Responses of single cells in the medial geniculate body of awake squirrel monkeys

Summary Response properties of 142 medial geniculate (MGB) cells were investigated in the awake and undrugged squirrel monkey (Saimiri sciureus). Using Jordan's (1973) parcellation of this complex nucleus, cells were assigned to 3 major subdivisions a, b and c MGB and compared for their general characteristics and response properties, b MBG cells had significantly higher rates of spontaneous firing and longer latency periods than a and c MGB cells. With regard to responsiveness to various auditory stimuli, response patterns, and tuning characteristics, cells in all 3 subdivisions were statistically similar and were thus treated as one cell population. About 95% of the cells responded to broadband white noise, steady tone bursts and frequency modulated (FM) tones. Click activated only 69% of the responding cells. Various through-stimulus responses comprised about 80% of the responses. Among the tonesensitive cells, 90% responded with complex patterns, out of which 50% were frequency-dependent. About 62% of the cells (for which tuning properties were determined) were quite broadly tuned (Q_10dB <2) and had either single or multi-peaked response areas. The other 38% were quite narrowly tuned (Q_10dB > 2) and had single-peaked, symmetrical or tailed response areas. Different inhibitory and excitatory response components of individual cells had different characteristic frequencies and response thresholds. The c MGB, which is tonotopically organized in a latero-medial orientation, appears to be homologous to the cat pars lateralis of the ventral MGB. The tonotopical organization of the b MGB, which is probably homologous to the cat's medial or magnocellular subdivision, is less clear. Most of the cells which were activated by FM tones disclosed direction sensitivity with different degrees of pattern complexity. It is suggested that pitch resolution in the MGB is based on spatio-temporal mechanisms.This study was supported by a research grant to Z. Wollberg from the Israel National Academy of Sciences and Humanities, The Commission for Basic ResearchPart of a Ph.D. thesis carried out under the supervision of the third author and submitted to the Tel Aviv University     

Auditory responsive cortex in the squirrel monkey: neural responses to amplitude-modulated sounds

The neural response to amplitude-modulated sinus sounds (AM sound) was investigated in the auditory cortex and insula of the awake squirrel monkey. It was found that 78.1% of all acoustically driven neurons encoded the envelope of the AM sound; the remaining 21.9% displayed simple On, On/Off or Off responses at the beginning or the end of the stimulus sound. Those neurons with AM coding were able to encode the AM sound frequency in two different ways: (1) the spikes followed the amplitude modulation envelopes in a phaselocked manner; (2) the spike rate changed significantly with changing modulation frequencies. As reported in other species, the modulation transfer functions for rate showed higher modulation frequencies than the phaselocked response. Both AM codings exhibited a filter characteristic for AM sound. Whereas 46.6% of all neurons had the same filter characteristic for both the spike discharge and the phase-locked response, the remaining neurons displayed combinations of different filter types. The discharge pattern of a neuron to simple tone or noise bursts suggests the behaviour of this neuron when AM sound is used as the stimulus. Neurons with strong onset responses to tone/noise bursts tended to have higher phase-locked AM responses than neurons with weak onset responses. The spike rate maxima for AM sound showed no relation to the tone/noise burst discharge patterns. Varying modulation depth was encoded by the neuron's ability to follow the envelope cycles and not by the non-phase-locked spike rate frequency. The organization of the squirrel monkey's auditory cortex has previously been established by an anatomical study. We have added two new fields using physiological parameters. All fields investigated showed a clear functional separation for time-critical information processing. The best temporal resolution was shown by the primary auditory field (AI), the first-temporal field (T1) and the parainsular au ditory field (Pi). The neural data in these fields and the amplitude modulation frequency range of squirrel monkey calls suggest a similar correlation between vocalization and perception as in human psychophysical data for speech and hearing sensation. The anterior fields in particular failed to follow the AM envelopes. For the first time in a primate, the insula was tested with different sound parameters ranging from simple tone bursts to AM sound. It is suggested that this cortical region plays a role in time-critical aspects of acoustic information processing. The observed best frequencies covered the same spectrum as AI. As in the auditory fields, most neurons in the insula encoded AM sound with different filter types. The high proportion of neurons unable to encode AM sound (40.6%) and the low mean best modulation frequency (9.9 Hz) do not support a prominent role of the insula in temporal information processing.     

