Stimulus-specific adaptation in the inferior colliculus of the anesthetized rat

Adaptation to stimulus statistics in the perception and neural representation of auditory space

Sensory systems are known to adapt their coding strategies to the statistics of their environment, but little is still known about the perceptual implications of such adjustments. We investigated how auditory spatial processing adapts to stimulus statistics by presenting human listeners and anesthetized ferrets with noise sequences in which interaural level differences (ILD) rapidly fluctuated according to a Gaussian distribution. The mean of the distribution biased the perceived laterality of a subsequent stimulus, whereas the distribution's variance changed the listeners' spatial sensitivity. The responses of neurons in the inferior colliculus changed in line with these perceptual phenomena. Their ILD preference adjusted to match the stimulus distribution mean, resulting in large shifts in rate-ILD functions, while their gain adapted to the stimulus variance, producing pronounced changes in neural sensitivity. Our findings suggest that processing of auditory space is geared toward emphasizing relative spatial differences rather than the accurate representation of absolute position.

On the importance of anchoring and the consequences of its impairment in dyslexia

One of the main impediments of individuals with reading difficulties and individuals with language difficulties is poor working memory. Typically measured using verbal stimuli, working memory deficits have often been considered as one aspect of the phonological difficulty putatively underlying dyslexia. Over the years it has been shown that a broad range of auditory discrimination abilities are also mildly impaired. Here we present evidence that a domain general, rather than a phonology specific, deficits in the ability to implicitly use contextual information, which we term anchoring, can account for both types of deficits. We propose that anchoring ability, which reflects a basic biological mechanism for replacing effortful mechanisms of explicit working memory with automatic mechanisms of implicit memory, and consequently boost performance in both perceptual and cognitive tasks, is a crucial factor in our ability to be expert users of oral and written language.

Neural adaptation to tone sequences in the songbird forebrain: patterns, determinants, and relation to the build-up of auditory streaming

Neural responses to tones in the mammalian primary auditory cortex (A1) exhibit adaptation over the course of several seconds. Important questions remain about the taxonomic distribution of multi-second adaptation and its possible roles in hearing. It has been hypothesized that neural adaptation could explain the gradual "build-up" of auditory stream segregation. We investigated the influence of several stimulus-related factors on neural adaptation in the avian homologue of mammalian A1 (field L2) in starlings (Sturnus vulgaris). We presented awake birds with sequences of repeated triplets of two interleaved tones (ABA-ABA-...) in which we varied the frequency separation between the A and B tones (DeltaF), the stimulus onset asynchrony (time from tone onset to onset within a triplet), and tone duration. We found that stimulus onset asynchrony generally had larger effects on adaptation compared with DeltaF and tone duration over the parameter range tested. Using a simple model, we show how time-dependent changes in neural responses can be transformed into neurometric functions that make testable predictions about the dependence of the build-up of stream segregation on various spectral and temporal stimulus properties.

Interactions between stimulus-specific adaptation and visual auditory integration in the forebrain of the barn owl

Neural adaptation and visual auditory integration are two well studied and common phenomena in the brain, yet little is known about the interaction between them. In the present study, we investigated a visual forebrain area in barn owls, the entopallium (E), which has been shown recently to encompass auditory responses as well. Responses of neurons to sequences of visual, auditory, and bimodal (visual and auditory together) events were analyzed. Sequences comprised two stimuli, one with a low probability of occurrence and the other with a high probability. Neurons in the E tended to respond more strongly to low probability visual stimuli than to high probability stimuli. Such a phenomenon is known as stimulus-specific adaptation (SSA) and is considered to be a neural correlate of change detection. Responses to the corresponding auditory sequences did not reveal an equivalent tendency. Interestingly, however, SSA to bimodal events was stronger than to visual events alone. This enhancement was apparent when the visual and auditory stimuli were presented from matching locations in space (congruent) but not when the bimodal stimuli were spatially incongruent. These findings suggest that the ongoing task of detecting unexpected events can benefit from the integration of visual and auditory information.

Auditory color constancy: calibration to reliable spectral properties across nonspeech context and targets

Brief experience with reliable spectral characteristics of a listening context can markedly alter perception of subsequent speech sounds, and parallels have been drawn between auditory compensation for listening context and visual color constancy. In order to better evaluate such an analogy, the generality of acoustic context effects for sounds with spectral-temporal compositions distinct from speech was investigated. Listeners identified nonspeech sounds-extensively edited samples produced by a French horn and a tenor saxophone-following either resynthesized speech or a short passage of music. Preceding contexts were "colored" by spectral envelope difference filters, which were created to emphasize differences between French horn and saxophone spectra. Listeners were more likely to report hearing a saxophone when the stimulus followed a context filtered to emphasize spectral characteristics of the French horn, and vice versa. Despite clear changes in apparent acoustic source, the auditory system calibrated to relatively predictable spectral characteristics of filtered context, differentially affecting perception of subsequent target nonspeech sounds. This calibration to listening context and relative indifference to acoustic sources operates much like visual color constancy, for which reliable properties of the spectrum of illumination are factored out of perception of color.

