Frequency organization and cellular lamination in the medial geniculate body of the rabbit

Plasticity of Multidimensional Receptive Fields in Core Rat Auditory Cortex Directed by Sound Statistics

Sensory cortical neurons can nonlinearly integrate a wide range of inputs. The outcome of this nonlinear process can be approximated by more than one receptive field component or filter to characterize the ensuing stimulus preference. The functional properties of multidimensional filters are, however, not well understood. Here we estimated two spectrotemporal receptive fields (STRFs) per neuron using maximally informative dimension analysis. We compared their temporal and spectral modulation properties and determined the stimulus information captured by the two STRFs in core rat auditory cortical fields, primary auditory cortex (A1) and ventral auditory field (VAF). The first STRF is the dominant filter and acts as a sound feature detector in both fields. The second STRF is less feature specific, preferred lower modulations, and had less spike information compared to the first STRF. The information jointly captured by the two STRFs was larger than that captured by the sum of the individual STRFs, reflecting nonlinear interactions of two filters. This information gain was larger in A1. We next determined how the acoustic environment affects the structure and relationship of these two STRFs. Rats were exposed to moderate levels of spectrotemporally modulated noise during development. Noise exposure strongly altered the spectrotemporal preference of the first STRF in both cortical fields. The interaction between the two STRFs was reduced by noise exposure in A1 but not in VAF. The results reveal new functional distinctions between A1 and VAF indicating that (i) A1 has stronger interactions of the two STRFs than VAF, (ii) noise exposure diminishes modulation parameter representation contained in the noise more strongly for the first STRF in both fields, and (iii) plasticity induced by noise exposure can affect the strength of filter interactions in A1. Taken together, ascertaining two STRFs per neuron enhances the understanding of cortical information processing and plasticity effects in core auditory cortex.

The homotopic connectivity of the functional brain: a meta-analytic approach

Homotopic connectivity (HC) is the connectivity between mirror areas of the brain hemispheres. It can exhibit a marked and functionally relevant spatial variability, and can be perturbed by several pathological conditions. The voxel-mirrored homotopic connectivity (VMHC) is a technique devised to enquire this pattern of brain organization, based on resting state functional connectivity. Since functional connectivity can be revealed also in a meta-analytical fashion using co-activations, here we propose to calculate the meta-analytic homotopic connectivity (MHC) as the meta-analytic counterpart of the VMHC. The comparison between the two techniques reveals their general similarity, but also highlights regional differences associated with how HC varies from task to rest. Two main differences were found from rest to task: (i) regions known to be characterized by global hubness are more similar than regions displaying local hubness; and (ii) medial areas are characterized by a higher degree of homotopic connectivity, while lateral areas appear to decrease their degree of homotopic connectivity during task performance. These findings show that MHC can be an insightful tool to study how the hemispheres functionally interact during task and rest conditions.

Reconsidering Tonotopic Maps in the Auditory Cortex and Lemniscal Auditory Thalamus in Mice

The auditory thalamus and auditory cortex (AC) are pivotal structures in the central auditory system. However, the thalamocortical mechanisms of processing sounds are largely unknown. Investigation of this process benefits greatly from the use of mice because the mouse is a powerful animal model in which various experimental techniques, especially genetic tools, can be applied. However, the use of mice has been limited in auditory research, and thus even basic anatomical knowledge of the mouse central auditory system has not been sufficiently collected. Recently, optical imaging combined with morphological analyses has enabled the elucidation of detailed anatomical properties of the mouse auditory system. These techniques have uncovered fine AC maps with multiple frequency-organized regions, each of which receives point-to-point thalamocortical projections from different origins inside the lemniscal auditory thalamus, the ventral division of the medial geniculate body (MGv). This precise anatomy now provides a platform for physiological research. In this mini review article, we summarize these recent achievements that will facilitate physiological investigations in the mouse auditory system.

