Ultrastructural characteristics of callosal neurons in deep layers of cat primary auditory cortex (AI)

We have carried out an electron microscopic investigation of retrogradely HRP-labeled nonpyramidal neurons in layers V and VI of the primary auditory cortex (AI), which are sources of transcallosal projections. We have established that on average 15.8±1.7% of the perikaryon surface of these cells is occupied by axo-somatic synapses. We detected in ultrathin sections from two to nine synapses on the profiles of the perikaryon of callosal neurons. All of these axo-somatic synapses are formed by axon terminals containing small flat synaptic vesicles and are characterized by symmetrical contacts. The length of the cross section of the contacts was on average 1.6±0.1 µm. The axon terminals of callosal fibers, antegradely labeled by the enzyme, form in the deep layers of the cortex asymmetrical synapses on the spines and stems of the dendrites. A possible functional significance of the axo-somatic synapses in the production of the impulse activity of callosal neurons in the deep layers of the AI region, is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 5, pp. 549–556, May, 1991.     

Quantitative and qualitative characteristics of synapses in different layers of the auditory cortex

An electron-microscopic study was made of 4520 synapses in different layers of the cat auditory cortex. Of the total number of synapses 53% were located on dendritic spines, 37% on dendrites, and 10% on neuron bodies; 91% of the synapses belonged to Gray's type I, 9% to type II. Most of the type I synapses were located on dendrites and dendritic spines, whereas the type II synapses were distributed on neuron bodies, axon hillocks, and large dendrites. Signs of degeneration were discovered 60 h after complete neuronal isolation of an area of the auditory cortex in 22.8% of synapses. No degenerating type II synapses were found. This indicates that they are formed by axons of intracortical neurons. The quantitative and qualitative composition of the synapses were shown to differ in different layers of the auditory cortex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 2, pp. 131–137, March–April, 1980.     

Unveiling functions of the visual cortex using task-specific deep neural networks

The human visual cortex enables visual perception through a cascade of hierarchical computations in cortical regions with distinct functionalities. Here, we introduce an AI-driven approach to discover the functional mapping of the visual cortex. We related human brain responses to scene images measured with functional MRI (fMRI) systematically to a diverse set of deep neural networks (DNNs) optimized to perform different scene perception tasks. We found a structured mapping between DNN tasks and brain regions along the ventral and dorsal visual streams. Low-level visual tasks mapped onto early brain regions, 3-dimensional scene perception tasks mapped onto the dorsal stream, and semantic tasks mapped onto the ventral stream. This mapping was of high fidelity, with more than 60% of the explainable variance in nine key regions being explained. Together, our results provide a novel functional mapping of the human visual cortex and demonstrate the power of the computational approach.     

Neuronal selectivity to complex vocalization features emerges in the superficial layers of primary auditory cortex

Early in auditory processing, neural responses faithfully reflect acoustic input. At higher stages of auditory processing, however, neurons become selective for particular call types, eventually leading to specialized regions of cortex that preferentially process calls at the highest auditory processing stages. We previously proposed that an intermediate step in how nonselective responses are transformed into call-selective responses is the detection of informative call features. But how neural selectivity for informative call features emerges from nonselective inputs, whether feature selectivity gradually emerges over the processing hierarchy, and how stimulus information is represented in nonselective and feature-selective populations remain open question. In this study, using unanesthetized guinea pigs (GPs), a highly vocal and social rodent, as an animal model, we characterized the neural representation of calls in 3 auditory processing stages—the thalamus (ventral medial geniculate body (vMGB)), and thalamorecipient (L4) and superficial layers (L2/3) of primary auditory cortex (A1). We found that neurons in vMGB and A1 L4 did not exhibit call-selective responses and responded throughout the call durations. However, A1 L2/3 neurons showed high call selectivity with about a third of neurons responding to only 1 or 2 call types. These A1 L2/3 neurons only responded to restricted portions of calls suggesting that they were highly selective for call features. Receptive fields of these A1 L2/3 neurons showed complex spectrotemporal structures that could underlie their high call feature selectivity. Information theoretic analysis revealed that in A1 L4, stimulus information was distributed over the population and was spread out over the call durations. In contrast, in A1 L2/3, individual neurons showed brief bursts of high stimulus-specific information and conveyed high levels of information per spike. These data demonstrate that a transformation in the neural representation of calls occurs between A1 L4 and A1 L2/3, leading to the emergence of a feature-based representation of calls in A1 L2/3. Our data thus suggest that observed cortical specializations for call processing emerge in A1 and set the stage for further mechanistic studies.

