Corticocortical connections of cat primary auditory cortex (AI): laminar organization and identification of supragranular neurons projecting to area AII

Development of Auditory Cortex Circuits

The ability to process and perceive sensory stimuli is an essential function for animals. Among the sensory modalities, audition is crucial for communication, pleasure, care for the young, and perceiving threats. The auditory cortex (ACtx) is a key sound processing region that combines ascending signals from the auditory periphery and inputs from other sensory and non-sensory regions. The development of ACtx is a protracted process starting prenatally and requires the complex interplay of molecular programs, spontaneous activity, and sensory experience. Here, we review the development of thalamic and cortical auditory circuits during pre- and early post-natal periods.

Regional brain volume predicts response to methylphenidate treatment in individuals with ADHD

BACKGROUND

Despite the effectiveness of methylphenidate for treating ADHD, up to 30% of individuals with ADHD show poor responses to methylphenidate. Neuroimaging biomarkers to predict medication responses remain elusive. This study characterized neuroanatomical features that differentiated between clinically good and poor methylphenidate responders with ADHD.

METHODS

Using a naturalistic observation design selected from a larger cohort, we included 79 drug-naive individuals (aged 6-42 years) with ADHD without major psychiatric comorbidity, who had acceptable baseline structural MRI data quality. Based on a retrospective chart review, we defined responders by individuals' responses to at least one-month treatment with methylphenidate. A nonparametric mass-univariate voxel-based morphometric analysis was used to compare regional gray matter volume differences between good and poor responders. A multivariate pattern recognition based on the support vector machine was further implemented to identify neuroanatomical indicators to predict an individual's response.

RESULTS

63 and 16 individuals were classified in the good and poor responder group, respectively. Using the small-volume correction procedure based on the hypothesis-driven striatal and default-mode network masks, poor responders had smaller regional volumes of the left putamen as well as larger precuneus volumes compared to good responders at baseline. The machine learning approach identified that volumetric information among these two regions alongside the left frontoparietal regions, occipital lobes, and posterior/inferior cerebellum could predict clinical responses to methylphenidate in individuals with ADHD.

CONCLUSION

Our results suggest regional striatal and precuneus gray matter volumes play a critical role in mediating treatment responses in individuals with ADHD.

Fast temporal dynamics and causal relevance of face processing in the human temporal cortex

We measured the fast temporal dynamics of face processing simultaneously across the human temporal cortex (TC) using intracranial recordings in eight participants. We found sites with selective responses to faces clustered in the ventral TC, which responded increasingly strongly to marine animal, bird, mammal, and human faces. Both face-selective and face-active but non-selective sites showed a posterior to anterior gradient in response time and selectivity. A sparse model focusing on information from the human face-selective sites performed as well as, or better than, anatomically distributed models when discriminating faces from non-faces stimuli. Additionally, we identified the posterior fusiform site (pFUS) as causally the most relevant node for inducing distortion of conscious face processing by direct electrical stimulation. These findings support anatomically discrete but temporally distributed response profiles in the human brain and provide a new common ground for unifying the seemingly contradictory modular and distributed modes of face processing.

Comparison of the Upper Marginal Neurons of Cortical Layer 2 with Layer 2/3 Pyramidal Neurons in Mouse Temporal Cortex

Layer 2/3 (L2/3) excitatory neurons in the neocortex make major contributions to corticocortical connections and therefore function to integrate information across cortical areas and hemispheres. Recent evidence suggests that excitatory neurons in L2/3 can have different properties. Sparse evidence from previous studies suggests that L2 neurons located at the border between L1 and L2 (referred to as L2 marginal neurons, L2MNs), have a morphology distinct from a typical pyramidal neuron. However, whether the membrane properties and input/output properties of L2MNs are different from those of typical pyramidal neurons in L2/3 is unknown. Here we addressed these questions in a slice preparation of mouse temporal cortex. We found that L2MNs were homogeneous in intrinsic membrane properties but appeared diverse in morphology. In agreement with previous studies, L2MNs either had oblique apical dendrites or had no obvious apical dendrites. The tufts of both apical and basal dendrites of these neurons invaded L1 extensively. All L2MNs showed a regular firing pattern with moderate adaptation. Compared with typical L2/3 pyramidal neurons that showed regular spiking (RS) activity (neurons), L2MNs showed a higher firing rate, larger sag ratio, and higher input resistance. No difference in the amplitude of excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively), evoked by stimulation of L1, was found between the two types of neurons, but the IPSPs in L2MNs had a slower time course than those in L2/3 RS cells. In paired recordings, unitary EPSPs showed no significant differences between synapses formed by L2MNs and those formed by L2/3 RS neurons. However, short-term synaptic depression (STSD) examined with a L2MN as the presynaptic neuron was greater when another L2MN was the postsynaptic neuron than when a L2/3 RS neuron was the postsynaptic neuron. The distinct morphological features of L2MNs found here have developmental implications, and the differences in electrophysiological properties between L2MNs and other L2/3 pyramidal neurons suggest that they play different functional roles in cortical networks.

Sublaminar Subdivision of Mouse Auditory Cortex Layer 2/3 Based on Functional Translaminar Connections

The cerebral cortex is subdivided into six layers based on morphological features. The supragranular layers 2/3 (L2/3) contain morphologically and genetically diverse populations of neurons, suggesting the existence of discrete classes of cells. In primates and carnivores L2/3 can be subdivided morphologically, but cytoarchitectonic divisions are less clear in rodents. Nevertheless, discrete classes of cells could exist based on their computational requirement, which might be linked to their associated functional microcircuits. Through in vitro slice recordings coupled with laser-scanning photostimulation we investigated whether L2/3 of male mouse auditory cortex contains discrete subpopulations of cells with specific functional microcircuits. We use hierarchical clustering on the laminar connection patterns to reveal the existence of multiple distinct classes of L2/3 neurons. The classes of L2/3 neurons are distinguished by the pattern of their laminar and columnar inputs from within A1 and their location within L2/3. Cells in superficial L2 show more extensive columnar integration than deeper L3 cells. Moreover, L3 cells receive more translaminar input from L4. In vivo imaging in awake mice revealed that L2 cells had higher bandwidth than L3 cells, consistent with the laminar differences in columnar integration. These results suggest that similar to higher mammals, rodent L2/3 is not a homogenous layer but contains several parallel microcircuits.SIGNIFICANCE STATEMENT Layer 2/3 of auditory cortex is functionally diverse. We investigated whether L2/3 cells form classes based on their functional connectivity. We used in vitro whole-cell patch-clamp recordings with laser-scanning photostimulation and performed unsupervised clustering on the resulting excitatory and inhibitory connection patterns. Cells within each class were located in different sublaminae. Superficial cells showed wider integration along the tonotopic axis and the amount of L4 input varied with sublaminar location. To identify whether sensory responses varied with sublaminar location, we performed in vivo Ca2+ imaging and found that L2 cells were less frequency-selective than L3 cells. Our results show that the diversity of receptive fields in L2/3 is likely due to diversity in the underlying functional circuits.

