A model of synaptic vesicle-pool depletion and replenishment can account for the interspike interval distributions and nonrenewal properties of spontaneous spike trains of auditory-nerve fibers.

In mammalian auditory systems, the spiking characteristics of each primary afferent (type I auditory-nerve fiber; ANF) are mainly determined by a single ribbon synapse in a single receptor cell (inner hair cell; IHC). ANF spike trains therefore provide a window into the operation of these synapses and cells. It was demonstrated previously (Heil et al., 2007) that the distribution of interspike intervals (ISIs) of cat ANFs during spontaneous activity can be modeled as resulting from refractoriness operating on a non-Poisson stochastic point process of excitation (transmitter release events from the IHC). Here, we investigate nonrenewal properties of these cat-ANF spontaneous spike trains, manifest as negative serial ISI correlations and reduced spike-count variability over short timescales. A previously discussed excitatory process, the constrained failure of events from a homogeneous Poisson point process, can account for these properties, but does not offer a parsimonious explanation for certain trends in the data. We then investigate a three-parameter model of vesicle-pool depletion and replenishment and find that it accounts for all experimental observations, including the ISI distributions, with only the release probability varying between spike trains. The maximum number of units (single vesicles or groups of simultaneously released vesicles) in the readily releasable pool and their replenishment time constant can be assumed to be constant (∼4 and 13.5 ms, respectively). We suggest that the organization of the IHC ribbon synapses not only enables sustained release of neurotransmitter but also imposes temporal regularity on the release process, particularly when operating at high rates.

Effects of chronic cochlear electrical stimulation after an extended period of profound deafness on primary auditory cortex organization in cats.

Extended periods of deafness have profound effects on central auditory system function and organization. Neonatal deafening results in loss of the normal cochleotopic organization of the primary auditory cortex (AI), but environmentally-derived intracochlear electrical stimulation, via a cochlear implant, initiated shortly after deafening, can prevent this loss. We investigated whether such stimulation initiated after an extended period of deafness can restore cochleotopy. In two groups of neonatally-deafened cats, a multi-channel intracochlear electrode array was implanted at 8 weeks of age. One group received only minimal stimulation, associated with brief recordings at 4-6-week intervals, over the following 6 months to check the efficacy of the implant. In the other group, this 6-month period was followed by 6 months of near-continuous intracochlear electrical stimulation from a modified clinical cochlear implant system. We recorded multi-unit clusters in the auditory cortex and used two different methods to define the region of interest in the putative AI. There was no evidence of cochleotopy in any of the minimally stimulated animals, confirming our earlier finding. In three of six chronically stimulated cats there was clear evidence of AI cochleotopy, and in a fourth cat in which the majority of penetrations were in the anterior auditory field there was clear evidence of cochleotopy in that field. The finding that chronic intracochlear electrical stimulation after an extended period of deafness is able to restore cochleotopy in some (but not all) cases has implications for the performance of patients implanted after an extended period of deafness.

A probabilistic model of absolute auditory thresholds and its possible physiological basis.

Detection thresholds for auditory stimuli, specified in terms of their -amplitude or level, depend on the stimulus temporal envelope and decrease with increasing stimulus duration. The neural mechanisms underlying these fundamental across-species observations are not fully understood. Here, we present a "continuous look" model, according to which the stimulus gives rise to stochastic neural detection events whose probability of occurrence is proportional to the 3rd power of the low-pass filtered, time-varying stimulus amplitude. Threshold is reached when a criterion number of events have occurred (probability summation). No long-term integration is required. We apply the model to an extensive set of thresholds measured in humans for tones of different envelopes and durations and find it to fit well. Subtle differences at long durations may be due to limited attention resources. We confirm the probabilistic nature of the detection events by analyses of simple reaction times and verify the exponent of 3 by validating model predictions for binaural thresholds from monaural thresholds. The exponent originates in the auditory periphery, possibly in the intrinsic Ca(2+) cooperativity of the Ca(2+) sensor involved in exocytosis from inner hair cells. It results in growth of the spike rate of auditory-nerve fibers (ANFs) with the 3rd power of the stimulus amplitude before saturating (Heil et al., J Neurosci 31:15424-15437, 2011), rather than with its square (i.e., with stimulus intensity), as is commonly assumed. Our work therefore suggests a link between detection thresholds and a key biochemical reaction in the receptor cells.

