Temporal processing in sensory systems

The speed of sight: Individual variation in critical flicker fusion thresholds

The critical flicker fusion threshold is a psychophysical measure commonly used to quantify visual temporal resolution; the fastest rate at which a visual system can discriminate visual signals. Critical flicker fusion thresholds vary substantially among species, reflecting different ecological niches and demands. However, it is unclear how much variation exists in flicker fusion thresholds between healthy individuals of the same species, or how stable this attribute is over time within individuals. In this study, we assessed both inter- and intra-individual variation in critical flicker fusion thresholds in a cohort of healthy human participants within a specific age range, using two common psychophysical methods and three different measurements during each session. The resulting thresholds for each method were highly correlated. We found a between-participant maximum difference of roughly 30 Hz in flicker fusion thresholds and we estimated a 95% prediction interval of 21 Hz. We used random-effects models to compare between- and within-participant variance and found that approximately 80% of variance was due to between-individual differences, and about 10% of the variance originated from within-individual differences over three sessions. Within-individual thresholds did not differ significantly between the three sessions in males, but did in females (P<0.001 for two methods and P<0.05 for one method), indicating that critical flicker fusion thresholds may be more variable in females than in males.

Neural hyperactivity and altered envelope encoding in the central auditory system: Changes with advanced age and hearing loss

Temporal modulations are ubiquitous features of sound signals that are important for auditory perception. The perception of temporal modulations, or temporal processing, is known to decline with aging and hearing loss and negatively impact auditory perception in general and speech recognition specifically. However, neurophysiological literature also provides evidence of exaggerated or enhanced encoding of specifically temporal envelopes in aging and hearing loss, which may arise from changes in inhibitory neurotransmission and neuronal hyperactivity. This review paper describes the physiological changes to the neural encoding of temporal envelopes that have been shown to occur with age and hearing loss and discusses the role of disinhibition and neural hyperactivity in contributing to these changes. Studies in both humans and animal models suggest that aging and hearing loss are associated with stronger neural representations of both periodic amplitude modulation envelopes and of naturalistic speech envelopes, but primarily for low-frequency modulations (<80 Hz). Although the frequency dependence of these results is generally taken as evidence of amplified envelope encoding at the cortex and impoverished encoding at the midbrain and brainstem, there is additional evidence to suggest that exaggerated envelope encoding may also occur subcortically, though only for envelopes with low modulation rates. A better understanding of how temporal envelope encoding is altered in aging and hearing loss, and the contexts in which neural responses are exaggerated/diminished, may aid in the development of interventions, assistive devices, and treatment strategies that work to ameliorate age- and hearing-loss-related auditory perceptual deficits.

An information theoretic method to resolve millisecond-scale spike timing precision in a comprehensive motor program

Sensory inputs in nervous systems are often encoded at the millisecond scale in a precise spike timing code. There is now growing evidence in behaviors ranging from slow breathing to rapid flight for the prevalence of precise timing encoding in motor systems. Despite this, we largely do not know at what scale timing matters in these circuits due to the difficulty of recording a complete set of spike-resolved motor signals and assessing spike timing precision for encoding continuous motor signals. We also do not know if the precision scale varies depending on the functional role of different motor units. We introduce a method to estimate spike timing precision in motor circuits using continuous MI estimation at increasing levels of added uniform noise. This method can assess spike timing precision at fine scales for encoding rich motor output variation. We demonstrate the advantages of this approach compared to a previously established discrete information theoretic method of assessing spike timing precision. We use this method to analyze the precision in a nearly complete, spike resolved recording of the 10 primary wing muscles control flight in an agile hawk moth, Manduca sexta. Tethered moths visually tracked a robotic flower producing a range of turning (yaw) torques. We know that all 10 muscles in this motor program encode the majority of information about yaw torque in spike timings, but we do not know whether individual muscles encode motor information at different levels of precision. We demonstrate that the scale of temporal precision in all motor units in this insect flight circuit is at the sub-millisecond or millisecond-scale, with variation in precision scale present between muscle types. This method can be applied broadly to estimate spike timing precision in sensory and motor circuits in both invertebrates and vertebrates.