Processing of frequency and location in human subcortical auditory structures

To date it remains largely unknown how fundamental aspects of natural sounds, such as their spectral content and location in space, are processed in human subcortical structures. Here we exploited the high sensitivity and specificity of high field fMRI (7 Tesla) to examine the human inferior colliculus (IC) and medial geniculate body (MGB). Subcortical responses to natural sounds were well explained by an encoding model of sound processing that represented frequency and location jointly. Frequency tuning was organized in one tonotopic gradient in the IC, whereas two tonotopic maps characterized the MGB reflecting two MGB subdivisions. In contrast, no topographic pattern of preferred location was detected, beyond an overall preference for peripheral (as opposed to central) and contralateral locations. Our findings suggest the functional organization of frequency and location processing in human subcortical auditory structures, and pave the way for studying the subcortical to cortical interaction required to create coherent auditory percepts.

Response features across the auditory midbrain reveal an organization consistent with a dual lemniscal pathway

The central auditory system has traditionally been divided into lemniscal and nonlemniscal pathways leading from the midbrain through the thalamus to the cortex. This view has served as an organizing principle for studying, modeling, and understanding the encoding of sound within the brain. However, there is evidence that the lemniscal pathway could be further divided into at least two subpathways, each potentially coding for sound in different ways. We investigated whether such an interpretation is supported by the spatial distribution of response features in the central nucleus of the inferior colliculus (ICC), the part of the auditory midbrain assigned to the lemniscal pathway. We recorded responses to pure tone stimuli in the ICC of ketamine-xylazine-anesthetized guinea pigs and used three-dimensional brain reconstruction techniques to map the location of the recording sites. Compared with neurons in caudal-and-medial regions within an isofrequency lamina of the ICC, neurons in rostral-and-lateral regions responded with shorter first-spike latencies with less spiking jitter, shorter durations of spiking responses, a higher proportion of spikes occurring near the onset of the stimulus, lower thresholds, and larger local field potentials with shorter latencies. Further analysis revealed two distinct clusters of response features located in either the caudal-and-medial or the rostral-and-lateral parts of the isofrequency laminae of the ICC. Thus we report substantial differences in coding properties in two regions of the ICC that are consistent with the hypothesis that the lemniscal pathway is made up of at least two distinct subpathways from the midbrain up to the cortex.

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.

Distinct core thalamocortical pathways to central and dorsal primary auditory cortex

The cat primary auditory cortex (AI) is usually assumed to form one continuous functional region. However, the dorsal and central parts of the AI iso-frequency domain contain neurons that have distinct response properties to acoustic stimuli. In this study, we asked whether neurons projecting to dorsal versus central regions of AI originate in different parts of the medial geniculate body (MGB). Spike rate responses to variations in the sound level and frequency of pure tones were used to measure characteristic frequency (CF) and frequency resolution. These were mapped with high spatial density in order to place retrograde tracers into matching frequency regions of the central narrow-band region (cNB) and dorsal AI. Labeled neurons projecting to these two parts of AI were concentrated in the middle and rostral thirds of the MGB, respectively. There was little evidence that differences in dorsal and central AI function could be due to convergent input from cells outside the ventral division of the MGB (MGBv). Instead, inputs arising from different locations along the caudal-to-rostral dimension of MGBv represent potential sources of response differences between central and dorsal sub-regions of AI.

Thalamic label patterns suggest primary and ventral auditory fields are distinct core regions