A study of the neuronal representations elicited in guinea pigs by conspecific calls at different auditory processing stages reveals insights into where call-selective neuronal responses emerge; the transformation from nonselective to call-selective responses occurs in the superficial layers of the primary auditory cortex.     

Prior expectations evoke stimulus-specific activity in the deep layers of the primary visual cortex

The way we perceive the world is strongly influenced by our expectations. In line with this, much recent research has revealed that prior expectations strongly modulate sensory processing. However, the neural circuitry through which the brain integrates external sensory inputs with internal expectation signals remains unknown. In order to understand the computational architecture of the cortex, we need to investigate the way these signals flow through the cortical layers. This is crucial because the different cortical layers have distinct intra- and interregional connectivity patterns, and therefore determining which layers are involved in a cortical computation can inform us on the sources and targets of these signals. Here, we used ultra-high field (7T) functional magnetic resonance imaging (fMRI) to reveal that prior expectations evoke stimulus-specific activity selectively in the deep layers of the primary visual cortex (V1). These findings are in line with predictive processing theories proposing that neurons in the deep cortical layers represent perceptual hypotheses and thereby shed light on the computational architecture of cortex.

The way we perceive the world is strongly influenced by our expectations, but the neural circuitry through which the brain achieves this remains unknown. A study using ultra-high field fMRI reveals that prior expectations evoke stimulus-specific signals in the deep layers of the primary visual cortex.     

Heterogeneous effects of cholecystokinin on neuronal response properties in deep layers of rat barrel cortex

Abstract

Cholecystokinin (CCK) is one of the most studied neuropeptides in the brain. In this study, we investigated the effects of CCK-8s and LY225910 (CCK₂ receptor antagonist) on properties of neuronal response to natural stimuli (whisker deflection) in deep layers of rat barrel cortex. This study was done on 20 male Wistar rats, weighing 230–260?g. CCK-8s (300?nmol/rat) and LY225910 (1?µmol/rat) were administered intracerebroventricularly (ICV). Neuronal responses to deflection of principal (PW) and adjacent (AW) whiskers were recorded in the barrel cortex using tungsten microelectrodes. Computer controlled mechanical displacement was used to deflect whiskers individually or in combination at 30?ms inter-stimulus intervals. ON and OFF responses for PW and AW deflections were measured. A condition-test ratio (CTR) was computed to quantify neuronal responses to whisker interaction. ICV administration of CCK-8s and LY225910 had heterogeneous effects on neuronal spontaneous activity, ON and OFF responses to PW and/or AW deflections, and CTR for both ON and OFF responses. The results of this study demonstrated that CCK-8s can modulate neuronal response properties in deep layers of rat barrel cortex probably via CCK₂ receptors.     

Spread of excitation in upper layers of the cat auditory cortex with participation of intracortical interneuronal connections

In chronically isolated slabs of the cat auditory cortex with additional transection of lower layers and preservation of the structural integrity of one, two, or three upper layers of cortex just under the pial membrane, impulse responses of slab neurons to stimulation applied at the additionally undercut section were studied. High effectiveness of axodendritic and axospinal excitatory contacts formed by nerve elements of intracortical origin in upper cortical layers was demonstrated. The participation of geniculocortical fibers in spread of excitation in the cortex through synaptic contacts in layer I with dendrites of underlying-layer pyramidal neurons is discussed. The capacity for generation of polysynaptic excitation responses by the neurons indicates preservation of complex interneuronal interactions in the isolated cortex slab preparations with their undercut lower layers.I. I. Mechnikov State University of Odessa, Odessa. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 80–87, January–February, 1991.     

Functional architecture of auditory cortex

Three complementary approaches demonstrate new types of organization in rodent, feline and primate auditory cortex, as well as differences in processing between auditory and visual cortex. First, connectional work reveals patterns of thalamocortical and corticocortical input unique to the auditory cortex. Second, physiological studies find multiple, interleaved auditory processing modules related to corticocortical connections and embedded in the isofrequency gradient. Third, functional analyses demonstrate independent processing streams for sound localization and identification analogous to the 'what' and 'where' streams in visual cortex, although the modular arrangements are modality-specific. Taken together, these data show that the auditory cortex has common and unique functional substrates.     