Neural readout of a latency code in the active electrosensory system

The latency of spikes relative to a stimulus conveys sensory information across modalities. However, in most cases, it remains unclear whether and how such latency codes are utilized by postsynaptic neurons. In the active electrosensory system of mormyrid fish, a latency code for stimulus amplitude in electroreceptor afferent nerve fibers (EAs) is hypothesized to be read out by a central reference provided by motor corollary discharge (CD). Here, we demonstrate that CD enhances sensory responses in postsynaptic granular cells of the electrosensory lobe but is not required for reading out EA input. Instead, diverse latency and spike count tuning across the EA population give rise to graded information about stimulus amplitude that can be read out by standard integration of converging excitatory synaptic inputs. Inhibitory control over the temporal window of integration renders two granular cell subclasses differentially sensitive to information derived from relative spike latency versus spike count.

Nicotine Exposure during Adolescence Leads to Changes of Synaptic Plasticity and Intrinsic Excitability of Mice Insular Pyramidal Cells at Later Life

To find satisfactory treatment for nicotine addiction, synaptic and cellular mechanisms should be investigated comprehensively. Synaptic transmission, plasticity and intrinsic excitability in various brain regions are known to be altered by acute nicotine exposure. However, it has not been addressed whether and how nicotine exposure during adolescence alters these synaptic events and intrinsic excitability in the insular cortex in adulthood. To address this question, we performed whole-cell patch-clamp recordings to examine the effects of adolescent nicotine exposure on synaptic transmission, plasticity and intrinsic excitability in layer V pyramidal neurons (PNs) of the mice insular cortex five weeks after the treatment. We found that excitatory synaptic transmission and potentiation were enhanced in these neurons. Following adolescent nicotine exposure, insular layer V PNs displayed enhanced intrinsic excitability, which was reflected in changes in relationship between current strength and spike number, inter-spike interval, spike current threshold and refractory period. In addition, spike-timing precision evaluated by standard deviation of spike timing was decreased following nicotine exposure. Our data indicate that adolescent nicotine exposure enhances synaptic transmission, plasticity and intrinsic excitability in layer V PNs of the mice insular cortex at later life, which might contribute to severe nicotine dependence in adulthood.

Ringing Decay of Gamma Oscillations and Transcranial Magnetic Stimulation Therapy in Autism Spectrum Disorder

Research suggest that in autism spectrum disorder (ASD) a disturbance in the coordinated interactions of neurons within local networks gives rise to abnormal patterns of brainwave activity in the gamma bandwidth. Low frequency transcranial magnetic stimulation (TMS) over the dorsolateral prefrontal cortex (DLPFC) has been proven to normalize gamma oscillation abnormalities, executive functions, and repetitive behaviors in high functioning ASD individuals. In this study, gamma frequency oscillations in response to a visual classification task (Kanizsa figures) were analyzed and compared in 19 ASD (ADI-R diagnosed, 14.2 ± 3.61 years old, 5 girls) and 19 (14.8 ± 3.67 years old, 5 girls) age/gender matched neurotypical individuals. The ASD group was treated with low frequency TMS (1.0 Hz, 90% motor threshold, 18 weekly sessions) targeting the DLPFC. In autistic subjects, as compared to neurotypicals, significant differences in event-related gamma oscillations were evident in amplitude (higher) pre-TMS. In addition, recordings after TMS treatment in our autistic subjects revealed a significant reduction in the time period to reach peak amplitude and an increase in the decay phase (settling time). The use of a novel metric for gamma oscillations. i.e., envelope analysis, and measurements of its ringing decay allowed us to characterize the impedance of the originating neuronal circuit. The ringing decay or dampening of gamma oscillations is dependent on the inhibitory tone generated by networks of interneurons. The results suggest that the ringing decay of gamma oscillations may provide a biomarker reflective of the excitatory/inhibitory balance of the cortex and a putative outcome measure for interventions in autism.

