Discharge properties of single units in auditory part of reticular nucleus of thalamus in cat

1. Extracellular recordings from 153 single units were obtained in the auditory part of the reticular nucleus (RE) of the thalamus of anesthetized cats. 2. In absence of acoustic stimulation, the majority of RE units (75%) had a spontaneous activity characterized by bursts of spikes lasting over 30 ms, alternating with isolated discharges; in contrast, only 30% of units in the medial geniculate body (MGB) showed these long bursts during spontaneous activity. 3. For a majority of RE units (65%), activity evoked by noise bursts consisted in complex response patterns with multiple excitatory and/or inhibitory components. For this stimulus, most units (79%) showed nonmonotonic rate-intensity functions, and median response latency to the first excitatory component was 16 ms, which is slightly longer than that obtained in the anterior part of the ventral division of the MGB for the same conditions. 4. Frequency tuning to pure tones in RE was usually broad: the median value of the width of response ranges was five octaves in RE, as compared to two octaves in the ventral division of the MGB and three octaves in the medial division of the MGB. Responses to tones were complex, usually characterized by frequent changes of response pattern with frequency. No preferential arrangement of units as a function of their best frequency was found along a rostrocaudal or a dorsolateral to ventromedial axis. 5. The present study demonstrates that units in the auditory part of RE have discharge properties clearly different from the main auditory thalamic nucleus, the MGB.     

Auditory corticocortical interconnections in the cat: evidence for parallel and hierarchical arrangement of the auditory cortical areas

Summary The origin and laminar arrangement of the homolateral and callosal projections to the anterior (AAF), primary (AI), posterior (PAF) and secondary (AII) auditory cortical areas were studied in the cat by means of electrophysiological recording and WGA-HRP tracing techniques. The transcallosal projections to AAF, AI, PAF and AII were principally homotypic since the major source of input was their corresponding area in the contralateral cortex. Heterotypic transcallosal projections to AAF and AI were seen, originating from the contralateral AI and AAF, respectively. PAF received heterotypic commissural projections from the opposite ventroposterior auditory cortical field (VPAF). Heterotypic callosal inputs to AII were rare, originating from AAF and AI. The neurons of origin of the transcallosal connections were located mainly in layers II and III (70–92%), and less frequently in deep layers (V and VI, 8–30%). Single unit recordings provided evidence that both homotypic and heterotypic transcallosal projections connect corresponding frequency regions of the two hemispheres. The regional distribution of the anterogradely labeled terminals indicated that the homotypic and heterotypic auditory transcallosal projections are reciprocal. The present data suggest that the transcallosal auditory interconnections are segregated in 3 major parallel components (AAF-AI, PAF-VPAF and AII), maintaining a segregation between parallel functional channels already established for the thalamocortical auditory interconnections. For the intrahemispheric connections, the analysis of the retrograde tracing data revealed that AAF and AI receive projections from the homolateral cortical areas PAF, VPAF and AII, whose neurons of origin were located mainly in their deep (V and VI) cortical layers. The reciprocal interconnections between the homolateral AAF and AI did not show a preferential laminar arrangement since the neurons of origin were distributed almost evenly in both superficial (II and III) and deep (V and VI) cortical layers. On the contrary, PAF received inputs from the homolateral cortical fields AAF, AI, AII and VPAF, originating predominantly from their superficial (II and III) layers. The homolateral projections reaching AII originated mainly from the superficial layers of AAF and AI, but from the deep layers of VPAF and PAF. The laminar distribution of anterogradely labeled terminal fields, when they were dense enough for a confident identification, was systematically related to the laminar arrangement of neurons of origin of the reciprocal projection: a projection originating from deep layers was associated with a reciprocal projection terminating mainly in layer IV, whereas a projection originating from superficial layers was associated with a reciprocal projection terminating predominantly outside layer IV. This laminar distribution indicates that the homolateral auditory cortical interconnections have a feed-forward/feed-back organization, corresponding to a hierarchical arrangement of the auditory cortical areas, according to criteria previously established in the visual system of primates. The principal auditory cortical areas could be ranked into 4 distinct hierarchical levels. The tonotopically organized areas AAF and AI represent the lowest level. The second level corresponds to the non-tonotopically organized area AII. Higher, the tonotopically organized areas VPAF and PAF occupy the third and fourth hierarchical levels, respectively.Abbreviations AAF anterior auditory cortical area - AI primary auditory cortical area - AII secondary auditory cortical area - BF best frequency - C cerebral cortex - CA caudate nucleus - CL claustrum - D dorsal nucleus of the dorsal division of the MGB - ea anterior ectosylvian sulcus - ep posterior ectosylviansulcus - IC internal capsule - LGN lateral geniculate nucleus - LV pars lateralis of the ventral division of the MGB - LVe lateral ventricule - M pars magnocellularis of the medial division of the MGB - MGB medial geniculate body - MGBv ventral division of the MGB - OT optic tract - OV pars ovoidea of the ventral division of the MGB - PAF posterior auditory cortical area - PH parahippocampal cortex - PO lateral part of the posterior group of thalamic nuclei - PU putamen - RE reticular complex of thalamus - rs rhinal sulcus - SG suprageniculate nucleus of the dorsal division of the MGB - ss suprasylvian sulcus - TMB tetrametylbenzidine - VBX ventrobasal complex - VLa ventrolateral complex - VL ventro-lateral nucleus of the ventral division of the MGB - WGA-HRP wheat germ agglutinin conjugated to horse-radish peroxidase - WM white matter - VPAF ventro-posterior auditory cortical area     

