeybee brain in response to foraging. These new findings are consistent with research from earlier studies implicating cholinergic signaling in associative learning as well as in the response to an enriched environment in mammals, which suggests that cholinergic signaling may play a critical role in learning and memory mechanisms across phyla.

PLASTICITY IN THE ADULT CENTRAL AUDITORY SYSTEM

The central auditory system retains into adulthood a remarkable capacity for plastic changes in the response characteristics of single neurons and the functional organization of groups of neurons. The most dramatic examples of this plasticity are provided by changes in frequency selectivity and organization as a consequence of either partial hearing loss or procedures that alter the significance of particular frequencies for the organism. Changes in temporal resolution are also seen as a consequence of altered experience. These forms of plasticity are likely to contribute to the improvements exhibited by cochlear implant users in the post-implantation period.

Prospective contributions of transgenic mouse models to central auditory research

Neuroscientists are increasingly embracing mice as a means to address central nervous system questions at a molecular level. Examples abound from sensory systems like olfaction and vision. The use of mice to study central auditory processing, however, has remained relatively limited. In this commentary, I draw on some of the successes from other fields to highlight directions in which mouse models may contribute valuable and otherwise unattainable insights into the neural circuitry and plasticity within central auditory stations. Efforts towards this are beginning and would benefit from increased collaboration to generate useful transgenic mouse models for such studies.

Translational Studies in Neurorehabilitation: From Bench to Bedside

Stroke is the leading cause of adult disability in the western world. Consensus has built over the last few years regarding the usefulness of training to improve motor disability resulting from stroke. Until recently, there were no accepted strategies to enhance the beneficial effects of training. However, the combination of basic and clinical science data over the last few years is changing this picture, and is highly relevant to the field of neurorehabilitation. Human studies in both healthy individuals and patients after brain damage demonstrate as a proof of principle that somatosensory input, cortical stimulation, interhemispheric interactions, and pharmacologic interventions can modulate cortical plasticity in neurorehabilitation after stroke. These findings strongly suggest directions in the development of novel strategies to enhance training effects on motor recovery. The intent of this review is to describe these strategies, the basic science principles on which they are based, and the clinical applications that have emerged so far.

Nonauditory events of a behavioral procedure activate auditory cortex of highly trained monkeys

A central tenet in brain research is that early sensory cortex is modality specific, and, only in exceptional cases, such as deaf and blind subjects or professional musicians, is influenced by other modalities. Here we describe extensive cross-modal activation in the auditory cortex of two monkeys while they performed a demanding auditory categorization task: after a cue light was turned on, monkeys could initiate a tone sequence by touching a bar and then earn a reward by releasing the bar on occurrence of a falling frequency contour in the sequence. In their primary auditory cortex and posterior belt areas, we found many acoustically responsive neurons whose firing was synchronized to the cue light or to the touch or release of the bar. Of 315 multiunits, 45 exhibited cue light-related firing, 194 exhibited firing that was related to bar touch, and 268 exhibited firing that was related to bar release. Among 60 single units, we found one neuron with cue light-related firing, 21 with bar touch-related firing, and 36 with release-related firing. This firing disappeared at individual sites when the monkeys performed a visual detection task. Our findings corroborate and extend recent findings on cross-modal activation in the auditory cortex and suggests that the auditory cortex can be activated by visual and somatosensory stimulation and by movements. We speculate that the multimodal corepresentation in the auditory cortex has arisen from the intensive practice of the subjects with the behavioral procedure and that it facilitates the performance of audiomotor tasks in proficient subjects.

Acetylcholine release in the hippocampus and striatum during place and response training

These experiments examined the release of acetylcholine in the hippocampus and striatum when rats were trained, within single sessions, on place or response versions of food-rewarded mazes. Microdialysis samples of extra-cellular fluid were collected from the hippocampus and striatum at 5-min increments before, during, and after training. These samples were later analyzed for ACh content using HPLC methods. In Experiment 1, ACh release in both the hippocampus and striatum increased during training on both the place and response tasks. The magnitude of increase of training-related ACh release in the striatum was greater in rats trained on the response task than in rats trained on the place task, while the magnitude of ACh release in the hippocampus was comparable in the two tasks. Experiment 2 tested the possibility that the hippocampus was engaged and participated in learning the response task, as well as the place task, because of the availability of extra-maze cues. Rats were trained on a response version of a maze under either cue-rich or cue-poor conditions. The findings indicate that ACh release in the hippocampus increased similarly under both cue conditions, but declined during training on the cue-poor condition, when spatial processing by the hippocampus would not be suitable for solving the maze. In addition, high baseline levels of ACh release in the hippocampus predicted rapid learning in the cue-rich condition and slow learning in the cue-poor condition. These findings suggest that ACh release in the hippocampus augments response learning when extra-maze cues can be used to solve the maze but impairs response learning when extra-maze cues are not available for use in solving the maze.