Neurophysiological correlates of brightness discrimination in the lateral geniculate nucleus of the squirrel monkey

Summary Incremental brightness thresholds (DI) were psychophysically determined at several background illumination intensities for three squirrel monkeys. Gross asymmetrical electrodes were then chronically implanted in the lateral geniculate nucleus of the same animals, and activity was recorded in stimulus conditions identical to behavioral testing. Overall activity, recorded through an integrating voltmeter, showed 1. a tendency to decrease as steady background illumination increased, and 2. an abrupt transient increase at both onset and offset to DI test flashes, directly proportional to test flash intensity. Background illumination, in proportion to its intensity, depressed response to a superimposed test flash. Test flashes below intensity DI at the various levels of background illumination produced no measurable response. The quantity DI was shown to be a function of the depressive or inhibitory effect of background illumination on the capacity of the system to respond to transient stimulation. A secondary determinant of DI appeared to be the amount of variability in ongoing neural activity upon which the DI flash is imposed.The author is indebted to the supervisor of her dissertation, Dr. L. R. Pinneo, for introducing her to the recording technique and for his help towards the completion of this work.Now Yerkes Regional Primate Research Center of Emory University, Atlanta, Georgia 30322.This research constitutes the author's Doctoral Dissertation at Florida State University. It was supported by USPHS Grants H=5691, FR=00165 and FR=05235 to Yerkes Laboratories, USPHS Grant FR-00164 and NB 04951-01.     

Frequency modulated sweep responses in the medial geniculate nucleus

A basic feature of communication signals is a dynamic change in frequency. One stimulus that lends itself well to investigating the frequency changes contained in these signals is the frequency modulated (FM) sweep. While many studies have investigated FM sweep responses in the auditory midbrain and cortex, relatively few have examined them in the thalamus. To this end, we investigated the responses of single units in the ventral division of the medial geniculate nucleus (MGNv) of the rat to FM sweeps. Both upward- (changing from low to high frequency) and downward-directed (changing from high to low frequency) FM sweeps were presented at four rates of frequency modulation (i.e., speed). Results showed that the majority (76%) of the cells preferred fast or medium FM sweeps. For direction selectivity, just under half of the units (47%) exhibited a preference for the direction of FM sweep. The results suggest that there is a greater degree of direction but not speed selectivity at progressively higher levels in the auditory pathway.     

Types of interneurons and their participation in the neuronal network of the medial geniculate body

Summary Three different types of interneurons can be separated in the Golgi picture, and many of their details can be identified under the electron microscope, in the medial geniculate body (MGB) of the cat: (1) typical short axon Golgi II. cells of the thalamic type, (2) somewhat larger Golgi type II cells with medium range axon, and (3) spidery neurogliform short axon cells. The most distinctive features of the two first types (1) and (2) are their irregular drumstick shape appendages, increasing in number as well as in length and irregularity of their stalks towards the periphery of the dendrites. These appendages form the vast majority of synaptic profiles in the aggregations of synaptic neuropil (glomeruli) of the nuclei, and they are both presynaptic and postsynaptic by the usual standards applied for the evaluation of the polarity of synapses. The characteristic beaded dendrites of the (3) neurogliform cell type can be recognised particularly easily in the electron microscope picture. They are both presynaptic and postsynaptic in structural polarity. All identified process profiles of interneurons contain flattened (F.-type) or pleomorphic synaptic vesicles. Membrane contacts, in which the interneurons appear to be presynaptic are either of the symmetric (Gray type II) or of an intermediate type. The membrane contacts of postsynaptic portions of the interneurons are usually of the asymmetric type (Gray type I) and the presynaptic profiles contain round (R-type) vesicles. The larger one have been shown already earlier to be derived from specific sensory (inferior collicular) afferents, while many of the smaller ones could be identified in the present study as being derived from cortico-geniculate descending pathways, arising from the auditory areas. Some of the synaptic contacts of the interneurons are apparently derived from other interneurons, the presynaptic profiles being often equivocal or more likely of axonal origin (all interneurons have clear axons in the Golgi picture). The occurrence of three distinct types of interneurons — probably all of inhibitory nature — the complexity in synaptic arrangement, and more particularly in the dendritic linkage of numerous synaptic sites does not favour such simple explanations as surround inhibition by forward or by backward inhibition, but suggests more sophisticated modes of impulse processing in the MGB.     