Interconnections of the auditory cortical fields of the cat with the cingulate and parahippocampal cortices

Summary The interconnections of the auditory cortex with the parahippocampal and cingulate cortices were studied in the cat. Injections of the anterograde and retrograde tracer WGA-HRP were performed, in different cats (n = 9), in electrophysiologically identified auditory cortical fields. Injections in the posterior zone of the auditory cortex (PAF or at the PAF/AI border) labeled neurons and axonal terminal fields in the cingulate gyrus, mainly in the ventral bank of the splenial sulcus (a region that can be considered as an extension of the cytoarchitectonic area Cg), and posteriorly in the retrosplenial area. Labeling was also present in area 35 of the perirhinal cortex, but it was sparser than in the cingulate gyrus. Following WGA-HRP injection in All, no labeling was found in the cingulate gyrus, but a few neurons and terminals were labeled in area 35. In contrast, no or very sparse labeling was observed in the cingulate and perirhinal cortices after WGA-HRP injections in the anterior zone of the auditory cortex (AI or AAF). A WGA-HRP injection in the cingulate gyrus labeled neurons in the posterior zone of the auditory cortex, between the posterior ectosylvian and the posterior suprasylvian sulci, but none was found more anteriorly in regions corresponding to AI, AAF and AII. The present data indicate the existence of preferential interconnections between the posterior auditory cortex and the limbic system (cingulate and parahippocampal cortices). This specialization of posterior auditory cortical areas can be related to previous observations indicating that the anterior and posterior regions of the auditory cortex differ from each other by their response properties to sounds and their pattern of connectivity with the auditory thalamus and the claustrum.Abbreviations AAF anterior auditory cortical field - aes anterior ectosylvian sulcus - AI primary auditory cortical field - AII secondary auditory cortical field - ALLS anterior-lateral lateral suprasylvian visual area - BF best frequency - C cerebral cortex - CC corpus callosum - CIN cingulate cortex - CL claustrum - DLS dorsal lateral suprasylvian visual area - DP dorsoposterior auditory area - E entorhinal cortex - IC inferior colliculus - LGN lateral geniculate nucleus - LV pars lateralis of the ventral division of the MGB - LVe lateral ventricule - MGB medial geniculate body - OT optic tract - OV pars ovoidea of the ventral division of the MGB - PAF posterior auditory cortical field - pes posterior ectosylvian sulcus - PLLS posterior-lateral lateral suprasylvian visual area - PS posterior suprasylvian visual area - PU putamen - RE reticular complex of thalamus - rs rhinal sulcus - SC superior colliculus - SS suprasylvian sulcus - T temporal auditory cortical field - TMB tetramethylbenzidine - VBX ventrobasal complex of thalamus, external nucleus - VL pars ventrolateralis of the ventral division of the MGB - VLS ventrolateral suprasylvian visual area - VPAF ventroposterior auditory cortical field - WGA-HRP wheat germ agglutinin labeled with horseradish peroxidase - wm white matter     

Effects of changes in cortical arousal and of auditory cortex cooling on neuronal activity in the medial geniculate body

