Corticofugal modulation of the information processing in the auditory thalamus of the cat

Summary Single unit activity of 355 cells was recorded in the auditory thalamus of anesthetized cats before, during, and after the inactivation by cooling of the ipsilateral primary auditory cortex (AI). Most of the units (n = 288) showed similar functional characteristics of firing before and after the cryogenic blockade of AI. The spontaneous firing rate remained unchanged by cooling in 20% of the units and decreased in the majority of them (60%). In some regions, i.e. dorsal division of the medial geniculate body (MGB), lateral part of the posterior group of the thalamus, and auditory sector of the reticular nucleus of the thalamus, the maximum firing rate evoked by white noise bursts was generally affected by cooling in the same direction and to the same extent as the spontaneous activity. Units in the ventral division of MGB showed a characteristic increase of signal-to-noise ratio during cortical cooling. The corticofugal modulation led to the appearance or disappearance of the best frequency of tuning in 51 units and changed it by more than 0.5 octave in 34 units. The bandwidths of different response patterns to pure tones stimulation were used to define a set of functional properties. During cryogenic blockade of AI, two cortically modulated sub-populations of units were usually distinguished that exhibited changes for a given functional property. The complexity and diversity of the effects of cortical inactivation suggest that the corticothalamic projection may be the support for selective operations such as an adaptive filtering of the incoming acoustic signal at the thalamic level adjusted as a function of cortical activity.     

Sleep: A prerequisite for reflex milk ejection in the rat

Summary Electroencephalographic activity (EEG) was recorded from the frontal cortex of unanaesthetized and urethane-anaesthetized lactating rats and analysed in relation to the pattern of milk ejection evoked by the nursing pups.The EEG of the anaesthetized rat fluctuated without experimental intervention between three distinctive patterns defined as synchronized, desynchronized, and stage III activity, whilst reflex milk ejection recurred at intervals of about 6 min (range 2->20 min) throughout the 1–4 h period the pups were left attached to the nipples. For > 10 s before and for up to 60 s after each milk ejection, as judged from recordings of intramammary pressure and pup behaviour, the EEG was invariably synchronized throughout. Conversely, milk ejection (n>300) was never observed during long periods of desynchronized, or stage III EEG activity. The vigorous increase in the sucking of the pups at milk ejection failed to produce a desynchronization (arousal) of the EEG as observed with other forms of sensory stimulation. Indeed, the sucking of the pups appeared to produce a soporific change in the maternal EEG for spontaneous periods of desynchronization were not observed in the 30–60 min following the initial attachment of the pups to the nipples.Similar EEG patterns were seen in the unanaesthetized rat, though arousal from the synchronized state was more easily produced, e.g., by weak auditory signals. Milk ejection, as judged from the behaviour of the pups, recurred at intervals of 2 min or more during each 20–80 min period of nursing. The rat appeared somnolent for most of the nursing period and the EEG was always synchronized for > 10 s before each milk ejection (n>200), though her eyes usually remained open. Arousal and desynchronization of the EEG was invariably observed in association with the increased pup behaviour at milk ejection.From these observations and the knowledge that oxytocin release from the neurohypophysis occurs about 10 s before milk ejection, we conclude that a synchronized EEG pattern, akin to that associated with slow-wave sleep, is an essential prerequisite for the expression of the milk-ejection reflex in the rat.     

Neural representations of temporally modulated signals in the auditory thalamus of awake primates

In sensory systems, the thalamus has historically been considered a relay station. Neural representations of temporal modulations in the auditory system undergo considerable changes as they pass from the inferior colliculus (IC) to the auditory cortex. We sought to determine in awake primates the extent to which auditory thalamic neurons contribute to these transformations. We tested the temporal processing capabilities of medial geniculate body (MGB) neurons in awake marmoset monkeys using repetitive click stimuli. MGB neurons were able to synchronize to periodic clicks at repetition rates significantly higher than auditory cortex neurons. Unlike responses in the MGB of anesthetized animals, >40% of MGB neurons in awake marmosets displayed nonsynchronized discharges when stimulated by high click rates (short interclick intervals). Such nonsynchronized MGB responses typically occurred at higher repetition rates than those observed in auditory cortex. In contrast to auditory cortex neurons, many MGB neurons exhibited both synchronized and nonsynchronized discharge patterns. In both MGB and auditory cortex, synchronized and nonsynchronized responses represented complementary ranges of interclick intervals (1/click rate). Furthermore, the temporal processing abilities of some MGB neurons were sensitive to the spectrotemporal parameters of the click stimuli used. Together, these findings suggest that MGB neurons participate in active transformations of the neural representations of temporal modulations from IC to auditory cortex. In particular, the MGB appears to be the first station in the auditory ascending pathway in which substantial nonsynchronized responses emerge.     

