Corticothalamic feedback enhances stimulus response precision in the visual system

Sensory deprivation differentially impacts the dendritic development of pyramidal versus non-pyramidal neurons in layer 6 of mouse barrel cortex

Early postnatal sensory experience can have profound impacts on the structure and function of cortical circuits affecting behavior. Using the mouse whisker-to-barrel system we chronically deprived animals of normal sensory experience by bilaterally trimming their whiskers every other day from birth for the first postnatal month. Brain tissue was then processed for Golgi staining and neurons in layer 6 of barrel cortex were reconstructed in three dimensions. Dendritic and somatic parameters were compared between sensory-deprived and normal sensory experience groups. Results demonstrated that layer 6 non-pyramidal neurons in the chronically deprived group showed an expansion of their dendritic arbors. The pyramidal cells responded to sensory deprivation with increased somatic size and basilar dendritic arborization but overall decreased apical dendritic parameters. In sum, sensory deprivation impacted on the neuronal architecture of pyramidal and non-pyramidal neurons in layer 6, which may provide a substrate for observed physiological and behavioral changes resulting from whisker trimming.

Trial-to-trial correlation between thalamic sensory response and global EEG activity

Thalamic gating of sensory inputs to the cortex varies with behavioral conditions, such as sleep-wake cycles, or with different stages of anesthesia. Behavioral conditions in turn are accompanied by stereotypic spectral content of the EEG. In the rodent somatosensory system, the receptive field size of the ventral posteromedial thalamic nucleus (VPM) shrinks when anesthesia is deepened. Here we examined whether evoked thalamic responses are correlated with global EEG activity on a fine time scale of a few seconds. Trial-by-trial analysis of responses of VPM cells to whisker stimulation in lightly anesthetized rats indicated that increased EEG power in the delta band (1-4 Hz) was accompanied by a small, but highly significant, reduction in spontaneous and evoked thalamic firing. The opposite effect was found for the gamma EEG band (30-50 Hz). These significant correlations were not accompanied by an apparent change in the size of the receptive fields and were not EEG phase-related. The correlation between EEG and firing rate was observed only in neurons that responded to multiple whiskers and was higher for the non-principal whiskers. Importantly, the contributions of the two EEG bands to the modulation of VPM responses were to a large extent independent of each other. Our findings suggest that information conveyed by different whiskers can be rapidly modulated according to the global brain activity.

Gain control by layer six in cortical circuits of vision

After entering the cerebral cortex, sensory information spreads through six different horizontal neuronal layers that are interconnected by vertical axonal projections. It is believed that through these projections layers can influence each other's response to sensory stimuli, but the specific role that each layer has in cortical processing is still poorly understood. Here we show that layer six in the primary visual cortex of the mouse has a crucial role in controlling the gain of visually evoked activity in neurons of the upper layers without changing their tuning to orientation. This gain modulation results from the coordinated action of layer six intracortical projections to superficial layers and deep projections to the thalamus, with a substantial role of the intracortical circuit. This study establishes layer six as a major mediator of cortical gain modulation and suggests that it could be a node through which convergent inputs from several brain areas can regulate the earliest steps of cortical visual processing.

Diversity of GABAergic interneurons in layer VIa and VIb of mouse barrel cortex

Neocortical layer VI modulates the thalamocortical transfer of information and has a significant impact on sensory processing. This function implicates local γ-aminobutyric acidergic (GABAergic) interneurons that have only been partly described at the present time. Here, we characterized 85 layer VI GABAergic interneurons in acute slices of mouse somatosensory barrel cortex, using whole-cell current-clamp recordings, single-cell reverse transcription-polymerase chain reaction, and biocytin labeling followed by Neurolucida reconstructions. Unsupervised clustering based on electrophysiological molecular and morphological properties disclosed 4 types of interneurons. The 2 major classes were fast-spiking cells transcribing parvalbumin (PV) (51%) and adapting interneurons transcribing somatostatin (SOM) (26%). The third population (18%) transcribed neuropeptide Y (NPY) and appeared very similar to neurogliaform cells. The last class (5%) was constituted by well-segregated GABAergic interneurons transcribing vasoactive intestinal peptide (VIP). Using transgenic mice expressing GFP under the control of the glutamic acid decarboxylase 67k (GAD67) promoter, we investigated the densities of GABAergic cells immunolabeled against PV, SOM, VIP, and NPY through the depth of layer VI. This analysis revealed that PV and NPY translating interneurons concentrate in the upper and lower parts of layer VI, respectively. This study provides an extensive characterization of the properties of layer VI interneurons.