Summary The activity of cells in the medial geniculate body (MGB) of adult cats was recorded during different states of cortical arousal with and without cooling of the auditory cortex. In the absence of auditory cortex cooling, the overall mean unit spontaneous discharge rate was 49% higher during desynchronized Electrocorticogram (ECoG) periods (high cortical arousal) than during synchronized periods (low cortical arousal). Responses to sound were somewhat more prominent vis-à-vis the spontaneous activity during periods of high arousal. Changes in spontaneous discharge rate associated with arousal shifts were significantly reduced during auditory cortex cooling. When the ECoG changed from desynchronized to synchronized activity, MGB cells showed a change in discharge pattern, typically characterized by an increase in both high-rate bursts and long-interval pauses. These changes were duplicated for most cells by cooling of the auditory cortex. Corticofugal fiber discharge thus has an effect on MGB neuronal activity which is dependent on the level of cortical arousal. This effect is most likely a result of direct corticogeniculate activity, though indirect auditory cortex — brainstem — MGB routes may also be involved.Supported by a grant from the University of Illinois Campus Research Board     

Correlation of neural response properties with auditory thalamus subdivisions in the awake marmoset

As the information bottleneck of nearly all auditory input that reaches the cortex, the auditory thalamus serves as the basis for establishing auditory cortical processing streams. The functional organization of the primary and nonprimary subdivisions of the auditory thalamus is not well characterized, particularly in awake primates. We have recorded from neurons in the auditory thalamus of awake marmoset monkeys and tested their responses to tones, band-pass noise, and temporally modulated stimuli. We analyzed the spectral and temporal response properties of recorded neurons and correlated those properties with their locations in the auditory thalamus, thereby forming the basis for parallel output channels. Three medial geniculate body (MGB) subdivisions were identified and studied physiologically and anatomically, although other medial subdivisions were also identified anatomically. Neurons in the ventral subdivision (MGV) were sharply tuned for frequency, preferred narrowband stimuli, and were able to synchronize to rapid temporal modulations. Anterodorsal subdivision (MGAD) neurons appeared well suited for temporal processing, responding similarly to tone or noise stimuli but able to synchronize to the highest modulation frequencies and with the highest temporal precision among MGB subdivisions. Posterodorsal subdivision (MGPD) neurons differed substantially from the other two subdivisions, with many neurons preferring broadband stimuli and signaling changes in modulation frequency with nonsynchronized changes in firing rate. Most neurons in all subdivisions responded to increases in tone sound level with nonmonotonic changes in firing rate. MGV and MGAD neurons exhibited responses consistent with provision of thalamocortical input to core regions, whereas MGPD neurons were consistent with provision of input to belt regions.     

Modulatory effect of cortical activation on the lemniscal auditory thalamus of the Guinea pig

In the present study, we investigated the point-to-point modulatory effects from the auditory cortex to the thalamus in the guinea pig. Corticofugal modulation on thalamic neurons was studied by electrical activation of the auditory cortex. The modulation effect was sampled along the frontal or sagittal planes of the auditory thalamus, focusing on the ventral division (MGv) of the medial geniculate body (MGB). Electrical activation was targeted at the anterior and dorsocaudal auditory fields, to which the MGv projects and from which it assumptively receives reciprocal projections. Of the 101 MGv neurons examined by activation of the auditory cortex through passing pulse trains of 100-200 microA current into one after another of the three implanted electrodes (101 neurons x 3 stimulation sites = 303 cases), 208 cases showed a facilitatory effect, 85 showed no effect, and only 10 cases (7 neurons) showed an inhibitory effect. Among the cases of facilitation, 63 cases showed a facilitatory effect >100%, and 145 cases showed a facilitatory effect from 20-100%. The corticofugal modulatory effect on the MGv of the guinea pig showed a widespread, strong facilitatory effect and very little inhibitory effect. The MGv neurons showed the greatest facilitations to stimulation by the cortical sites, with the closest correspondence in BF. Six of seven neurons showed an elevation of the rate-frequency functions when the auditory cortex was activated. The comparative results of the corticofugal modulatory effects on the MGv of the guinea pig and the cat, together with anatomical findings, hint that the strong facilitatory effect is generated through the strong corticothalamic direct connection and that the weak inhibitory effect might be mainly generated via the interneurons of the MGv. The temporal firing pattern of neuronal response to auditory stimulus was also modulated by cortical stimulation. The mean first-spike latency increased significantly from 15.7 +/- 5.3 ms with only noise-burst stimulus to 18.3 +/- 4.9 ms (n = 5, P < 0.01, paired t-test), while the auditory cortex was activated with a train of 10 pulses. Taking these results together with those of previous experiments conducted on the cat, we speculate that the relatively weaker inhibitory effect compared with that in the cat could be due to the smaller number of interneurons in the guinea pig MGB. The corticofugal modulation of the firing pattern of the thalamic neurons might enable single neurons to encode more auditory information using not only the firing rate but also the firing pattern.     