Neuronal activity in the medial geniculate body of the cat during monaural and binaural stimulation

Summary The activity of 93 medial geniculate body (MGB) neurons was examined in anesthetized cats in response to monaural and binaural click stimuli. Three types of reaction were found with single clicks: an initial discharge (latency: 6–30 msec); an initial and a late discharge (latency of late discharge: 0,5-4 sec), and only a late discharge. Under the conditions of lateralisation (interaural time or intensity difference of the binaurally applied stimuli) significant changes as of the initial as of the late discharge were found. Using sound signals simulating a moving sound source a number of neurons were found to react specifically to the direction of this movement. An assumption is made concerning the role of the late discharge for the fixation of information about position of a sound source in space.     

Specific patterns of neuron arrangement and of synaptic articulation in the medial geniculate body

Summary Golgi and electron microscopic analysis of the known cellular layers in concentric shells of the ventro-lateral portion of the medial geniculate body revealed a flat grid of high density neuropil filling the space between the geniculocortical relay cells, forming essentially a single cell layer in each lamina. The skeleton of this neuropil grid is made up by the interdigitating dendritic tufts of the geniculocortical relay cells, joined together by a rich system of desmosomoid adhesion plaques. The holes of the skeleton are filled in by the multilobed dendritic appendages of Golgi type II interneurons and the grape-like terminals of the inferior collicular specific afferents. Additional axon terminals of other sources — terminals of descending corticogenicular fibers, axons of the Golgi type II interneurons and terminals of the initial collaterals of the geniculocortical relay cells — contribute only to a very insignificant fraction of neuropil volume. The Golgi type II interneurons are oriented in perpendicular direction to the cell layers so that they may bridge with their dendrites several successive layers.Although the general expression synaptic glomeruli used in other relay nuclei for this type of specific synaptic arrangement is hardly applicable to this grid-like neuropil, the essential synaptic articulation pattern of all thalamic relay nuclei is well maintained. The specific inferior collicular afferents are presynaptic to both relay cell dendrites and to the multilobed dendritic appendages of Golgi type II cells, which in turn are presynaptic to the same dendritic regions of the relay cells receiving the bulk of the specific afferents.     

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     

Synaptic architecture in the medial geniculate body (ventral division)