Arousal a prerequisite for suckling in the conscious rabbit?

Electrocorticographic activity (ECoG) was recorded from the frontal cortex of unanaesthetized, lactating rabbits, and analysed in relation to the suckling behaviour of the animals. Suckling lasted 2–5 minutes each day and was always associated with a desynchronized ECoG. Milk ejection was never seen during slow wave sleep. Mild sedation of the doe caused a dose-dependent block of milk yield but did not affect her willingness to suckle and the ECoG was desynchronized during nursing. In contrast to the rat, but similar to the pig, sleep is not a prerequisite for reflex milk ejection in the rabbit. Evidence presented implies that ECoG arousal may be important for this reflex in the rabbit.     

Physiological plasticity of single neurons in auditory cortex of the cat during acquisition of the pupillary conditioned response: II. Secondary field (AII)

The discharges of 22 single neurons were recorded in the secondary auditory cortical field (AII) during acquisition of the pupillary dilation conditioned defensive response in chronically prepared cats. All 22 neurons developed discharge plasticity in background activity, and 21/22 cells developed plasticity in their responses to the acoustic conditioned stimulus (CS). Nonassociative factors were ruled out by the use of a sensitization phase (CS and US [unconditioned stimulus] unpaired) preceding the conditioning phase and by ensuring stimulus constancy at the periphery by neuromuscular paralysis. Changes in background neuronal activity were related to measures of behavioral learning or to changes in the level of arousal. Specifically, decreases in background activity (17/22 cells) developed at the time that subjects began to display conditioned responses. Increases in background activity (5/22) developed in animals that became more tonically aroused during conditioning. However, both increases (11/22) and decreases (10/22) in evoked activity developed independently of the rate of pupillary learning, tonic arousal level, or changes in background activity. These findings indicate that changes in background activity are closely related to behavioral processes of learning and arousal whereas stimulus-evoked discharge plasticity develops solely as a consequence of stimulus pairing. A comparative analysis of the effects of conditioning on secondary and primary (AI) auditory cortex indicates that both regions develop neuronal discharge plasticity early in the conditioning phase and that increases in background activity in primary auditory cortex are also associated with elevated levels of tonic arousal. In addition, the overall incidence of single neurons developing learning-related discharge plasticity is significantly greater in AII than in AI. The relevance of these findings is discussed in terms of parallel processing in sensory systems and multiple sensory cortical fields.     

Correlation between postsynaptic activity and surface potentials in the gigantopyramidal area of the cat cortex

Cats were anesthetized with chloralose and pentobarbital and immobilized with muscle relaxants. Parallel macroelectrode recordings of the spontaneous electrocorticogram (ECoG) and cortical surface potentials — primary and association responses (PRs and ARs) — evoked by peripheral stimuli and microelectrode intracellular recordings of unit activity in the gigantopyramidal area were obtained. Both spontaneous and evoked changes in membrane potential in many cases correlated clearly with certain components of the ECoG, PR and AR. The most obvious and stable correlation was that between postsynaptic depolarization in the neurons of all three groups isolated (pyramidal tract, unidentified corticofugal neurons, and interneurons) and the first positive component of the PR and AR. A tendency was observed for synchronous development of the inhibitory postsynaptic potential (IPSP) and negative phase of the evoked potentials. No specific correlates were found in the ECoG for spike discharges in single neurons. The possibility of assessing cellular mechanisms of the genesis of PR and AR components is discussed on the basis of correlation observed between PSPs and global cortical surface potentials. The factor deciding the probability of appearance of an ECoG wave is evidently the degree of synchronization of PSP generation in large populations of cortical neurons.     

Spontaneous activity of neurones of nucleus reticularis thalami in freely moving cats