Microsecond interaural time difference discrimination restored by cochlear implants after neonatal deafness

Spatial hearing in cochlear implant (CI) patients remains a major challenge, with many early deaf users reported to have no measurable sensitivity to interaural time differences (ITDs). Deprivation of binaural experience during an early critical period is often hypothesized to be the cause of this shortcoming. However, we show that neonatally deafened (ND) rats provided with precisely synchronized CI stimulation in adulthood can be trained to lateralize ITDs with essentially normal behavioral thresholds near 50 μs. Furthermore, comparable ND rats show high physiological sensitivity to ITDs immediately after binaural implantation in adulthood. Our result that ND-CI rats achieved very good behavioral ITD thresholds, while prelingually deaf human CI patients often fail to develop a useful sensitivity to ITD raises urgent questions concerning the possibility that shortcomings in technology or treatment, rather than missing input during early development, may be behind the usually poor binaural outcomes for current CI patients.

Synaptic inhibition in the neocortex: Orchestration and computation through canonical circuits and variations on the theme

The neocortex plays a crucial role in all basic and abstract cognitive functions. Conscious mental processes are achieved through a correct flow of information within and across neocortical networks, whose particular activity state results from a tight balance between excitation and inhibition. The proper equilibrium between these indissoluble forces is operated with multiscale organization: along the dendro-somatic axis of single neurons and at the network level. Fast synaptic inhibition is assured by a multitude of inhibitory interneurons. During cortical activities, these cells operate a finely tuned division of labor that is epitomized by their detailed connectivity scheme. Recent results combining the use of mouse genetics, cutting-edge optical and neurophysiological approaches have highlighted the role of fast synaptic inhibition in driving cognition-related activity through a canonical cortical circuit, involving several major interneuron subtypes and principal neurons. Here we detail the organization of this cortical blueprint and we highlight the crucial role played by different neuron types in fundamental cortical computations. In addition, we argue that this canonical circuit is prone to many variations on the theme, depending on the resolution of the classification of neuronal types, and the cortical area investigated. Finally, we discuss how specific alterations of distinct inhibitory circuits can underlie several devastating brain diseases.

Efficient calculation of heterogeneous non-equilibrium statistics in coupled firing-rate models

Understanding nervous system function requires careful study of transient (non-equilibrium) neural response to rapidly changing, noisy input from the outside world. Such neural response results from dynamic interactions among multiple, heterogeneous brain regions. Realistic modeling of these large networks requires enormous computational resources, especially when high-dimensional parameter spaces are considered. By assuming quasi-steady-state activity, one can neglect the complex temporal dynamics; however, in many cases the quasi-steady-state assumption fails. Here, we develop a new reduction method for a general heterogeneous firing-rate model receiving background correlated noisy inputs that accurately handles highly non-equilibrium statistics and interactions of heterogeneous cells. Our method involves solving an efficient set of nonlinear ODEs, rather than time-consuming Monte Carlo simulations or high-dimensional PDEs, and it captures the entire set of first and second order statistics while allowing significant heterogeneity in all model parameters.

Functional Development of Principal Neurons in the Anteroventral Cochlear Nucleus Extends Beyond Hearing Onset

Sound information is transduced into graded receptor potential by cochlear hair cells and encoded as discrete action potentials of auditory nerve fibers. In the cochlear nucleus, auditory nerve fibers convey this information through morphologically distinct synaptic terminals onto bushy cells (BCs) and stellate cells (SCs) for processing of different sound features. With expanding use of transgenic mouse models, it is increasingly important to understand the in vivo functional development of these neurons in mice. We characterized the maturation of spontaneous and acoustically evoked activity in BCs and SCs by acquiring single-unit juxtacellular recordings between hearing onset (P12) and young adulthood (P30) of anesthetized CBA/J mice. In both cell types, hearing sensitivity and characteristic frequency (CF) range are mostly adult-like by P14, consistent with rapid maturation of the auditory periphery. In BCs, however, some physiological features like maximal firing rate, dynamic range, temporal response properties, recovery from post-stimulus depression, first spike latency (FSL) and encoding of sinusoid amplitude modulation undergo further maturation up to P18. In SCs, the development of excitatory responses is even more prolonged, indicated by a gradual increase in spontaneous and maximum firing rates up to P30. In the same cell type, broadly tuned acoustically evoked inhibition is immediately effective at hearing onset, covering the low- and high-frequency flanks of the excitatory response area. Together, these data suggest that maturation of auditory processing in the parallel ascending BC and SC streams engages distinct mechanisms at the first central synapses that may differently depend on the early auditory experience.

