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PubMed1995

Representation of a voice onset time continuum in primary auditory cortex of the cat.

The representation of voice onset time (VOT) for 197 single units in cat primary auditory cortex was studied for a /ba/-/pa/ continuum in which VOT was varied in 5-ms steps from 0 to 70 ms. The effect of stimulus intensity, characteristic frequency of the neurons, and age of the animals was investigated. The minimum VOT represented in onset responses to both the voiceless and voiced parts of the sound (a "double-on" response) was dependent on overall stimulus level. An interaction was found between the efficacy of the burst in evoking neural activity and the size of the subsequent response to the onset of voicing. There was only a minor difference in the mean values for the minimal neural VOT for young (42 ms), juvenile (36 ms), and adult animals (46 ms), albeit that for individual young and juvenile animals more frequently values close to 10-15 ms were found. The cumulative distribution for the adult group showed a relative lack of neural VOTs around 30-40 ms. No other cues in the single unit and local neuronal group firing rate representation of VOT were found that were related to the categorical perception boundary.

Animals↗

PubMed1982

Neural representation of target distance in auditory cortex of the echolocating bat Myotis lucifugus.

1. Single- and multiunit recordings were obtained from neurons in the auditory cortex of the echolocating bat Myotis lucifugus, while trains of stimuli designed to simulate the bat's frequency-modulated (FM) orientation pulse and its returning echo were delivered. It was found that many neurons in the cortex responded selectively to pulse-echo pairs only if the time delay separating the artificial pulse and the echo was within a certain range. This response property is called "delay-dependent facilitation." Since echolating bats are known to utilize echo-delay information for the determination of target distance, it is postulated that these neurons are involved in the process of distance perception. 2. Two types of delay-sensitive neurons were characterized on the basis of their response patterns. P-type units had short maximum response delays, narrow delay response functions, and response latencies for pulse-echo pairs that were similar to their response latencies for single loud FM pulses. E-type units had longer maximum response delays, wide delay response functions, and pulse-echo pair response latencies that were time-locked to the echo. Another important difference between these two classes was that changes in the amplitude of the artificial echo caused systematic changes in the delay response of E-type units but not of P-type units. 3. The sharpness and stability of the delay response functions of P-type units suggested that they may encode target distance by responding at discrete echo delays. In contrast, delay tuning may not be an unambiguous determinant of echo delay in E-type units. Here, the most consistent and reliable response parameter for echo delay is the time at which the responses occurred. This suggested that echo delay could be encoded by the temporal pattern of responses in E-type units in relation to the responses evoked by the outgoing orientation cry. The different range of delay of delay sensitivity of P-type and E-type units indicates that these two mechanisms could be operating at different ranges of target distance. 4. P-type and E-type responses may not be due to different populations of neurons but to different response properties of the same population under different conditions. Evidence for this proposition was obtained by showing that in some recordings, decreases in the amplitude of the artificial pulse caused a switch in response from a long best delay, E-type response to a short best delay, P-type response. This suggested that the delay sensitivity of cortical neurons could be under the bat's control based on the intensity of its pulse emissions.

Animals↗

PubMed2005

Microstimulation in auditory cortex provides a substrate for detailed behaviors.

Sensory cortical prostheses have potential to aid people suffering from blindness, deafness and other sensory deficits. However, research to date has shown that sensation thresholds via epicortical stimulation are surprisingly large. These thresholds result in potentially deleterious electrical currents, as well as large activation volumes. Large activation volumes putatively limit the corresponding number of independent stimulation channels in a neural prosthesis. In this study, penetrating stimulation of the auditory cortex was tested for its ability to transmit salient information to behaving rat subjects. Here, we show that subjects that were previously trained to discriminate natural stimuli immediately discriminated different microstimulation cues more accurately and with shorter response latencies than the natural stimuli. Additionally, the cortical microstimulation resulted in a generalization gradient across locations within the cortex. The results demonstrate the efficacy of using closely spaced cortical microstimulation to efficiently transmit highly salient and discriminable information to a behaving subject.

Acoustic Stimulation↗

PubMed2004

Associative learning shapes the neural code for stimulus magnitude in primary auditory cortex.