1. Fifty-four neurones of the caudal part of the nucleus reticularis thalami (nuc. ret.) were recorded during different phases of sleep and wakefulness in unanaesthetized freely moving cats.2. During wakefulness the activity of the neurones was characterized by a continuous, well-spaced discharge. The mean firing rate was 35.58 +/- 15.06 spikes/sec (average +/- S.D.).3. During sleep with synchronized e.e.g. (S-sleep) the neurones fired in high frequency bursts with long pauses in between. Each burst was formed of 10-15 spikes. Often the bursts were followed by prolonged discharges formed of spikes well spaced one from the other. Bursts followed by prolonged activity were more commonly observed at the beginning of S-sleep and during the S-sleep periods preceding sleep with desynchronized e.e.g., whereas bursts immediately followed by silence were more frequent in the S-sleep periods with e.e.g. delta waves. The long periods of silence between the bursts usually lasted over 200 msec and values greater than 1 sec were frequently found. The mean firing rate of neurones during S-sleep was 19.22 +/- 10.50 spikes/sec.4. During sleep with desynchronized e.e.g. (D-sleep) the activity of the neurones was, as during wakefulness, characterized by a continuous, well spaced, unclustered discharge. The mean firing rate was 40.00 +/- 18.74 spikes/sec. During the rapid eye movements of this phase most units increased the frequency of their discharge, which, nevertheless, maintained the unclustered feature proper to the desynchronized phase of sleep.5. Interspike interval distribution was similar during wakefulness and sleep with desynchronized e.e.g., whereas that for sleep with synchronized e.e.g. was markedly different from those for both the other stages.6. The implications of the striking similarity between the activity of reticularis neurones during wakefulness and sleep with desynchronized e.e.g. are discussed.     

Acoustically evoked potentials in the rat during sleep and waking

Summary Click-evoked potentials were recorded from the rat's auditory cortex (AC), medial geniculate body (MGB), reticular formation (RF), and hippocampus (HIPP) during natural sleep and waking. Various amplitude measurements made on sliding averages by a computer provided essentially continuous measurement of evoked responses in long experiments. Changes in AC responses were simplest in potentials recorded from a depth of 1.0–1.5 mm, all components of the average waveform being larger during slow-wave sleep (SS) than in waking (W) and low-voltage fast sleep (LVFS). More complex changes in cortical surface responses included increases in the first positive wave, in a second positive deflection, and in a late negative wave during drowsiness (D) and SS. The second positive wave remained large during LVFS, while the late negativity changed form and peaked earlier. Early components of both surface and deep responses were not consistently different in W and LVFS. The only clear change in MGB responses was a reduction in amplitude during LVFS. Late positive waves in RF and HIPP responses were large during SS and small during W and LVFS. In general, increases in the very late components of responses at all sites distinguished periods of cortical synchrony from periods of cortical activation. The first negative deflection and a succeeding positive wave in the HIPP response were markedly increased during arousal.This work was supported principally by the National Institutes of Health (Grant 5 POl GM-14940-03), and in part by the Joint Services Electronics Program (Contract DA 28-043-AMC-02536 (E)). One of us (A.A.B.) was supported in part by a grant from the Swiss National Foundation for Scientific Research. Preliminary reports were presented at the Annual Meeting of the Eastern Psychological Association, April 18, 1969 in Washington, D.C. and in the Quarterly Progress Report of the Research Laboratory of Electronics, No. 90, July 15, 1968.     

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.     

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     

Comparison of spontaneous cortical unit activity in several brain areas of unanesthetized cats

Spontaneous unit activity in association area 5 and some projection areas of the cortex (first somatosensory, first and second auditory areas) were studied in cats immobilized with D-tubocurarine in which the index of specific spontaneous activity, the mean frequency, types of spontaneous activity, and statistical parameters — distribution of interspike intervals and autocorrelation function — were determined. The results showed that spontaneous unit activity in the association area differs from that in the projection areas in both intensity and character. A special feature of the spontaneous activity of the auditory areas was a well-marked volley distribution of activity. In the somatosensory area the level of spontaneous activity as reflected in all indices was the lowest. In the association cortex the largest number of neurons with spontaneous activity lay at a depth of 500–1000 corresponding to cortical layers III–IV. In the first auditory area neurons with spontaneous activity were concentrated at a depth of 1400 (layer V) and in the somatosensory area at a depth of 1000–1400 (layers IV–V). The possible functional significance of these differences is discussed.Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 13–21, January–February, 1976.     

Click train encoding in primary and non-primary auditory cortex of anesthetized macaque monkeys

We studied encoding of temporally modulated sounds in 28 multiunits in the primary auditory cortical field (AI) and in 35 multiunits in the secondary auditory cortical field (caudomedial auditory cortical field, CM) by presenting periodic click trains with click rates between 1 and 300 Hz lasting for 2-4 s. We found that all multiunits increased or decreased their firing rate during the steady state portion of the click train and that all except two multiunits synchronized their firing to individual clicks in the train. Rate increases and synchronized responses were most prevalent and strongest at low click rates, as expressed by best modulation frequency, limiting frequency, percentage of responsive multiunits, and average rate response and vector strength. Synchronized responses occurred up to 100 Hz; rate response occurred up to 300 Hz. Both auditory fields responded similarly to low click rates but differed at click rates above approximately 12 Hz at which more multiunits in AI than in CM exhibited synchronized responses and increased rate responses and more multiunits in CM exhibited decreased rate responses. These findings suggest that the auditory cortex of macaque monkeys encodes temporally modulated sounds similar to the auditory cortex of other mammals. Together with other observations presented in this and other reports, our findings also suggest that AI and CM have largely overlapping sensitivities for acoustic stimulus features but encode these features differently.     