Salicylate-induced cochlear impairments, cortical hyperactivity and re-tuning, and tinnitus

High doses of sodium salicylate (SS) have long been known to induce temporary hearing loss and tinnitus, effects attributed to cochlear dysfunction. However, our recent publications reviewed here show that SS can induce profound, permanent, and unexpected changes in the cochlea and central nervous system. Prolonged treatment with SS permanently decreased the cochlear compound action potential (CAP) amplitude in vivo. In vitro, high dose SS resulted in a permanent loss of spiral ganglion neurons and nerve fibers, but did not damage hair cells. Acute treatment with high-dose SS produced a frequency-dependent decrease in the amplitude of distortion product otoacoustic emissions and CAP. Losses were greatest at low and high frequencies, but least at the mid-frequencies (10-20 kHz), the mid-frequency band that corresponds to the tinnitus pitch measured behaviorally. In the auditory cortex, medial geniculate body and amygdala, high-dose SS enhanced sound-evoked neural responses at high stimulus levels, but it suppressed activity at low intensities and elevated response threshold. When SS was applied directly to the auditory cortex or amygdala, it only enhanced sound evoked activity, but did not elevate response threshold. Current source density analysis revealed enhanced current flow into the supragranular layer of auditory cortex following systemic SS treatment. Systemic SS treatment also altered tuning in auditory cortex and amygdala; low frequency and high frequency multiunit clusters up-shifted or down-shifted their characteristic frequency into the 10-20 kHz range thereby altering auditory cortex tonotopy and enhancing neural activity at mid-frequencies corresponding to the tinnitus pitch. These results suggest that SS-induced hyperactivity in auditory cortex originates in the central nervous system, that the amygdala potentiates these effects and that the SS-induced tonotopic shifts in auditory cortex, the putative neural correlate of tinnitus, arises from the interaction between the frequency-dependent losses in the cochlea and hyperactivity in the central nervous system.

Connectivity changes underlying spectral EEG changes during propofol-induced loss of consciousness

The mechanisms underlying anesthesia-induced loss of consciousness remain a matter of debate. Recent electrophysiological reports suggest that while initial propofol infusion provokes an increase in fast rhythms (from beta to gamma range), slow activity (from delta to alpha range) rises selectively during loss of consciousness. Dynamic causal modeling was used to investigate the neural mechanisms mediating these changes in spectral power in humans. We analyzed source-reconstructed data from frontal and parietal cortices during normal wakefulness, propofol-induced mild sedation, and loss of consciousness. Bayesian model selection revealed that the best model for explaining spectral changes across the three states involved changes in corticothalamic interactions. Compared with wakefulness, mild sedation was accounted for by an increase in thalamic excitability, which did not further increase during loss of consciousness. In contrast, loss of consciousness per se was accompanied by a decrease in backward corticocortical connectivity from frontal to parietal cortices, while thalamocortical connectivity remained unchanged. These results emphasize the importance of recurrent corticocortical communication in the maintenance of consciousness and suggest a direct effect of propofol on cortical dynamics.

Dynamic causal models and physiological inference: a validation study using isoflurane anaesthesia in rodents

Generative models of neuroimaging and electrophysiological data present new opportunities for accessing hidden or latent brain states. Dynamic causal modeling (DCM) uses Bayesian model inversion and selection to infer the synaptic mechanisms underlying empirically observed brain responses. DCM for electrophysiological data, in particular, aims to estimate the relative strength of synaptic transmission at different cell types and via specific neurotransmitters. Here, we report a DCM validation study concerning inference on excitatory and inhibitory synaptic transmission, using different doses of a volatile anaesthetic agent (isoflurane) to parametrically modify excitatory and inhibitory synaptic processing while recording local field potentials (LFPs) from primary auditory cortex (A1) and the posterior auditory field (PAF) in the auditory belt region in rodents. We test whether DCM can infer, from the LFP measurements, the expected drug-induced changes in synaptic transmission mediated via fast ionotropic receptors; i.e., excitatory (glutamatergic) AMPA and inhibitory GABA(A) receptors. Cross- and auto-spectra from the two regions were used to optimise three DCMs based on biologically plausible neural mass models and specific network architectures. Consistent with known extrinsic connectivity patterns in sensory hierarchies, we found that a model comprising forward connections from A1 to PAF and backward connections from PAF to A1 outperformed a model with forward connections from PAF to A1 and backward connections from A1 to PAF and a model with reciprocal lateral connections. The parameter estimates from the most plausible model indicated that the amplitude of fast glutamatergic excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) behaved as predicted by previous neurophysiological studies. Specifically, with increasing levels of anaesthesia, glutamatergic EPSPs decreased linearly, whereas fast GABAergic IPSPs displayed a nonlinear (saturating) increase. The consistency of our model-based in vivo results with experimental in vitro results lends further validity to the capacity of DCM to infer on synaptic processes using macroscopic neurophysiological data.

Reciprocal modulatory influences between tonotopic and nontonotopic cortical fields in the cat

Functional and anatomical studies suggest that acoustic signals are processed hierarchically in auditory cortex. Although most regions of acoustically responsive cortex are not tonotopically organized, all previous electrophysiological investigations of interfield interactions have only examined tonotopically represented areas. The purpose of the present study was to investigate the functional interactions between tonotopically and nontonotopically organized fields in auditory cortex. We accomplished this goal by examining the bidirectional contributions between the cochleotopically organized primary auditory cortex (A1) and the noncochleotopically organized second auditory field (A2). Multiunit acute recording procedures in combination with reversible cooling deactivation techniques were used in eight mature cats. The synaptic activity of A1 or A2 was suppressed while the neuronal response to tonal stimuli of the noninactivated area (A1 or A2) was measured. Response strength, neuronal threshold, receptive field bandwidths, and latency measures were collected at each recorded site before, during, and after cooling deactivation epochs. Our analysis revealed comparable changes in A1 and A2 neuronal response properties. Specifically, significant decreases in neuronal response strength, increases in neuronal threshold, and shortening of response latency were found in both fields during periods of cooling deactivation. The weak anatomical connections between the two fields investigated make these findings unexpected. Furthermore, the observed neuronal changes suggest a model of corticocortical interaction among auditory fields in which neither differences in the magnitude of anatomical projections nor cortical representation of sensory stimuli are reliable determinants of modulatory functions.

Columnar connectivity and laminar processing in cat primary auditory cortex

BACKGROUND

Radial intra- and interlaminar connections form a basic microcircuit in primary auditory cortex (AI) that extracts acoustic information and distributes it to cortical and subcortical networks. Though the structure of this microcircuit is known, we do not know how the functional connectivity between layers relates to laminar processing.

METHODOLOGY/PRINCIPAL FINDINGS

We studied the relationships between functional connectivity and receptive field properties in this columnar microcircuit by simultaneously recording from single neurons in cat AI in response to broadband dynamic moving ripple stimuli. We used spectrotemporal receptive fields (STRFs) to estimate the relationship between receptive field parameters and the functional connectivity between pairs of neurons. Interlaminar connectivity obtained through cross-covariance analysis reflected a consistent pattern of information flow from thalamic input layers to cortical output layers. Connection strength and STRF similarity were greatest for intralaminar neuron pairs and in supragranular layers and weaker for interlaminar projections. Interlaminar connection strength co-varied with several STRF parameters: feature selectivity, phase locking to the stimulus envelope, best temporal modulation frequency, and best spectral modulation frequency. Connectivity properties and receptive field relationships differed for vertical and horizontal connections.

CONCLUSIONS/SIGNIFICANCE

Thus, the mode of local processing in supragranular layers differs from that in infragranular layers. Therefore, specific connectivity patterns in the auditory cortex shape the flow of information and constrain how spectrotemporal processing transformations progress in the canonical columnar auditory microcircuit.

Laminar diversity of dynamic sound processing in cat primary auditory cortex

For primary auditory cortex (AI) laminae, there is little evidence of functional specificity despite clearly expressed cellular and connectional differences. Natural sounds are dominated by dynamic temporal and spectral modulations and we used these properties to evaluate local functional differences or constancies across laminae. To examine the layer-specific processing of acoustic modulation information, we simultaneously recorded from multiple AI laminae in the anesthetized cat. Neurons were challenged with dynamic moving ripple stimuli and we subsequently computed spectrotemporal receptive fields (STRFs). From the STRFs, temporal and spectral modulation transfer functions (tMTFs, sMTFs) were calculated and compared across layers. Temporal and spectral modulation properties often differed between layers. On average, layer II/III and VI neurons responded to lower temporal modulations than those in layer IV. tMTFs were mainly band-pass in granular layer IV and became more low-pass in infragranular layers. Compared with layer IV, spectral MTFs were broader and their upper cutoff frequencies higher in layers V and VI. In individual penetrations, temporal modulation preference was similar across layers for roughly 70% of the penetrations, suggesting a common, columnar functional characteristic. By contrast, only about 30% of penetrations showed consistent spectral modulation preferences across layers, indicative of functional laminar diversity or specialization. Since local laminar differences in stimulus preference do not always parallel the main flow of information in the columnar cortical microcircuit, this indicates the influence of additional horizontal or thalamocortical inputs. AI layers that express differing modulation properties may serve distinct roles in the extraction of dynamic sound information, with the differing information specific to the targeted stations of each layer.