Social Skills Deficits in Autism Spectrum Disorder: Potential Biological Origins and Progress in Developing Therapeutic Agents

Autism spectrum disorder is defined by two core symptoms: a deficit in social communication and the presence of repetitive behaviors and/or restricted interests. Currently, there is no US Food and Drug Administration-approved drug for these core symptoms. This article reviews the biological origins of the social function deficit associated with autism spectrum disorder and the drug therapies with the potential to treat this deficit. A review of the history of autism demonstrates that a deficit in social interaction has been the defining feature of the concept of autism from its conception. Abnormalities identified in early social skill development and an overview of the pathophysiology abnormalities associated with autism spectrum disorder are discussed as are the abnormalities in brain circuits associated with the social function deficit. Previous and ongoing clinical trials examining agents that have the potential to improve social deficits associated with autism spectrum disorder are discussed in detail. This discussion reveals that agents such as oxytocin and propranolol are particularly promising and undergoing active investigation, while other agents such as vasopressin agonists and antagonists are being activity investigated but have limited published evidence at this time. In addition, agents such as bumetanide and manipulation of the enteric microbiome using microbiota transfer therapy appear to have promising effects on core autism spectrum disorder symptoms including social function. Other pertinent issues associated with developing treatments in autism spectrum disorder, such as disease heterogeneity, high placebo response rates, trial design, and the most appropriate way of assessing effects on social skills (outcome measures), are also discussed.

Decoding sound level in the marmoset primary auditory cortex

Neurons that respond favorably to a particular sound level have been observed throughout the central auditory system, becoming steadily more common at higher processing areas. One theory about the role of these level-tuned or nonmonotonic neurons is the level-invariant encoding of sounds. To investigate this theory, we simulated various subpopulations of neurons by drawing from real primary auditory cortex (A1) neuron responses and surveyed their performance in forming different sound level representations. Pure nonmonotonic subpopulations did not provide the best level-invariant decoding; instead, mixtures of monotonic and nonmonotonic neurons provided the most accurate decoding. For level-fidelity decoding, the inclusion of nonmonotonic neurons slightly improved or did not change decoding accuracy until they constituted a high proportion. These results indicate that nonmonotonic neurons fill an encoding role complementary to, rather than alternate to, monotonic neurons.NEW & NOTEWORTHY Neurons with nonmonotonic rate-level functions are unique to the central auditory system. These level-tuned neurons have been proposed to account for invariant sound perception across sound levels. Through systematic simulations based on real neuron responses, this study shows that neuron populations perform sound encoding optimally when containing both monotonic and nonmonotonic neurons. The results indicate that instead of working independently, nonmonotonic neurons complement the function of monotonic neurons in different sound-encoding contexts.

Neuropathological Mechanisms of Seizures in Autism Spectrum Disorder

This manuscript reviews biological abnormalities shared by autism spectrum disorder (ASD) and epilepsy. Two neuropathological findings are shared by ASD and epilepsy: abnormalities in minicolumn architecture and γ-aminobutyric acid (GABA) neurotransmission. The peripheral neuropil, which is the region that contains the inhibition circuits of the minicolumns, has been found to be decreased in the post-mortem ASD brain. ASD and epilepsy are associated with inhibitory GABA neurotransmission abnormalities including reduced GABA_A and GABA_B subunit expression. These abnormalities can elevate the excitation-to-inhibition balance, resulting in hyperexcitablity of the cortex and, in turn, increase the risk of seizures. Medical abnormalities associated with both epilepsy and ASD are discussed. These include specific genetic syndromes, specific metabolic disorders including disorders of energy metabolism and GABA and glutamate neurotransmission, mineral and vitamin deficiencies, heavy metal exposures and immune dysfunction. Many of these medical abnormalities can result in an elevation of the excitatory-to-inhibitory balance. Fragile X is linked to dysfunction of the mGluR5 receptor and Fragile X, Angelman and Rett syndromes are linked to a reduction in GABA_A receptor expression. Defects in energy metabolism can reduce GABA interneuron function. Both pyridoxine dependent seizures and succinic semialdehyde dehydrogenase deficiency cause GABA deficiencies while urea cycle defects and phenylketonuria cause abnormalities in glutamate neurotransmission. Mineral deficiencies can cause glutamate and GABA neurotransmission abnormalities and heavy metals can cause mitochondrial dysfunction which disrupts GABA metabolism. Thus, both ASD and epilepsy are associated with similar abnormalities that may alter the excitatory-to-inhibitory balance of the cortex. These parallels may explain the high prevalence of epilepsy in ASD and the elevated prevalence of ASD features in individuals with epilepsy.