Endocannabinoid CB1 receptors modulate visual output from the thalamus

RATIONALE

Endocannabinoids have emerged as a modulatory brain system affecting different types of synapses, broadly distributed throughout the CNS, which explain the diverse psychophysical effects observed following activation of the endocannabinoid system.

OBJECTIVES AND METHODS

The present study aimed to characterize the effect of CB1-mediated activity in the visual thalamus. In vivo single-unit extracellular recordings were performed in anaesthetized adult pigmented rats, measuring visual and spontaneous activity, combined with application of CB1 receptor agonists (anandamide, 2-AG, and O2545) and one antagonist, AM251.

RESULTS

CB1 receptors activation revealed two cellular populations, with excitatory effects on ∼28% of cells and inhibitory in ∼72%, actions which were blocked by the antagonist AM251. The agonist action significantly altered both spontaneous and visual activity, shifting the signal-to-noise ratio (S/N), with accompanying changes in the variability within the visual response. Increased responses by agonist application were accompanied by a decrease in S/N and an increase in variability, while those cells inhibited by the agonist showed an increase in S/N and a decrease in variability. There was no obvious correlation between the two effects and any other response property suggesting a more general role in modulating all information passing from LGN to cortex.

CONCLUSIONS

Our data support a role for CB1 at the level of the thalamus acting as a dynamic modulator of visual information being sent to the cortex, apparently maintaining the salience of the signal within upper and lower boundaries. This may account for some of the behavioral effects of cannabis.

The functional roles of feedback projections in the visual system

Neurons in the nervous system make connections with ascending feedforward projections and descending feedback projections, as well as projections from neural structures at the identical hierarchical level. These neurons form extremely complicated neural networks and pathways. Compared with the role of the feedforward projection, much less is known concerning the functional roles of the feedback projection. Visual cortex is a good model for studying functional roles of cortical feedback projections which involve many high functions, such as attention, searching and cognition. The present review mainly focused on the functional roles of feedback projections in the visual system.

Feedback enhances feedforward figure-ground segmentation by changing firing mode

In the visual cortex, feedback projections are conjectured to be crucial in figure-ground segregation. However, the precise function of feedback herein is unclear. Here we tested a hypothetical model of reentrant feedback. We used a previous developed 2-layered feedforwardspiking network that is able to segregate figure from ground and included feedback connections. Our computer model data show that without feedback, neurons respond with regular low-frequency (∼9 Hz) bursting to a figure-ground stimulus. After including feedback the firing pattern changed into a regular (tonic) spiking pattern. In this state, we found an extra enhancement of figure responses and a further suppression of background responses resulting in a stronger figure-ground signal. Such push-pull effect was confirmed by comparing the figure-ground responses withthe responses to a homogenous texture. We propose that feedback controlsfigure-ground segregation by influencing the neural firing patterns of feedforward projecting neurons.

Cortical feedback regulation of input to visual cortex: role of intrageniculate interneurons

Neurons in the dorsal lateral geniculate nucleus (dLGN) process and transmit visual signals from retina to visual cortex. The processing is dynamically regulated by cortical excitatory feedback to neurons in dLGN, and synaptic short-term plasticity (STP) has an important role in this regulation. It is known that corticogeniculate synapses on thalamocortical (TC) projection-neurons are facilitating, but type and characteristics of STP of synapses on inhibitory interneurons in dLGN are unknown. We studied STP at corticogeniculate synapses on interneurons and compared the results with STP-characteristics of corticogeniculate synapses on TC neurons to gain insights into the dynamics of cortical regulation of processing in dLGN. We studied neurons in thalamic slices from glutamate decarboxylase 67 (GAD67)–green fluorescent protein (GFP) knock-in mice and made whole-cell recordings of responses evoked by electrical paired-pulse and pulse train stimulation of cortical afferents. We found that cortical excitations of interneurons and TC neurons have distinctly different properties. A single pulse evoked larger EPSCs in interneurons than in TC neurons. However, repetitive stimulation induced frequency-dependent depression of interneurons in contrast to the facilitation of TC neurons. Thus, through these differences of STP mechanisms, the balance of cortical excitation of the two types of neurons could change during stimulation from strongest excitation of interneurons to strongest excitation of TC neurons depending on stimulus frequency and duration, and thereby contribute to activity-dependent cortical regulation of thalamocortical transmission between net depression and net facilitation. Studies of postsynaptic response patterns of interneurons to train stimulation demonstrated that cortical input can activate different types of neuronal integration mechanisms that in addition to the STP mechanisms may change the output from dLGN. Lower stimulus intensity, presumably activating few cortical afferents, or moderate frequencies, elicited summation of graded EPSPs reflecting synaptic depression. However, strong activation through higher intensity or frequency, elicited complex response patterns in interneurons caused at least partly by activation of calcium conductances.