Glutamatergic nonpyramidal neurons from neocortical layer VI and their comparison with pyramidal and spiny stellate neurons

The deeper part of neocortical layer VI is dominated by nonpyramidal neurons, which lack a prominent vertically ascending dendrite and predominantly establish corticocortical connections. These neurons were studied in rat neocortical slices using patch-clamp, single-cell reverse transcription-polymerase chain reaction, and biocytin labeling. The majority of these neurons expressed the vesicular glutamate transporter but not glutamic acid decarboxylase, suggesting that a high proportion of layer VI nonpyramidal neurons are glutamatergic. Indeed, they exhibited numerous dendritic spines and established asymmetrical synapses. Our sample of glutamatergic nonpyramidal neurons displayed a wide variety of somatodendritic morphologies and a subset of these cells expressed the Nurr1 mRNA, a marker for ipsilateral, but not commissural corticocortical projection neurons in layer VI. Comparison with spiny stellate and pyramidal neurons from other layers showed that glutamatergic neurons consistently exhibited a low occurrence of GABAergic interneuron markers and regular spiking firing patterns. Analysis of electrophysiological diversity using unsupervised clustering disclosed three groups of cells. Layer V pyramidal neurons were segregated into a first group, whereas a second group consisted of a subpopulation of layer VI neurons exhibiting tonic firing. A third heterogeneous cluster comprised spiny stellate, layer II/III pyramidal, and layer VI neurons exhibiting adaptive firing. The segregation of layer VI neurons in two different clusters did not correlate either with their somatodendritic morphologies or with Nurr1 expression. Our results suggest that electrophysiological similarities between neocortical glutamatergic neurons extend beyond layer positioning, somatodendritic morphology, and projection specificity.

Cortical reorganization in children with cochlear implants

Congenital deafness leads to atypical organization of the auditory nervous system. However, the extent to which auditory pathways reorganize during deafness is not well understood. We recorded cortical auditory evoked potentials in normal hearing children and in congenitally deaf children fitted with cochlear implants. High-density EEG and source modeling revealed principal activity from auditory cortex in normal hearing and early implanted children. However, children implanted after a critical period of seven years revealed activity from parietotemporal cortex in response to auditory stimulation, demonstrating reorganized cortical pathways. Reorganization of central auditory pathways is limited by the age at which implantation occurs, and may help explain the benefits and limitations of implantation in congenitally deaf children.

Sound localization deficits during reversible deactivation of primary auditory cortex and/or the dorsal zone

We examined the contributions of primary auditory cortex (A1) and the dorsal zone of auditory cortex (DZ) to sound localization behavior during separate and combined unilateral and bilateral deactivation. From a central visual fixation point, cats learned to make an orienting response (head movement and approach) to a 100-ms broadband noise burst emitted from a central speaker or one of 12 peripheral sites (located in front of the animal, from left 90 degrees to right 90 degrees, at 15 degrees intervals) along the horizontal plane. Following training, each cat was implanted with separate cryoloops over A1 and DZ bilaterally. Unilateral deactivation of A1 or DZ or simultaneous unilateral deactivation of A1 and DZ (A1/DZ) resulted in spatial localization deficits confined to the contralateral hemifield, whereas sound localization to positions in the ipsilateral hemifield remained unaffected. Simultaneous bilateral deactivation of both A1 and DZ resulted in sound localization performance dropping from near-perfect to chance (7.7% correct) across the entire field. Errors made during bilateral deactivation of A1/DZ tended to be confined to the same hemifield as the target. However, unlike the profound sound localization deficit that occurs when A1 and DZ are deactivated together, deactivation of either A1 or DZ alone produced partial and field-specific deficits. For A1, bilateral deactivation resulted in higher error rates (performance dropping to approximately 45%) but relatively small errors (mostly within 30 degrees of the target). In contrast, bilateral deactivation of DZ produced somewhat fewer errors (performance dropping to only approximately 60% correct), but the errors tended to be larger, often into the incorrect hemifield. Therefore individual deactivation of either A1 or DZ produced specific and unique sound localization deficits. The results of the present study reveal that DZ plays a role in sound localization. Along with previous anatomical and physiological data, these behavioral data support the view that A1 and DZ are distinct cortical areas. Finally, the findings that deactivation of either A1 or DZ alone produces partial sound localization deficits, whereas deactivation of either posterior auditory field (PAF) or anterior ectosylvian sulcus (AES) produces profound sound localization deficits, suggests that PAF and AES make more significant contributions to sound localization than either A1 or DZ.

Frequency-modulation encoding in the primary auditory cortex of the awake owl monkey

Many communication sounds, such as New World monkey twitter calls, contain frequency-modulated (FM) sweeps. To determine how this prominent vocalization element is represented in the auditory cortex we examined neural responses to logarithmic FM sweep stimuli in the primary auditory cortex (AI) of two awake owl monkeys. Using an implanted array of microelectrodes we quantitatively characterized neuronal responses to FM sweeps and to random tone-pip stimuli. Tone-pip responses were used to construct spectrotemporal receptive fields (STRFs). Classification of FM sweep responses revealed few neurons with high direction and speed selectivity. Most neurons responded to sweeps in both directions and over a broad range of sweep speeds. Characteristic frequency estimates from FM responses were highly correlated with estimates from STRFs, although spectral receptive field bandwidth was consistently underestimated by FM stimuli. Predictions of FM direction selectivity and best speed from STRFs were significantly correlated with observed FM responses, although some systematic discrepancies existed. Last, the population distributions of FM responses in the awake owl monkey were similar to, although of longer temporal duration than, those in the anesthetized squirrel monkeys.

Functional Organization of Squirrel Monkey Primary Auditory Cortex: Responses to Frequency-Modulation Sweeps

The squirrel monkey twitter call is an exemplar of a broad class of species-specific vocalizations that contain naturally voiced frequency-modulated (FM) sweeps. To investigate how this prominent communication call element is represented in primary auditory cortex (AI), neuronal receptive field properties to pure-tone and synthetic, logarithmically spaced FM-sweep stimuli in 3 barbiturate-anesthetized squirrel monkeys are studied. Responses to pure tones are assessed by using standard measures of frequency response areas, whereas responses to FM sweeps are classified according to direction selectivity, best speed, and speed tuning preferences. Most neuronal clusters respond to FM sweeps in both directions and over a range of FM speeds. Center frequencies calculated from the average of high and low trigger frequency edges of FM response profiles are highly correlated with pure-tone characteristic frequencies (CFs). However, bandwidth estimates are only weakly correlated with their pure-tone counterparts. CF and direction selectivity are negatively correlated. Best speed maps reveal idiosyncratically positioned spatial aggregation of similar values. In contrast, direction selectivity maps show unambiguous spatial organization. Neuronal clusters selective for upward-directed FM sweeps are located in ventral–caudal AI, where CFs range from 0.5 to 1 kHz. Combinations of pure-tone and FM response parameters form 2 significant factors to account for response variations. These results are interpreted in the context of earlier FM investigations and neuronal encoding of dynamic sounds.