A hierarchical scheme proposed by Kaas and colleagues suggests that primate auditory cortex can be divided into core and belt regions based on anatomic connections with thalamus and distinctions among response properties. According to their model, core auditory cortex receives predominantly unimodal sensory input from the ventral nucleus of the medial geniculate body (MGBv); whereas belt cortex receives predominantly cross-modal sensory input from nuclei outside the MGBv. We previously characterized distinct response properties in rat primary (A1) versus ventral auditory field (VAF) cortex; however, it has been unclear whether VAF should be categorized as a core or belt auditory cortex. The current study employed high-resolution functional imaging to map intrinsic metabolic responses to tones and to guide retrograde tracer injections into A1 and VAF. The size and density of retrogradely labeled somas in the medial geniculate body (MGB) were examined as a function of their position along the caudal-to-rostral axis, subdivision of origin, and cortical projection target. A1 and VAF projecting neurons were found in the same subdivisions of the MGB but in rostral and caudal parts, respectively. Less than 3% of the cells projected to both regions. VAF projecting neurons were smaller than A1 projecting neurons located in dorsal (MGBd) and suprageniculate (SG) nuclei. Thus, soma size varied with both caudal-rostral position and cortical target. Finally, the majority (>70%) of A1 and VAF projecting neurons were located in MGBv. These MGB connection profiles suggest that rat auditory cortex, like primate auditory cortex, is made up of multiple distinct core regions.

The medial geniculate, not the amygdala, as the root of auditory fear conditioning

The neural basis of auditory fear conditioning (AFC) is almost universally believed to be the amygdala, where auditory fear memories are reputedly acquired and stored. This widely-accepted amygdala model holds that the auditory conditioned stimulus (CS) and the nociceptive unconditioned stimulus (US) first converge in the lateral nucleus of the amygdala (AL), and are projected independently to it from the medial division of the medial geniculate nucleus (MGm) and the adjacent posterior intralaminar nucleus (PIN), which serve merely as sensory relays. However, the four criteria that are used to support the AL model, (a) CS-US convergence, (b) associative plasticity, (c) LTP and (d) lesion-induced learning impairment, are also met by the MGm/PIN. Synaptic and molecular approaches supporting the AL also implicate the MGm/PIN. As both the AL and its preceding MGm/PIN are critically involved, we propose that the latter be considered the "root" of AFC.

Axonal projections of auditory cells with short and long response latencies in the medial geniculate nucleus: distinct topographies in the connection with the thalamic reticular nucleus

The thalamic reticular nucleus (TRN) is a crucial anatomical node of thalamocortical connectivity for sensory processing. In the rat auditory system, we determined features of thalamic projections to the TRN, using juxtacellular recording and labeling techniques. Two types of auditory cells (short latency, SL, and long latency, LL), exhibiting unit discharges to noise burst stimuli (duration, 100 ms) with short (< 50 ms) and long (> 100 ms) response latencies, were obtained from the ventral division of the medial geniculate nucleus (MGV). Both SL and LL cells had a propensity to exhibit reverberatory discharges in response to sound stimuli. The primary discharges of SL cells were mostly single spikes while the non-primary discharges of SL cells and the whole discharges of LL cells were mostly burst spikes. SL cells sent topographic projections to the TRN along the dorsoventral and rostrocaudal neural axes while LL cells only along the rostrocaudal axis. As tonotopy-related cortical projections to the TRN are topographic primarily along the dorsoventral extent of the TRN and the MGV is tonotopically organized along the dorsoventral axis, SL cells, directly activated by ascending auditory inputs, may be closely involved in tonotopic thalamocortical connectivity. On the other hand, LL cells, which are suppressed by ascending inputs and could be driven to discharge by corticofugal inputs, are assumed to activate the TRN in a manner less related to tonotopic organization. There may exist heterogeneous projections from the MGV to the TRN, which, in conjunction with corticofugal connections, could constitute distinct channels of auditory processing.

Two thalamic pathways to primary auditory cortex

Neurons in the center of cat primary auditory cortex (AI) respond to a narrow range of sound frequencies and the preferred frequencies in local neuron clusters are closely aligned in this central narrow bandwidth region (cNB). Response preferences to other input parameters, such as sound intensity and binaural interaction, vary within cNB; however, the source of this variability is unknown. Here we examined whether input to the cNB could arise from multiple, anatomically independent subregions in the ventral nucleus of the medial geniculate body (MGBv). Retrograde tracers injected into cNB labeled discontinuous clusters of neurons in the superior (sMGBv) and inferior (iMGBv) halves of the MGBv. Most labeled neurons were in the sMGBv and their density was greater, iMGBv somata were significantly larger. These findings suggest that cNB projection neurons in superior and iMGBv have distinct anatomic and possibly physiologic organization.