Dysfunctional thalamus-related networks in schizophrenia

Thalamus abnormalities are common in neurological and psychiatric illnesses. Therefore, it is essential to understand the properties of the thalamus-related networks. The thalamic reticular nucleus (TRN) is a thin GABAergic layer interface strategically located between the thalamus and the neocortex. It is, at the very beginning of life, an essential neurodevelopmental guide for the accurate build up of reciprocal anatomical glutamatergic connections between the thalamus and neocortex. It is more than the mediator of selective attention. It appears as a combinatorial matrix because it holds and can combine multiple functional modalities. TRN cells work like integrators, thanks to their extraordinary intrinsic electrophysiological properties, under the contextual and leading influence of corticothalamic inputs. The TRN and thalamus principally form 2-neuron open-loop circuits (no reciprocal connection). The major functioning principle of such GABAergic-glutamatergic circuits is lateral inhibition, which is a gold standard device to set up, via differential amplifications, coherent structured thalamocortical activity patterns. Thereby, it selects relevant streams of information and deletes distractors during action, resting states, and information integration, including during consciousness, cognition, emotion, and thought. Disruption of thalamic lateral inhibition may contribute to a lack of coordination in activity between brain regions, as observed in psychiatric disorders like schizophrenia.

Seizure-like thalamocortical rhythms initiate in the deep layers of the cortex in a co-culture model

The oscillatory rhythms underlying many physiological and pathological states, including absence seizures, require both the thalamus and cortices for full expression. A co-culture preparation combining cortical and thalamic explants provides a unique model for investigating how such oscillations initiate and spread. Here we investigated the dynamics of synchronized thalamocortical activity by simultaneous measurement of field-potential recordings and rapid imaging of Ca(2+) transients by fluorescence methods. Spontaneous sustained hypersynchronized "seizure-like" oscillations required reciprocal cortico-thalamocortical connections. Isolated cortical explants can independently develop brief discharges, while thalamic explants alone were unable to do so. Rapid imaging of Ca(2+) transients demonstrated deep-layer cortical initiation of oscillatory network activity in both connected and isolated explants. Further, cortical explants derived from a rat model of genetic absence epilepsy showed increased bursting duration consistent with an excitable cortex. We propose that thalamocortical oscillatory network activity initiates in deep layers of the cortex with reciprocal thalamic interconnections enabling sustained hyper-synchronization.

Dynamics of stimulus-evoked spike timing correlations in the cat lateral geniculate nucleus

Precisely synchronized neuronal activity has been commonly observed in the mammalian visual pathway. Spike timing correlations in the lateral geniculate nucleus (LGN) often take the form of phase synchronized oscillations in the high gamma frequency range. To study the relations between oscillatory activity, synchrony, and their time-dependent properties, we recorded activity from multiple single units in the cat LGN under stimulation by stationary spots of light. Autocorrelation analysis showed that approximately one third of the cells exhibited oscillatory firing with a mean frequency ∼80 Hz. Cross-correlation analysis showed that 30% of unit pairs showed significant synchronization, and 61% of these pairs consisted of synchronous oscillations. Cross-correlation analysis assumes that synchronous firing is stationary and maintained throughout the period of stimulation. We tested this assumption by applying unitary events analysis (UEA). We found that UEA was more sensitive to weak and transient synchrony than cross-correlation analysis and detected a higher incidence (49% of cell pairs) of significant synchrony (unitary events). In many unit pairs, the unitary events were optimally characterized at a bin width of 1 ms, indicating that neural synchrony has a high degree of temporal precision. We also found that approximately one half of the unit pairs showed nonstationary changes in synchrony that could not be predicted by the modulation of firing rates. Population statistics showed that the onset of synchrony between LGN cells occurred significantly later than that observed between retinal afferents and LGN cells. The synchrony detected among unit pairs recorded on separate tetrodes tended to be more transient and have a later onset than that observed between adjacent units. These findings show that stimulus-evoked synchronous activity within the LGN is often rhythmic, highly nonstationary, and modulated by endogenous processes that are not tightly correlated with firing rate.