Branched projections in the auditory thalamocortical and corticocortical systems

Branched axons (BAs) projecting to different areas of the brain can create multiple feature-specific maps or synchronize processing in remote targets. We examined the organization of BAs in the cat auditory forebrain using two sensitive retrograde tracers. In one set of experiments (n=4), the tracers were injected into different frequency-matched loci in the primary auditory area (AI) and the anterior auditory field (AAF). In the other set (n=4), we injected primary, non-primary, or limbic cortical areas. After mapped injections, percentages of double-labeled cells (PDLs) in the medial geniculate body (MGB) ranged from 1.4% (ventral division) to 2.8% (rostral pole). In both ipsilateral and contralateral areas AI and AAF, the average PDLs were <1%. In the unmapped cases, the MGB PDLs ranged from 0.6% (ventral division) after insular cortex injections to 6.7% (dorsal division) after temporal cortex injections. Cortical PDLs ranged from 0.1% (ipsilateral AI injections) to 3.7% in the second auditory cortical area (AII) (contralateral AII injections). PDLs within the smaller (minority) projection population were significantly higher than those in the overall population. About 2% of auditory forebrain projection cells have BAs and such cells are organized differently than those in the subcortical auditory system, where BAs can be far more numerous. Forebrain branched projections follow different organizational rules than their unbranched counterparts. Finally, the relatively larger proportion of visual and somatic sensory forebrain BAs suggests modality specific rules for BA organization.     

Differential distribution of burst and single-spike responses in auditory thalamus

The medial geniculate body (MGB) of the auditory thalamus comprises lemniscal and nonlemniscal neurons that project to the primary auditory cortex and limbic structures, respectively. Here we show that in anesthetized guinea pigs, MGB responses to a noise-burst stimulus exhibit distinct and synaptic pathway-specific firing patterns. The majority of nonlemniscal MGB cells exhibited bursting responses, whereas lemniscal neurons discharged mainly single or spike doublets. The burst firing is delayed in nonlemniscal neurons and exhibited several features that are characteristics of those mediated by low-threshold Ca(2+) spikes. Such a synaptic pathway-specific allocation of bursting and single-spike firing patterns is consistent with the notion of parallel processing of auditory information in thalamocortical system.     

Modulation of thalamic auditory neurons by the primary auditory cortex

The central auditory system consists of the lemniscal and nonlemniscal pathways or systems, which are anatomically and physiologically different from each other. In the thalamus, the ventral division of the medial geniculate body (MGBv) belongs to the lemniscal system, whereas its medial (MGBm) and dorsal (MGBd) divisions belong to the nonlemniscal system. Lemniscal neurons are sharply frequency-tuned and provide highly frequency-specific information to the primary auditory cortex (AI), whereas nonlemniscal neurons are generally broadly frequency-tuned and project widely to cortical auditory areas including AI. These two systems are presumably different not only in auditory signal processing, but also in eliciting cortical plastic changes. Electric stimulation of narrowly frequency-tuned MGBv neurons evokes the shift of the frequency-tuning curves of AI neurons toward the tuning curves of the stimulated MGBv neurons (tone-specific plasticity). In contrast, electric stimulation of broadly frequency-tuned MGBm neurons augments the auditory responses of AI neurons and broadens their frequency-tuning curves (nonspecific plasticity). In our current studies, we found that electric stimulation of AI evoked tone-specific plastic changes of the MGBv neurons, whereas it degraded the frequency tuning of MGBm neurons by inhibiting their auditory responses. AI apparently modulates the lemniscal and nonlemniscal thalamic neurons in quite different ways. High MGBm activity presumably makes AI neurons less favorable for fine auditory signal processing, whereas high MGBv activity makes AI neurons more suitable for fine processing of specific auditory signals and reduces MGBm activity.     