An improved model for the rate-level functions of auditory-nerve fibers.

Acoustic information is conveyed to the brain by the spike patterns in auditory-nerve fibers (ANFs). In mammals, each ANF is excited via a single ribbon synapse in a single inner hair cell (IHC), and the spike patterns therefore also provide valuable information about those intriguing synapses. Here we reexamine and model a key property of ANFs, the dependence of their spike rates on the sound pressure level of acoustic stimuli (rate-level functions). We build upon the seminal model of Sachs and Abbas (1974), which provides good fits to experimental data but has limited utility for defining physiological mechanisms. We present an improved, physiologically plausible model according to which the spike rate follows a Hill equation and spontaneous activity and its experimentally observed tight correlation with ANF sensitivity are emergent properties. We apply it to 156 cat ANF rate-level functions using frequencies where the mechanics are linear and find that a single Hill coefficient of 3 can account for the population of functions. We also demonstrate a tight correspondence between ANF rate-level functions and the Ca(2+) dependence of exocytosis from IHCs, and derive estimates of the effective intracellular Ca(2+) concentrations at the individual active zones of IHCs. We argue that the Hill coefficient might reflect the intrinsic, biochemical Ca(2+) cooperativity of the Ca(2+) sensor involved in exocytosis from the IHC. The model also links ANF properties with properties of psychophysical absolute thresholds.

Provision of interaural time difference information in chronic intracochlear electrical stimulation enhances neural sensitivity to these differences in neonatally deafened cats.

Although performance with bilateral cochlear implants is superior to that with a unilateral implant, bilateral implantees have poor performance in sound localisation and in speech discrimination in noise compared to normal hearing subjects. Studies of the neural processing of interaural time differences (ITDs) in the inferior colliculus (IC) of long-term deaf animals, show substantial degradation compared to that in normal hearing animals. It is not known whether this degradation can be ameliorated by chronic cochlear electrical stimulation, but such amelioration is unlikely to be achieved using current clinical speech processors and cochlear implants, which do not provide good ITD cues. We therefore developed a custom sound processor to deliver salient ITDs for chronic bilateral intra-cochlear electrical stimulation in a cat model of neonatal deafness, to determine if long-term exposure to salient ITDs would prevent degradation of ITD processing. We compared the sensitivity to ITDs in cochlear electrical stimuli of neurons in the IC of cats chronically stimulated with our custom ITD-aware sound processor with sensitivity in acutely deafened cats with normal hearing development and in cats chronically stimulated with a clinical stimulator and sound processor. Animals that experienced stimulation with our custom ITD-aware sound processor had significantly higher neural sensitivity to ITDs than those that received stimulation from clinical sound processors. There was no significant difference between animals received no stimulation and those that received stimulation from clinical sound processors, consistent with findings from clinical cochlear implant users. This result suggests that development and use of clinical ITD-aware sound processing strategies from a young age may promote ITD sensitivity in the clinical population.

Effects of restricted basilar papillar lesions and hair cell regeneration on auditory forebrain frequency organization in adult European starlings.

The frequency organization of neurons in the forebrain Field L complex (FLC) of adult starlings was investigated to determine the effects of hair cell (HC) destruction in the basal portion of the basilar papilla (BP) and of subsequent HC regeneration. Conventional microelectrode mapping techniques were used in normal starlings and in lesioned starlings either 2 d or 6-10 weeks after aminoglycoside treatment. Histological examination of the BP and recordings of auditory brainstem evoked responses confirmed massive loss of HCs in the basal portion of the BP and hearing losses at frequencies >2 kHz in starlings tested 2 d after aminoglycoside treatment. In these birds, all neurons in the region of the FLC in which characteristic frequencies (CFs) normally increase from 2 to 6 kHz had CF in the range of 2-4 kHz. The significantly elevated thresholds of responses in this region of altered tonotopic organization indicated that they were the residue of prelesion responses and did not reflect CNS plasticity. In the long-term recovery birds, there was histological evidence of substantial HC regeneration. The tonotopic organization of the high-frequency region of the FLC did not differ from that in normal starlings, but the mean threshold at CF in this frequency range was intermediate between the values in the normal and lesioned short-recovery groups. The recovery of normal tonotopicity indicates considerable stability of the topography of neuronal connections in the avian auditory system, but the residual loss of sensitivity suggests deficiencies in high-frequency HC function.