Visual stimulation decorrelates neuronal activity

The accuracy of neuronal encoding depends on the response statistics of individual neurons and the correlation of the activity between different neurons. Here, the dynamics of the neuronal response statistics in the anterior superior temporal sulcus of the macaque monkey is described. A transient reduction in the normalized trial-by-trial variability and decorrelation of the responses with both the activity of other neurons and previous activity of the same neuron are found at response onset. The variability of neuronal activity and its correlation structure return to the levels observed in the resting state 50-100 ms after response onset, except for marked increases in the signal correlation between neurons. The transient changes in the response statistics are seen even if there is little or no stimulus-elicited activity, indicating the effect is due to network properties rather than to activity changes per se. Modeling also indicates that the observed variations in response variability and correlation structure of the neuronal activity over time cannot be attributed to changes in firing rate. However, a reset of the underlying spike-generating process, possibly due to the driving input changing from recurrent to feedforward inputs, captures most of the observed changes. The nonstationarity indicated by the changes in correlation structure around response onset increases coding efficiency: compared with the mutual information calculated without regard to the transitory changes, the decorrelation increases the information conveyed by the initial response of modeled neuronal pairs by ≤ 4% and suggests that an integration time of as little as 50 ms is sufficient to extract 95% the available information during the initial response period.

Spectral integration in primary auditory cortex attributable to temporally precise convergence of thalamocortical and intracortical input

Primary sensory cortex integrates sensory information from afferent feedforward thalamocortical projection systems and convergent intracortical microcircuits. Both input systems have been demonstrated to provide different aspects of sensory information. Here we have used high-density recordings of laminar current source density (CSD) distributions in primary auditory cortex of Mongolian gerbils in combination with pharmacological silencing of cortical activity and analysis of the residual CSD, to dissociate the feedforward thalamocortical contribution and the intracortical contribution to spectral integration. We found a temporally highly precise integration of both types of inputs when the stimulation frequency was in close spectral neighborhood of the best frequency of the measurement site, in which the overlap between both inputs is maximal. Local intracortical connections provide both directly feedforward excitatory and modulatory input from adjacent cortical sites, which determine how concurrent afferent inputs are integrated. Through separate excitatory horizontal projections, terminating in cortical layers II/III, information about stimulus energy in greater spectral distance is provided even over long cortical distances. These projections effectively broaden spectral tuning width. Based on these data, we suggest a mechanism of spectral integration in primary auditory cortex that is based on temporally precise interactions of afferent thalamocortical inputs and different short- and long-range intracortical networks. The proposed conceptual framework allows integration of different and partly controversial anatomical and physiological models of spectral integration in the literature.

A generalized linear model of the impact of direct and indirect inputs to the lateral geniculate nucleus

Relay neurons in the lateral geniculate nucleus (LGN) receive direct visual input predominantly from a single retinal ganglion cell (RGC), in addition to indirect input from other sources including interneurons, thalamic reticular nucleus (TRN), and the visual cortex. To address the extent of influence of these indirect sources on the response properties of the LGN neurons, we fit a Generalized Linear Model (GLM) to the spike responses of cat LGN neurons driven by spatially homogeneous spots that were rapidly modulated by a pseudorandom luminance sequence. Several spot sizes were used to probe the spatial extent of the indirect visual effects. Our extracellular recordings captured both the LGN spikes and the incoming RGC input (S potentials), allowing us to divide the inputs to the GLM into two categories: the direct RGC input and the indirect input to which we have access through the luminance of the visual stimulus. For spots no larger than the receptive field center, the effect of the indirect input is negligible, while for larger spots its effect can, on average, account for 5% of the variance of the data and for as much as 25% in some cells. The polarity of the indirect visual influence is opposite to that of the linear receptive field of the neurons. We conclude that the indirect source of response modulation of the LGN relay neurons arises from inhibitory sources, compatible with thalamic interneurons or TRN.

Corticogeniculate feedback and visual processing in the primate

Corticogeniculate neurones make more synapses in the lateral geniculate nucleus (LGN) than retinal ganglion cells, yet we know relatively little about the functions of corticogeniculate feedback for visual processing. In primates, feedforward projections from the retina to the LGN and from the LGN to primary visual cortex are organized into anatomically and physiologically distinct parallel pathways. Recent work demonstrates a close relationship between these parallel streams of feedforward projections and the corticogeniculate feedback pathway. Here, we review the evidence for stream-specific feedback in the primate and consider the implications of parallel streams of feedback for vision.

A comparison of visual responses in the lateral geniculate nucleus of alert and anaesthetized macaque monkeys

Despite the increasing use of alert animals for studies aimed at understanding visual processing in the cerebral cortex, relatively little attention has been focused on quantifying the response properties of neurons that provide input to the cortex. Here, we examine the response properties of neurons in the lateral geniculate nucleus (LGN) of the thalamus in the alert macaque monkey and compare these responses to those in the anaesthetized animal. Compared to the anaesthetized animal, we show that magnocellular and parvocellular neurons in the alert animal respond to visual stimuli with significantly higher firing rates. This increase in responsiveness is not accompanied by a change in the shape of neuronal contrast response functions or the strength of centre–surround antagonism; however, it is accompanied by an increased ability of neurons to follow stimuli drifting at higher spatial and temporal frequencies.

