RESUMEN
Functional columns of primary auditory cortex (AI) are arranged in layers, each composed of highly connected fine-scale networks. The basic response properties and interactions within these local subnetworks have only begun to be assessed. We examined the functional diversity of neurons within the laminar microarchitecture of cat AI to determine the relationship of spectrotemporal processing between neighboring neurons. Neuronal activity was recorded across the cortical layers while presenting a dynamically modulated broadband noise. Spectrotemporal receptive fields (STRFs) and their nonlinear input/output functions (nonlinearities) were constructed for each neuron and compared for pairs of neurons simultaneously recorded at the same contact site. Properties of these local neuron pairs showed greater similarity than non-paired neurons within the same column for all considered parameters including firing rate, envelope-phase precision, preferred spectral and temporal modulation frequency, as well as for the threshold and transition of the response nonlinearity. This higher functional similarity of paired versus non-paired neurons was most apparent in infragranular neuron pairs, and less for local supragranular and granular pairs. The functional similarity of local paired neurons for firing rate, best temporal modulation frequency and two nonlinearity aspects was laminar dependent, with infragranular local pair-wise differences larger than for granular or supragranular layers. Synchronous spiking events between pairs of neurons revealed that simultaneous 'Bicellular' spikes, in addition to carrying higher stimulus information than non-synchronized spikes, encoded faster modulation frequencies. Bicellular functional differences to the best matched of the paired neurons could be substantial. Bicellular nonlinearities showed that synchronous spikes act to transmit stimulus information with higher fidelity and precision than non-synchronous spikes of the individual neurons, thus, likely enhancing stimulus feature selectivity in their target neurons. Overall, the well-correlated and temporally precise processing within local subnetworks of cat AI showed laminar-dependent functional diversity in spectrotemporal processing, despite high intra-columnar congruity in frequency preference.
Asunto(s)
Corteza Auditiva/fisiología , Vías Auditivas/fisiología , Percepción Auditiva/fisiología , Red Nerviosa/fisiología , Neuronas/fisiología , Estimulación Acústica , Potenciales de Acción/fisiología , Animales , Corteza Auditiva/citología , Gatos , Análisis de Fourier , Técnicas In Vitro , Modelos Neurológicos , Dinámicas no Lineales , Psicoacústica , Factores de TiempoRESUMEN
BACKGROUND: To dissect the intricate workings of neural circuits, it is essential to gain precise control over subsets of neurons while retaining the ability to monitor larger-scale circuit dynamics. This requires the ability to both evoke and record neural activity simultaneously with high spatial and temporal resolution. NEW METHOD: In this paper we present approaches that address this need by combining micro-electrocorticography (µECoG) with optogenetics in ways that avoid photovoltaic artifacts. RESULTS: We demonstrate that variations of this approach are broadly applicable across three commonly studied mammalian species - mouse, rat, and macaque monkey - and that the recorded µECoG signal shows complex spectral and spatio-temporal patterns in response to optical stimulation. COMPARISON WITH EXISTING METHODS: While optogenetics provides the ability to excite or inhibit neural subpopulations in a targeted fashion, large-scale recording of resulting neural activity remains challenging. Recent advances in optical physiology, such as genetically encoded Ca(2+) indicators, are promising but currently do not allow simultaneous recordings from extended cortical areas due to limitations in optical imaging hardware. CONCLUSIONS: We demonstrate techniques for the large-scale simultaneous interrogation of cortical circuits in three commonly used mammalian species.