Basal forebrain cholinergic input is not essential for lesion-induced plasticity in mature auditory cortex.

The putative role of the basal forebrain cholinergic system in mediating lesion-induced plasticity in topographic cortical representations was investigated. Cholinergic immunolesions were combined with unilateral restricted cochlear lesions in adult cats, demonstrating the consequence of cholinergic depletion on lesion-induced plasticity in primary auditory cortex (AI). Immunolesions almost eliminated the cholinergic input to AI, while cochlear lesions produced broad high-frequency hearing losses. The results demonstrate that the near elimination of cholinergic input does not disrupt reorganization of the tonotopic representation of the lesioned (contralateral) cochlea in AI and does not affect the normal representation of the unlesioned (ipsilateral) cochlea. It is concluded that cholinergic basal forebrain input to AI is not essential for the occurrence of lesion-induced plasticity in AI.

Spontaneous activity of auditory-nerve fibers: insights into stochastic processes at ribbon synapses.

In several sensory systems, the conversion of the representation of stimuli from graded membrane potentials into stochastic spike trains is performed by ribbon synapses. In the mammalian auditory system, the spiking characteristics of the vast majority of primary afferent auditory-nerve (AN) fibers are determined primarily by a single ribbon synapse in a single inner hair cell (IHC), and thus provide a unique window into the operation of the synapse. Here, we examine the distributions of interspike intervals (ISIs) of cat AN fibers under conditions when the IHC membrane potential can be considered constant and the processes generating AN fiber activity can be considered stationary, namely in the absence of auditory stimulation. Such spontaneous activity is commonly thought to result from an excitatory Poisson point process modified by the refractory properties of the fiber, but here we show that this cannot be the case. Rather, the ISI distributions are one to two orders of magnitude better and very accurately described as a result of a homogeneous stochastic process of excitation (transmitter release events) in which the distribution of interevent times is a mixture of an exponential and a gamma distribution with shape factor 2, both with the same scale parameter. Whereas the scale parameter varies across fibers, the proportions of exponentially and gamma distributed intervals in the mixture, and the refractory properties, can be considered constant. This suggests that all of the ribbon synapses operate in a similar manner, possibly just at different rates. Our findings also constitute an essential step toward a better understanding of the spike-train representation of time-varying stimuli initiated at this synapse, and thus of the fundamentals of temporal coding in the auditory pathway.

Directed attention eliminates 'change deafness' in complex auditory scenes.

In natural environments that contain multiple sound sources, acoustic energy arising from the different sources sums to produce a single complex waveform at each of the listener's ears. The auditory system must segregate this waveform into distinct streams to permit identification of the objects from which the signals emanate [1]. Although the processes involved in stream segregation are now reasonably well understood [1, 2 and 3], little is known about the nature of our perception of complex auditory scenes. Here, we examined complex scene perception by having listeners detect a discrete change to an auditory scene comprising multiple concurrent naturalistic sounds. We found that listeners were remarkably poor at detecting the disappearance of an individual auditory object when listening to scenes containing more than four objects, but they performed near perfectly when their attention was directed to the identity of a potential change. In the absence of directed attention, this "change deafness" [4] was greater for objects arising from a common location in space than for objects separated in azimuth. Change deafness was also observed for changes in object location, suggesting that it may reflect a general effect of the dependence of human auditory perception on attention.

Cochlear implants and brain plasticity.

Cochlear implants have been implanted in over 110,000 deaf adults and children worldwide and provide these patients with important auditory cues necessary for auditory awareness and speech perception via electrical stimulation of the auditory nerve (AN). In 1942, Woolsey and Walzl presented the first report of cortical responses to localised electrical stimulation of different sectors of the AN in normal hearing cats, and established the cochleotopic organization of the projections to primary auditory cortex. Subsequently, individual cortical neurons in normal hearing animals have been shown to have well characterized input-output functions for electrical stimulation and decreasing response latencies with increasing stimulus strength. However, the central auditory system is not immutable, and has a remarkable capacity for plastic change, even into adulthood, as a result of changes in afferent input. This capacity for change is likely to contribute to the ongoing clinical improvements observed in speech perception for cochlear implant users. This review examines the evidence for changes of the response properties of neurons in, and consequently the functional organization of, the central auditory system produced by chronic, behaviourally relevant, electrical stimulation of the AN in profoundly deaf humans and animals.