Neocortical layer 6, a review

This review attempts to summarise some of the major areas of neocortical research as it pertains to neocortical layer 6. After a brief summary of the development of this intriguing layer, the major pyramidal cell classes to be found in layer 6 are described and compared. The connections made and received by these different classes of neurones are then discussed and the possible functions of these connections, with particular reference to the shaping of responses in visual cortex and thalamus. Inhibition in layer 6 is discussed where appropriate, but not in great detail. Many types of interneurones are to be found in each cortical layer and layer 6 is no exception, but the functions of each type remain to be elucidated (Gonchar et al., 2007).

Variability of visual responses of superior colliculus neurons depends on stimulus velocity

Visually responding neurons in the superficial, retinorecipient layers of the cat superior colliculus receive input from two primarily parallel information processing channels, Y and W, which is reflected in their velocity response profiles. We quantified the time-dependent variability of responses of these neurons to stimuli moving with different velocities by Fano factor (FF) calculated in discrete time windows. The FF for cells responding to low-velocity stimuli, thus receiving W inputs, increased with the increase in the firing rate. In contrast, the dynamics of activity of the cells responding to fast moving stimuli, processed by Y pathway, correlated negatively with FF whether the response was excitatory or suppressive. These observations were tested against several types of surrogate data. Whereas Poisson description failed to reproduce the variability of all collicular responses, the inclusion of secondary structure to the generating point process recovered most of the observed features of responses to fast moving stimuli. Neither model could reproduce the variability of low-velocity responses, which suggests that, in this case, more complex time dependencies need to be taken into account. Our results indicate that Y and W channels may differ in reliability of responses to visual stimulation. Apart from previously reported morphological and physiological differences of the cells belonging to Y and W channels, this is a new feature distinguishing these two pathways.

Neocortical layer 6, a review

This review attempts to summarise some of the major areas of neocortical research as it pertains to neocortical layer 6. After a brief summary of the development of this intriguing layer, the major pyramidal cell classes to be found in layer 6 are described and compared. The connections made and received by these different classes of neurones are then discussed and the possible functions of these connections, with particular reference to the shaping of responses in visual cortex and thalamus. Inhibition in layer 6 is discussed where appropriate, but not in great detail. Many types of interneurones are to be found in each cortical layer and layer 6 is no exception, but the functions of each type remain to be elucidated (Gonchar et al., 2007).

Surround suppression sharpens orientation tuning in the cat primary visual cortex

In the primary visual cortex (V1), the response of a neuron to stimulation of its classical receptive field (CRF) is suppressed by concurrent stimulation of the extraclassical receptive field (ECRF), a phenomenon termed 'surround suppression'. It is also known that the orientation tuning of V1 neurons becomes sharper as the size of the stimulus increases beyond the CRF. However, there have been few quantitative investigations of the relationship between sharpening of orientation tuning and surround suppression. We examined this relationship in 73 V1 neurons recorded from anesthetized and paralysed cats using sinusoidal grating patches as stimuli. We found that sharpening of orientation tuning was significantly correlated with the strength of surround suppression for large stimuli that cover both CRF and ECRF. Furthermore, simulation analysis using a variety of tuning widths and most suppressive orientation of orientation-tuned surround suppression demonstrated that broadly orientation-tuned surround suppression sharpens orientation tuning for large gratings without shift in optimal orientation. Our findings suggest that one of the functional roles of surround suppression in V1 is enhancement of orientation discrimination for large and uniformly patterned objects.

Parallel processing in the corticogeniculate pathway of the macaque monkey

Although corticothalamic feedback is ubiquitous across species and modalities, its role in sensory processing is unclear. This study provides a detailed description of the visual physiology of corticogeniculate neurons in the primate. Using electrical stimulation to identify corticogeniculate neurons, we distinguish three groups of neurons with response properties that closely resemble those of neurons in the magnocellular, parvocellular, and koniocellular layers of their target structure, the lateral geniculate nucleus (LGN) of the thalamus. Our results indicate that corticogeniculate feedback in the primate is stream specific, and provide strong evidence in support of the view that corticothalamic feedback can influence the transmission of sensory information from the thalamus to the cortex in a stream-selective manner.