Since the dawn of experimental psychology, researchers have sought an understanding of the fundamental relationship between the amplitude of sensory stimuli and the magnitudes of their perceptual representations. Contemporary theories support the view that magnitude is encoded by a linear increase in firing rate established in the primary afferent pathways. In the present study, we have investigated sound intensity coding in the rat primary auditory cortex (AI) and describe its plasticity by following paired stimulus reinforcement and instrumental conditioning paradigms. In trained animals, population-response strengths in AI became more strongly nonlinear with increasing stimulus intensity. Individual AI responses became selective to more restricted ranges of sound intensities and, as a population, represented a broader range of preferred sound levels. These experiments demonstrate that the representation of stimulus magnitude can be powerfully reshaped by associative learning processes and suggest that the code for sound intensity within AI can be derived from intensity-tuned neurons that change, rather than simply increase, their firing rates in proportion to increases in sound intensity.

Acoustic Stimulation↗

PubMed2005

Spectro-temporal processing during speech perception involves left posterior auditory cortex.

This functional magnetic resonance imaging study investigates the neural underpinnings of spectro-temporal integration during speech perception. Participants performed an auditory discrimination task on a set of sine-wave analogues that could be perceived as either nonspeech or speech. Behavioural results revealed a difference in the processing mode; spectro-temporal integration occurred during speech perception, but not when stimuli were perceived as nonspeech. In terms of neuroimaging, we observed an activation increase in the left posterior primary and secondary auditory cortex, namely Heschl's gyrus and planum temporale encroaching onto the superior temporal sulcus, reflecting a shift from auditory to speech perception. This finding demonstrates that the left posterior superior temporal lobe is essential for spectro-temporal processing during speech perception.

Acoustic Stimulation↗

PubMed2002

Selectivity for echo spectral interference and delay in the auditory cortex of the big brown bat Eptesicus fuscus.

The acoustic environment for an echolocating bat can contain multiple objects that reflect echoes so closely separated in time that they are almost completely overlapping. This results in a single echo with a spectrum characterized by deep notches due to interference. The object of this study was to document the possible selectivity, or lack thereof, of auditory neurons to the temporal separation of biosonar signals on a coarse (ms) and fine (micros) temporal scale. We recorded single-unit activity from the auditory cortex of big brown bats while presenting four protocol designs using wideband FM signals. The protocols simulated a pair of partially overlapping echoes where the separation between the first and second echo varied between 0 and 72 micros, a pulse followed by a single echo at varying delay from 0 to 30 ms, a pulse followed at a fixed delay by a pair of partially overlapping echoes that had a varying temporal separation of 0-72 micros, and a pulse followed, with a varying delay between 0 and 30 ms, by a pair of echoes that themselves had a fixed temporal separation on a microsecond time scale. About half of the cortical units showed increased spike counts to pairs of partially overlapping echoes at particular separations (6-72 micros) compared with a baseline stimulus at 0-micros separation. For many neurons tested with a pulse followed by two overlapping echoes, we observed a sensitivity to the coarse delay between the pulse and pair of overlapping echoes and to the separation between the two echoes themselves. The sensitivity to the partial overlap between the two echoes was not tuned to a single temporal separation. For bats, this means that the absolute range to the closest reflector and range between reflectors may be jointly encoded across a small population of single units. There are several possible neuronal mechanisms for encoding the separation between two nearby echoes based on the sensitivity to spectral notches.

Acoustic Stimulation↗

PubMed1985

[Relation between responses of auditory cortex neurons in the cat and the signal significance of acoustic stimuli in the instrumental alimentary reflex].

Responses of 93 neurons to single sound clicks and to a series of 10 clicks with frequency 1000 c as well as reactions of 66 neurons after working out of conditioned reflex were studied in chronic cats. The clicks became a positive conditioned stimulus and the series of clicks were used as a negative differential stimulus in the reflex. It was shown that the working out and realization of the differentiation did not lead to amplification of inhibition in the auditory cortex neurons. Responses of neurons to positive as well as to negative conditioned stimuli underwent the same changes after learning that can be considered as evidence of the similar role of auditory neurons in the instrumental reflex realization and its differential inhibition. The existence of neuronal groups responding by excitation and inhibition only to stimulus with definite signal meaning is supposed to be important for the process of differentiation.