Asunto(s)
Electrocorticografía/métodos , Optogenética/métodos , Animales , Artefactos , Percepción Auditiva/fisiología , Corteza Cerebral/fisiología , Diseño Asistido por Computadora , Impedancia Eléctrica , Electrocorticografía/instrumentación , Electrodos Implantados , Diseño de Equipo , Potenciales Evocados/fisiología , Macaca mulatta , Masculino , Ratones Transgénicos , Inhibición Neural/fisiología , Neuronas/fisiología , Optogenética/instrumentación , Estimulación Luminosa/métodos , Ratas Long-Evans , Compuestos de EstañoRESUMEN
Neurons in the center of cat primary auditory cortex (AI) respond to a narrow range of sound frequencies and the preferred frequencies in local neuron clusters are closely aligned in this central narrow bandwidth region (cNB). Response preferences to other input parameters, such as sound intensity and binaural interaction, vary within cNB; however, the source of this variability is unknown. Here we examined whether input to the cNB could arise from multiple, anatomically independent subregions in the ventral nucleus of the medial geniculate body (MGBv). Retrograde tracers injected into cNB labeled discontinuous clusters of neurons in the superior (sMGBv) and inferior (iMGBv) halves of the MGBv. Most labeled neurons were in the sMGBv and their density was greater, iMGBv somata were significantly larger. These findings suggest that cNB projection neurons in superior and iMGBv have distinct anatomic and possibly physiologic organization.
Asunto(s)
Corteza Auditiva/citología , Vías Auditivas/citología , Mapeo Encefálico , Neuronas/citología , Tálamo/citología , Estimulación Acústica , Animales , GatosRESUMEN
Noncoplanar polychlorinated biphenyls (PCBs) are widely dispersed in human environment and tissues. Here, an exemplar noncoplanar PCB was fed to rat dams during gestation and throughout three subsequent nursing weeks. Although the hearing sensitivity and brainstem auditory responses of pups were normal, exposure resulted in the abnormal development of the primary auditory cortex (A1). A1 was irregularly shaped and marked by internal nonresponsive zones, its topographic organization was grossly abnormal or reversed in about half of the exposed pups, the balance of neuronal inhibition to excitation for A1 neurons was disturbed, and the critical period plasticity that underlies normal postnatal auditory system development was significantly altered. These findings demonstrate that developmental exposure to this class of environmental contaminant alters cortical development. It is proposed that exposure to noncoplanar PCBs may contribute to common developmental disorders, especially in populations with heritable imbalances in neurotransmitter systems that regulate the ratio of inhibition and excitation in the brain. We conclude that the health implications associated with exposure to noncoplanar PCBs in human populations merit a more careful examination.
Asunto(s)
Corteza Auditiva/efectos de los fármacos , Exposición Materna , Plasticidad Neuronal/efectos de los fármacos , Bifenilos Policlorados/farmacología , Animales , Electrofisiología , Femenino , Audición/efectos de los fármacos , Ratas , Ratas Sprague-Dawley , Sensibilidad y EspecificidadRESUMEN
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.
Asunto(s)
Corteza Auditiva/citología , Inhibición Neural/fisiología , Neuronas/fisiología , Sinapsis/fisiología , Estimulación Acústica/métodos , Animales , Conducta Animal , Condicionamiento Operante/fisiología , Relación Dosis-Respuesta en la Radiación , Femenino , Técnicas In Vitro , Potenciales de la Membrana/fisiología , Potenciales de la Membrana/efectos de la radiación , Técnicas de Placa-Clamp/métodos , Ratas , Ratas Sprague-Dawley , Tiempo de Reacción/fisiología , Tiempo de Reacción/efectos de la radiación , Factores de TiempoRESUMEN
Combined physiological and connectional studies show significant non-topographic extrinsic projections to frequency-specific domains in the cat auditory cortex. These frequency-mismatched loci in the thalamus, ipsilateral cortex, and commissural system complement the predicted topographic and tonotopic projections. Two tonotopic areas, the primary auditory cortex (AI) and the anterior auditory field (AAF), were electrophysiologically characterized by their frequency organization. Next, either cholera toxin beta subunit or cholera toxin beta subunit gold conjugate was injected into frequency-matched locations in each area to reveal the projection pattern from the thalamus and cortex. Most retrograde labeling was found at tonotopically appropriate locations within a 1 mm-wide strip in the thalamus and a 2-3 mm-wide expanse of cortex (approximately 85%). However, approximately 13-30% of the neurons originated from frequency-mismatched locations far from their predicted positions in thalamic nuclei and cortical areas, respectively. We propose that these heterotopic projections satisfy at least three criteria that may be necessary to support the magnitude and character of plastic changes in physiological studies. First, they are found in the thalamus, ipsilateral and commissural cortex; since this reorganization could arise from any of these routes and may involve each, such projections ought to occur in them. Second, they originate from nuclei and areas with or without tonotopy; it is likely that plasticity is not exclusively shaped by spectral influences and not limited to cochleotopic regions. Finally, the projections are appropriate in magnitude and sign to plausibly support such rearrangements; given the rapidity of some aspects of plastic changes, they should be mediated by substantial existing connections. Alternative roles for these heterotopic projections are also considered.