Spontaneous local gamma oscillation selectively enhances neural network responsiveness

Synchronized oscillation is very commonly observed in many neuronal systems and might play an important role in the response properties of the system. We have studied how the spontaneous oscillatory activity affects the responsiveness of a neuronal network, using a neural network model of the visual cortex built from Hodgkin-Huxley type excitatory (E-) and inhibitory (I-) neurons. When the isotropic local E-I and I-E synaptic connections were sufficiently strong, the network commonly generated gamma frequency oscillatory firing patterns in response to random feed-forward (FF) input spikes. This spontaneous oscillatory network activity injects a periodic local current that could amplify a weak synaptic input and enhance the network's responsiveness. When E-E connections were added, we found that the strength of oscillation can be modulated by varying the FF input strength without any changes in single neuron properties or interneuron connectivity. The response modulation is proportional to the oscillation strength, which leads to self-regulation such that the cortical network selectively amplifies various FF inputs according to its strength, without requiring any adaptation mechanism. We show that this selective cortical amplification is controlled by E-E cell interactions. We also found that this response amplification is spatially localized, which suggests that the responsiveness modulation may also be spatially selective. This suggests a generalized mechanism by which neural oscillatory activity can enhance the selectivity of a neural network to FF inputs.

In the nervous system, information is delivered and processed digitally via voltage spikes transmitted between cells. A neural system is characterized by its input/output spike signal patterns. Generally, a network of neurons shows a very different response pattern than that of a single neuron. In some cases, a neural network generates interesting population activities, such as synchronized oscillations, which are thought to modulate the response properties of the network. However, the exact role of these neural oscillations is unknown. We investigated the relationship between the oscillatory activity and the response modulation in neural networks using computational simulation modeling. We found that the response of the system is significantly modified by the oscillations in the network. In particular, the responsiveness to weak inputs is remarkably enhanced. This suggests that the oscillation can differentially amplify sensory information depending on the input signal conditions. We conclude that a neural network can dynamically modify its response properties by the selective amplification of sensory signals due to oscillation activity, which may explain some experimental observations and help us to better understand neural systems.

Stimulus-specific and stimulus-nonspecific firing synchrony and its modulation by sensory adaptation in the whisker-to-barrel pathway

The stimulus-evoked response of a cortical neuron depends on both details of the afferent signal and the momentary state of the larger network in which it is embedded. Consequently, identical sensory stimuli evoke highly variable responses. Using simultaneous recordings of thalamic barreloid and/or cortical barrel neurons in the rat whisker-to-barrel pathway, we determined the extent to which the responses of pairs of cells covary on a trial-by-trial basis. In the thalamus and cortical layer IV, a substantial component of trial-to-trial variability is independent of the specific parameters of the stimulus, probed here using deflection angle. These stimulus-nonspecific effects resulted in greater-than-chance similarities in trial-averaged angular tuning among simultaneously recorded pairs of barrel neurons. Such effects were not observed among simultaneously recorded thalamic and cortical barrel neurons, suggesting strong intracortical mechanisms of synchronization. Sensory adaptation produced by prior whisker deflections reduced response magnitudes and enhanced the joint angular tuning of simultaneously recorded neurons. Adaptation also decorrelated stimulus-evoked responses, rendering trial-by-trial responses of neuron pairs less similar to each other. Adaptation-induced decorrelation coupled with sharpened joint tuning could enhance the saliency of cells within thalamus or cortex that continue to fire synchronously during ongoing tactile stimulation associated with active touch.

Task-dependent modulation of medial geniculate body is behaviorally relevant for speech recognition

Recent work has shown that responses in first-order sensory thalamic nuclei are modulated by cortical areas [1–5]. However, the functional role of such corticothalamic modulation and its relevance for human perception is still unclear. Here, we show in two functional magnetic resonance imaging (fMRI) studies that the neuronal response in the first-order auditory thalamus, the medial geniculate body (MGB), is increased when rapidly varying spectrotemporal features of speech sounds are processed, as compared to processing slowly varying spectrotemporal features of the same sounds. The strength of this task-dependent modulation is positively correlated with the speech recognition scores of individual subjects. These results show that task-dependent modulation of the MGB serves the processing of specific features of speech sounds and is behaviorally relevant for speech recognition. Our findings suggest that the first-order auditory thalamus is not simply a nonspecific gatekeeper controlled by attention [6]. Together with studies in nonhuman mammals [4, 5], our findings imply a mechanism in which the first-order auditory thalamus, possibly by corticothalamic modulation, reacts adaptively to features of sensory input.