Animals↗

PubMed2004

Within- and between-channel gap detection in the human auditory cortex.

We examined the neural correlates associated with a short gap between two identical pure tones (within-channel) and between two different tones (between-channel) in an odd-ball paradigm. Gap durations were selected such that a gap between identical tones was as discriminable as a gap between two different tones. Spatio-temporal dipole source modeling of electrophysiological data revealed a significant difference between standard and deviant gap stimuli, with mismatch negativity responses that were comparable in amplitude and latency for within- and between-channel conditions. Therefore, the ability to automatically register discontinuity (i.e., gap) within and between channels is comparable despite significant differences in gap size. The dipole source modeling suggests that both within- and between-gap signals are represented in or near the primary auditory cortex.

Acoustic Stimulation↗

PubMed2003

Lateralization of auditory-cortex functions.

In the present review, we summarize the most recent findings and current views about the structural and functional basis of human brain lateralization in the auditory modality. Main emphasis is given to hemodynamic and electromagnetic data of healthy adult participants with regard to music- vs. speech-sound encoding. Moreover, a selective set of behavioral dichotic-listening (DL) results and clinical findings (e.g., schizophrenia, dyslexia) are included. It is shown that human brain has a strong predisposition to process speech sounds in the left and music sounds in the right auditory cortex in the temporal lobe. Up to great extent, an auditory area located at the posterior end of the temporal lobe (called planum temporale [PT]) underlies this functional asymmetry. However, the predisposition is not bound to informational sound content but to rapid temporal information more common in speech than in music sounds. Finally, we obtain evidence for the vulnerability of the functional specialization of sound processing. These altered forms of lateralization may be caused by top-down and bottom-up effects inter- and intraindividually In other words, relatively small changes in acoustic sound features or in their familiarity may modify the degree in which the left vs. right auditory areas contribute to sound encoding.

Acoustic Stimulation↗

PubMed1988

Muscarinic agonists modulate spontaneous and evoked unit discharge in auditory cortex of cat.

The present experiments studied the effects of cholinergic agonists and antagonists on the spontaneous and acoustic-evoked discharge of auditory cortical neurons and examined whether these effects were mediated by muscarinic cholinergic receptors. A primary focus of this report is the analysis of specific effects of these agents on the spontaneous and tone-evoked discharge and on different temporal components of the evoked discharge. Single neurons were recorded in the auditory cortex of chronically prepared, awake cats with multibarrel micropipette electrodes. The responses to acoustic stimuli were obtained before, during, and following continuous ejection of cholinergic agonist or antagonists by micropressure. The mean rate of discharge of the neurons was analyzed quantitatively for spontaneous discharge and for different peaks of the tone-evoked PSTH corresponding to tone "on," "through," and "off" responses. Acetylcholine (ACh) and acetyl-beta-methacholine (MCh) produced significant effects on spontaneous activity in 72% and 68% of neurons tested, respectively. Tone-evoked responses were effected in 92% and 82% of cells tested, respectively. The ability of these agonists to modify spontaneous or evoked activity was dose-dependent. Agonist effects on spontaneous and evoked activity were often different in the same cell; however, effects on spontaneous activity did predict effects on "through" responses. The most common effect of ACh or MCh on evoked activity was facilitation of the tone "on" response. For neurons with multicomponent discharge patterns in response to tones, the agonists had nonuniform effects on different response components. However, the effects of ACh on the "on" and "off" responses covaried. Hence cholinergic agonists produce heterogeneous, selective effects on different components of the responses of auditory cortical neurons rather than simple increases or decreases in discharge level. The effects of cholinergic agonists were modified in the presence of atropine. The effects of MCh were blocked by atropine in a higher proportion of cases than those of ACh.

Acetylcholine↗

PubMed1986

Role of the dog's auditory cortex in discrimination of sound signals simulating sound source movement.