Asunto(s)
Corteza Auditiva/fisiología , Vías Auditivas/fisiología , Mapeo Encefálico , Animales , Corteza Auditiva/anatomía & histología , Vías Auditivas/anatomía & histología , Gatos , Toxina del Cólera/metabolismo , Relación Dosis-Respuesta en la Radiación , Estimulación Eléctrica/métodos , Femenino , Lateralidad Funcional/fisiología , Masculino , Redes Neurales de la Computación , Tálamo/anatomía & histología , Tálamo/fisiologíaRESUMEN
Amplitude modulation (AM) is a temporal feature of most natural acoustic signals. A long psychophysical tradition has shown that AM is important in a variety of perceptual tasks, over a range of time scales. Technical possibilities in stimulus synthesis have reinvigorated this field and brought the modulation dimension back into focus. We address the question whether specialized neural mechanisms exist to extract AM information, and thus whether consideration of the modulation domain is essential in understanding the neural architecture of the auditory system. The available evidence suggests that this is the case. Peripheral neural structures not only transmit envelope information in the form of neural activity synchronized to the modulation waveform but are often tuned so that they only respond over a limited range of modulation frequencies. Ascending the auditory neuraxis, AM tuning persists but increasingly takes the form of tuning in average firing rate, rather than synchronization, to modulation frequency. There is a decrease in the highest modulation frequencies that influence the neural response, either in average rate or synchronization, as one records at higher and higher levels along the neuraxis. In parallel, there is an increasing tolerance of modulation tuning for other stimulus parameters such as sound pressure level, modulation depth, and type of carrier. At several anatomical levels, consideration of modulation response properties assists the prediction of neural responses to complex natural stimuli. Finally, some evidence exists for a topographic ordering of neurons according to modulation tuning. The picture that emerges is that temporal modulations are a critical stimulus attribute that assists us in the detection, discrimination, identification, parsing, and localization of acoustic sources and that this wide-ranging role is reflected in dedicated physiological properties at different anatomical levels.
Asunto(s)
Percepción Auditiva/fisiología , Encéfalo/fisiología , Neuronas/fisiología , Sonido , Animales , HumanosRESUMEN
Action potentials are a universal currency for fast information transfer in the nervous system, yet few studies address how some spikes carry more information than others. We focused on the transformation of sensory representations in the lemniscal (high-fidelity) auditory thalamocortical network. While stimulating with a complex sound, we recorded simultaneously from functionally connected cell pairs in the ventral medial geniculate body and primary auditory cortex. Thalamic action potentials that immediately preceded or potentially caused a cortical spike were more selective than the average thalamic spike for spectrotemporal stimulus features. This net improvement of thalamic signaling indicates that for some thalamic cells, spikes are not propagated through cortex independently but interact with other inputs onto the same target cell. We then developed a method to identify the spectrotemporal nature of these interactions and found that they could be cooperative or antagonistic to the average receptive field of the thalamic cell. The degree of cooperativity with the thalamic cell determined the increase in feature selectivity for potentially causal thalamic spikes. We therefore show how some thalamic spikes carry more receptive field information than average and how other inputs cooperate to constrain the information communicated through a cortical cell.