Towards a unifying basis of auditory thresholds: distributions of the first-spike latencies of auditory-nerve fibers.

Detecting sounds in quiet is the simplest task performed by the auditory system, but the neural mechanisms underlying perceptual detection thresholds for sounds in quiet are still not understood. Heil and Neubauer [Heil, P., Neubauer, H., 2003. A unifying basis of auditory thresholds based on temporal summation. Proc. Natl. Acad. Sci. USA 100, 6151-6156] have provided evidence for a simple probabilistic model according to which the stimulus, at any point in time, has a certain probability of exceeding threshold and being detected. Consequently, as stimulus duration increases, the cumulative probability of detection events increases, performance improves, and threshold amplitude decreases. The origin of these processes was traced to the first synapse in the auditory system, between the inner hair cell and the afferent auditory-nerve fiber (ANF). Here we provide further support for this probabilistic "continuous-look" model. It is derived from analyses of the distributions of the latencies of the first spikes of cat ANFs with very low spontaneous discharge rates to tones of different amplitudes. The first spikes in these fibers can be considered detection events. We show that, as predicted, the distributions can be explained by the joint probability of the occurrence of three independent sub-events, where the probability of each of those occurring is proportional to the low-pass filtered stimulus amplitude. The "temporal integration" functions of individual ANFs, derived from their first-spike latencies, are remarkably similar in shape to "temporal integration" functions, which relate threshold sound pressure level at the perceptual level to stimulus duration. This further supports a close link between the mechanisms determining the timing of the first (and other) evoked spikes at the level of the auditory nerve and detection thresholds at the perceptual level. The possible origin and some functional consequences of the expansive power-law non-linearity are discussed.

The role of spatial location in auditory search.

The majority of research findings to date indicate that spatial cues play a minor role in enhancing listeners' ability to parse and detect a sound of interest when it is presented in a complex auditory scene comprising multiple simultaneous sounds. Frequency and temporal differences between sound streams provide more reliable cues for scene analysis as well as for directing attention to relevant auditory 'objects' in complex displays. The present study used naturalistic sounds with varying spectro-temporal profiles to examine whether spatial separation of sound sources can enhance target detection in an auditory search paradigm. The arrays of sounds were presented in virtual auditory space over headphones. The results of Experiment 1 suggest that target detection is enhanced when sound sources are spatially separated relative to when they are presented at the same location. Experiment 2 demonstrated that this effect is most prominent within the first 250 ms of exposure to the array of sounds. These findings suggest that spatial cues may be effective for enhancing early processes such as stream segregation, rather than simply directing attention to objects that have already been segmented.

Auditory perceptual learning and changes in the conceptualization of auditory cortex.

Perceptual learning, improvement in discriminative ability as a consequence of training, is one of the forms of sensory system plasticity that has driven profound changes in our conceptualization of sensory cortical function. Psychophysical and neurophysiological studies of auditory perceptual learning have indicated that the characteristics of the learning, and by implication the nature of the underlying neural changes, are highly task specific. Some studies in animals have indicated that recruitment of neurons to the population responding to the training stimuli, and hence an increase in the so-called cortical "area of representation" of those stimuli, is the substrate of improved performance, but such changes have not been observed in other studies. A possible reconciliation of these conflicting results is provided by evidence that changes in area of representation constitute a transient stage in the processes underlying perceptual learning. This expansion - renormalization hypothesis is supported by evidence from studies of the learning of motor skills, another form of procedural learning, but leaves open the nature of the permanent neural substrate of improved performance. Other studies have suggested that the substrate might be reduced response variability - a decrease in internal noise. Neuroimaging studies in humans have also provided compelling evidence that training results in long-term changes in auditory cortical function and in the auditory brainstem frequency-following response. Musical training provides a valuable model, but the evidence it provides is qualified by the fact that most such training is multimodal and sensorimotor, and that few of the studies are experimental and allow control over confounding variables. More generally, the overwhelming majority of experimental studies of the various forms of auditory perceptual learning have established the co-occurrence of neural and perceptual changes, but have not established that the former are causally related to the latter. Important forms of perceptual learning in humans are those involved in language acquisition and in the improvement in speech perception performance of post-lingually deaf cochlear implantees over the months following implantation. The development of a range of auditory training programs has focused interest on the factors determining the extent to which perceptual learning is specific or generalises to tasks other than those used in training. The context specificity demonstrated in a number of studies of perceptual learning suggests a multiplexing model, in which learning relating to a particular stimulus attribute depends on a subset of the diverse inputs to a given cortical neuron being strengthened, and different subsets being gated by top-down influences. This hypothesis avoids the difficulty of balancing system stability with plasticity, which is a problem for recruitment hypotheses. The characteristics of auditory perceptual learning reflect the fact that auditory cortex forms part of distributed networks that integrate the representation of auditory stimuli with attention, decision, and reward processes.