Improvement-related functional plasticity following pitch memory training

Functional activation patterns of an auditory working memory task were examined prior to and after 5 days of training (1 h/day). A control group with no training was scanned twice at the same intervals to assess test-retest effects. Based on behavioral improvement scores, the training group (n = 14) was divided into "Strong-Learners (SL)" and "Weak-Learners (WL)". No significant functional or structural brain differences were seen between the SL and WL groups prior to training. Imaging contrasts comparing post- with pre-training sessions showed a significant signal increase in the left Heschl's gyrus (HG) as well as in the left posterior superior temporal and supramarginal gyrus for the SL group, while the WL group showed significant signal increases in the left HG and anterior insular cortex as well as in a lingual-orbitofrontal-parahippocampal network. The test-retest analysis in the control group revealed only minimal signal increases in a right dorsolateral prefrontal region. A random effects analysis comparing the SL group with the WL group using the post- and pre-training contrast images showed increased activation only in the left supramarginal gyrus but not in HG. The importance of HG in pitch discrimination has been established in previous studies. The pitch memory component differentiated our task from a straight pitch discrimination task. It is most likely that the activation of the SMG reflects its importance in the short-term storage of auditory material, and it was this activation that best differentiated between subjects' levels of performance.

Induction of LTP in the human auditory cortex by sensory stimulation

High-frequency, repetitive, auditory stimulation was used to determine whether induction of a long-lasting increase of the human auditory evoked potential (AEP) was possible. Recording non-invasively with electroencephalogram scalp electrodes, stable increases in amplitude were observed in the N1 component of the AEP, which is thought to reflect activity within auditory cortex (N1). The increase was maintained over an hour and was shown to be independent of alterations in the state of arousal. This is the first demonstration of the induction of long-lasting plastic changes in AEPs, and suggest that this represents the first direct demonstration of long-term potentiation in the auditory cortex of normal, intact humans.

Consolidation of auditory cortex-dependent memory requires N-methyl-D-aspartate receptor activation

The pharmacological basis of sensory cortex-dependent learning and associated cortical reorganizations is only partially understood. In the Mongolian gerbil, the auditory cortex is critical for discriminating the directions of modulation of linearly frequency-modulated tones (FMs). To examine the role of N-methyl-D-aspartate (NMDA)-type glutamate receptors in FM discrimination learning, selective antagonists were used. Compared to vehicle-treated controls, both systemic administration of MK-801 before but not after training, and infusion of D-AP-5 into the auditory cortex after training caused retention deficits detectable 24h later. The amnesic actions were reversible and in a close temporal relation to memory formation. Acquisition performance and performance of an established FM discrimination reaction were not affected. These findings suggest that NMDA receptor activation is required for long-term memory consolidation in auditory cortex-dependent learning.

Stimulation of muscarinic receptors mimics experience-dependent plasticity in the honey bee brain

Honey bees begin life working in the hive. At approximately 3 weeks of age, they shift to visiting flowers to forage for pollen and nectar. Foraging is a complex task associated with enlargement of the mushroom bodies, a brain region important in insects for certain forms of learning and memory. We report here that foraging bees had a larger volume of mushroom body neuropil than did age-matched bees confined to the hive. This result indicates that direct experience of the world outside the hive causes mushroom body neuropil growth in bees. We also show that oral treatment of caged bees with pilocarpine, a muscarinic agonist, induced an increase in the volume of the neuropil similar to that seen after a week of foraging experience. Effects of pilocarpine were blocked by scopolamine, a muscarinic antagonist. Our results suggest that signaling in cholinergic pathways couples experience to structural brain plasticity.