Asunto(s)
Potenciales de Acción/fisiología , Corteza Auditiva/fisiología , Vías Auditivas/fisiología , Interneuronas/fisiología , Tálamo/fisiología , Estimulación Acústica , Animales , Gatos , Cuerpos Geniculados/fisiología , Tiempo de Reacción/fisiologíaRESUMEN
One of the brain's fundamental tasks is to construct and transform representations of an animal's environment, yet few studies describe how individual neurons accomplish this. Our results from correlated pairs in the auditory thalamocortical system show that cortical excitatory receptive field regions can be directly inherited from thalamus, constructed from smaller inputs, and assembled by the cooperative activity of neuronal ensembles. The prevalence of functional thalamocortical connectivity is strictly governed by tonotopy, but connection strength is not. Finally, spectral and temporal modulation preferences in cortex may differ dramatically from the thalamic input. Our observations reveal a radical reconstruction of response properties from auditory thalamus to cortex, and illustrate how some properties are propagated with great fidelity while others are significantly transformed or generated intracortically.
Asunto(s)
Corteza Auditiva/citología , Corteza Auditiva/fisiología , Cuerpos Geniculados/citología , Cuerpos Geniculados/fisiología , Estimulación Acústica , Potenciales de Acción/fisiología , Anestésicos Disociativos , Animales , Vías Auditivas/citología , Vías Auditivas/fisiología , Gatos , Electrofisiología , Ketamina , Inhibición Neural/fisiologíaRESUMEN
Many response properties in primary auditory cortex (AI) are segregated spatially and organized topographically as those in primary visual cortex. Intensive study has not revealed an intrinsic, anatomical organizing principle related to an AI functional topography. We used retrograde anatomic tracing and topographic physiologic mapping of acoustic response properties to reveal long-range (> or = 1.5 mm) convergent intrinsic horizontal connections between AI subregions with similar bandwidth and characteristic frequency selectivity. This suggests a modular organization for processing spectral bandwidth in AI.
Asunto(s)
Corteza Auditiva/anatomía & histología , Corteza Auditiva/fisiología , Animales , Gatos , Procesamiento de Imagen Asistido por ComputadorRESUMEN
The cortical representation of the sensory environment is continuously modified by experience. Changes in spatial (receptive field) and temporal response properties of cortical neurons underlie many forms of natural learning. The scale and direction of these changes appear to be determined by specific features of the behavioral tasks that evoke cortical plasticity. The neural mechanisms responsible for this differential plasticity remain unclear partly because important sensory and cognitive parameters differ among these tasks. In this report, we demonstrate that differential sensory experience directs differential plasticity using a single paradigm that eliminates the task-specific variables that have confounded direct comparison of previous studies. Electrical activation of the basal forebrain (BF) was used to gate cortical plasticity mechanisms. The auditory stimulus paired with BF stimulation was systematically varied to determine how several basic features of the sensory input direct plasticity in primary auditory cortex (A1) of adult rats. The distributed cortical response was reconstructed from a dense sampling of A1 neurons after 4 wk of BF-sound pairing. We have previously used this method to show that when a tone is paired with BF activation, the region of the cortical map responding to that tone frequency is specifically expanded. In this report, we demonstrate that receptive-field size is determined by features of the stimulus paired with BF activation. Specifically, receptive fields were narrowed or broadened as a systematic function of both carrier-frequency variability and the temporal modulation rate of paired acoustic stimuli. For example, the mean bandwidth of A1 neurons was increased (+60%) after pairing BF stimulation with a rapid train of tones and decreased (-25%) after pairing unmodulated tones of different frequencies. These effects are consistent with previous reports of receptive-field plasticity evoked by natural learning. The maximum cortical following rate and minimum response latency were also modified as a function of stimulus modulation rate and carrier-frequency variability. The cortical response to a rapid train of tones was nearly doubled if BF stimulation was paired with rapid trains of random carrier frequency, while no following rate plasticity was observed if a single carrier frequency was used. Finally, we observed significant increases in response strength and total area of functionally defined A1 following BF activation paired with certain classes of stimuli and not others. These results indicate that the degree and direction of cortical plasticity of temporal and receptive-field selectivity are specified by the structure and schedule of inputs that co-occur with basal forebrain activation and suggest that the rules of cortical plasticity do not operate on each elemental stimulus feature independently of others.