A sustained deviance response evoked by the auditory oddball paradigm

OBJECTIVE

Previous studies have suggested that the MMN(m) is related to selective adaptation of the N(1m). Since selective adaptation has also been reported for the sustained field, we hypothesized a second deviance response in addition to the MMN(m). The present study evaluated the existence of this wave.

METHODS

Magnetoencephalography was used to record deviance responses for pure tones of 1000 and 1050Hz. Tone duration was 50, 150, or 600ms in separate sets. Our hypothesis was that a sustained deviance response would increase with tone duration.

RESULTS

The data revealed a sustained deviance response with a similar source configuration as the main MMN(m), but a distinct time course. The sustained deviance response increased with the tone duration, but less than the standard sustained field. Moreover, the sustained deviance response was already present for short (50ms) tones.

CONCLUSIONS

The MMN(m) is followed by a sustained deviance response in the oddball paradigm. While some characteristics of the response coincide with the sustained field, its growth with tone duration differs. The response could possibly be related to automatic orienting of attention, but further studies are required to explore its functional role.

SIGNIFICANCE

The sustained deviance response is a separate component--distinct from the MMN(m) and P3--that needs to be considered in the evaluation of data obtained with the auditory oddball paradigm.

Spectral and temporal modulation tradeoff in the inferior colliculus

The cochlea encodes sounds through frequency-selective channels that exhibit low-pass modulation sensitivity. Unlike the cochlea, neurons in the auditory midbrain are tuned for spectral and temporal modulations found in natural sounds, yet the role of this transformation is not known. We report a distinct tradeoff in modulation sensitivity and tuning that is topographically ordered within the central nucleus of the inferior colliculus (CNIC). Spectrotemporal receptive fields (STRFs) were obtained with 16-channel electrodes inserted orthogonal to the isofrequency lamina. Surprisingly, temporal and spectral characteristics exhibited an opposing relationship along the tonotopic axis. For low best frequencies (BFs), units were selective for fast temporal and broad spectral modulations. A systematic progression was observed toward slower temporal and finer spectral modulation sensitivity at high BF. This tradeoff was strongly reflected in the arrangement of excitation and inhibition and, consequently, in the modulation tuning characteristics. Comparisons with auditory nerve fibers show that these trends oppose the pattern imposed by the peripheral filters. These results suggest that spectrotemporal preferences are reordered within the tonotopic axis of the CNIC. This topographic organization has profound implications for the coding of spectrotemporal features in natural sounds and could underlie a number of perceptual phenomena.

Projections from auditory cortex to midbrain cholinergic neurons that project to the inferior colliculus

We have shown that auditory cortex projects to cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and laterodorsal tegmental nucleus (LDT). PPT and LDT are the sources of cholinergic projections to the inferior colliculus, but it is not known if the cortical inputs contact the cholinergic cells that project to the inferior colliculus. We injected FluoroRuby into auditory cortex in pigmented guinea pigs to label cortical projections to PPT and LDT. In the same animals, we injected Fast Blue into the left or right inferior colliculus to label PPT and LDT cells that project to the inferior colliculus. We processed the brain to identify cholinergic cells with an antibody to choline acetyltransferase, which was visualized with a green fluorescent marker distinguishable from both FluoroRuby and Fast Blue. We then examined the PPT and LDT to determine whether boutons of FluoroRuby-labeled cortical axons were in close contact with cells that were double-labeled with the retrograde tracer and the immunolabel. Apparent contacts were observed ipsilateral and, less often, contralateral to the injected cortex. On both sides, the contacts were more numerous in PPT than in LDT. The results indicate that auditory cortex projects directly to brainstem cholinergic cells that innervate the ipsilateral or contralateral inferior colliculus. This suggests that cortical projections could elicit cholinergic effects on both sides of the auditory midbrain.

Correlating stimulus-specific adaptation of cortical neurons and local field potentials in the awake rat

Changes in the sensory environment are good indicators for behaviorally relevant events and strong triggers for the reallocation of attention. In the auditory domain, violations of a pattern of repetitive stimuli precipitate in the event-related potentials as mismatch negativity (MMN). Stimulus-specific adaptation (SSA) of single neurons in the auditory cortex has been proposed to be the cellular substrate of MMN (Nelken and Ulanovsky, 2007). However, until now, the existence of SSA in the awake auditory cortex has not been shown. In the present study, we recorded single and multiunits in parallel with evoked local field potentials (eLFPs) in the primary auditory cortex of the awake rat. Both neurons and eLFPs in the awake animal adapted in a stimulus-specific manner, and SSA was controlled by stimulus probability and frequency separation. SSA of isolated units was significant during the first stimulus-evoked "on" response but not in the following inhibition and rebound of activity. The eLFPs exhibited SSA in the first negative deflection and, to a lesser degree, in a slower positive deflection but no MMN. Spike adaptation correlated closely with adaptation of the fast negative deflection but not the positive deflection. Therefore, we conclude that single neurons in the auditory cortex of the awake rat adapt in a stimulus-specific manner and contribute to corresponding changes in eLFP but do not generate a late deviant response component directly equivalent to the human MMN. Nevertheless, the described effect may reflect a certain part of the process needed for sound discrimination.