Plasticity in the auditory system.

Over the last 30 years a wide range of manipulations of auditory input and experience have been shown to result in plasticity in auditory cortical and subcortical structures. The time course of plasticity ranges from very rapid stimulus-specific adaptation to longer-term changes associated with, for example, partial hearing loss or perceptual learning. Evidence for plasticity as a consequence of these and a range of other manipulations of auditory input and/or its significance is reviewed, with an emphasis on plasticity in adults and in the auditory cortex. The nature of the changes in auditory cortex associated with attention, memory and perceptual learning depend critically on task structure, reward contingencies, and learning strategy. Most forms of auditory system plasticity are adaptive, in that they serve to optimize auditory performance, prompting attempts to harness this plasticity for therapeutic purposes. However, plasticity associated with cochlear trauma and partial hearing loss appears to be maladaptive, and has been linked to tinnitus. Three important forms of human learning-related auditory system plasticity are those associated with language development, musical training, and improvement in performance with a cochlear implant. Almost all forms of plasticity involve changes in synaptic excitatory - inhibitory balance within existing patterns of connectivity. An attractive model applicable to a number of forms of learning-related plasticity is dynamic multiplexing by individual neurons, such that learning involving a particular stimulus attribute reflects a particular subset of the diverse inputs to a given neuron being gated by top-down influences. The plasticity evidence indicates that auditory cortex is a component of complex distributed networks that integrate the representation of auditory stimuli with attention, decision and reward processes.

Association between auditory and visual symptoms of unilateral spatial neglect.

Unilateral spatial neglect due to right brain damage (RBD) can occur in several different sensory modalities in the same patient. Previous studies of the association between auditory and visual neglect have yielded conflicting outcomes. Most such studies have compared performance on relatively simple clinical measures of visual neglect, such as target cancellation, with that on more sophisticated measures of auditory perception. This is problematic because such tasks are typically not matched for the cognitive processes they exercise. We overcame this limitation by using equivalent visual and auditory versions of extinction and temporal-order judgment (TOJ) tasks. RBD patients demonstrated lateralized deficits on both visual and auditory tasks when compared with same-aged, healthy controls. Critically, a significant association between the severity of visual and auditory deficits was apparent on the TOJ task but not the extinction task, suggesting that even when task demands are matched across modalities, dissociations between visual and auditory neglect can be apparent. Across the auditory tasks, patients showed more pronounced deficits for verbal stimuli than for non-verbal stimuli. These findings have implications for recent models proposed to explain the role of spatial attention in multimodal perception.

Auditory cortical plasticity: does it provide evidence for cognitive processing in the auditory cortex?

The past 20 years have seen substantial changes in our view of the nature of the processing carried out in auditory cortex. Some processing of a cognitive nature, previously attributed to higher-order "association" areas, is now considered to take place in auditory cortex itself. One argument adduced in support of this view is the evidence indicating a remarkable degree of plasticity in the auditory cortex of adult animals. Such plasticity has been demonstrated in a wide range of paradigms, in which auditory input or the behavioural significance of particular inputs is manipulated. Changes over the same time period in our conceptualization of the receptive fields of cortical neurons, and well-established mechanisms for use-related changes in synaptic function, can account for many forms of auditory cortical plasticity. On the basis of a review of auditory cortical plasticity and its probable mechanisms, it is argued that only plasticity associated with learning tasks provides a strong case for cognitive processing in auditory cortex. Even in this case the evidence is indirect, in that it has not yet been established that the changes in auditory cortex are necessary for behavioural learning and memory. Although other lines of evidence provide convincing support for cognitive processing in auditory cortex, that provided by auditory cortical plasticity remains equivocal.