Divergent response properties of layer-V neurons in rat primary auditory cortex

Layer-V pyramidal cells comprise a major output of primary auditory cortex (A1). At least two cell types displaying different morphology, projections and in vitro physiology have been previously identified in layer-V. The focus of the present study was to characterize extracellular receptive field properties of layer-V neurons to determine whether a similar breakdown of responses can be found in vivo. Recordings from 105 layer-V neurons revealed two predominant receptive field types. Thirty-two percent displayed strong excitatory V/U-shaped receptive field maps and spiking patterns with shorter stimulus-driven interspike intervals (ISIs), reminiscent of the bursting cells discussed in the in vitro literature. V/U-shaped maps remained relatively unchanged across the three sequential repetitions of the map run on each neuron. Neurons with V/U-shaped maps were also easily depolarized with extracellular current pulse stimulation. In contrast, 47% of the neurons displayed Complex receptive field maps characterized by weak and/or inconsistent excitatory regions and were difficult to depolarize with current pulses. These findings suggest that V/U-shaped receptive fields could correspond to previously described intrinsic bursting (IB) cells with corticotectal projections, and that neurons with Complex receptive fields might represent the regular spiking (RS) cells with their greater inhibitory input and corticocortical/corticostriatal projection pattern.

Principles Governing Auditory Cortex Connections

Topographic maps are common constituents of the primary auditory, visual, and somatic sensory cortex. However, in most cortical areas, no such maps have yet been identified, posing a conceptual problem for theories of cortical function centered on topography. What principle guides the organization of these other areas? We investigated this issue in cat auditory cortex. The connectional topography of five tonotopic areas and eight non-tonotopic areas was assessed using retrograde tract tracing and quantified by three metrics: clustering, dispersion, and separation. Clustering measures the spatial density of labeled neurons, dispersion provides an index of their spread, and separation serves as a scaling metric. These parameters each show that all auditory cortical regions receive precise and equally topographic connections from thalamic, corticocortical, and commissural sources. This isotropic principle suggests a common substrate for coordinating communication across the cortex and may reflect common mechanisms related to the developmental patterning of connections. This unifying principle extends to auditory and prefrontal cortex, and perhaps to other neocortical areas.

Connections of the posterior thalamic region with the auditory ectosylvian cortex in the dog

The purpose of the present study was to define auditory cortical areas in the dog on the basis of thalamocortical connectivity patterns. Connections between the posterior thalamic region and auditory ectosylvian cortex were studied using axonally transported tracers: fluorochromes and biotinylated dextran amine. Cyto- and chemoarchitecture provided grounds for the division of the posterior thalamic region into three complexes, medial geniculate body (MGB), posterior nuclei (Po), and lateromedial and suprageniculate nuclei (LM-Sg). Distinctive cytoarchitectonic features and the distribution of dominant thalamocortical connections (determined quantitatively) allowed us to define four ectosylvian areas: middle (EM), anterior (EA), posterior (EP), and composite (CE). We found that each area was a place of convergence for projections from five to eleven nuclei of the three thalamic complexes, with dominant projections derived from one or two nuclei. Dominant topographical projections from the ventral nucleus to area EM confirmed physiological reports that it may be considered a primary auditory area (AI). We found the anterior part of the EM to be distinct in having unique strong connections with the deep dorsal MGB nucleus. Area EA, which receives dominant projections from the lateral Po (Pol) and medial MGB nuclei, as well as area EP, which receives dominant connections from the dorsal caudal MGB nucleus, compose two parasensory areas. Area CE receives dominant projections from the extrageniculate nuclei, anterior region of the LM-Sg, and Pol, supplemented with an input from the somatosensory VP complex, and may be considered a polymodal association area.

Gamma-aminobutyric acid circuits shape response properties of auditory cortex neurons

Neurons containing gamma aminobutyric acid (GABA) are widely distributed throughout the primary auditory cortex (AI). We investigated the effects of endogenous GABA by comparing response properties of 110 neurons in chinchilla AI before and after iontophoresis of bicuculline, a GABA(A) receptor antagonist, and/or CGP35348, a GABA(B) receptor antagonist. GABA(A) receptor blockade significantly increased spontaneous and driven discharge rates, dramatically decreased the thresholds of many neurons, and constricted the range of thresholds across the neural population. Some neurons with 'non-onset' temporal discharge patterns developed an onset pattern that was followed by a long pause. Interestingly, the excitatory response area typically expanded on both sides of the characteristic frequency; this expansion exceeded one octave in a third of the sample. Although GABA(B) receptor blockade had little effect alone, the combination of CGP35348 and bicuculline produced greater increases in driven rate and expansion of the frequency response area than GABA(A) receptor blockade alone, suggesting a modulatory role of local GABA(B) receptors. The results suggest that local GABA inhibition contributes significantly to intensity and frequency coding by controlling the range of intensities over which cortical neurons operate and the range of frequencies to which they respond. The inhibitory circuits that generate nonmonotonic rate-level functions are separate from those that influence other response properties of AI neurons.

Widespread Horizontal Connections Arising from Layer 5/6 Border Inverted Cells in Rabbit Visual Cortex

Herein we describe the inverted cells [defined as those projection neurons having a major dendritic shaft abpially oriented (Bueno-López et al., Eur. J. Neurosci., 3, 415, 1991)] originating a unique set of cortical connections characterized by extraordinarily widespread horizontal distribution. Single and multiple injections of wheatgerm agglutinin - horseradish peroxidase were made in areas 17 and 18 and the resulting retrograde labelling in the cortex was analysed. The findings were assessed in independent control experiments in which Fluoro-Gold was used as retrograde tracer. Following single injections in area 17 several separate patches of labelled cells comprising layers 2 - 6 were consistently found in area 18. In addition to these associational cells a number of labelled cells appeared at the layer 5/6 border but were distributed over most of the tangential extent of the visual occipital cortex. This widespread pattern was particularly striking in brains after multiple injections. In these brains a conspicuous band of labelled cells at the 5/6 border radiated from the injection sites, making up an apparently continuous horizontal sheet that intersected the striate - extrastriate boundary and merged with the patches of labelled cells in area 18 and beyond. Most of the cells in the 5/6 border band were inverted cells (82%; n=2081). Injections in area 18 failed to produce such a widespread set of labelled cells in area 17. The functional significance of these connections furnished by the 5/6 border inverted cells remains to be determined, but their distribution would allow for convergent/divergent binding interactions both intra-areally (within area 17) and inter-areally (from area 18 to area 17).

Layer V in cat primary auditory cortex (AI): cellular architecture and identification of projection neurons

The cytoarchitectonic organization and the structure of layer V neuronal populations in cat primary auditory cortex (AI) were analyzed in Golgi, Nissl, immunocytochemical, and plastic-embedded preparations from mature specimens. The major cell types were characterized as a prelude to identifying their connections with the thalamus, midbrain, and cerebral cortex using axoplasmic transport methods. The goal was to describe the structure and connections of layer V neurons more fully. Layer V has three sublayers based on the types of neuron and their sublaminar projections. Four types of pyramidal and three kinds of nonpyramidal cells were present. Classic pyramidal cells had a long apical dendrite, robust basal arbors, and an axon with both local and corticofugal projections. Only the largest pyramidal cell apical dendrites reached the supragranular layers, and their somata were found mainly in layer Vb. Three types departed from the classic pattern; these were the star, fusiform, and inverted pyramidal neurons. Nonpyramidal cells ranged from large multipolar neurons with radiating dendrites, to Martinotti cells, with smooth dendrites and a primary trunk oriented toward the white matter. Many nonpyramidal cells were multipolar, of which three subtypes (large, medium, and small) were identified; bipolar and other types also were seen. Their axons formed local projections within layer V, often near pyramidal neurons. Several features distinguish layer V from other layers in AI. The largest pyramidal neurons were in layer V. Layer V neuronal diversity aligns it with layer VI (Prieto and Winer [1999] J. Comp. Neurol. 404:332--358), and it is consistent with the many connectional systems in layer V, each of which has specific sublaminar and neuronal origins. The infragranular layers are the source for several parallel descending systems. There were significant differences in somatic size among these projection neurons. This finding implies that diverse corticofugal roles in sensorimotor processing may require a correspondingly wide range of neuronal architecture.