Cells in the rat auditory system have sensory-delay correlates during the performance of an auditory working memory task

Single unit activity was recorded from rat auditory cortex (AC), medial geniculate body (MGB), and inferior colliculus (IC) during performance of a continuous nonmatching-to-sample task. The rats made go and no-go responses to indicate whether the current tone was the same as (match) or different from (nonmatch) the preceding tone. Between 31% and 55% of the units from AC, MGB, and IC showed sensory correlates (differences in activity to the two types of tones), indicating an involvement in sensory discrimination. Twenty percent of the units from AC and MGB had delay correlates (sustained differential activity during the delay immediately after the tones), indicating an involvement in retention. Most of the units with delay correlates also had sensory correlates. These results suggest that the auditory system, especially AC and MGB, discriminates and retains auditory stimuli in an auditory working memory.     

Overlapping projections to the amygdala and striatum from auditory processing areas of the thalamus and cortex.

The purpose of this study was to advance our understanding of the anatomical organization of sensory projections to the amygdala, and specifically to identify potential interactions within the amygdala between thalamic and cortical sensory projections of a single sensory modality. Thus, interconnections between the amygdala and acoustic processing areas of the thalamus and cortex were examined in the rat using WGA-HRP as an anterograde and a retrograde axonal tracer. Injections placed in medial aspects of the medial geniculate body (MGB) produced anterograde transport to the lateral nucleus of the amygdala and to adjacent areas of the striatum. Injections of primary auditory cortex (TE1) produced no transport to amygdala. In contrast, injections ventral to TE1 involving TE3 and perirhinal periallocortex (PRh) produced anterograde transport in the subcortical forebrain that was indistinguishable from that produced by the MGB injections. The TE3 and PRh injections also resulted in retrograde transport to primary auditory cortex and to MGB, thus confirming the involvement of these ventral cortical areas in auditory functions. Injections of the lateral nucleus of the amygdala resulted in retrograde transport back to the medial areas of MGB and to temporal cortical areas PRh, TE3, and the ventral most part of TE1. Thus, auditory processing regions of the thalamus and cortex give rise to overlapping (possibly convergent) projections to the lateral nucleus of the amygdala. These projections may allow diverse auditory signals to act on common ensembles of amygdaloid neurons and may therefore play a role in the integration of sensory messages leading to emotional reactions.     

Dexamethasone induced changes of neural activity in the auditory cortex of rats

Previous studies have suggested that elevation of glucocorticoid level can alter auditory processing and may have relevance for auditory hallucinations. However, the neural mechanism underlying the glucocorticoid induced sensory change remains unclear. To examine the effects of glucocorticoid on the neuronal spike activities of sensory cortex, we topically applied dexamethasone (DEX), a glucocorticoid receptor agonist, to the auditory cortical surface of rats while recording single-unit extracellular spike activities in response to sound stimuli. Our major findings are: (1) a topical administration of DEX increased the cortical neural responses to pure-tone stimuli from 10 to 60 min after administration, while the peak response enhancement occurred at 20–30 min; (2) DEX not only markedly increased the magnitude of tone-evoked responses, but also extended the response duration and the frequency range of the neural responses; (3) the enhancement of neural responses was more salient at the higher frequency band; (4) the ratio of spontaneous firing rate between post- and pre-administration was negatively correlated with the unit's spontaneous firing rate before treatment. Our data confirm that DEX can modulate the neural activity at the cortical level and provide more information for understanding the mechanism of glucocorticoid-induced alterations in auditory processing.     

Corticofugal modulation of the auditory thalamic reticular nucleus of the guinea pig

Neuronal responses to auditory stimuli and electrical stimulation were examined in 104 neurones in the auditory sector of thalamic reticular nucleus (TRN) and nine medial geniculate (MGB) neurones from anaesthetized guinea pigs. TRN neurones showed rhythmic spontaneous activities. TRN neurones changed firing pattern over time, from tonic to burst in a time interval of several seconds to tens of seconds. One-third of the TRN neurones (25/76) responded to the acoustic stimulus in a slow oscillation mode, either producing a spike burst at one time and responded with nothing another time, or producing a spike burst at one time and a single spike at the other. Thirty-two of 40 neurones received a corticofugal modulation effect. Nineteen of 32 neurones responded directly to electrical stimulation of the cortex with an oscillation of the same rhythm (7–14 Hz) as its auditory-evoked oscillation. Six neurones changed their firing pattern from burst to tonic when the auditory cortex was activated. As the TRN applied inhibition to the MGB, the oscillatory nature of inhibition would affect the fidelity of MGB relays. Thus, it was unlikely that the MGB was in relay mode when the TRN was in a slow oscillation mode. These results hint at a possible mechanism for the modulation of states of vigilance through the corticofugal pathway via the TRN.     