Influences of cortico-tectal and intertectal connections on visual responses in the cat's superior colliculus

Summary Our experiments, utilizing electrical shocks applied to the lateral- or supra-sylvian gyrus of the cortex, demonstrate an initially excitatory (latency 2–10 msec) but predominantly inhibitory influence of cortico-tectal afferents on the discharge of tectal neurons. Primary or secondary inhibition in tectal cells after cortical stimulation suppressed spontaneous or visually driven activity and limited the frequency of stimulation which tectal neurons could follow.The main influence of the contralateral colliculus on visual responses of tectal cells is inhibitory but again some principally monosynaptic intertectal connections evoked initial excitation (latency 3–10 msec) after electrical stimulation of the contralateral optic tract.Removal of the visual areas 17, 18 and 19 did not cause a loss of movement- or direction-selectivity in neurons of the superior colliculus. Cooling of the occipital cortex, while recording from direction-selective tectal neurons did not alter their essential response characteristics. The response to cortical shocks disappeared in tectal neurons during cooling but could be restored by rewarming of the cortex.It could not be confirmed in our experiments that excitation and movement- or direction-selectivity of neurons in the superior colliculus depend on a specific input from areas 17, 18 and 19 of the cortex.     

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     

Delayed synchronization of activity in cortex and subthalamic nucleus following cortical stimulation in the rat

Oscillations may play a role in the functional organization of cortico-basal ganglia-thalamocortical circuits, and it is important to understand their underlying mechanisms. The cortex often drives basal ganglia (BG) activity, and particularly, oscillatory activity in the subthalamic nucleus (STN). However, the STN may also indirectly influence cortex. The aim of this study was to characterize the delayed (>200 ms) responses of STN neurons to synchronized cortical inputs, focusing on their relationship with oscillatory cortical activity. We recorded the short-latency and delayed responses of STN units and frontal electrocorticogram (ECoG) to cortical stimulation in anaesthetized rats. Similar to previous studies, stimulation of ipsilateral frontal cortex, but not temporal cortex, evoked a short-latency triphasic response, followed by a sustained reduction or pause in firing, in rostral STN units. Caudal STN units did not show the short-latency triphasic response but often displayed a prolonged firing reduction. Oscillations in STN unit activity and ECoG were common after this sustained firing reduction, particularly between 200 and 600 ms after frontal cortical stimulation. These delayed oscillations were significantly coherent in a broad frequency band of 5–30 Hz. Coherence with ECoG at 5–15 Hz was observed throughout STN, though coherence at 15–30 Hz was largely restricted to rostral STN. Furthermore, oscillatory responses at 5–30 Hz in rostral STN predominantly led those in cortex (mean latency of 29 ms) after frontal cortical stimulation. These findings suggest that STN neurons responding to corticosubthalamic inputs may provide a delayed input to cortex, via BG output nuclei, and thence, thalamocortical pathways.     

Corticofugal modulation on both ON and OFF responses in the nonlemniscal auditory thalamus of the guinea pig

Corticofugal modulation on both ON and OFF responses in various nuclei in the medial geniculate body (MGB) was examined by locally activating the auditory cortex and looking for effects on the neuronal responses to acoustic stimuli. In contrast with a major corticofugal facilitatory effect on the ON neurons in the lemniscal nucleus of the MGB of the guinea pigs, of 132 ON neurons tested in three conditions with cortical activation through each of three implanted electrodes, the majority of the tested conditions (319/396) that were sampled from the nonlemniscal nuclei of the MGB received inhibitory modulation from the activated cortex. This inhibitory effect was >50% for 99 cases while the auditory cortex was activated. Most of the OFF and ON-OFF MGB neurons (44/54) showed a facilitatory effect of 111.4 +/- 99.9%, and three showed a small inhibitory effect of 25.7 +/- 5.8% on their OFF responses. Thirty neurons in the border region between the lemniscal and nonlemniscal MGB showed mainly facilitatory corticofugal effects on both ON and OFF responses. Meanwhile, cortical stimulation induced almost exclusive inhibitory effects on the ON response and facilitatory effects on the OFF response in the MGcm. It is suggested that the OFF response is produced as a disinhibition from the inhibitory input of the auditory stimulus. The present results provide a possible explanation for selective gating of the auditory information through the lemniscal MGB while switching off other unwanted sensory signals and the interference from the limbic system, leaving the other auditory cortex prepared to process only the auditory signal.     