Active listening: task-dependent plasticity of spectrotemporal receptive fields in primary auditory cortex

Listening is an active process in which attentive focus on salient acoustic features in auditory tasks can influence receptive field properties of cortical neurons. Recent studies showing rapid task-related changes in neuronal spectrotemporal receptive fields (STRFs) in primary auditory cortex of the behaving ferret are reviewed in the context of current research on cortical plasticity. Ferrets were trained on spectral tasks, including tone detection and two-tone discrimination, and on temporal tasks, including gap detection and click-rate discrimination. STRF changes could be measured on-line during task performance and occurred within minutes of task onset. During spectral tasks, there were specific spectral changes (enhanced response to tonal target frequency in tone detection and discrimination, suppressed response to tonal reference frequency in tone discrimination). However, only in the temporal tasks, the STRF was changed along the temporal dimension by sharpening temporal dynamics. In ferrets trained on multiple tasks, distinctive and task-specific STRF changes could be observed in the same cortical neurons in successive behavioral sessions. These results suggest that rapid task-related plasticity is an ongoing process that occurs at a network and single unit level as the animal switches between different tasks and dynamically adapts cortical STRFs in response to changing acoustic demands.

Spectral integration in auditory cortex: mechanisms and modulation

Auditory cortex contributes to the processing and perception of spectrotemporally complex stimuli. However, the mechanisms by which this is accomplished are not well understood. In this review, we examine evidence that single cortical neurons receive input covering much of the audible spectrum. We then propose an anatomical framework by which spectral information converges on single neurons in primary auditory cortex, via a combination of thalamocortical and intracortical "horizontal" pathways. By its nature, the framework confers sensitivity to specific, spectrotemporally complex stimuli. Finally, to address how spectral integration can be regulated, we show how one neuromodulator, acetylcholine, could act within the hypothesized framework to alter integration in single neurons. The results of these studies promote a cellular understanding of information processing in auditory cortex.

Structural Brain Changes in Tinnitus

Tinnitus is a common but poorly understood disorder characterized by ringing or buzzing in the ear. Central mechanisms must play a crucial role in generating this auditory phantom sensation as it persists in most cases after severing the auditory nerve. One hypothesis states that tinnitus is caused by a reorganization of tonotopic maps in the auditory cortex, which leads to an overrepresentation of tinnitus frequencies. Moreover, the participation of the limbic system in generating tinnitus has been postulated. Here we aimed at identifying brain areas that display structural change in tinnitus. We compared tinnitus sufferers with healthy controls by using high-resolution magnetic resonance imaging and voxel-based morphometry. Within the auditory pathways, we found gray-matter increases only at the thalamic level. Outside the auditory system, gray-matter decrease was found in the subcallosal region including the nucleus accumbens. Our results suggest that reciprocal involvement of both sensory and emotional areas are essential in the generation of tinnitus.

Modulation of P2 auditory-evoked responses by the spectral complexity of musical sounds

We investigated whether N1 and P2 auditory-evoked responses are modulated by the spectral complexity of musical sounds in pianists and non-musicians. Study participants were presented with three variants of a C4 piano tone equated for temporal envelope but differing in the number of harmonics contained in the stimulus. A fourth tone was a pure tone matched to the fundamental frequency of the piano tones. A simultaneous electroencephalographic/magnetoencephalographic recording was made. P2 amplitude was larger in musicians and increased with spectral complexity preferentially in this group, but N1 did not. The results suggest that P2 reflects the specific features of acoustic stimuli experienced during musical practice and point to functional differences in P2 and N1 that relate to their underlying mechanisms.

Learning-induced plasticity in animal and human auditory cortex

Recent data on learning-related changes in animal and human auditory cortex indicate functions beyond mere stimulus representation and simple recognition memory for stimuli. Rather, auditory cortex seems to process and represent stimuli in a task-dependent fashion. This implies plasticity in neural processing, which can be observed at the level of single neuron firing and the level of spatiotemporal activity patterns in cortical areas. Auditory cortex is a structure in which behaviorally relevant aspects of stimulus processing are highly developed because of the fugitive nature of auditory stimuli.

Modulation of the C1 visual event-related component by conditioned stimuli: evidence for sensory plasticity in early affective perception

Previous research has demonstrated optimized processing of motivationally significant stimuli early in perception. In the present study, the time course and underlying mechanisms for such fast differentiation are of interest. We investigated the involvement of the primary visual cortex in affective evaluation of conditioned stimuli (CSs). In order to elicit learning within the visual system we chose affective pictures as unconditioned stimuli and used laterally presented gratings as CSs. Using high-density electroencephalography, we demonstrated modulation of the C1 visual event-related component for threat-related stimuli versus neutral stimuli, which increased with continuing acquisition of affective meaning. The differentiation between aversive and neutral visual stimuli occurred as early as 65-90 ms after stimulus onset and suggested involvement of the primary visual areas in affective evaluation. As an underlying mechanism, we discuss short-term reorganization in visual cortex, enabling sensory amplification of specific visual features that are related to motivationally relevant information.