A discontinuous tonotopic organization in the inferior colliculus of the rat

Audible frequencies of sound are encoded in a continuous manner along the length of the cochlea, and frequency is transmitted to the brain as a representation of place on the basilar membrane. The resulting tonotopic map has been assumed to be a continuous smooth progression from low to high frequency throughout the central auditory system. Here, physiological and anatomical data show that best frequency is represented in a discontinuous manner in the inferior colliculus, the major auditory structure of the midbrain. Multiunit maps demonstrate a distinct stepwise organization in the order of best frequency progression. Furthermore, independent data from single neurons show that best frequencies at octave intervals of approximately one-third are more prevalent than others. These data suggest that, in the inferior colliculus, there is a defined space of tissue devoted to a given frequency, and input within this frequency band may be pooled for higher-level processing.

Multiple topographically organized projections connect the central nucleus of the inferior colliculus to the ventral division of the medial geniculate nucleus in the gerbil, Meriones unguiculatus

The ventral division of the medial geniculate nucleus (MGv) receives almost all of its ascending input from the ipsilateral central nucleus of the inferior colliculus (CNIC). In a previous study (Cant and Benson [2006] J. Comp. Neurol. 495:511-528), we made injections of biotinylated dextran amine into the CNIC of the gerbil and demonstrated that it can be divided into two parts. One part (zone 1) receives almost all of its ascending input from the cochlear nuclei, the nuclei of the lateral lemniscus, and the main nuclei of the superior olivary complex; the other part (zone 2) receives inputs from the cochlear nuclei and nuclei of the lateral lemniscus but few or no inputs from the main olivary nuclei. Here we show that these two parts of the CNIC project differentially to the MGv. Axons labeled anterogradely by injections in zone 1 project throughout the rostral two-thirds of the MGv, whereas axons from zone 2 project to the caudal third of the MGv. Throughout much of their extent, the terminal fields do not appear to overlap, although both parts of the CNIC project to medial and dorsal parts of the MGv, and there may be overlap in the most ventral part as well. The results indicate that two parallel pathways arising in the CNIC remain largely separate in the medial geniculate nucleus of the gerbil. It seems most likely that the neurons in the two terminal zones in the MGv perform different functions in audition.

Multiparametric Auditory Receptive Field Organization Across Five Cortical Fields in the Albino Rat

The auditory cortex of the rat is becoming an increasingly popular model system for studies of experience-dependent receptive field plasticity. However, the relative position of various fields within the auditory core and the receptive field organization within each field have yet to be fully described in the normative case. In this study, the macro- and micro-organizational features of the auditory cortex were studied in pentobarbital-anesthetized adult rats with a combination of physiological and anatomical methods. Dense microelectrode mapping procedures were used to identify the relative position of five tonotopically organized fields within the auditory core: primary auditory cortex (AI), the posterior auditory field (PAF), the anterior auditory field (AAF), the ventral auditory field (VAF), and the suprarhinal auditory field (SRAF). AI and AAF both featured short-latency, sharply tuned responses with predominantly monotonic intensity-response functions. SRAF and PAF were both characterized by longer-latency, broadly tuned responses. VAF directly abutted the ventral boundary of AI but was almost exclusively composed of low-threshold nonmonotonic intensity-tuned responses. Dual injection of retrograde tracers into AI and VAF was used to demonstrate that the sources of thalamic input from the medial geniculate body to each area were essentially nonoverlapping. An analysis of receptive field parameters beyond characteristic frequency revealed independent spatially ordered representations for features related to spectral tuning, intensity tuning, and onset response properties in AI, AAF, VAF, and SRAF. These data demonstrate that despite its greatly reduced physical scale, the rat auditory cortex features a surprising degree of organizational complexity and detail.