Localization of prolyl oligopeptidase in the thalamic and cortical projection neurons: a retrograde neurotracing study in the rat brain

Prolyl oligopeptidase (POP) is a serine endopeptidase which hydrolyses proline-containing peptides shorter than 30-mer. POP is believed to be associated with cognitive functions via neuropeptide cleavage. POP has been also connected to the inositol 1,4,5-triphosphate (IP(3)) signalling but the effects of POP-inhibition to the IP(3) accumulation in vivo are still unclear. However, little is known about the physiological role of POP in the brain. We have previously found that in the rat brain POP was specifically expressed in the pyramidal neurons of the cerebral cortex, particularly in the primary motor and somatosensory cortices, and corresponding projection areas in thalamus. Using a retrograde neurotracer we have now visualized the localization of POP in thalamocortical and corticothalamic projection neurons in ventrobasal complex and medial geniculate nucleus of thalamus and somatosensory/motor and auditory cortices. We observed that both in thalamus and cortex over 50% of projection neurons contained POP. These results support the hypothesis that POP is involved in thalamocortical and corticothalamic signal processing. We also propose, based on our neuroanatomical findings and literature, that POP may take part in the thalamocortical oscillations by interacting with IP(3) signalling in cells.

Quantifying neural coding of event timing

Single-neuron firing is often analyzed relative to an external event, such as successful task performance or the delivery of a stimulus. The perievent time histogram (PETH) examines how, on average, neural firing modulates before and after the alignment event. However, the PETH contains no information about the single-trial reliability of the neural response, which is important from the perspective of a target neuron. In this study, we propose the concept of using the neural activity to predict the timing of the occurrence of an event, as opposed to using the event to predict the neural response. We first estimate the likelihood of an observed spike train, under the assumption that it was generated by an inhomogeneous gamma process with rate profile similar to the PETH shifted by a small time. This is used to generate a probability distribution of the event occurrence, using Bayes’ rule. By an information theoretic approach, this method yields a single value (in bits) that quantifies the reduction in uncertainty regarding the time of an external event following observation of the spike train. We show that the approach is sensitive to the amplitude of a response, to the level of baseline firing, and to the consistency of a response between trials, all of which are factors that will influence a neuron's ability to code for the time of the event. The technique can provide a useful means not only of determining which of several behavioral events a cell encodes best, but also of permitting objective comparison of different cell populations.

Rapid changes in thalamic firing synchrony during repetitive whisker stimulation

Thalamic firing synchrony is thought to ensure selective transmission of relevant sensory information to the recipient cortical neurons by rendering them more responsive to temporally correlated input spikes. However, direct evidence for a synchrony code in the thalamus is limited. Here, we directly measure thalamic firing synchrony and its stimulus-induced modulation over time, using simultaneous single unit recordings from individual thalamic barreloids in the rat somatosensory whisker/barrel system. Employing whisker deflections varying in velocity or frequency and a cross-correlation approach, we find systematic changes in both time course and strength of thalamic firing synchrony as a function of stimulus parameters and sensory adaptation. Synchrony develops faster and is greater with higher velocity deflections. Greater firing synchrony reflects stimulus-dependent increases in instantaneous firing rates, greater spike time precision relative to stimulus onset as well as common input that likely arises from divergent trigeminothalamic and corticothalamic neurons. With adaptation, synchrony decreases and takes longer to develop but is more dependent on the cells' common inputs. Rapid, sharp increases in thalamic synchrony mirroring quick increases in whisker velocity occur also during ongoing random, high-frequency whisker vibrations. Together, results demonstrate millisecond by millisecond changes in thalamic near-synchronous firing during complex patterns of ongoing vibrissa movements that may ensure transmission of preferred sensory information in local thalamocortical circuits during whisking and active touch.

Emerging views of corticothalamic function

Although it is now generally accepted that the thalamus is more than a simple relay of sensory signals to the cortex, we are just beginning to gain an understanding of how corticothalamic feedback influences sensory processing. Results from an increasing number of studies across sensory systems and different species reveal effects of feedback both on the receptive fields of thalamic neurons and on the transmission of sensory information between the thalamus and cortex. Importantly, these studies demonstrate that the cortico-thalamic projection cannot be viewed in isolation, but must be considered as an integral part of a thalamo-corticothalamic circuit which intimately interconnects the thalamus and cortex for sensory processing.

The twinkle aftereffect is pre-cortical and is independent of filling-in

A real or artificial scotoma within a dynamic noise field fills in within a few seconds. When the dynamic noise is replaced with a homogenous field, a twinkling after effect (TwAE) is induced exclusively in the location of the former scotoma. We are employing the appearance of the TwAE to perform rapid perimetry in patients with retinal scotomas. To analyze the loci within the visual system and the mechanisms of filling-in and the TwAE, we examined their orientation tuning, inter-ocular transfer, and threshold versus contrast functions by measuring contrast detection thresholds for stimuli presented in areas that were filled-in or contained the TwAE. For filling-in, detection thresholds were narrowly tuned for orientation, transferred interocularly, and rose monotonically with the contrast of a surround pattern. These results indicate that surround suppression and filling-in involve inhibitory processes originating at cortical stages of visual processing. Threshold versus contrast functions were weakly dipper-shaped for the TwAE, did not transfer inter-ocularly, and were not tuned for orientation. These results indicate that the TwAE involves additive noise that is pre-cortical in origin and that it is distinct from filling-in.