Are there critical periods for musical development?

A critical period can be defined as a developmental window during which specific experience has a greater effect than at other times. Musical behavior involves many skills, including the basic encoding of pitch and time information, understanding scale and harmonic structure, performance, interpretation, and composition. We review studies of genetics, behavior, and brain structure and function in conjunction with the experiences of auditory deprivation and musical enrichment, and conclude that there is more supporting evidence for critical periods for basic than for more complex aspects of musical pitch acquisition. Much remains unknown about the mechanisms of interaction between genetic and experiential factors that create critical periods, but it is clear that there are multiple pathways for achieving musical expertise.

Effects of Agonists and Antagonists of NMDA and ACh Receptors on Plasticity of Bat Auditory System Elicited by Fear Conditioning

In big brown bats, tone-specific plastic changes [best frequency (BF) shifts] of cortical and collicular neurons can be evoked by auditory fear conditioning, repetitive acoustic stimuli or cortical electric stimulation. It has been shown that acetylcholine (ACh) plays an important role in evoking large long-term cortical BF shifts. However, the role of N-methyl-d-aspartate (NMDA) receptors in evoking BF shifts has not yet been studied. We found 1) NMDA applied to the auditory cortex (AC) or inferior colliculus (IC) augmented the auditory responses, as ACh did, whereas 2-amino-5-phosphovalerate (APV), an antagonist of NMDA receptors, reduced the auditory responses, as atropine did; 2) although any of these four drugs did not evoke BF shifts, they influenced the development of the long-term cortical and short-term collicular BF shifts elicited by conditioning; 3) like ACh, NMDA augmented the cortical and collicular BF shifts regardless of whether it was applied to the AC or IC; 4) endogenous ACh of the AC and IC is necessary to produce the long-term cortical and short-term collicular BF shifts; 5) blockade of collicular NMDA receptors by APV abolished the development of the collicular BF shift and made the cortical BF shift small and short-term; 6) blockade of cortical NMDA receptors by APV reduced the cortical and collicular BF shifts and made the cortical BF shift short-term; and 7) conditioning with NMDA + atropine applied to the AC evoked the small, short-term cortical BF shift, whereas conditioning with APV + ACh applied to the AC evoked the small, but long-term cortical BF shift.

Long-Lasting Modulation by Stimulus Context in Primate Auditory Cortex

A sound embedded in an acoustic stream cannot be unambiguously segmented and identified without reference to its stimulus context. To understand the role of stimulus context in cortical processing, we investigated the responses of auditory cortical neurons to 2-sound sequences in awake marmosets, with a focus on stimulus properties other than carrier frequency. Both suppressive and facilitatory modulations of cortical responses were observed by using combinations of modulated tone and noise stimuli. The main findings are as follows. 1) Preceding stimuli could suppress or facilitate responses to succeeding stimuli for durations >1 s. These long-lasting effects were dependent on the duration, sound level, and modulation parameters of the preceding stimulus, in addition to the carrier frequency. They occurred regardless of whether the 2 stimuli were separated by a silent interval. 2) Suppression was often tuned such that preceding stimuli whose parameters were similar to succeeding stimuli produced the strongest suppression. However, the responses of many units could be suppressed, although often weaker, even when the 2 stimuli were dissimilar. In some cases, only a dissimilar preceding stimulus produced suppression in the responses to the succeeding stimulus. 3) In contrast to suppression, facilitation of responses to succeeding stimuli by the preceding stimulus was usually strongest when the 2 stimuli were dissimilar. 4) There was no clear correlation between the firing rate evoked by the preceding stimulus and the change in the firing rate evoked by the succeeding stimulus, indicating that the observed suppression was not simply a result of habituation or spike adaptation. These results demonstrate that persistent modulations of the responses of an auditory cortical neuron to a given stimulus can be induced by preceding stimuli. Decreases or increases of responses to the succeeding stimuli are dependent on the spectral, temporal, and intensity properties of the preceding stimulus. This indicates that cortical auditory responses to a sound are not static, but instead depend on the stimulus context in a stimulus-specific manner. The long-lasting impact of stimulus context and the prevalence of facilitation suggest that such cortical response properties are important for auditory processing beyond forward masking, such as for auditory streaming and segregation.