Carbamazepine induced pitch shift and octave space representation

Octave-circular pitch perception, the repetition of pitch scale qualities when surpassing the octave interval, has been observed in behavioral data from humans and monkeys, but the underlying anatomy and physiology is still unknown. Here we analyze octave circularity in a concert pianist with absolute pitch, both under medication with the neurotropic drug carbamazepine (CBZ) and without medication. Analysis of 4619 responses in a pitch identification task revealed an internal tone-scale representation, based on the norm-tone scale re A4=440 Hz, with an octave-circular pattern of strongly and weakly represented tones. CBZ caused a global down-shift of pitch (ca. 1 semitone at 500 Hz), but no down-shift of the octave-circular pattern of tone characteristics. This pattern was similar in the six tested octave ranges (32.7-2093 Hz), both under the control and the CBZ condition. Pattern repetition always occurred at octave intervals and did not reflect the stretched octaves of piano tuning. The results indicate that CBZ influences pitch detection peripheral of an octave-circular pitch representation. Thus they support previous evidence for pitch detection in the auditory midbrain and for octave-circular pitch mapping in the auditory thalamus.

Topography of projections from the primary and non-primary auditory cortical areas to the medial geniculate body and thalamic reticular nucleus in the rat

The functional significance of parallel and redundant information processing by multiple cortical auditory fields remains elusive. A possible function is that they may exert distinct corticofugal modulations on thalamic information processing through their parallel connections with the medial geniculate body and thalamic reticular nucleus. To reveal the anatomical framework for this function, we examined corticothalamic projections of tonotopically comparable subfields in the primary and non-primary areas in the rat auditory cortex. Biocytin was injected in and around cortical area Te1 after determining best frequency at the injection site on the basis of epicortical field potentials evoked by pure tones. The rostral part of area Te1 (primary auditory area) and area temporal cortex, area 2, dorsal (Te2D) (posterodorsal auditory area) dorsal to the caudal end of area Te1, which both exhibited high best frequencies, projected to the ventral zone of the ventral division of the medial geniculate body. The caudal end of area Te1 (auditory area) and the rostroventral part of area Te1 (a part of anterior auditory field), which both exhibited low best frequencies, projected to the dorsal zone of the ventral division of the medial geniculate body. In contrast to the similar topography in the projections to the ventral division of the medial geniculate body, collateral projections to the thalamic reticular nucleus terminated in the opposite dorsal and ventral zones of the lateral and middle tiers of the nucleus in each pair of the tonotopically comparable cortical subfields. In addition, the projections of the non-primary cortical subfields further arborized in the medial tier of the thalamic reticular nucleus. The results suggest that tonotopically comparable primary and non-primary subfields in the auditory cortex provide corticofugal excitatory effects to the same part of the ventral division of the medial geniculate body. On the other hand, corticofugal inhibition via the thalamic reticular nucleus may operate in different parts of the ventral division of the medial geniculate body or different thalamic nuclei. The primary and non-primary cortical auditory areas are presumed to subserve distinct gating functions for auditory attention.

Auditory thalamic organization: Cellular slabs, dendritic arbors and tectothalamic axons underlying the frequency map

A model of auditory thalamic organization is presented incorporating cellular laminae, oriented dendritic arbors and tectothalamic axons as a basis for the tonotopic map at this level of the central auditory system. The heart of this model is the laminar organization of neuronal somata in the ventral division of the medial geniculate body (MGV) of the rabbit, visible in routine Nissl stains. Microelectrode studies have demonstrated a step-wise ascending progression of best frequencies perpendicular to the cell layers. The dendritic arbors of MGV neurons are aligned parallel to the cellular laminae and dendritic tree width along the frequency axis corresponds closely with the frequency steps seen in microelectrode studies. In the laminated subdivision, tectothalamic axons terminate in the form of bands closely aligned with the laminae and dendritic arbors of thalamic relay neurons. The bands of tectothalamic axons extend in the anterior-posterior (A-P) plane forming a dorsal-ventral series of stacked frequency slabs. In the pars ovoidea region, the homologous spiraling of somata, dendritic fields and tectothalamic axons appear to represent a low-frequency area in this species. At least two types of tectothalamic terminals were found within the bands: large boutons frequently arranged in a glomerular pattern and smaller boutons arising from fine caliber axons. We propose that the rabbit is an ideal model to investigate the structural-functional basis of functional maps in the mammalian auditory forebrain.