Extracortical descending projections to the rat inferior colliculus

Feedback controlling is an important element in the sensory processing in the auditory system. It has been long recognized that the inferior colliculus (IC) sends direct ascending projections to the medial geniculate body (MGB), but receives feedback regulation from the auditory cortex. In the present study we probed the shorter extracortical projections to the IC, including the direct descending pathway from the MGB. In the rat, the fluorescence retrograde tracers Fluorogold, True Blue or Rhodamine latex microspheres were injected into the IC, and the auditory thalamus and surrounding regions were examined for fluorescent neurones. We did not find any retrograde labelling in the ventral division of the MGB. However, retrogradely labelled neurones were found in the medial and suprageniculate nuclei of the MGB. We also observed densely packed groups of fluorescent neurones in the peripeduncular nucleus and numerous labelled neurones in the nucleus of the brachium of the IC. The existence of a direct descending pathway to the IC from at least some auditory thalamic nuclei challenges the perception of the colliculo-thalamic relationship as one-way traffic and suggests more direct involvement of the auditory thalamus in the feedback regulation of the incoming acoustic signals.     

Neuronal connections in the medial geniculate body of the guinea-pig

The spontaneous and evoked activities of individual pairs of single units were recorded simultaneously with the same microelectrode in the medial geniculate body (MGB) of ketamine-xylazine-anaesthetised guinea-pigs. Cross-correlograms (CCGs) of spike train pairs were computed and divided on the basis of correlation peak shape into four classes [a unilateral narrow (UN) peak, a centrally positioned wide (CW) peak, a complex peak and no significant peak] interpreted in terms of the functional connection between neighbouring neurones. The shift predictor procedure was applied with the aim of removing the effect of the stimulus on the final CCG shape. The occurrence of correlation peak types and the distribution of correlation coefficients were found to be similar for the spontaneous activity during silent periods following acoustical stimulation and for the long-lasting recording of spontaneous activity. CCGs in 38% of pairs computed during silent interstimulus intervals contained a UN peak, suggesting a monosynaptic excitatory connection. Almost 20% of all pairs expressed a CCG shape typical for a common input, i.e. a CW peak. In 5% of cases multiple, so-called complex peaks, were found. About 20% of the CCGs contained no significant correlation peak in the interstimulus period, which is typical for a very weak or absent functional connection between recorded neurones. No inhibitory interaction (groove in the CCGs) between recorded pairs was observed. The distribution of correlation peak shapes was similar when calculated during acoustical stimulation and during silent interstimulus intervals. CCGs computed during presentation of four acoustical stimuli (pure tone bursts, noise bursts, natural call whistle and artificially inverted whistle) showed most frequently a UN peak (28-37%) followed by CCGs with no significant peak (18-28%) and with a UN/CW peak (14-23%). On average, the occurrence of UN peaks tended to be less frequent during stimulus presentation than in silent conditions, but the difference was not statistically significant. The most frequent occurrence of clear UN peaks was found in the medial part of the MGB (from 52-64% of pairs depending on the type of acoustical stimulus), while the least was observed in the ventral part of the MGB (12-22%). In contrast, CW peaks were most frequently expressed in pairs located in the ventral part of the MGB (18-33%), while neuronal pairs in the medial part revealed a very low occurrence of CW peaks (0-7%). The occurrence of independently firing neurones was lowest in the medial part of the MGB (8-20% of pairs) in comparison with the ventral (31-39%) and dorsal (12-41%) parts. In 20% of pairs acoustical stimulation produced a change in the type of correlation peak present during spontaneous activity. Most frequently, a CW peak (shared input) changed to a flat CCG, which represents independently firing neurones. In some pairs higher connection strengths (as expressed by the value of the correlation coefficient) were found for silent interstimulus intervals than for acoustical stimulation. This suggests that in the MGB the stimulus may desynchronise the spontaneous activity of simultaneously firing units in neuronal pairs.     