Synchrony between orientation-selective neurons is modulated during adaptation-induced plasticity in cat visual cortex

BACKGROUND

Visual neurons respond essentially to luminance variations occurring within their receptive fields. In primary visual cortex, each neuron is a filter for stimulus features such as orientation, motion direction and velocity, with the appropriate combination of features eliciting maximal firing rate. Temporal correlation of spike trains was proposed as a potential code for linking the neuronal responses evoked by various features of a same object. In the present study, synchrony strength was measured between cells following an adaptation protocol (prolonged exposure to a non-preferred stimulus) which induce plasticity of neurons' orientation preference.

RESULTS

Multi-unit activity from area 17 of anesthetized adult cats was recorded. Single cells were sorted out and (1) orientation tuning curves were measured before and following 12 min adaptation and 60 min after adaptation (2) pairwise synchrony was measured by an index that was normalized in relation to the cells' firing rate. We first observed that the prolonged presentation of a non-preferred stimulus produces attractive (58%) and repulsive (42%) shifts of cell's tuning curves. It follows that the adaptation-induced plasticity leads to changes in preferred orientation difference, i.e. increase or decrease in tuning properties between neurons. We report here that, after adaptation, the neuron pairs that shared closer tuning properties display a significant increase of synchronization. Recovery from adaptation was accompanied by a return to the initial synchrony level.

CONCLUSION

We conclude that synchrony reflects the similarity in neurons' response properties, and varies accordingly when these properties change.

Populations of Tightly Coupled Neurons: The RGC/LGN System

A mathematical model, of general character for the dynamic description of coupled neural oscillators is presented. The population approach that is employed applies equally to coupled cells as to populations of such coupled cells. The formulation includes stochasticity and preserves details of precisely firing neurons. Based on the generally accepted view of cortical wiring, this formulation is applied to the retinal ganglion cell (RGC)/lateral geniculate nucleus (LGN) relay cell system, of the early mammalian visual system. The smallness of quantal voltage jumps at the retinal level permits a Fokker-Planck approximation for the RGC contribution; however, the LGN description requires the use of finite jumps, which for fast synaptic dynamics appears as finite jumps in the membrane potential.

Analyses of equilibrium spiking behavior for both the deterministic and stochastic cases are presented. Green's function methods form the basis for the asymptotic and exact results that are presented. This determines the spiking ratio (i.e., the number of RGC arrivals per LGN spike), which is the reciprocal of the transfer ratio, under wide circumstances. Criteria for spiking regimes, in terms of the relatively few parameters of the model, are presented.

Under reasonable hypotheses, it is shown that the transfer ratio is ≤1/2, in the absence of input from other areas. Thus, the model suggests that the LGN/RGC system may be a relatively unsophisticated spike editor. In the absence of other input, the system is designed to fire an LGN spike only when two or more RGC spikes appear in a relatively short time. Transfer ratios that briefly exceed 1/2 (but are less than 1) have been recorded in the laboratory. Inclusion of brain stem input has been shown to provide a signal that elevates the transfer ratio (Ozaki & Kaplan, 2006). A model that includes this contribution is also presented.

A simple model of retina-LGN transmission

To gain a deeper understanding of the transmission of visual signals from retina through the lateral geniculate nucleus (LGN), we have used a simple leaky integrate and-fire model to simulate a relay cell in the LGN. The simplicity of the model was motivated by two questions: (1) Can an LGN model that is driven by a retinal spike train recorded as synaptic ('S') potentials, but does not include a diverse array of ion channels, nor feedback inputs from the cortex, brainstem, and thalamic reticular nucleus, accurately simulate the LGN discharge on a spike-for-spike basis? (2) Are any special synaptic mechanisms, beyond simple summation of currents, necessary to model experimental recordings? We recorded cat relay cell responses to spatially homogeneous small or large spots, with luminance that was rapidly modulated in a pseudo-random fashion. Model parameters for each cell were optimized with a Simplex algorithm using a short segment of the recording. The model was then tested on a much longer, distinct data set consisting of responses to numerous repetitions of the noisy stimulus. For LGN cells that spiked in response to a sufficiently large fraction of retinal inputs, we found that this simplified model accurately predicted the firing times of LGN discharges. This suggests that modulations of the efficacy of the retino-geniculate synapse by pre-synaptic facilitation or depression are not necessary in order to account for the LGN responses generated by our stimuli, and that post-synaptic summation is sufficient.