Topography of corticothalamic projections from the auditory cortex of the rat

Corticothalamic projections from cortical auditory field to the medial geniculate body (MG) in the rat were systematically examined by making small injections of biocytin in cortical area Te1. All injections, confined to 400 microm in diameter, resulted in two projections terminating in the ventral (MGV) and dorsal divisions (MGD) of the MG. The projections to the MGV were evidently topographic. The rostral and caudal portions of area Te1 projected to the ventromedial and dorsolateral parts of the MGV, respectively, forming narrow bands of terminal axons that extended in the mediolateral direction in the coronal plane of the MGV. The minimum dorsoventral width of the bands ranged approximately from 100 to 300 microm. Besides, the more rostral portion of area Te1 tended to project to the more rostral side of the MGV. The projections to the MGD consistently arborized in its ventral margin made up of the deep dorsal nucleus of the MGD. A similar weak topography along the rostrocaudal direction was observed in the projections to the MGD. Large terminals were occasionally found in the MGD after the injections involving cortical layer V. The distribution of large terminals also appeared topographic along with small terminals that were the major component of labeling. Collaterals of labeled axons produced slabs of terminal field in the thalamic reticular nucleus, which also exhibited a weak topography of distribution. These results provide insights into the structural basis of corticofugal modulations related to the tonotopic organizations in the cortex and MG.

Mutation in Mitochondrial DNA as a Cause of Presbyacusis

Much of the hearing loss that occurs in old age is likely to be due to the long-term deterioration of the mitochondria in the different structures of the cochlea. The current review surveys some of the basic information on mitochondria and mitochondrial DNA, as a background to their possible involvement in presbyacusis. It is likely that oxygen radicals damage mitochondrial DNA and other components of the mitochondria, such as their proteins and lipids. This further compromises both oxidative phosphorylation and the repair processes in mitochondria, setting up a vicious cycle of degradation. Evidence is presented from inherited point mutations on the possibly most critical sites for mutations in mitochondrial DNA associated with hearing loss. It is suggested that random sorting and clonal expansion of mutations both maintain the integrity of the pool of mitochondrial DNA molecules and give rise to the apoptosis that leads to loss of vulnerable cells, and hence to deafness. It is moreover suggested that apoptosis of the vulnerable cells of the inner ear may to some extent be preventable, or at least delayed.

Dendritic orientation and laminar architecture in the rabbit auditory thalamus

A laminar organization composed of the dendritic fields of principal neurons and afferent axonal arbors has been proposed as the anatomical substrate for the frequency map at several levels of the mammalian central auditory system, including the inferior colliculus and medial geniculate body (MGB). In contrast to the auditory thalamus in most mammals, the ventral division of the rabbit medial geniculate body (MGV) has cellular laminae visible in routine Nissl stains, allowing a direct comparison of the laminar organization with the dendritic architecture and frequency organization. In total 30 presumptive relay neurons in the MGV were labeled with the juxtacellular recording method, and their dendritic arbors were fully reconstructed from serial sections with the aid of a computer microscope. The spatial organization of MGV dendritic fields was analyzed using the dendritic prism, dendritic stick, and fan-in projection methods. Quantitative spatial analyses revealed that, for MGV neurons in the central pars lateralis subdivision, the major axis of the dendritic fields (approximately 29 degrees relative to the horizontal plane) was closely aligned with that of the Nissl laminae (approximately 25 degrees). Both were oriented orthogonally to the tonotopic axis. In contrast, cells in the pars ovoidea had their major axis of orientation parallel to the anteroposterior axis of the brain. Although a bitufted dendritic field was the norm, it was not uncommon for MGV neurons to have pronounced spatial asymmetries in their dendritic fields. A model is presented that incorporates cellular laminae and oriented dendritic growth to form frequency-related slabs within the MGV.