Asunto(s)
Corteza Auditiva/citología , Corteza Auditiva/fisiología , Potenciales Evocados Auditivos/fisiología , Plasticidad Neuronal/fisiología , Estimulación Acústica , Animales , Mapeo Encefálico , Estimulación Eléctrica , Electrodos Implantados , Electrofisiología , Ratas , Tiempo de Reacción/fisiologíaRESUMEN
Response properties of the middle layers of feline primary auditory cortex neurons to simple sounds were compared for isoflurane versus pentobarbital anesthesia in a within subject study control design. Initial microelectrode recordings were made under isoflurane anesthesia. After a several hour washout period, recordings were repeated at spatially matched locations in the same animal under pentobarbital. The median spatial separation between matched recording locations was 50 microns. Excitatory frequency tuning curves (n=71 pairs) to tone bursts and entrainment to click train sequences (n=64 pairs) ranging from 2 to 38 Hz were measured. Characteristic frequency and BW10 and BW30 were not different under either anesthetic. The spontaneous rate was slightly decreased (P<0.05) for isoflurane (median 4.2 spikes/s) compared to pentobarbital (median 5.8 spikes/s). Minimum median threshold and latency were elevated by 12 dB and 2 ms, respectively, under isoflurane. Entrainment to click sequences assumed a lowpass filter profile under both anesthetics, but was markedly impoverished under isoflurane. Responses to click sequences under isoflurane were phasic to the first click but had very poor following to subsequent elements. Compared to pentobarbital, isoflurane appears to have a profound impact on response sensitivity and temporal response properties of auditory cortical neurons.
Asunto(s)
Anestesia , Anestésicos por Inhalación , Corteza Auditiva/fisiología , Hipnóticos y Sedantes , Isoflurano , Neuronas Aferentes/fisiología , Pentobarbital , Estimulación Acústica/métodos , Animales , Corteza Auditiva/citología , Umbral Auditivo , Gatos , Electrofisiología , Tiempo de ReacciónRESUMEN
The spatial organization of response parameters in squirrel monkey primary auditory cortex (AI) accessible on the temporal gyrus was determined with the excitatory receptive field to pure tone stimuli. Dense, microelectrode mapping of the temporal gyrus in four animals revealed that characteristic frequency (CF) had a smooth, monotonic gradient that systematically changed from lower values (0.5 kHz) in the caudoventral quadrant to higher values (5--6 kHz) in the rostrodorsal quadrant. The extent of AI on the temporal gyrus was approximately 4 mm in the rostrocaudal axis and 2--3 mm in the dorsoventral axis. The entire length of isofrequency contours below 6 kHz was accessible for study. Several independent, spatially organized functional response parameters were demonstrated for the squirrel monkey AI. Latency, the asymptotic minimum arrival time for spikes with increasing sound pressure levels at CF, was topographically organized as a monotonic gradient across AI nearly orthogonal to the CF gradient. Rostral AI had longer latencies (range = 4 ms). Threshold and bandwidth co-varied with the CF. Factoring out the contribution of the CF on threshold variance, residual threshold showed a monotonic gradient across AI that had higher values (range = 10 dB) caudally. The orientation of the threshold gradient was significantly different from the CF gradient. CF-corrected bandwidth, residual Q10, was spatially organized in local patches of coherent values whose loci were specific for each monkey. These data support the existence of multiple, overlying receptive field gradients within AI and form the basis to develop a conceptual framework to understand simple and complex sound coding in mammals.