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.

Second spatial derivative analysis of cortical surface potentials recorded in cat primary auditory cortex using thin film surface arrays: Comparisons with multi-unit data.

BACKGROUND: Current source density analysis of recordings from penetrating electrode arrays has traditionally been used to examine the layer- specific cortical activation and plastic changes associated with changed afferent input. We report on a related analysis, the second spatial derivative (SSD) of surface local field potentials (LFPs) recorded using custom designed thin-film polyimide substrate arrays. RESULTS: SSD analysis of tone- evoked LFPs generated from the auditory cortex under the recording array demonstrated a stereotypical single local minimum, often flanked by maxima on both the caudal and rostral sides. In contrast, tone-pips at frequencies not represented in the region under the array, but known (on the basis of normal tonotopic organization) to be represented caudal to the recording array, had a more complex pattern of many sources and sinks. COMPARISON WITH EXISTING METHODS: Compared to traditional analysis of LFPs, SSD analysis produced a tonotopic map that was more similar to that obtained with multi-unit recordings in a normal-hearing animal. Additionally, the statistically significant decrease in the number of acoustically responsive cortical locations in partially deafened cats following 6 months of cochlear implant use compared to unstimulated cases observed with multi-unit data (p=0.04) was also observed with SSD analysis (p=0.02), but was not apparent using traditional analysis of LFPs (p=0.6). CONCLUSIONS: SSD analysis of surface LFPs from the thin-film array provides a rapid and robust method for examining the spatial distribution of cortical activity with improved spatial resolution compared to more traditional LFP recordings.

Origin and immunolesioning of cholinergic basal forebrain innervation of cat primary auditory cortex.

Numerous studies have implicated the cholinergic basal forebrain (cBF) in the modulation of auditory cortical responses. This study aimed to accurately define the sources of cBF input to primary auditory cortex (AI) and to assess the efficacy of a cholinergic immunotoxin in cat. Three anaesthetized cats received multiple injections of horseradish-peroxidase conjugated wheatgerm-agglutin into physiologically identified AI. Following one to two days survival, tetramethylbenzidine histochemistry revealed the greatest number of retrogradely labeled cells in ipsilateral putamen, globus pallidus and internal capsule, and smaller numbers in more medial nuclei of the basal forebrain (BF). Concurrent choline acetyltransferase immunohistochemistry showed that almost 80% of the retrogradely labeled cells in BF were cholinergic, with the vast majority of these cells arising from the more lateral BF nuclei identified above. In the second part of the study, unilateral intraparenchymal injections of the cholinergic immunotoxin ME20.4-SAP were made into the putamen/globus pallidus nuclei of six cats. Immuno- and histochemistry revealed a massive reduction in the number of cholinergic cells in and around the targeted area, and a corresponding reduction in the density of cholinergic fibers in auditory cortex. These results are discussed in terms of their implications for investigations of the role of the cBF in cortical plasticity.

Effects of restricted cochlear lesions in adult cats on the frequency organization of the inferior colliculus.

Restricted cochlear lesions in adult animals result in plastic changes in the representation of the lesioned cochlea, and thus in the frequency map, in the contralateral auditory cortex and thalamus. To examine the contribution of subthalamic changes to this reorganization, the effects of unilateral mechanical cochlear lesions on the frequency organization of the central nucleus of the inferior colliculus (ICC) were examined in adult cats. Lesions typically resulted in a broad high-frequency hearing loss extending from a frequency in the range 15-22 kHz. After recovery periods of 2.5-18 months, the frequency organization of ICC contralateral to the lesioned cochlea was determined separately for the onset and late components of multiunit responses to tone-burst stimuli. For the late response component in all but one penetration through the ICC, and for the onset response component in more than half of the penetrations, changes in frequency organization in the lesion projection zone were explicable as the residue of prelesion responses. In half of the penetrations exhibiting nonresidue type changes in onset-response frequency organization, the changes appeared to reflect the unmasking of normally inhibited inputs. In the other half it was unclear whether the changes reflected unmasking or a dynamic process of reorganization. Thus, most of the observed changes were explicable as passive consequences of the lesion, and there was limited evidence for plasticity in the ICC. The implications of the data with respect to the primary locus of the changes and to the manner in which they contribute to thalamocortical reorganization are considered.