Processing of twitter-call fundamental frequencies in insula and auditory cortex of squirrel monkeys

 Amplitude-modulated (AM) and frequency-modulated (FM) elements are prominent periodic sound features of squirrel monkeys’ twitter calls. To investigate how the periodic FM elements are represented in the spike activity of cortical neurons, single units in the insula, primary auditory field (AI) and rostral auditory field (R) were recorded. In five monkeys, 566 units (insula, n=181; AI, n=221; R, n=164) were exposed to synthesized fundamental frequencies and one natural twitter call. Neuronal encoding of periodic FM elements takes place by phase-locking to either the up- or the down-directed FM sweeps. The phase-locking was strongly influenced by the FM-period repetition rate. The ability of neurons in both auditory fields and the insula to encode all periodic FM elements showed a marked reduction at 16 Hz FM-period repetition rate. The neurons’ best frequency (BF) influenced the quality of periodicity encoding, but neurons with BFs outside the frequency range of the fundamentals also responded with periodic discharge rates. Even neurons in AI (6.8%) and the insula (22.6%) that did not respond to pure tones showed clear periodic FM encoding. The percentage of neurons able to encode all periodic FM elements within the twitter fundamental was significantly higher in field R than in AI and the insula. From 58 simultaneously recorded pairs of units in AI and the insula that had positive cross-correlation coefficients of spontaneous activity, the influence of the FM-period repetition rate on neuronal correlation was investigated. Correlated firing of AI and insula neurons seems limited to low-period repetition rates. The cross-correlation coefficients obtained for spontaneous activity and six different periodic FM sounds showed a band-pass characteristic. The natural twitter call evoked stronger neuronal responses in all fields than the synthesized fundamental frequencies with corresponding bi-directional FM sweeps. The better encoding of the transient features in the natural call can be attributed to the amplitude modulation added to the FM elements in the natural call. These amplitude modulations divide the FM elements of twitter calls into syllable-like sound elements. It is probable that encoding the complex pattern in the time and frequency domains of a call must undergo some integration at a cortical level. Additionally, these data provide the first evidence that insula neurons contribute to the encoding of complex FM signals. Received: 11 April 1997/ Accepted: 6 May 1998     

Tonotopic organization in lateral part of posterior group of thalamic nuclei in the cat

Responses of single units and clusters of units to tone-burst stimulation were recorded at 100-micron intervals along vertical electrode penetrations through the lateral part of the posterior group of thalamic nuclei (Po) in five barbiturate-anesthetized cats. Best frequencies and minimum response latencies to tone-burst stimulation were studied at each location along a penetration. Most of Po is located rostral to the medial geniculate body (MGB) and is contiguous with the ventral nucleus and medial division. Po is characterized physiologically by narrowly tuned, short-latency (less than 40 ms) responses. Considerable scatter of best frequencies occurs along electrode penetrations, although a clear tonotopic organization is apparent in the distribution of best frequencies obtained from several electrode penetrations located in the same frontal plane of an individual brain. A "single" frequency is represented as an irregularly shaped lamina. A three-dimensional "block" model of the tonotopic organization of Po is described in which the highest best frequencies are located caudally, and the lowest best frequencies are located rostrally within the nucleus. The high-frequency representation of Po is contiguous with the high-frequency representation of the ventral nucleus of the MGB. The low- and middle-frequency representations of the ventral nucleus and Po are discontinuous. The ventral nucleus and Po have similar physiological properties and together constitute the tonotopic division of the auditory thalamus in the cat. Neurons in the medial division adjacent to the medial border of Po are larger than neurons in Po, lack tonotopic organization, and respond at short latencies.     