The ability of seven dogs to discriminate signal simulating sound source movement was studied using the avoidance technique. It was found that dogs can differentiate moving and stationary sound sources, and also discern the direction of the sound source movement. In addition, this study has defined the limits of the conditions under which sound source movement perception occurs. In each dog, unilateral ablation of the auditory cortex was followed by a localization deficit on the side contralateral to the ablation. Bilateral cortical lesions led to complete absence of the ability to discriminate source movement, simulated by changing stimulus interaural time differences. However, the dogs' ability to discriminate the movement after unilateral ablation by detecting interaural intensity differences was preserved, although their discriminative ability was lower than that of intact dogs.

Acoustic Stimulation↗

PubMed2001

Auditory evoked potentials from auditory cortex, medial geniculate nucleus, and inferior colliculus during sleep-wake states and spike-wave discharges in the WAG/Rij rat.

OBJECTIVE: Click auditory evoked potentials (AEP) were simultaneously recorded from the auditory cortex (ACx), the medial geniculate nucleus (MGN), and the inferior colliculus (IC) in the freely moving WAG/Rij rat, to investigate state-dependent changes of the AEP in different anatomical locations along the auditory pathway. METHODS: AEPs obtained during active (AW) and passive wakefulness (PW), slow wave sleep (SWS), rapid-eye-movement sleep (REM) and generalized spike-wave discharges (SWD; a specific trait of the WAG/Rij rat, a genetic model for absence epilepsy), were compared. RESULTS: The early components in ACx, MGN and IC were stable throughout the sleep-wake cycle and SWD, apart from a slight increase in the IC during SWD. At all three locations a prominent enlargement of a later component (i.e., N32 in IC, N33 in MGN, and N44 in ACx) was found during SWS and SWD. CONCLUSIONS: The early AEP components are not modulated by the normal sleep-wake states, and are not impaired during SWD. A strong state-dependent modulation of a later AEP component occurs at all three anatomical locations investigated. This suggests that apart from the thalamic burst firing mode, additional mechanisms must exist for the enlargement of the AEP during EEG-synchronized states at the prethalamic and cortical level.

Action Potentials↗

PubMed1997

Field-specific responses in the auditory cortex of the unanaesthetized Mongolian gerbil to tones and slow frequency modulations.

Responses of multi-units in the auditory cortex (AC) of unanaesthetized Mongolian gerbils to pure tones and to linearly frequency modulated (FM) sounds were analysed. Three types of responses to pure tones could be clearly distinguished on the basis of spectral tuning properties, response latencies and overall temporal response pattern. In response to FM sweeps these three types discharged in a temporal pattern similar to tone responses. However, for all type-1 units the latencies of some phasic response components shifted systematically as a function of range and/or speed of modulation. Measurements of response latencies to FMs revealed that such responses were evoked whenever the modulation reached a particular instantaneous frequency (Fi). Effective Fi was: (1) independent of modulation range and speed, (2) always reached before the modulation arrived at a local maximum of the frequency response function (FRF) and consequently differed for downward and upward sweeps, and (3) was correlated with the steepest slope of that FRF maximum. The three different types of units were found in discrete and separate fields or regions of the AC. It is concluded that gross temporal response properties are one of the key features distinguishing auditory cortical regions in the Mongolian gerbil.

Acoustic Stimulation↗

PubMed1980

[Relationship between background and induced activity of cat auditory cortex neurons during elaboration of a defensive conditioned reflex to acoustic stimulation].

Using glass microelectrodes, the impulse activity of auditory cortex neurons was studied in chronic experiments on cats. It was shown that an increase in frequency of the background activity occurring in many neurons during conditioning was often observed prior to the appearance of conditioned motor reactions. Comparison of mean frequencies of the background activity and corresponding evoked activity shows, on the one hand, the nonlinear dependence between them and, on the other hand, the prevalence of activation reactions as compared with inhibitory ones. Some levels of background activity were distinguished corresponding to mean evoked activity frequency ranges. Background activity analysis performed prior to the presentation of the conditioned stimulus and after its cessation (aftereffect) shows that at stable conditioned reflex mean frequencies in these periods are close to each other.

Animals↗

PubMedDate not supplied

Interhemispheric anatomical differences in human primary auditory cortex: probabilistic mapping and volume measurement from magnetic resonance scans.