Dissociation of psychophysical and EEG steady-state response measures of cross-modal temporal correspondence for amplitude modulated acoustic and vibrotactile stimulation

Research examining multisensory integration suggests that the correspondence of stimulus characteristics across modalities (cross-modal correspondence) can have a dramatic influence on both neurophysiological and perceptual responses to multimodal stimulation. The current study extends prior research by examining the cross-modal correspondence of amplitude modulation rate for simultaneous acoustic and vibrotactile stimulation using EEG and perceptual measures of sensitivity to amplitude modulation. To achieve this, psychophysical thresholds and steady-state responses (SSRs) were measured for acoustic and vibrotactile amplitude modulated (AM) stimulation for 21 and 40 Hz AM rates as a function of the cross-modal correspondence. The study design included three primary conditions to determine whether the changes in the SSR and psychophysical thresholds were due to the cross-modal temporal correspondence of amplitude modulated stimuli: NONE (AM in one modality only), SAME (the same AM rate for each modality) and DIFF (different AM rates for each modality). The results of the psychophysical analysis showed that AM detection thresholds for the simultaneous AM conditions (i.e., SAME and DIFF) were significantly higher (i.e., lower sensitivity) than AM detection thresholds for the stimulation of a single modality (i.e., NONE). SSR results showed significant effects of SAME and DIFF conditions on SSR activity. The different pattern of results for perceptual and SSR measures of cross-modal correspondence of AM rate indicates a dissociation between entrained cortical activity (i.e., SSR) and perception.

Variations in interpulse interval of double action potentials during propagation in single neurons

In this work, we analyzed the interpulse interval (IPI) of doublets and triplets in single neurons of three biological models. Pulse trains with two or three spikes originate from the process of sensory mechanotransduction in neurons of the locust femoral nerve, as well as through spontaneous activity both in the abdominal motor neurons and the caudal photoreceptor of the crayfish. We show that the IPI for successive low-frequency single action potentials, as recorded with two electrodes at two different points along a nerve axon, remains constant. On the other hand, IPI in doublets either remains constant, increases or decreases by up to about 3 ms as the pair propagates. When IPI increases, the succeeding pulse travels at a slower speed than the preceding one. When IPI is reduced, the succeeding pulse travels faster than the preceding one and may exceed the normal value for the specific neuron. In both cases, IPI increase and reduction, the speed of the preceding pulse differs slightly from the normal value, therefore the two pulses travel at different speeds in the same nerve axon. On the basis of our results, we may state that the effect of attraction or repulsion in doublets suggests a tendency of the spikes to reach a stable configuration. We strongly suggest that the change in IPI during spike propagation of doublets opens up a whole new realm of possibilities for neural coding and may have major implications for understanding information processing in nervous systems.

Combined LTP and LTD of modulatory inputs controls neuronal processing of primary sensory inputs

A hallmark of brain organization is the integration of primary and modulatory pathways by principal neurons. However, the pathway interactions that shape primary input processing remain unknown. We investigated this problem in mouse dorsal cochlear nucleus (DCN) where principal cells integrate primary, auditory nerve input with modulatory, parallel fiber input. Using a combined experimental and computational approach, we show that combined LTP and LTD of parallel fiber inputs to DCN principal cells and interneurons, respectively, broaden the time window within which synaptic inputs summate. Enhanced summation depolarizes the resting membrane potential and thus lowers the response threshold to auditory nerve inputs. Combined LTP and LTD, by preserving the variance of membrane potential fluctuations and the membrane time constant, fixes response gain and spike latency as threshold is lowered. Our data reveal a novel mechanism mediating adaptive and concomitant homeostatic regulation of distinct features of neuronal processing of sensory inputs.