Phase-locked responses to pure tones in the auditory thalamus

Accurate temporal coding of low-frequency tones by spikes that are locked to a particular phase of the sine wave (phase-locking), occurs among certain groups of neurons at various processing levels in the brain. Phase-locked responses have previously been studied in the inferior colliculus and neocortex of the guinea pig and we now describe the responses in the auditory thalamus. Recordings were made from 241 single units, 32 (13%) of which showed phase-locked responses. Units with phase-locked responses were mainly (82%) located in the ventral division of the medial geniculate body (MGB), and also the medial division (18%), but were not found in the dorsal or shell divisions. The upper limiting frequency of phase-locking varied greatly between units (60-1,100 Hz) and between anatomical divisions. The upper limit in the ventral division was 520 Hz and in the medial was 1,100 Hz. The range of steady-state delays calculated from phase plots also varied: ventral division, 8.6-14 ms (mean 11.1 ms; SD 1.56); medial division, 7.5-11 ms (mean 9.3 ms; SD 1.5). Taken together, these measurements are consistent with the medial division receiving a phase-locked input directly from the brain stem, without an obligatory relay in the inferior colliculus. Cells in both the ventral and medial divisions of the MGB showed a response that phase-locked to the fundamental frequency of a guinea pig purr and may be involved in analyzing communication calls.     

Note on the tonotopic organization in the cat medial geniculate body: influence of sampling of units

Summary The tonotopic organization observed in the present study for the pars lateralis (LV) of the medial geniculate body (MGB) in nitrous oxide anesthetized cats is generally consistent with that previously reported under barbiturate anesthesia. The present data, however, provide evidence for local deviations in characteristic frequency (CF) using appropriate sampling procedures of single units. Although the majority of pairs of units recorded simultaneously with the same microelectrode showed comparable CFs, a few pairs of such neighbouring units displayed CF disparities of up to 1.5 octaves. In addition, some units characterized by an elevated threshold had a CF deviating significantly from the general CF progression observed for the majority of units having low thresholds. This study points out the influence of the sampling procedure on the quality of the tonotopic organization observed in the MGB in addition to a possible effect of the level of anesthesia.Abbreviations BIC brachium of the inferior colliculus - BIN nucleus of the brachium of the inferior colliculus - CF characteristic frequency - D dorsal nucleus of the dorsal division of the MGB - DD deep dorsal nucleus of the dorsal division of the MGB - EL electrode track - LGN lateral geniculate nucleus - LV pars lateralis of the ventral division of the MGB - M pars magnocellularis of the medial division of the MGB - MGB medial geniculate body - OV pars ovoidea of the ventral division of the MGB - SG suprageniculate nucleus of the dorsal division of the MGB     

Directionality derived from differential sensitivity to monaural and binaural cues in the cat's medial geniculate body

Azimuth tuning of high-frequency neurons in the primary auditory cortex (AI) is known to depend on binaural disparity and monaural spectral (pinna) cues present in broadband noise bursts. Single-unit response patterns differ according to binaural interactions, strength of monaural excitatory input from each ear, and azimuth sensitivity to monaural stimulation. The latter characteristic has been used as a gauge of neural sensitivity to monaural spectral directional cues. Azimuth sensitivity may depend predominantly on binaural disparity cues, exclusively on monaural spectral cues, or on both. The primary goal of this study was to determine whether each cortical response pattern corresponds to a similar pattern in the medial geniculate body (MGB) or whether some patterns are unique to the cortex. Single-unit responses were recorded from the ventral nucleus (Vn) and lateral part of the posterior group of thalamic nuclei (Po), tonotopic subdivisions of the MGB. Responses to free-field presentation of noise bursts that varied in azimuth and sound pressure level were obtained using methods identical to those used previously in field AI. Many units were azimuth sensitive, i.e., they responded well at some azimuths, and poorly, if at all, at others. These were studied further by obtaining responses to monaural noise stimulation, approximated by reversible plugging of one ear. Monaural directional (MD) cells were sensitive to the azimuth of monaural noise stimulation, whereas binaural directional (BD) cells were either insensitive to its azimuth or monaurally unresponsive. Thus BD and MD cells show differential sensitivity to monaural spectral cues. Monaural azimuth sensitivity could not be used to interpret the spectral sensitivity of predominantly binaural cells that exhibited strong binaural facilitation because they were either unresponsive or poorly responsive to monaural stimulation. The available evidence suggests that some such cells are sensitive to spectral cues. The results do not indicate the presence of any response types in AI that are not present in the MGB. Vn and Po contain similar classes of MD and BD cells. Because Po neurons project to the anterior auditory field, neurons in this cortical area also are likely to exhibit differential sensitivity to binaural disparity and monaural spectral cues. Comparison of these MGB data with a published report of cochlear nucleus (CN) single-unit azimuth tuning shows that MGB sensitivity to spectral cues is considerably stronger than CN sensitivity.