Morphology of the cat auditory cortex

The ultrastructural features of the primary auditory cortex of the cats and the character of the endings of geniculo-cortical afferent fibers in the early stages of experimental degeneration evoked by destruction of the medial geniculate body were studied. In all layers of the cortex asymmetrical synapses with round synaptic vesicles on dendritic spines and on thin dendritic branches of pyramidal and nonpyramidal neurons are predominant. Symmetrical synapses with flattened or polymorphic vesicles are distributed chiefly on the bodies of the neurons and their large dendrites. Because there are few symmetrical synapses which could be regarded as inhibitory it is postulated that inhibitory influences may also be transmitted through asymmetrical synapses with round vesicles. Other types of contacts between the bodies of neurons, dendrites, and glial processes also were found in the auditory cortex. Degenerating terminals of geniculo-cortical fibers were shown to terminate chiefly in layer IV of the cortex on pyramidal and nonpyramidal neurons. Degeneration was of the dark type in asymmetrical synapses with round vesicles. The results are dicussed in connection with electrophysiological investigations of the auditory cortex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 5, No. 5, pp. 519–524, September–October, 1973.     

Integration of touch and sound in auditory cortex

To form a coherent percept of the environment, our brain combines information from different senses. Such multisensory integration occurs in higher association cortices; but supposedly, it also occurs in early sensory areas. Confirming the latter hypothesis, we unequivocally demonstrate supra-additive integration of touch and sound stimulation at the second stage of the auditory cortex. Using high-resolution fMRI of the macaque monkey, we quantified the integration of auditory broad-band noise and tactile stimulation of hand and foot in anaesthetized animals. Integration was found posterior to and along the lateral side of the primary auditory cortex in the caudal auditory belt. Integration was stronger for temporally coincident stimuli and obeyed the principle of inverse effectiveness: greater enhancement for less effective stimuli. These findings demonstrates that multisensory integration occurs early and close to primary sensory areas and--because it occurs in anaesthetized animals--suggests that this integration is mediated by preattentive bottom-up mechanisms.     

Autoradiographic localization of kappa opiate receptors to deep layers of the cerebral cortex may explain unique sedative and analgesic effects

The pharmacologically defined kappa drug 3H-ethylketazocine (3H-EKC) and 3H-bremazocine bind to unique sites, but also to mu and delta receptors. By displacing mu and delta interactions with morphine and D-Ala2, D-Leu5-enkephalin (DADL) respectively we have visualized selective receptors for 3H-EKC and 3H-bremazocine. These two kappa ligands are localized to sites different from mu and delta receptors labeled with 3H-dihydromorphine (3H-DHM) and 3H-DADL. The highest density and most selective localization of putative kappa receptors occurs in layers V and VI of the cerebral cortex. Cells in these layers project to the thalamus, regulating sensory input to the cortex. These deep cortical kappa receptors may account for the unique sedative and analgesic actions of kappa opiates.     

Learning-induced plasticity in animal and human auditory cortex

Recent data on learning-related changes in animal and human auditory cortex indicate functions beyond mere stimulus representation and simple recognition memory for stimuli. Rather, auditory cortex seems to process and represent stimuli in a task-dependent fashion. This implies plasticity in neural processing, which can be observed at the level of single neuron firing and the level of spatiotemporal activity patterns in cortical areas. Auditory cortex is a structure in which behaviorally relevant aspects of stimulus processing are highly developed because of the fugitive nature of auditory stimuli.     

Reliability and representational bandwidth in the auditory cortex

It is unclear why there are so many more neurons in sensory cortex than in the sensory periphery. One possibility is that these "extra" neurons are used to overcome cortical noise and faithfully represent the acoustic stimulus. Another possibility is that even after overcoming cortical noise, there is "excess representational bandwidth" available and that this bandwidth is used to represent conjunctions of auditory and nonauditory information for computation. Here, we discuss recent data about neuronal reliability in auditory cortex showing that cortical noise may not be as high as was previously believed. Although at present, the data suggest that auditory cortex neurons can be more reliable than those in the visual cortex, we speculate that the principles governing cortical computation are universal and that visual and other cortical areas can also exploit strategies based on similarly high-fidelity activity.     

Functional organization of the human auditory cortex

We investigated the functional organization of the human auditory cortex using a novel electrophysiological recording technique combined with an advanced brain magnetic resonance imaging (MRI) technique. Tonotopic mapping data were obtained during single unit recording along the Heschl’s gyrus. Most of the units studied (73%) demonstrated sharply tuned excitatory responses. A tonotopic pattern was observed with the best frequencies systematically increasing as more medial-caudal recording sites were sampled. Additionally, a new auditory field along the posterior aspect of the superior temporal gyrus has been identified using a high spatial resolution direct recording technique. Results obtained during electrical stimulation demonstrate functional connectivity between the primary auditory cortex and the auditory field in the posterior superior temporal gyrus.