Regulation of response properties and operating range of the AFD thermosensory neurons by cGMP signaling

BACKGROUND

The neuronal mechanisms that encode specific stimulus features in order to elicit defined behavioral responses are poorly understood. C. elegans forms a memory of its cultivation temperature (T(c)) and exhibits distinct behaviors in different temperature ranges relative to T(c). In particular, C. elegans tracks isotherms only in a narrow temperature band near T(c). T(c) memory is in part encoded by the threshold of responsiveness (T∗(AFD)) of the AFD thermosensory neuron pair to temperature stimuli. However, because AFD thermosensory responses appear to be similar at all examined temperatures above T∗(AFD), the mechanisms that generate specific behaviors in defined temperature ranges remain to be determined.

RESULTS

Here, we show that the AFD neurons respond to the sinusoidal variations in thermal stimuli followed by animals during isothermal tracking (IT) behavior only in a narrow temperature range near T(c). We find that mutations in the AFD-expressed gcy-8 receptor guanylyl cyclase (rGC) gene result in defects in the execution of IT behavior and are associated with defects in the responses of the AFD neurons to oscillating thermal stimuli. In contrast, mutations in the gcy-18 or gcy-23 rGCs alter the temperature range in which IT behavior is exhibited. Alteration of intracellular cGMP levels via rGC mutations or addition of cGMP analogs shift the lower and upper ranges of the temperature range of IT behavior in part via alteration in T∗(AFD).

CONCLUSIONS

Our observations provide insights into the mechanisms by which a single sensory neuron type encodes features of a given stimulus to generate different behaviors in defined zones.

Competing streams at the cocktail party: exploring the mechanisms of attention and temporal integration

Processing of complex acoustic scenes depends critically on the temporal integration of sensory information as sounds evolve naturally over time. It has been previously speculated that this process is guided by both innate mechanisms of temporal processing in the auditory system, as well as top-down mechanisms of attention and possibly other schema-based processes. In an effort to unravel the neural underpinnings of these processes and their role in scene analysis, we combine magnetoencephalography (MEG) with behavioral measures in humans in the context of polyrhythmic tone sequences. While maintaining unchanged sensory input, we manipulate subjects' attention to one of two competing rhythmic streams in the same sequence. The results reveal that the neural representation of the attended rhythm is significantly enhanced in both its steady-state power and spatial phase coherence relative to its unattended state, closely correlating with its perceptual detectability for each listener. Interestingly, the data reveal a differential efficiency of rhythmic rates of the order of few hertz during the streaming process, closely following known neural and behavioral measures of temporal modulation sensitivity in the auditory system. These findings establish a direct link between known temporal modulation tuning in the auditory system (particularly at the level of auditory cortex) and the temporal integration of perceptual features in a complex acoustic scene, while mediated by processes of attention.

Low-Frequency Repetitive Transcranial Magnetic Stimulation (rTMS) Modulates Evoked-Gamma Frequency Oscillations in Autism Spectrum Disorder (ASD)