Auditory thalamocortical projections in the cat: laminar and areal patterns of input

Thalamocortical projections were studied in adult cats using biotinylated dextran amines, wheat germ agglutinin conjugated to horseradish peroxidase, and autoradiography with tritiated leucine and/or proline. The input from 7 architectonically defined nuclei to 14 auditory cortical fields was characterized qualitatively and quantitatively. The principal results were that 1) every thalamic nucleus projected to more than 1 field (range, 4-14 fields; mean, 7 fields); 2) only the projection from the ventral division to some primary fields (primary auditory cortex and posterior auditory cortex) had a periodic, clustered distribution, whereas the input from other divisions to nonprimary areas was continuous; 3) layers III-V received >85% of the total axonal profiles; 4) in most experiments, five or more layers were labeled; 5) the projections to nonprimary auditory areas had many laterally oriented axons; 6) the heaviest input to layer I in all experiments was usually in its upper half, suggesting a sublaminar arrangement; 7) the largest axonal trunks (up to 6 microm in diameter) arose from the medial division and ended in layer Ia, where they ran laterally for long distances; 8) there were three projection patterns: type 1 had its peak in layers III-IV with little input to layer I, and it arose from the ventral division and the dorsal superficial, dorsal, and suprageniculate nuclei of the dorsal division; type 2 had heavy labeling in layer I and less in layers III-IV, arising from the dorsal division nuclei primarily, especially the caudal dorsal and deep dorsal nuclei; and type 3 was a trimodal concentration in layers I, III-IV, and VI that originated chiefly in the medial division and had the lowest density of labeling; and 9) the quantitative profiles with the three methods were very similar. The results suggest that the subdivisions of the auditory thalamus have consistent patterns of laminar distribution to different cortical areas, that an average of five or more layers receive significant input in a specific area, that a given thalamic nucleus can influence areas as far as 20 mm apart, that the first information to arrive at the cortex may reach layer I by virtue of the giant axons, and that several laminar patterns of auditory thalamocortical projection exist. The view that the auditory thalamus (and perhaps other thalamic nuclei) serves mainly a relay function underestimates its many modes for influencing the cortex on a laminar basis.

Congenital auditory deprivation reduces synaptic activity within the auditory cortex in a layer-specific manner

The present study investigates the functional deficits of naive auditory cortices in adult congenitally deaf cats. For this purpose, their auditory system was stimulated electrically using cochlear implants. Synaptic currents in cortical layers were revealed using current source density analyses. They were compared with synaptic currents found in electrically stimulated hearing cats. The naive auditory cortex showed significant deficits in synaptic activity in infragranular cortical layers. Furthermore, there was also a deficit of synaptic activities at longer latencies (>30 ms). The 'cortical column' was not activated in the well-defined sequence found in normal hearing cats. These results demonstrate functional deficits as a consequence of congenital auditory deprivation. Similar deficits are likely in congenitally deaf children.

Organization of connections underlying the processing of auditory information in the dog

1. The canine temporal cortex includes the ectosylvian, composite posterior and sylvian gyri. 2. The distinctive features of the canine temporal cortex include the ectosylvian sulcus closed in its dorsal side and the substantial development of neocortex located within the posterior composite gyrus. 3. Thalamofugal connections from particular nuclei of the medial geniculate body, posterior thalamus and lateromedial-suprageniculate complex project to specific areas of the canine temporal cortex and are arranged as dominant and non-dominant projections. 4. Local intracortical connections distinguish the ectosylvian and posterior composite areas as unimodal auditory cortex. Long distant connections and polymodal convergence indicate that the composite ectosylvian area of the anterior ectosylvian gyrus and the anterodorsal sylvian areas are higher order association cortex. 5. Analysis of both thalamo-cortical and intracortical connections indicate that auditory processing in the cortex occurs in successive, hierarchically organized stages and in two main, anterior and ventral pathways.

Cross-modal reorganization of callosal connectivity without altering thalamocortical projections

Mammalian cerebral cortex is composed of a multitude of different areas that are each specialized for a unique purpose. It is unclear whether the activity pattern and modality of sensory inputs to cortex play an important role in the development of cortical regionalization. The modality of sensory inputs to cerebral cortex can be altered experimentally. Neonatal diversion of retinal axons to the auditory thalamus (cross-modal rewiring) results in a primary auditory cortex (AI) that resembles the primary visual cortex in its visual response properties and topography. Functional reorganization could occur because the visual inputs use existing circuitry in AI, or because the early visual inputs promote changes in AI's circuitry that make it capable of constructing visual receptive field properties. The present study begins to distinguish between these possibilities by exploring whether the callosal connectivity of AI is altered by early visual experience. Here we show that early visual inputs to auditory thalamus can reorganize callosal connections in auditory cortex, causing both a reduction in their extent and a reorganization of the pattern. This result is distinctly different from that in deafened animals, which have widespread callosal connections, as in early postnatal development. Thus, profound changes in cortical circuitry can result simply from a change in the modality of afferent input. Similar changes may underlie cortical compensatory processes in deaf and blind humans.

Layer VI in cat primary auditory cortex: Golgi study and sublaminar origins of projection neurons

The organization of layer VI in cat primary auditory cortex (AI) was studied in mature specimens. Golgi-impregnated neurons were classified on the basis of their dendritic and somatic form. Ipsilateral and contralateral projection neurons and the corticogeniculate cells of origin were labeled with retrograde tracers and their profiles were compared with the results from Golgi studies. Layer VI was divided into a superficial half (layer VIa) with many pyramidal neurons and a deeper part (layer VIb) that is dominated by horizontal cells. Nine types of neuron were identified; four classes had subvarieties. Classical pyramidal cells and star, fusiform, tangential, and inverted pyramidal cells occur. Nonpyramidal neurons were Martinotti, multipolar stellate, bipolar, and horizontal cells. This variety of neurons distinguished layer VI from other AI layers. Pyramidal neuron dendrites contributed to the vertical, modular organization in AI, although their apical processes did not project beyond layer IV. Their axons had vertical, intrinsic processes as well as corticofugal branches. Horizontal cell dendrites extended laterally up to 700 microm and could integrate thalamic input across wide expanses of the tonotopic domain. Connectional experiments confirmed the sublaminar arrangement seen in Nissl material. Commissural cells were concentrated in layer VIa, whereas corticocortical neurons were more numerous in layer VIb. Corticothalamic cells were distributed more equally. The cytological complexity and diverse connections of layer VI may relate to a possible role in cortical development. Layer VI contained most of the neuronal types found in other layers in AI, and these cells form many of the same intrinsic and corticofugal connections that neurons in other layers will assume in adulthood. Layer VI, thus, may play a fundamental ontogenetic role in the construction and early function of the cortex.

Connections of the dorsal zone of cat auditory cortex

The present study examined the anatomic connections of the dorsal zone of cat auditory cortex (DZ). The DZ was discriminated physiologically from the primary auditory field (AI) on the basis of neuronal responses with long latency and broad or multipeaked tuning curves. Wheat germ agglutinin-horseradish peroxidase was then injected either by pressure or iontophoretically. The thalamocortical and corticothalamic connections of the DZ were visualized by the presence of retrogradely labeled neurons and anterogradely labeled terminal fields in the thalamus; ipsilateral corticocortical projections from other cortical fields were visualized by the presence of retrogradely labeled cells. Injections of tracer into the DZ retrogradely labeled cells mainly in the lateral division of posterior complex (Po) and in the dorsal division (MGd) of the medial geniculate body (MGB); fewer labeled cells were found in the ventral (MGv) and medial (MGm) divisions of the MGB and in the suprageniculate nucleus. The DZ projection to Po, MGv, and MGd was heavy and was more diffuse than the reciprocal thalamocortical projection; the projection to MGm was light. The corticothalamic terminations and thalamocortical cells projecting to the same part of the DZ were not superimposed rigidly. The DZ received cortical projections from AI and from the second, anterior, and posterior auditory fields, and there were strong intra-DZ connections. Together with the physiological findings, the present results suggest that the DZ is a potentially separate auditory field from AI and is likely to be involved in both temporal and spectral integration of acoustic information.