The gyral morphology of the region of the primary auditory cortex (PAC) in the human brain is highly variable, and possible asymmetries between the hemispheres have been noted since the beginning of the century. We mapped the location and extent of PAC as identified from gross anatomical landmarks in magnetic resonance scans that had been transformed into Talairach-Tournoux stereotaxic space. Individual maps were averaged to produce a probabilistic map of the region which can be co-registered with any image of brain structure or function that has been similarly transformed. The map can be used to localize a region of interest, such as a lesion, or an activation focus from position omission tomography or functional magnetic resonance imaging, within a specified range of probability. We also measured the total volume of the region and found a significant L > R asymmetry both on average and in the majority of subjects. Automatic segmentation of the volumes into grey and white matter revealed larger volumes of white, but not grey matter on the left. This larger volume of cortical connecting fibres may be related to the known left-hemisphere dominance for speech, and a preferential role for left PAC in processing temporal aspects of auditory stimuli is suggested.

Adolescent↗

PubMed2004

Tonotopic organization of the human auditory cortex probed with frequency-modulated tones.

Using neuromagnetic source imaging, we investigated tonotopic representation and direction sensitivity in the auditory cortex of humans (N = 15). For this purpose, source analysis was undertaken at every single sampling point during the presentation of a frequency-modulated tone (FM) sweeping slowly downward or upward across periods of 3 s duration. Stimuli were selected to target response properties of the central part of the primary auditory cortical field, which has been shown to exhibit sensitivity to distinct FM-sound features as compared to the ventral and dorsal part. Linear mixed-effects model statistics confirm tonotopic gradients in medial-lateral and anterior-posterior directions. The high resolution provided by this method revealed that the relationship between frequency and spatial location of the responding neural tissue is nonlinear. The idea that neurons specifically sensitive to the employed sound characteristics (slow, downward modulation) were activated is supported by the fact that the upward sweep of identical duration produced a different pattern of functional organisation.

Acoustic Stimulation↗

PubMed1992

Spectral selectivity of FM-FM neurons in the auditory cortex of the echolocating bat, Myotis lucifugus.

1. Spectral sensitivity was examined in delay-sensitive neurons in the auditory cortex of the awake FM bat, Myotis lucifugus. FM stimuli sweeping 60 kHz downward in 4 ms were used as simulated pulse-echo pairs to measure delay-dependent responses. At each neuron's best delay, the pulse and/or echo were divided into 4 FM quarters (Ist, IInd, IIIrd, and IVth), each sweeping 15 kHz in 1 ms, and quarters essential for delay sensitivity were determined for both pulse and echo. 2. For the pulse, the IVth quarter was essential for delay sensitivity in the majority of neurons. For the echo, the essential quarter for most neurons was the IInd, IIIrd, or IVth. 3. Different quarters of the pulse and echo were essential for delay sensitivity in 68% of the neurons examined. 4. This study provides neurophysiological evidence linking both spectral and temporal processing in delay-sensitive neurons of Myotis. Since spectral cues can provide target-shape information, sensitivity to both spectral and temporal parameters in single neurons may endow these neurons in FM bats with the potential for target analysis other than echo-ranging.

Acoustic Stimulation↗

PubMed1988

Intracortical connections and their physiological correlates in the primary auditory cortex (AI) of the cat.

We studied the functional and anatomical properties of the intrinsic connections in the primary auditory cortex (AI) of the cat by using physiological mapping and retrograde tracing methods. Our results revealed that a focal microinjection of tracer labeled as many as five intracortical patches in AI. The patches contained labeled pyramidal and non-pyramidal cell types, most of which were clustered in the middle layers. A densely distributed anterograde-like reaction product was present in the superficial layers above the labeled cells. The distribution of the patches was anisotropic, with most patches occurring dorsal, ventral, and anterior to the injection site. We examined the correlation between the characteristic frequency (CF) and binaural response properties of the injected and labeled regions. We found local labeling in regions possessing CFs equivalent to or slightly greater than that of the injected area. This appears to be a specific connection since we were able to predict the general location of many of the patches on the basis of the organization of the isofrequency domains. Patches were more numerous dorsoposterior to the injection site when the isofrequency contours ran obliquely (i.e., dorsoposterior to ventroanterior) across AI. The binaural response properties of the injected and labelled regions, however, were unrelated.

Animals↗