INTRODUCTION: It has been reported that individuals with Autism Spectrum Disorder (ASD) have abnormal reactions to the sensory environment and visuo-perceptual abnormalities. Electrophysiological research has provided evidence that gamma band activity (30-80 Hz) is a physiological indicator of the co-activation of cortical cells engaged in processing visual stimuli and integrating different features of a stimulus. A number of studies have found augmented and indiscriminative gamma band power at early stages of visual processing in ASD; this may be related to decreased inhibitory processing and an increase in the ratio of cortical excitation to inhibition. Low frequency or 'slow' (≤1HZ) repetitive transcranial magnetic stimulation (rTMS) has been shown to increase inhibition of stimulated cortex by the activation of inhibitory circuits. METHODS: We wanted to test the hypothesis of gamma band abnormalities at early stages of visual processing in ASD by investigating relative evoked (i.e. ~ 100 ms) gamma power in 25 subjects with ASD and 20 age-matched controls using Kanizsa illusory figures. Additionally, we wanted to assess the effects of 12 sessions of bilateral 'slow' rTMS to the dorsolateral prefrontal cortex (DLPFC) on evoked gamma activity using a randomized controlled design. RESULTS: In individuals with ASD evoked gamma activity was not discriminative of stimulus type, whereas in controls early gamma power differences between target and non-target stimuli were highly significant. Following rTMS individuals with ASD showed significant improvement in discriminatory gamma activity between relevant and irrelevant visual stimuli. We also found significant improvement in the responses on behavioral questionnaires (i.e., irritability, repetitive behavior) as a result of rTMS. CONCLUSION: We proposed that 'slow' rTMS may have increased cortical inhibitory tone which improved discriminatory gamma activity at early stages of visual processing. rTMS has the potential to become an important therapeutic tool in ASD treatment and has shown significant benefits in treating core symptoms of ASD with few, if any side effects.

Aberrant high-frequency desynchronization of cerebellar cortices in early-onset psychosis

Sensorimotor integration deficits are routinely observed in both schizophreniform and mood-disordered psychoses. Neurobiological theories of schizophrenia and related psychoses have proposed that aberrations in large-scale cortico-thalamic-cerebellar-thalamic-cortical loops may underlie integration abnormalities, and that such dysfunctional connectivity may be central to the pathophysiology. In this study, we utilized a basic mechanoreception task to probe cortical-cerebellar circuitry in early-onset psychosis. Ten adolescents with psychosis and 10 controls completed unilateral tactile stimulation of the right and left index finger, as whole-head magnetoencephalography (MEG) data were acquired. MEG data were imaged in the frequency domain, using spatial filtering, and the resulting event-related synchronizations and desynchronizations (ERS/ERD) were subjected to voxel-wise analyses of group and task effects using statistical parametric mapping. Our results indicated bilateral ERD activation of cerebellar regions and postcentral gyri in both groups during stimulation of either hand. Interestingly, during left finger stimulations, adolescents with psychosis exhibited greater alpha and gamma ERD activity in right cerebellar cortices relative to controls. Subjects with psychosis also showed greater ERD in bilateral cerebellum and the right postcentral gyrus during right finger stimulation, and these differences were statistically stronger for higher frequency bins. Lastly, controls exhibited greater alpha ERS of the right postcentral gyrus during right finger stimulation. These findings provide new data on the neurodevelopmental trajectory of basic mechanoreception in adolescents, and also indicate aberrant cerebellar functioning in early-onset psychoses, especially in the right cerebellum, which may be the crucial dysfunctional node in cortico-thalamic-cerebellar-thalamic-cortical circuits.

Temporal-pattern recognition by single neurons in a sensory pathway devoted to social communication behavior

Sensory systems often encode stimulus information into the temporal pattern of action potential activity. However, little is known about how the information contained within these patterns is extracted by postsynaptic neurons. Similar to temporal coding by sensory neurons, social information in mormyrid fish is encoded into the temporal patterning of an electric organ discharge. In the current study, sensitivity to temporal patterns of electrosensory stimuli was found to arise within the midbrain posterior exterolateral nucleus (ELp). Whole-cell patch recordings from ELp neurons in vivo revealed three patterns of interpulse interval (IPI) tuning: low-pass neurons tuned to long intervals, high-pass neurons tuned to short intervals, and bandpass neurons tuned to intermediate intervals. Many neurons within each class also responded preferentially to either increasing or decreasing IPIs. Playback of electric signaling patterns recorded from freely behaving fish revealed that the IPI and direction tuning of ELp neurons resulted in selective responses to particular social communication displays characterized by distinct IPI patterns. The postsynaptic potential responses of many neurons indicated a combination of excitatory and inhibitory synaptic input, and the IPI tuning of ELp neurons was directly related to rate-dependent changes in the direction and amplitude of postsynaptic potentials. These results suggest that differences in the dynamics of short-term synaptic plasticity in excitatory and inhibitory pathways may tune central sensory neurons to particular temporal patterns of presynaptic activity. This may represent a general mechanism for the processing of behaviorally relevant stimulus information encoded into temporal patterns of activity by sensory neurons.