Multiple time scales of adaptation in auditory cortex neurons

Neurons in primary auditory cortex (A1) of cats show strong stimulus-specific adaptation (SSA). In probabilistic settings, in which one stimulus is common and another is rare, responses to common sounds adapt more strongly than responses to rare sounds. This SSA could be a correlate of auditory sensory memory at the level of single A1 neurons. Here we studied adaptation in A1 neurons, using three different probabilistic designs. We showed that SSA has several time scales concurrently, spanning many orders of magnitude, from hundreds of milliseconds to tens of seconds. Similar time scales are known for the auditory memory span of humans, as measured both psychophysically and using evoked potentials. A simple model, with linear dependence on both short-term and long-term stimulus history, provided a good fit to A1 responses. Auditory thalamus neurons did not show SSA, and their responses were poorly fitted by the same model. In addition, SSA increased the proportion of failures in the responses of A1 neurons to the adapting stimulus. Finally, SSA caused a bias in the neuronal responses to unbiased stimuli, enhancing the responses to eccentric stimuli. Therefore, we propose that a major function of SSA in A1 neurons is to encode auditory sensory memory on multiple time scales. This SSA might play a role in stream segregation and in binding of auditory objects over many time scales, a property that is crucial for processing of natural auditory scenes in cats and of speech and music in humans.

Unraveling the attentional functions of cortical cholinergic inputs: interactions between signal-driven and cognitive modulation of signal detection

Neurophysiological studies demonstrated that increases in cholinergic transmission in sensory areas enhance the cortical processing of thalamic inputs. Cholinergic activity also suppresses the retrieval of internal associations, thereby further promoting sensory input processing. Behavioral studies documented the role of cortical cholinergic inputs in attentional functions and capacities by demonstrating, for example, that the integrity of the cortical cholinergic input system is necessary for attentional performance, and that the activity of cortical cholinergic inputs is selectively enhanced during attentional performance. This review aims at integrating the neurophysiological and behavioral evidence on the functions of cortical cholinergic inputs and hypothesizes that the cortical cholinergic input system generally acts to optimize the processing of signals in attention-demanding contexts. Such signals 'recruit', via activation of basal forebrain corticopetal cholinergic projections, the cortical attention systems and thereby amplify the processing of attention-demanding signals (termed 'signal-driven cholinergic modulation of detection'). The activity of corticopetal cholinergic projections is also modulated by direct prefrontal projections to the basal forebrain and, indirectly, to cholinergic terminals elsewhere in the cortex; thus, cortical cholinergic inputs are also involved in the mediation of top-down effects, such as the knowledge-based augmentation of detection (see Footnote 1) of signals and the filtering of irrelevant information (termed 'cognitive cholinergic modulation of detection'). Thus, depending on the quality of signals and task characteristics, cortical cholinergic activity reflects the combined effects of signal-driven and cognitive modulation of detection. This hypothesis begins to explain signal intensity or duration-dependent performance in attention tasks, the distinct effects of cortex-wide versus prefrontal cholinergic deafferentation on attention performance, and it generates specific predictions concerning cortical acetylcholine (ACh) release in attention task-performing animals. Finally, the consequences of abnormalities in the regulation of cortical cholinergic inputs for the manifestation of the symptoms of major neuropsychiatric disorders are conceptualized in terms of dysregulation in the signal-driven and cognitive cholinergic modulation of detection processes.

Choline transporters, cholinergic transmission and cognition

Cholinergic projections to the cortex and hippocampus mediate fundamental cognitive processes. The capacity of the high-affinity choline uptake transporter (CHT) to import choline from the extracellular space to presynaptic terminals is essential for normal acetylcholine synthesis and therefore cholinergic transmission. The CHT is highly regulated, and the cellular mechanisms that modulate its capacity show considerable plasticity. Recent evidence links changes in CHT capacity with the ability to perform tasks that tax attentional processes and capacities. Abnormal regulation of CHT capacity might contribute to the cognitive impairments that are associated with neurodegenerative and neuropsychiatric disorders. Therefore, the CHT might represent a productive target for the development of new pharmacological treatments for these conditions.

Coordination of multiple memory systems

On the basis of lesions of different brain areas, several neural systems appear to be important for processing information regarding different types of learning and memory. This paper examines the development of pharmacological and neurochemical approaches to multiple memory systems from past studies of modulation of memory formation. The findings suggest that peripheral neuroendocrine mechanisms that regulate memory processing may target their actions toward those neural systems most engaged in the processing of learning and memory. In addition, measurements of acetylcholine release in different memory systems reveals extensive interactions between memory systems, some cooperative and some competitive. These results imply that many neural systems, often characterized as relatively independent, may in fact interact extensively, blurring the dependencies of different memory tasks on specific neural systems.