Asunto(s)
Corteza Auditiva/fisiología , Estimulación Acústica/métodos , Animales , Umbral Auditivo/fisiología , Mapeo Encefálico , Electrofisiología , Masculino , Tiempo de Reacción/fisiología , SaimiriRESUMEN
Properties of sequence-sensitive neurons in primary auditory cortex of cats were explored in detail. Stimuli were sequences of two tones, in which the frequency and intensity of the first tone and the temporal separation between the first and second, or probe, tone were parametrically varied. After presentation of the first tone, the responses of 32 single units and 48 multiunits to the probe tone were found to be enhanced up to 140-5270% (median 340%) above the response obtained in the single-tone condition. Probe tone enhancement was induced from a considerable number of sequence conditions and depended on the frequency and intensity of the first tone and on the temporal separation between the onsets of the first and the probe tone. On average, the maximally enhanced response occurred when the first tone was 1 octave below or above the probe tone and its intensity was 14 dB louder than the probe tone. The most effective temporal separation of the tones for an enhancement effect was approximately 100 ms. The range of enhancing tones was largely outside the excitatory tuning curve of a neuron. Results extend previous findings of properties of sequence-sensitive neurons in the auditory cortex of echolocating bats and non-echolocating mammals, and suggest that sequence-sensitive neurons are quite common and involved in the cortical representation of spectrotemporal patterns of acoustic signals.
Asunto(s)
Corteza Auditiva/fisiología , Neuronas/fisiología , Estimulación Acústica/métodos , Animales , Corteza Auditiva/citología , GatosRESUMEN
Neural systems operate in various dynamic states that determine how they process information (Livingstone and Hubel, 1981; Funke and Eysel, 1992; Morrow and Casey, 1992; Abeles et al., 1995; Guido et al., 1995; Mukherjee and Kaplan, 1995; Kenmochi and Eggermont, 1997; Wörgötter et al., 1998; Kisley and Gerstein, 1999). To investigate the function of a brain area, it is therefore crucial to determine the state of that system. One grave difficulty is that even under well controlled conditions, the thalamocortical network may undergo random dynamic state fluctuations which alter the most basic spatial and temporal response properties of the neurons. These uncontrolled state changes hinder the evaluation of state-specific properties of neural processing and, consequently, the interpretation of thalamocortical function. Simultaneous extracellular recordings were made in the auditory thalamus and cortex of the ketamine-anesthetized cat under several stimulus conditions. By considering the cellular and network mechanisms that govern state changes, we develop a complex stimulus that controls the dynamic state of the thalamocortical network. Traditional auditory stimuli have ambivalent effects on thalamocortical state, sometimes eliciting an oscillatory state prevalent in sleeping animals and other times suppressing it. By contrast, our complex stimulus clamps the network in a dynamic state resembling that observed in the alert animal. It thus allows evaluation of neural information processing not confounded by uncontrolled variations. Stimulus-based state control illustrates a general and direct mechanism whereby the functional modes of the brain are influenced by structural features of the external world.
Asunto(s)
Corteza Auditiva/fisiología , Vías Auditivas/fisiología , Tálamo/fisiología , Estimulación Acústica/métodos , Animales , Atención/fisiología , Relojes Biológicos/fisiología , Gatos , Red Nerviosa/fisiología , Percepción de la Altura Tonal/fisiología , Espectrografía del SonidoRESUMEN
Two fundamental aspects of frequency analysis shape the functional organization of primary auditory cortex. For one, the decomposition of complex sounds into different frequency components is reflected in the tonotopic organization of auditory cortical fields. Second, recent findings suggest that this decomposition is carried out in parallel for a wide range of frequency resolutions by neurons with frequency receptive fields of different sizes (bandwidths). A systematic representation of the range of frequency resolution and, equivalently, spectral integration shapes the functional organization of the iso-frequency domain. Distinct subregions, or "modules," along the iso-frequency domain can be demonstrated with various measures of spectral integration, including pure-tone tuning curves, noise masking, and electrical cochlear stimulation. This modularity in the representation of spectral integration is expressed by intrinsic cortical connections. This organization has implications for our understanding of psychophysical spectral integration measures such as the critical band and general cortical coding strategies.