Summary An electron and light microscope study of the ventral division of the medial geniculate body using Golgi techniques, neurofibrillar stains and experimentally induced secondary degeneration. Geniculo-cortical relay cells and Golgi type II interneurons are easily recognized in the Golgi picture; under the electron microscope the two cell types and their dendrites can be identified on the basis of their different plasma structure. Synaptic arrangement of this region is analysed by a detailed comparison between the EM structure and the Golgi picture of the dendrites and axonal arborizations, the neurofibrillar picture of synapses, as well as degeneration pictures after lesions of the inferior colliculus and of the auditory cortical fields. Criteria derived from the light microscope study concerning size and distribution of various nerve endings, as well as secondary degeneration, have been used to identify various axon endings under the EM. The synapses of specific auditory afferents are confined to synaptic clusters arranged around the interdigitating dendritic tufts of relay cells. Although these synaptic clusters resemble in many respects the synaptic glomeruli of the lateral geniculate body, they cannot be termed as such due to lack of a glial capsule and also to less regularity in topographic arrangement of their various axonal and dendritic elements. It became nevertheless possible to identify in the synaptic clusters (1) the terminals of the specific auditory afferents of inferior collicular origin, (2) the axon terminals of Golgi II type neurons and, with less certainty (3) another axonal ending that might belong to descending cortical fibers originating from the auditory region. All types of axons have contacts with the dendrites of relay cells. It is not quite clear, whether the dendrites of Golgi type II cells are involved in these synaptic clusters, but this is very probable. Axo-axonic synapses between type (1) and (2) are frequent, type (1) (the auditory afferent) being always presynaptic. Very numerous synaptic contacts on the distal dendrites of both main cell types are found outside the synaptic clusters. They do not belong to either type (1) or type (2), many look like type (3) and may undergo degeneration after lesions of auditory cortical regions.     

Tonotopic organization in the medial geniculate body (MGB) of lightly anesthetized cats

Summary In the medial geniculate body (MGB) of nitrous oxide anesthetized cats, the pars lateralis (PL) was the only nucleus to show a clear topographic arrangement of its neurons according to their characteristic frequency (CF). When compared to barbiturate anesthetized cats (Imig and Morel 1985a), the tonotopic organization in PL appeared less strict and was characterized by a significant local CF disparity. Furthermore, the degree of tonotopic organization varied along the rostrocaudal axis of the nucleus: it was lower in its caudal than in its rostral half. In the pars ovoidea, the rostral half of the pars magnocellularis (PM) and the suprageniculate nucleus, CF sequences and quantitative evaluations of the tonotopicity indicated the presence of some degree of tonotopic organization which was lower than in PL. No such organization was observed in the caudal part of PM nor in the ventrolateral nucleus, while in the dorsal nucleus, the proportion of toneresponding units was too low for a significant analysis.     

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     

Responses to acoustic stimuli of units in the auditory cortex of awake squirrel monkeys

Summary The responses of single units in the superior temporal gyrus of awake squirrel monkeys to tone pips, noise, clicks, and frequency-modulated sounds were recorded with extracellular microelectrodes. A majority of the units responded to acoustic stimulation, pure tone pips being the most effective in terms of the percentage of units responding (71%). No simple catalogue of response types could be elicited. Units varied in terms of the combinations of stimuli to which they were responsive, the frequency range over which pure tones were effective, and the temporal pattern of discharge to different frequencies and different types of stimuli. A substantial majority of units gave precise timed responses to the onset or offset of the stimuli, while a few introduced long delays between the stimulus and the response. With regard to the area studied, acoustic units could be found between stereotaxic coordinates A3 and A10, both on the lateral surface of the hemisphere and in the superior temporal plane, as well as in the caudal insular region. No precise tonotopic organization could be discerned.Peter Winter died in a skiing accident in March, 1972.     

Methods for estimating structure of trace reactions of neurons in medial geniculate body

The trace reactions of neurons in the medial geniculate body during the action of various auditory signals were investigated in anesthetized cats. Using the traditional peristimulus histogram (PSTH) method to analyze the impulse activity, trace reactions were found in some neurons that possessed a definite time structure with a varying degree of periodicity. Prolonged, diffuse changes in the level of activity were demonstrated in another group of neurons. The trace reactions of the second group of neurons were evaluated by averaging for a multitude of responses the individual PSTHs obtained for each response using the procedure of eliminating time shifts. Elimination in this way of the physiological variation in the time of reaction onset made it possible to establish the presence of a time structure in the trace reactions of a number of neurons in the second group.Translated from Fiziologicheskil Zhurnal SSSR imeni I. M. Sechenova, Vol. 71, No. 12, pp. 1531–1539, December, 1985.     