Modeling the auditory scene: predictive regularity representations and perceptual objects

Predictive processing of information is essential for goal-directed behavior. We offer an account of auditory perception suggesting that representations of predictable patterns, or 'regularities', extracted from the incoming sounds serve as auditory perceptual objects. The auditory system continuously searches for regularities within the acoustic signal. Primitive regularities may be encoded by neurons adapting their response to specific sounds. Such neurons have been observed in many parts of the auditory system. Representations of the detected regularities produce predictions of upcoming sounds as well as alternative solutions for parsing the composite input into coherent sequences potentially emitted by putative sound sources. Accuracy of the predictions can be utilized for selecting the most likely interpretation of the auditory input. Thus in our view, perception generates hypotheses about the causal structure of the world.

Projections from auditory cortex to cholinergic cells in the midbrain tegmentum of guinea pigs

Anterograde and retrograde tracing techniques were used to characterize projections from the auditory cortex to the pedunculopontine and laterodorsal tegmental nuclei (PPT and LDT, respectively) in the midbrain tegmentum in guinea pigs. For anterograde tracing, tetramethylrhodamine dextran (FluoroRuby) was injected at several sites within auditory cortex. After sufficient time for transport, the brain was processed for immunohistochemistry with anti-choline acetyltransferase to reveal presumptive cholinergic cells. Anterogradely labeled axons were observed ipsilaterally and, in smaller numbers, contralaterally, in both the pedunculopontine and laterodorsal tegmental nuclei. In all four nuclei, tracer-labeled boutons appeared to contact immunolabeled (i.e., cholinergic) cells. The contacts occurred on cell bodies and dendrites. The results were similar following injections that spread across multiple auditory cortical areas or injections that were within primary auditory cortex. In order to confirm the anterograde results, in a second series of experiments, retrograde tracers were deposited in the pedunculopontine tegmental nucleus. These injections labeled layer V pyramidal cells in the auditory cortex. The results suggest an excitatory projection from primary auditory cortex bilaterally to cholinergic cells in the midbrain tegmentum. Such a pathway could allow auditory cortex to activate brainstem cholinergic circuits, possibly including the cholinergic pathways associated with arousal and gating of acoustic stimuli.

Current source density profiles of stimulus-specific adaptation in rat auditory cortex

Neurons in primary auditory cortex (A1) are known to exhibit a phenomenon known as stimulus-specific adaptation (SSA), which means that, when tested with pure tones, they will respond more strongly to a particular frequency if it is presented as a rare, unexpected "oddball" stimulus than when the same stimulus forms part of a series of common, "standard" stimuli. Although SSA has occasionally been observed in midbrain neurons that form part of the paraleminscal auditory pathway, it is thought to be weak, rare, or nonexistent among neurons of the leminscal pathway that provide the main afferent input to A1, so that SSA seen in A1 is likely generated within A1 by local mechanisms. To study the contributions that neural processing within the different cytoarchitectonic layers of A1 may make to SSA, we recorded local field potentials in A1 of the rat in response to standard and oddball tones and subjected these to current source density analysis. Although our results show that SSA can be observed throughout all layers of A1, right from the earliest part of the response, there are nevertheless significant differences between layers, with SSA becoming significantly stronger as stimulus-related activity passes from the main thalamorecipient layers III and IV to layer V.

Stimulus-specific adaptation occurs in the auditory thalamus

Neurons in the primary auditory cortex respond less strongly to a commonly occurring "standard" tone than to the same tone when it is rare or "deviant." This phenomenon, called "stimulus-specific adaptation" (SSA), has been proposed as a possible single-neuron correlate of the mismatch negativity, a cortical evoked potential associated with stimulus novelty. Previous studies in cat did not observe SSA in single neurons in the auditory thalamus. However, these reports did not differentiate between the auditory thalamic subdivisions and did not examine the effects of changing the stimulus presentation rate. To explore the possibility of thalamic SSA more completely, we recorded extracellularly from 30 single units and 22 multiunit clusters in the ventral, medial, and dorsal subdivisions of the mouse medial geniculate body (MGB), while presenting the anesthetized animals with sequences of standard and deviant tones at interstimulus intervals of 400, 500 and 800 ms. We found SSA in the auditory thalamus at all three stimulus presentation rates, primarily in the medial subdivision but to a lesser degree also in the ventral MGB. Thalamic SSA was evident from the earliest onset of tone-evoked activity, although the latencies of responses to standard and deviant tones were not significantly different. Together with related findings of SSA in neurons of the "belt" regions of the inferior colliculus, these results demonstrate that SSA is present at subcortical levels, primarily in but not restricted to the nonlemniscal auditory pathway.