Asunto(s)
Corteza Auditiva/fisiología , Percepción Auditiva/fisiología , Animales , Vías Auditivas/fisiología , Mapeo EncefálicoRESUMEN
Cochlear prostheses for electrical stimulation of the auditory nerve ("electrical hearing") can provide auditory capacity for profoundly deaf adults and children, including in many cases a restored ability to perceive speech without visual cues. A fundamental challenge in auditory neuroscience is to understand the neural and perceptual mechanisms that make rehabilitation of hearing possible in these deaf humans. We have developed a feline behavioral model that allows us to study behavioral and physiological variables in the same deaf animals. Cats deafened by injection of ototoxic antibiotics were implanted with either a monopolar round window electrode or a multichannel scala tympani electrode array. To evaluate the effects of perceptually significant electrical stimulation of the auditory nerve on the central auditory system, an animal was trained to avoid a mild electrocutaneous shock when biphasic current pulses (0.2 ms/phase) were delivered to its implanted cochlea. Psychophysical detection thresholds and electrical auditory brain stem response (EABR) thresholds were estimated in each cat. At the conclusion of behavioral testing, acute physiological experiments were conducted, and threshold responses were recorded for single neurons and multineuronal clusters in the central nucleus of the inferior colliculus (ICC) and the primary auditory cortex (A1). Behavioral and neurophysiological thresholds were evaluated with reference to cochlear histopathology in the same deaf cats. The results of the present study include: 1) in the cats implanted with a scala tympani electrode array, the lowest ICC and A1 neural thresholds were virtually identical to the behavioral thresholds for intracochlear bipolar stimulation; 2) behavioral thresholds were lower than ICC and A1 neural thresholds in each of the cats implanted with a monopolar round window electrode; 3) EABR thresholds were higher than behavioral thresholds in all of the cats (mean difference = 6.5 dB); and 4) the cumulative number of action potentials for a sample of ICC neurons increased monotonically as a function of the amplitude and the number of stimulating biphasic pulses. This physiological result suggests that the output from the ICC may be integrated spatially across neurons and temporally integrated across pulses when the auditory nerve array is stimulated with a train of biphasic current pulses. Because behavioral thresholds were lower and reaction times were faster at a pulse rate of 30 pps compared with a pulse rate of 2 pps, spatial-temporal integration in the central auditory system was presumably reflected in psychophysical performance.
Asunto(s)
Umbral Auditivo/fisiología , Implantes Cocleares , Nervio Coclear/fisiología , Sordera/fisiopatología , Psicofísica , Factores de Edad , Animales , Conducta Animal/fisiología , Gatos , Nervio Coclear/citología , Condicionamiento Psicológico/fisiología , Modelos Animales de Enfermedad , Estimulación Eléctrica , Potenciales Evocados Auditivos del Tronco Encefálico/fisiología , Audición/fisiología , Colículos Inferiores/citología , Colículos Inferiores/fisiología , Microelectrodos , Neuronas Aferentes/fisiología , Tiempo de Reacción/fisiología , Ventana Redonda/fisiología , Rampa Timpánica , Ganglio Espiral de la Cóclea/citología , Ganglio Espiral de la Cóclea/fisiologíaRESUMEN
Regional differences in spectral integration of neurons in cat primary auditory cortex (AI) suggest that regions differ in effects of background noise on operating characteristics of neurons. Therefore, tone-response threshold, best level (peak-rate intensity), dynamic range, and sharpness of tuning in quiet and in continuous broadband noise were mapped for single neurons along the isofrequency domain of AI. Neurons did not show an excitatory response to the noise. Noise invariably increased the tone-response threshold and best levels. Consequently, the dynamic ranges and receptive fields shifted to higher intensity levels without changes of average sharpness of tuning. These shifts were linearly related to noise level and showed little inter-neuronal variability for neurons in the central, mostly sharply tuned part of AI. In more dorsal and ventral parts of AI, neurons were more variable in tone-response threshold, dynamic range and best level, and no systematic relationship between increase in noise level, threshold increase and best-level increase was observed. We conclude that linear shifts in the operating range of neurons in central AI in the presence of continuous noise backgrounds do not affect other response properties and may relate to the unaltered analysis and representation of spectral components of sounds. In contrast, neurons in dorsal and ventral AI change response properties in a non-predictable way in the presence of noise in accordance with the more complex receptive field properties in those areas.