Functional subregions in primary auditory cortex defined by thalamocortical terminal arbors: an electrophysiological and anterograde labeling study

Several functional maps have been described in primary auditory cortex, including those related to frequency, tuning, latency, binaurality, and intensity. Many of these maps are arranged in a discontinuous or patchy manner. Similarly, thalamocortical projections arising from the ventral division of the medial geniculate body to the primary auditory cortex are also patchy. We used anterograde labeling and electrophysiological methods to examine the relationship between thalamocortical patches and auditory cortical maps. Biotinylated dextran-amine was deposited into physiologically characterized sites in the ventral division of the medial geniculate body of New Zealand white rabbits. Approximately 7 d later, the animal was again anesthetized and the ipsilateral auditory cortex was mapped with tungsten microelectrodes. Multi-unit physiological data were obtained for the following characteristics: best frequency (BF), binaurality, response type, latency, sharpness of tuning, and threshold. Immunocytochemical methods were used to reveal the injection site in the ventral division of the medial geniculate body as well as the anterogradely labeled thalamocortical afferents in the auditory cortex. In 86% of the cases (12 of 14), entry into a thalamocortical patch was associated with a marked change in physiological responses. A consistent BF and binaural class were usually observed within a patch. The patches appear to innervate distinct functional regions coding frequency and binaurality. A model is presented showing how patchy thalamocortical projections participate in the formation of tonotopic and binaural maps in primary auditory cortex.

Cell types and response properties of neurons in the ventral division of the medial geniculate body of the rabbit

Although there is evidence for multiple classes of thalamic relay neurons in the auditory thalamus, correlative anatomical and physiological studies are lacking. We have used the juxtacellular labeling technique, in conjunction with Nissl, Golgi, and immunocytochemical methods, to study the morphology and response properties of cells in the ventral division of the medial geniculate body of the rabbit. Single units in the ventral division of the medial geniculate body (MGV) were characterized extracellularly with monaural and binaural tone and noise bursts (100- to 250-msec duration). Characterized units were filled with biocytin and visualized with an antibody enhanced diaminobenzidine reaction. A total of 31 neurons were physiologically characterized and labeled with the juxtacellular technique. Labeled neurons were fully reconstructed from serial sections by using a computer microscope system. Three subregions of the rabbit MGV were identified, each characterized by differences in Nissl architecture, calcium-binding protein expression, and by the dendritic orientation of tufted relay neurons. In general, the dendritic fields of relay neurons were closely aligned with the cellular laminae. Qualitative and quantitative analyses revealed two types of presumptive relay neurons within the MGV. Type I cells had thick dendrites with a greater total volume and morphologically diverse appendages compared with the Type II cells whose dendrites were thin with a moderate number of small spines. Both classes were acoustically responsive and exhibited a variety of response patterns, including onset, offset, and sustained responses. In terms of binaural characteristics, most (ca. 53%) labeled neurons were of the EE type, with the remaining cells classified as EO (27%) or EI (20%) response types. Two types of presumptive interneurons were also seen: bipolar neurons with large dendritic fields and a small neurogliaform variety. Cell types and dendritic orientation within the MGV are discussed in terms of the physiological organization of the rabbit auditory thalamus.

A role for purinergic receptors at the inner hair cell-afferent synapse?

Previously published evidence is reviewed for a functional role of ATP and its receptors at the chemical synapse between the mammalian inner hair cell and the primary afferent dendrites of the VIIIth nerve. New findings are also presented, both from gross cochlear potentials and single neurone recordings during intracochlear perfusion of ATP-receptor antagonists and agonists. Both the previous and present results are consistent with the notion that endogenous ATP may act to regulate the excitability of the primary afferent dendrite, possibly acting on P2X2 receptors.