The organization of the ipsilateral corticocortical projections of the middle sylvian gyrus (AII) in the cat cerebral cortex: an anterograde and retrograde tracing study

The primary auditory area and surrounding cortical areas including the secondary auditory area subserve auditory information processing. In this study, corticocortical ipsilateral connections of the secondary auditory area were investigated in the cat by injections of axonally transported tracers into this area. Retrogradely labeled cells and anterogradely labeled terminals were observed within the secondary auditory area itself, and the primary, posterior, and ventroposterior auditory areas, in the insular cortex and in the temporal auditory area. These labeled terminals appeared in patches or bands. In the ventroposterior auditory area, a transverse band of anterograde and retrograde labeling, which extended from the posterior ectosylvian sulcus to the surface of the posterior ectosylvian gyrus was observed. In addition, scattered labeling was detected in the posterior auditory area. Although different parts of the secondary auditory area were found to send axons to different zones within the temporal cortex, no precise topographical arrangement was detected. When two different tracers were injected into the secondary auditory area and the anterior auditory area, respectively, some terminal aggregates anterogradely labeled from these two areas converged within the posterior auditory area. The organization of these connections is discussed in relationship to the functional role of the auditory cortical areas.

Morphology and spatial distribution of corticothalamic terminals originating from the cat auditory cortex

In this paper we studied the morphology and spatial distribution of corticothalamic axons and terminals originating from the auditory cortical fields of the cat. The anterograde tracer biocytin was injected at electrophysiologically characterized loci in the primary (AI) (N = 2), anterior (AAF) (N = 1), posterior (PAF) (N = 1) and secondary (AII) (N = 2) auditory fields. In all cases, two different types of labeled terminals were found in the auditory thalamus: small spherical endings (1-2 microns) and giant, finger-like endings (5-10 microns). After biocytin injections in AI and AAF, the majority of anterogradely labeled axons terminated in the rostral half of the pars lateralis (LV) of the ventral division of the medial geniculate body (vMGB). In LV, the corticothalamic axons ramified profusely, giving rise to dense terminal fields forming well delineated curved stripes, with small spherical endings. Additional terminal fields formed by small endings were observed in the medial division of the medial geniculate body (mMGB). Giant endings were observed in a small area in the dorsal nucleus (D) of the dorsal division of the medial geniculate body (dMGB), near its border with the vMGB. PAF projections were located in the caudal half of vMGB and in mMGB, where only small terminals were found. Giant endings were seen in the superficial part of dMGB emerging from labeled corticothalamic axons oriented in parallel to the dorsal surface of the MGB. Projections from AII gave rise to a main terminal field of small endings in D; a second terminal field consisting of giant endings intermingled with small endings was found in the deep dorsal nucleus (DD) of dMGB. We conclude that small terminals serve the feedback projection to the thalamic nucleus from which the injected cortical field receives its main input, whereas giant terminals cross the borders between the parallel ascending auditory pathways.

Morphology and spatial distribution of GABAergic neurons in cat primary auditory cortex (AI)

This is a survey of the distribution, form, and proportion of neurons immunoreactive for gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase (GAD) in cat primary auditory cortex (AI). The cells were studied in adult animals and were classified with respect to their somatic size, shape, and laminar location, and with regard to the origins and branching pattern of their dendrites. These attributes were used to relate each of the GAD-positive neuronal types to their counterparts in Golgi preparations. Each layer had a particular set of GABAergic cell types that is unique to it. There were 10 different GABAergic cell types in AI. Some were specific to one layer, such as the horizontal cells in layer I or the extraverted multipolar cells in layer II, while other types, such as the small and medium-sized multipolar cells, were found in every layer. The number and proportion of GABAergic cells were determined by using postembedding immunocytochemistry. The proportion of GABAergic neurons was 24.6%. This was slightly higher than the values reported elsewhere in the neocortex. The laminar differences in density and proportion of GABAergic and non-GABAergic neurons were also comparable (though somewhat higher) to those found in other cortical areas: thus, 94% of layer I cells were GABAergic, while the values in other layers ranged from 27% (layer V) to 16% (layer VI). Layer VI had the most heterogeneous population of GABAergic neurons. The proportion of these cells across different regions within AI was studied. Since some receptive field properties such as sharpness of tuning and aurality are distributed non-uniformly across AI, these might be reflected by regional differences across the cerebral cortex. There were significantly more GABAergic somata in layers III and IV in the central part of AI, along the dorsoventral axis, where physiological studies report that the neurons are tuned most sharply (Schreiner and Mendelson [1990] J. Neurophysiol. 64:1442-1459). Thus, there may be a structural basis for certain aspects of local inhibitory neuronal organization.

Visual projections induced into the auditory pathway of ferrets: II. Corticocortical connections of primary auditory cortex

Although the development of corticocortical projections has been well studied, less is known about the role of sensory inputs in the specification of these connections. As part of an ongoing series of studies in our laboratory, we have examined the role of thalamic input modality in the development of corticocortical connections. These studies involve making unilateral lesions and inducing retinal inputs into the auditory thalamus (MGN) during early development in ferrets, thereby conferring visual responsiveness on primary auditory cortex (AI). In this way we can examine the role of input identity in cortical specification in general, and connectivity patterns specifically. A previous paper (Pallas et al. [1990] J. Comp. Neurol. 298:50-68) described the pattern of thalamocortical and corticothalamic connections of auditory cortex in normal and lesioned animals. This study compares the pattern of auditory corticocortical connections in normal and lesioned animals. We injected neuroanatomical tracers into AI and mapped out the distribution of retrogradely labelled cells in the cortex. We report that the cortical inputs to ferret AI resembled those in cats, and that the pattern of ipsi- and contralateral corticocortical connections of ferret AI with visual input was similar to the normal pattern. Auditory cortex with visual input did not make ectopic connections with visual cortex, but maintained its connections with other auditory cortical areas. These results suggest that the overall corticocortical connections of an area are not influenced by the modality or activity pattern of its inputs. In particular, altering the input activity to a cortical area does not seem to promote the formation of entirely new connections, although small changes in the strength of existing connections are possible (Sur et al. [1990] Trends Neurosci. 13:227-233).

The auditory cortex of the mouse: connections of the ultrasonic field

The cortical and subcortical connections of the ultrasonic field (UF) of the auditory cortex of the house mouse (Mus musculus) were studied by using retrograde and anterograde transport of horseradish peroxidase (HRP). Small amounts of HRP were locally injected into the electrophysiologically defined UF. Superficial (layer I-IV) and deep (layer IV-VI) injections were prepared. Superficial injections led to labelling of both cells (retrograde) and terminals (anterograde) in areas of the ipsilateral primary and secondary auditory cortex and in its dorsoposterior field, in an ipsilateral dorsal association area (patches of label), probably in ipsilateral secondary somatosensory cortex, in the contralateral homotopic UF, and in the ipsilateral medial geniculate body (MGBv, MGBd, and MGBm) and caudal posterior nucleus complex. Deep injections showed the same connectivities as superficial ones and, in addition, terminals in the very caudal caudatoputamen, in the nucleus limitans and the nucleus reticularis of the thalamus, in the rostral pole, the dorsomedial, and lateral nucleus of the inferior colliculus, in the stratum griseum intermediale of the superior colliculus, and in a pontine nucleus ventromedial of the lateral lemniscus. All these projections occurred only ipsilaterally. The majority of connections, except those with the nucleus limitans, superior colliculus and pontine nucleus, suggest that UF is part of the primary anditory cortex (AI) and/or of the anterior anditory field (AAF) of the auditory cortex. Since UF has no regular tonotopy, this has important implications for the functional role that AI/AAF can have in communication-sound analysis.