Brain state-dependency of coherent oscillatory activity in the cerebral cortex and basal ganglia of the rat

The nature of the coupling between neuronal assemblies in the cerebral cortex and basal ganglia (BG) is poorly understood. We tested the hypothesis that coherent population activity is dependent on brain state, frequency range, and/or BG nucleus using data from simultaneous recordings of electrocorticogram (ECoG) and BG local field potentials (LFPs) in anesthetized rats. The coherence between ECoG and LFPs simultaneously recorded from subthalamic nucleus (STN), globus pallidus (GP), and substantia nigra pars reticulata (SNr) was largely confined to slow- ( approximately 1 Hz) and spindle- (7-12 Hz) frequency oscillations during slow-wave activity (SWA). In contrast, during cortical activation, coherence was mostly restricted to high-frequency oscillations (15-60 Hz). The coherence between ECoG and LFPs also depended on BG recording site. Partial coherence analyses showed that, during SWA, STN and SNr shared the same temporal coupling with cortex, thereby forming a single functional axis. Cortex was also tightly, but independently, correlated with GP in a separate functional axis. During activation, STN, GP, and, to a lesser extent, SNr shared the same coherence with cortex as part of one functional axis. In addition, GP formed a second, independently coherent loop with cortex. These data suggest that coherent oscillatory activity is present at the level of LFPs recorded in cortico-basal ganglia circuits, and that synchronized population activity is dynamically organized according to brain state, frequency, and nucleus. These attributes further suggest that synchronized activity should be considered as one of a number of candidate mechanisms underlying the functional organization of these brain circuits.     

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.     

Reconstruction of intracortical whisker-evoked local field potential from electrocorticogram using a model trained for spontaneous activity in the rat barrel cortex

Electrocorticogram (ECoG) has provided neural information from the cortical surfaces, is widely used in clinical applications, and expected to be useful for brain–machine interfaces. Recent studies have defined the relationship between neural activity in deep layers of the cerebral cortex and ECoG. However, it is still unclear whether this relationship is shared across different brain states. In this study, spontaneous activity and whisker-evoked responses in the barrel cortex of anesthetized rats were recorded with a 32-channel ECoG electrode array and 32-channel linear silicon probe electrodes, respectively. Spontaneous local field potentials (LFPs) at various depths could be reconstructed with high accuracy (R > 0.9) by a linear weighted summation of spontaneous ECoG. Current source density analysis revealed that the reconstructed LFPs correctly represented laminar profiles of current sinks and sources as well as the raw LFP. Moreover, when we applied the spontaneous activity model to reconstruction of LFP from the whisker-related ECoG, high accuracy of reconstruction could be obtained (R > 0.9). Our results suggest that the ECoG carried rich information about synaptic currents in the deep layers of the cortex, and the same reconstruction model can be applied to estimate both spontaneous activity and whisker-evoked responses.     

Spectrotemporal receptive fields during spindling and non-spindling epochs in cat primary auditory cortex

It was often thought that synchronized rhythmic epochs of spindle waves disconnect thalamo-cortical system from incoming sensory signals. The present study addresses this issue by simultaneous extracellular action potential and local field potential (LFP) recordings from primary auditory cortex of ketamine-anesthetized cats during spindling activity. We compared cortical spectrotemporal receptive fields (STRF) obtained during spindling and non-spindling epochs. The basic spectro-temporal parameters of "spindling" and "non-spindling" STRFs were similar. However, the peak-firing rate at the best frequency was significantly enhanced during spindling epochs. This enhancement was mainly caused by the increased probability of a stimulus to evoke spikes (effectiveness of stimuli) during spindling as compared with non-spindling epochs. Augmented LFPs associated with effective stimuli and increased single-unit pair correlations during spindling epochs suggested higher synchrony of thalamo-cortical inputs during spindling that resulted in increased effectiveness of stimuli presented during spindling activity. The neuronal firing rate, both stimulus-driven and spontaneous, was higher during spindling as compared with non-spindling epochs. Overall, our results suggests that thalamic cells during spindling respond to incoming stimuli-related inputs and, moreover, cause more powerful stimulus-related or spontaneous activation of the cortex.