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     

Quantitative analyses of behavioral sequences elicited by automated telestimulation in squirrel monkeys

Summary The technique of telestimulation was employed in studying the social behavior of squirrel monkeys (Saimiri sciureus). Within a group of five monkeys an animal is electrically stimulated. Its stimulus response and the consecutively occurring reactions of the non-stimulated animals caused by the stimulus response form a sequence of actions.Using stimulus parameters which remained constant, the stimulation of the same electrode in the same animal was repeated automatically. The sequences of actions accumulated thereby were then analysed mathematically on the basis of probability theory.The frequency distributions, the transition matrices and the courses of the frequencies of the actions over the sequences provided the basis for a classification of the behavior units.The courses of the values of entropy and conditional entropy provided summarized information about the sequential process of the animals' behavior. The study of the structure of the sequences was approached by appropriate tests. The applicability of the mathematical methods employed to the material at hand is discussed.     

老年人内侧膝状体形态、血供和来源动脉的病理学改变

目的 :研究老年人内侧膝状体的形态、毗邻及供血动脉来源、分支、分布和病理改变。方法 :体视及手术显微镜下观察 60～ 80岁年龄的脑内侧膝状体的形态、毗邻和血供情况 ;取内侧膝状体来源动脉 (大脑后动脉 )光镜下观察动脉壁的病理改变情况。结果 :内侧膝状体呈半球形 ,动脉来源于大脑后动脉的分支 ,即丘脑膝状体动脉 ,脉络丛后内、外动脉和丘体动脉 ,每侧有小动脉 ( 6.8± 1 .5 )支 ,大脑后动脉粥样硬化改变者占 88.3 %。结论 :内侧膝状体动脉细小 ,仅由大脑后动脉供血 ,动脉硬化可致小动脉管腔狭窄 ,供血不足 ,可能是老年人听力下降的原因之一。     

Effect of the superior colliculus on function of the contralateral lateral geniculate body in cats

Laboratory of Neurophysiology of Brain Integrative Activity and Section Brain and Behavior,, A. I. Karaev Institute of Physiology, Academy of Sciences of the Azerbaijan SSR, Baku. (Presented by Academician of the Academy of Medical Sciences of the USSR O. S. Adrianov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 111, No. 3, pp. 227–229, March, 1991.     

Projections of the reticular complex of the thalamus onto physiologically characterized regions of the medial geniculate body

Afferents from the reticular complex of the thalamus (RE) to the subdivisions of the medial geniculate body (MGB) in the cat were studied by retrograde axonal transport of horseradish peroxidase injected in sites where single unit responses to tones had been characterized. All MGB subdivisions studied received afferents from the same region of RE corresponding to its ventral posterior third, characterized by large neurons. No obvious differences were seen in the localization of labelled neurons within RE according to which MGB subdivision was injected, except that pars lateralis afferents seemed to originate from somewhat more limited portions of RE.     

Participation of the lateral geniculate body in mechanisms of brain activation

The significance of the lateral geniculate body for nonspecific activation of the brain was elucidated in experiments on cats. It was established that when the connections of the lateral geniculate body remain intact, its stimulation elicits the usual activation of the EEG, but at higher threshold values of the current (120–190 A) than when the mesencephalic recticular formation of the medial center of the thalamus is stimulated (50–80 A). If only direct connections with the cortex remain, however, and the others are disrupted, the threshold for activation increases to 220–400 A. When the lateral geniculate body is coagulated, cortical activity occurs only in response to very bright light flashes (250–1000 1x). All of this indicates that, in addition to carrying out its principal function of processing and sending the basic flow of visual impulsation to higher optical centers, the lateral geniculate body may be the source of activating transmissions sent to the cortex. Non-specific stimulation that develops in the cortex is subsequently regulated by the cortex itself; the cortex plays a leading role in these processes.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 68, No. 7, pp. 960–965, July, 1982.