Intrinsic connections in cat visual cortex: a combined anterograde and retrograde tracing study

Area 18 of cat visual cortex was examined for intrinsic axons following small, columnar injections of an anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). Locally projecting axons radiated from the injection site and branched to form 10-15 discrete, approximately circular patches 500-750 microns in diameter consisting of many bouton-studded terminal arborizations. Labeled fibers and boutons ramified densely in layers I, II/II, V, and VI, and were noticeably less dense in layer IV. Afferent and efferent pathways originating from the same cortical columns were studied by injecting a mixture of PHA-L and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Between 10 and 15 patches of cells retrogradely labeled by WGA-HRP surrounded each injection site. Within a patch, labeled cells were found in all layers and included both pyramidal and non-pyramidal cells. The distribution of PHA-L labeling was similar to that obtained when PHA-L was injected alone. Most often, the labeled patches resulting from injections of such mixtures contained both anterograde and retrograde labeling. However, patches consisting of retrograde labeling alone and of anterograde labeling alone were also observed, indicating that the local connections linking neighboring cortical columns were not always reciprocal.

Morphology of the cells within the inferior temporal gyrus that project to the prefrontal cortex in the macaque monkey

The primate neocortex possesses an extraordinary degree of regional specialization. Virtually all cortical functions are dependent upon a complex system of reciprocal connections between related cortical regions that allow for distributed information processing. Although some aspects of the organization of these corticocortical projections are understood, little is known about the morphology and afferents to the cells of origin of long corticocortical projections in primates. We combined intracellular injection of Lucifer Yellow (LY) in fixed tissue with in vivo retrograde transport of fast blue to study the dendritic morphology of neurons within the inferior temporal gyrus (ITG) and the superior temporal sulcus (STS) that furnish corticocortical projections to the prefrontal cortex. The fast blue retrogradely labeled cells formed two clearly defined bands within the inferior temporal cortex: a supragranular band that corresponded to layer III, and an infragranular band that corresponded to layers V and VI. After Lucifer Yellow intracellular filling, these retrogradely labeled cells projecting to the prefrontal cortex were found to be morphologically very heterogeneous. Although all filled cells had spiny dendrites, they presented a wide range of cell body sizes and dendritic tree morphologies. In layer III, the majority of cells were typical pyramids of various sizes. In layers V-VI, numerous typical pyramidal cells were present. In addition, significant numbers of modified pyramidal forms were found, including vertical and horizontal fusiform cells, asymmetrical pyramids and multipolar cells. The entire dendritic arbor of individual subtypes in layers III, V, and VI was restricted to a few cortical layers, but as a group these cells had dendrites spanning the whole cortical depth. We suggest that corticocortically projecting cells are distinct from subcortically projecting cells and consist of a defined set of morphological and functional subgroups, each of which is driven by a distinct set of afferents and likely possesses different response properties.

Commissural connections between the auditory cortices of the rat

The pattern of commissural connections of the rat auditory cortex (AC) was investigated with injections of wheat germ agglutinated horseradish peroxidase into the AC. Homotopic and heterotopic patches of neurons were retrogradely labeled in the contralateral hemisphere. Each injection labeled neurons at the corresponding contralateral site, i.e. the homotopic site. In addition, retrogradely labeled neurons were found at non-corresponding locations in contralateral AC, i.e. at heterotopic locations. The pattern of heterotopic labeling changed systematically with the injections. Mapping rules were established that led to the parcellation of areas 41 and 36 into 6 fields. Four fields were defined in Krieg's area 41 (primary AC) and two fields in Krieg's area 36 (secondary AC). In area 41 the heterotopic connection is not reciprocal; in area 36, however, heterotopic projections are organized reciprocally. Contrary to the visual cortex, homotopic and heterotopic projection neurons were equally distributed across the cortical laminae. With double-label experiments it could be shown that a considerable number of the neurons in area 41 bifurcate and project to homotopic as well as to heterotopic sites in the contralateral hemisphere. We conclude that in the AC there are several subtypes of neurons projecting to the contralateral hemisphere; it would be of interest whether these anatomical differences are manifested by physiological differences.

Populations of GABAergic neurons and axons in layer I of rat auditory cortex

Neurons and axon terminals (puncta) immunostained by an antibody against glutamic acid decarboxylase were studied in layer I of adult rats in architectonically identified area 41 of auditory cortex. The borders of area 41 and the laminar subdivisions of cortex were established in normal material and in other studies of cortical connections. Vibratomed or frozen sections were immunostained. The objectives of the study were to classify the types of (i) glutamic acid decarboxylase-positive neurons and (ii) puncta, and (iii) to examine their spatial distribution within layer I. Control sections were devoid of specific immunostaining. More than 90% of layer I cells are glutamic acid decarboxylase-positive. Four types of neuron were identified in Golgi material, including small neurons with stellate dendritic fields, horizontal cells with laterally projecting arbors, medium-sized neurons with stellate, widely ramifying dendritic fields, and large neurons with broad dendritic fields spanning the depth of layer I or branching laterally. In the glutamic acid decarboxylase material, examples with a somatodendritic shape matching each of these types were found. The average somatic diameter of glutamic acid decarboxylase-positive neurons (mean = 59 microns2, S.D. = 21 microns2) suggests that the small and medium-sized neurons predominate. Glutamic acid decarboxylase-positive neurons occur throughout the depth of layer I, but are far more numerous in the deeper half (68% in layer Ib) than in the superficial part (32% in layer Ia). Glutamic acid decarboxylase-positive neurons form small clusters of three to five cells across the cortical surface, with a range of 0-9/100 microns across the cortex. Most glutamic acid decarboxylase-positive neuronal perikarya were intensely immunostained, and the dendrites of the medium-sized and large neurons were traced as far as 50-75 microns beyond their initial branching point. Glutamic acid decarboxylase-positive puncta also had variable shapes. Both small, fine puncta (less than 0.5 micron in diameter) and larger, coarser ones (greater than 1.5 micron in diameter) were present, though the former were much more common. In traverses from the pia to the layer II border, the puncta average about 40/100 microns2 (range: 20-80), and the shape of individual pia--layer II traverses is multipeaked, often with a slight trough at congruent to 80 microns depth, then rising slowly in number toward layer II.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of the association cortical afferent connections of area 5: a retrograde tracer study in the cat

The association (intrahemispheric) cortico-cortical afferent connections of area 5 were studied in the cat by means of retrograde tracing techniques involving horseradish peroxidase (HRP) free or wheat germ agglutinin-conjugated (WGA-HRP) or fluorochrome injections. Single or multiple injections were placed in different parts of areas 5a and 5b, the medial division of area 5 (5m), or in the anterior suprasylvian area (SsA). Labeled cells were plotted on projection drawings of the coronal sections and on two-dimensional "maps" of the cerebral cortex, which were produced according to an accurate and consistent procedure. The major findings of this study are: 1. All divisions of the anterior parietal cortex (areas 3a, 3b, 1, and 2) project to area 5 and to SsA. These projections, however, show marked differences in amount and topographical distribution, depending on the mediolateral and rostrocaudal location of the injections. 2. The motor cortex (areas 4 and 6) also projects heavily to area 5 and to SsA in a well-organized topographic fashion: Area 4 projects mainly upon areas 5a, 5m, SsA, and the medial part of 5b; area 6 projects mainly upon the lateral part of 5b and SsA. Moreover, the upper bank of the cruciate sulcus (areas 4 tau and 4 delta) projects to medial parts of area 5, and the lower bank (areas 4 tau, 6a alpha, and 6a beta) projects to lateral parts of area 5. 3. The somatosensory areas in the anterior ectosylvian gyrus and surrounding cortices (SIIm, SII, and SIV) are connected primarily with medial parts of area 5 (particularly 5a), and SsA. 4. Areas 7 and 7m and a number of visual areas (19, SVA, AmLS, PmLS, 21, 20, 18, ALS, and PLS) project in varying degrees to lateral parts of area 5b. Some of these areas also send weak to moderate projections to the medial part of 5b and the lateral part of 5a. 5. Sparse projections arising from the dorsolateral prefrontal, cingular, retrosplenial, granular insular, and suprasylvian fringe cortices were found to distribute in area 5 and SsA, particularly in lateral portions of 5b. 6. Quite abundant intrinsic connections also found, which were loosely organized according to a complex topographic pattern. On the basis of these and previous results (Avendaño et al., 1985), the identity of area 5 in the cat is discussed, and comparisons are made between this area and sectors of adjoining cortex of cat and primates.