Asunto(s)
Corteza Auditiva/fisiología , Ruido/efectos adversos , Estimulación Acústica , Animales , Corteza Auditiva/anatomía & histología , Umbral Auditivo/fisiología , Gatos , Potenciales Evocados Auditivos/fisiología , Neuronas Aferentes/fisiologíaRESUMEN
Psychophysical detection thresholds for unmodulated electrical pulse trains or for sinusoidally amplitude-modulated (SAM) pulse trains were estimated in deaf juvenile cats using a conditioned avoidance paradigm. Biphasic current pulses (0.2 ms/phase) were delivered by scala tympani electrodes consisting of 4-8 electrode contacts driven as bipolar pairs. Electrical auditory brainstem response (EABR) thresholds were obtained periodically, and at the conclusion of behavioral training, response thresholds were obtained for neurons in the inferior colliculus (IC) and the primary auditory cortex (A1) in acute physiological experiments in the same animals. The results of the study include: (1) detection thresholds for unmodulated pulse trains and for SAM pulse trains were virtually identical; (2) EABR thresholds and behavioral thresholds were significantly correlated, although EABR thresholds consistently overestimated behavioral thresholds; (3) the lowest thresholds in the IC and the A1 were significantly correlated with behavioral thresholds, and (4) mean lowest thresholds in the IC and the A1 were essentially the same as the mean psychophysical detection threshold in the trained deaf cats.
Asunto(s)
Cóclea/fisiología , Sordera/cirugía , Estimulación Eléctrica , Potenciales Evocados Auditivos del Tronco Encefálico/fisiología , Animales , Umbral Auditivo/fisiología , Gatos , Implantes Cocleares , Estimulación Eléctrica/instrumentación , Diseño de EquipoRESUMEN
The effects of auditory deprivation on the spatial distribution of cortical response thresholds to electrical stimulation of the adult cat cochlea were evaluated. Threshold distributions for single- and multiple-unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in three groups of animals: adult, acutely implanted animals ("acute group"); adult animals, 2 wk after deafening and implantation ("short-term group"); adult, neonatally deafened animals ("long-term group") implanted after 2-5 years of deafness. For all three groups, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The two low-threshold regions in the acute and the short-term group were arranged cochleotopically. This was reflected in a systematic shift of the cortical locations with minimum thresholds as a function of cochlear position of the radial and monopolar stimulation electrodes. By contrast, the long-term deafened animals maintained only weak or no signs of cochleotopicity. In some cases of this group, significant deviations from a simple tri-partition of the dorsoventral axis of AI was observed. Analysis of the spatial extent of the low-threshold regions revealed that the activated area in acute cases was significantly smaller than the long- and the short-term cases for both dorsal and ventral AI. There were no significant differences in the rostrocaudal extent of activation between long- and short-term deafening, although the total activated area in the short-term cases was larger than in long-term deafened animals. The width of the narrow high-threshold ridge that separated the dorsal and ventral low-threshold regions was the widest for the acute cases and the narrowest for the short-term deafened animals. The findings of relative large differences in cortical response distributions between the acute and short-term animals suggests that the effects observed in long-term deafened animals are not solely a consequence of loss of peripheral innervation density. The effects may reflect electrode-specific effects or reorganizational changes based on factors such as differences in excitatory and inhibitory balance.