Layer V in rat auditory cortex: projections to the inferior colliculus and contralateral cortex

This study compares the form and distribution within layer V of cells projecting to the inferior colliculus with that of commissural cells of origin in adult rat auditory cortex after horseradish peroxidase injections in the ipsilateral inferior colliculus or auditory cortex. The goal of this work was to determine whether every part of layer V participates equally in both projections, and if the cortical neurons in each pathway were similar. The types of neurons were defined in Golgi-Cox preparations and matched with the profiles of retrogradely labeled cells from architectonically defined cortical area 41. Inferior colliculus and commissural neurons form two populations that differ in their distribution in layer V, in somatic area, and in the form of their apical dendritic arbors. Corticocollicular neurons include the largest pyramidal cells, whose robustly filled apical dendrites ascend into layer II or farther. Commissural cells are smaller and have a more heterogeneous form. Their apical dendrites do not usually extend above layer IV, and a few of these cells may be non-pyramidal. Small pyramidal cells and inverted pyramidal cells project to the opposite cortex, but not to the inferior colliculus. Medium-sized pyramidal cells project in both systems. In addition, certain callosal cells of origin in layers V and III were morphologically similar. More than one-third of the commissural cells originate in the superficial part of layer V, where only 7% of the inferior colliculus projection neurons arise. Most corticocollicular cells lie deeper in layer V, where there are fewer commissural neurons. These findings suggest that the efferent systems projecting to telencephalic and mesencephalic targets are morphologically distinct and spatially segregated in layer V. However, the commissural projection includes similar cells in different cortical layers. The types of these efferent neurons may be more closely related to their target than to their laminar origin.

Visual and auditory association areas of the cat's posterior ectosylvian gyrus: cortical afferents

In a preceding report, we described patterns of thalamic retrograde labeling following 17 tracer deposits on the cat's posterior ectosylvian gyrus and concluded, on the basis of patterns of thalamic connectivity, that the posterior ectosylvian gyrus is composed of three major divisions: a tonotopic auditory zone located anteriorly, a belt of auditory association cortex occupying the gyral crown, and a visual belt located posteriorly. We describe here patterns of transcortical retrograde labeling obtained from tracer deposits in the three zones so defined. Our results indicate that the tonotopic auditory strip is innervated primarily by axons from low-order auditory areas (AAF, AI, P, VP, and V), that the auditory belt receives its strongest input from nontonotopic auditory fields (AII, temporal cortex, and other parts of the auditory belt), and that projections to the visual belt derive primarily from extrastriate visual areas (ALLS, PLLS, DLS, 19, 20, and 21) and from association areas affiliated with the visual system (insular cortex, posterior cingulate gyrus, area 7p, and frontal cortex). We discuss the results in relation to previous systems for parcellating the posterior ectosylvian gyrus of the cat and consider the possibility that divisions of the feline posterior ectosylvian gyrus correspond directly to areas making up the superior temporal gyrus in primates.

Visual and auditory association areas of the cat's posterior ectosylvian gyrus: thalamic afferents

The feline posterior ectosylvian gyrus contains a broad band of association cortex that is bounded anteriorly by tonotopic auditory areas and posteriorly by retinotopic visual areas. To characterize the possible functions of this cortex and to throw light on its pattern of internal divisions, we have carried out an analysis of its thalamic afferents. Deposits of differentiable retrograde tracers were placed at 17 cortical sites in nine cats. The deposit sites spanned the crown of the posterior ectosylvian gyrus and adjacent cortex in the suprasylvian sulcus. We compiled counts of retrogradely labeled neurons in 12 thalamic nuclei delineated by use of Nissl and acetylcholinesterase stains. We then employed a statistical clustering algorithm to identify groups of injections that gave rise to similar patterns of thalamic labeling. The results suggest that the posterior ectosylvian gyrus contains 3 fundamentally different cortical districts that have the form of parallel vertical bands. Very anterior cortex, overlapping previously identified tonotopic auditory areas (AI, P and VP) receives a dense projection from the laminated division of the medial geniculate body (MGl). An intermediate strip, to which we refer as the auditory belt, is innervated by axons from nontonotopic divisions of the medial geniculate body (MGds, MGvl, MGm, and MGd), from the lateral division of the posterior group (Pol), and from the posterior suprageniculate nucleus (SGp). A posterior strip, to which we refer as EPp, receives strong projections from the LM-SG complex (LM-SGa and LMp), and lighter projections from the intralaminar and lateroposterior (LPm and LPl) nuclei. On grounds of thalamic connectivity, EPp is not obviously distinguishable from adjacent retinotopic visual areas (PLLS, DLS, and VLS), and may be regarded as forming, together with these areas, a connectionally homogeneous visual belt.

Neurons accumulating [3H]gamma-aminobutyric acid (GABA) in supragranular layers of cat primary auditory cortex (AI)

The classes of neurons accumulating exogenously injected, tritiated gamma-aminobutyric acid [( 3H]GABA) were studied in the supragranular layers in the primary auditory field of the adult cat. The size, laminar locus, and somatodendritic profiles of labeled neurons were studied light microscopically in frozen- or Vibratome-sectioned, 30 micron thick material, and in semithin, 1-2 micron thick, plastic-embedded high-resolution autoradiographic preparations. The chief goals of the study were to determine which types of cells could be identified as accumulating [3H]GABA in layers I, II and III, and to establish possible relationships between these cells and neurons described in Golgi studies of these layers, and the neurons found, in parallel investigations of the connections of the primary auditory field, to participate as ipsilateral corticocortical and commissural cells of origin. The principal findings are: that neurons in every layer in the primary auditory field take up tritiated gamma-aminobutyric acid; that their Nissl-counterstained somata have a smaller average area, and a smaller range of areas, than do the unlabeled cells; that more than one type of labeled neuron-as defined by somatic size and shape, height:width ratios, and nuclear membrane morphology-could be identified in each layer; that none of the labeled neurons had a soma with a pyramidal configuration; that the labeled cells are comparable in size, shape, and laminar distribution to some populations of non-pyramidal ipsilateral corticocortical cells of origin in layers II and III, and perhaps to certain classes of commissurally projecting, layer III non-pyramidal neurons; and finally, that only a rather small proportion-perhaps 10% or less, except in layer I-of the supragranular cells appear to accumulate labeled material. With regard to the identity of particular classes of neurons accumulating silver grains above background in the individual layers, in layer I, 2 of the 4 types of neurons characterized in Golgi preparations take up gamma-aminobutyric acid and the remaining 2 types may also, and the relative number of labeled cells appears to be higher than in the other layers; in layer II, 2 of the 9 varieties are labeled, and 4 other types may also be; and in layer III, 2 of the 11 types take up gamma-aminobutyric acid, and 5 other varieties may as well. Three types of non-pyramidal layer II cells that project ipsilaterally from AI to the second auditory cortical field, AII, possibly accumulate gamma-aminobutyric acid; 3 types of commissural non-pyramidal cells of origin linking AI to AI appear to be labeled by gamma-aminobutyric acid.(ABSTRACT TRUNCATED AT 400 WORDS)