Divisive gain modulation with dynamic stimuli in integrate-and-fire neurons

The modulation of the sensitivity, or gain, of neural responses to input is an important component of neural computation. It has been shown that divisive gain modulation of neural responses can result from a stochastic shunting from balanced (mixed excitation and inhibition) background activity. This gain control scheme was developed and explored with static inputs, where the membrane and spike train statistics were stationary in time. However, input statistics, such as the firing rates of pre-synaptic neurons, are often dynamic, varying on timescales comparable to typical membrane time constants. Using a population density approach for integrate-and-fire neurons with dynamic and temporally rich inputs, we find that the same fluctuation-induced divisive gain modulation is operative for dynamic inputs driving nonequilibrium responses. Moreover, the degree of divisive scaling of the dynamic response is quantitatively the same as the steady-state responses—thus, gain modulation via balanced conductance fluctuations generalizes in a straight-forward way to a dynamic setting.

Many neural computations, including sensory and motor processing, require neurons to control their sensitivity (often termed ‘gain’) to stimuli. One common form of gain manipulation is divisive gain control, where the neural response to a specific stimulus is simply scaled by a constant. Most previous theoretical and experimental work on divisive gain control have assumed input statistics to be constant in time. However, realistic inputs can be highly time-varying, often with time-varying statistics, and divisive gain control remains to be extended to these cases. A widespread mechanism for divisive gain control for static inputs is through an increase in stimulus independent membrane fluctuations. We address the question of whether this divisive gain control scheme is indeed operative for time-varying inputs. Using simplified spiking neuron models, we employ accurate theoretical methods to estimate the dynamic neural response. We find that gain control via membrane fluctuations does indeed extend to the time-varying regime, and moreover, the degree of divisive scaling does not depend on the timescales of the driving input. This significantly increases the relevance of this form of divisive gain control for neural computations where input statistics change in time, as expected during normal sensory and motor behavior.

Time course of cortically induced fos expression in auditory thalamus and midbrain after bilateral cochlear ablation

Expression of c-fos in the medial geniculate body (MGB) and the inferior colliculus (IC) in response to bicuculline-induced corticofugal activation was examined in rats at different time points after bilateral cochlear ablation (4 h-30 days). Corticofugal activation was crucial in eliciting Fos expression in the MGB after cochlear ablation. The pars ovoidea (OV) of the medial geniculate body ventral division (MGv) showed dense Fos expression 4 h after cochlear ablation; the expression declined to very low levels at 24 h and thereafter. In turn, strong Fos expression was found in the pars lateralis (LV) of the MGv 24 h after cochlear ablation and dropped dramatically at 14 days. The dorsal division of the MGB (MGd) showed high Fos expression 7 days after cochlear ablation, which persisted for a period of time. Using multi-electrode recordings, neuronal activity of different MGB subnuclei was found to correlate well with Fos expressions. The temporal changes in cortically activated Fos expression in different MGB subnuclei after bilateral cochlear ablation indicate differential denervation hypersensitivities of these MGB neurons and likely point to differential dependence of these nuclei on both auditory ascending and corticofugal descending inputs. After bilateral cochlear ablation, significant increases in Fos-positive neurons were detected unilaterally in all IC subnuclei, ipsilateral to the bicuculline injection.

Facilitatory mechanisms underlying selectivity for the direction and rate of frequency modulated sweeps in the auditory cortex

Neurons selective for frequency modulated (FM) sweeps are common in auditory systems across different vertebrate groups and may underlie representation of species-specific vocalizations. Studies on mechanisms of FM sweep selectivity have primarily focused on sideband inhibition. Here, we present the first evidence for facilitatory mechanisms of FM sweep selectivity. Facilitatory interactions were found in 46 of 264 (17%) neurons tuned in the echolocation range (25-60 kHz) in the auditory cortex of the pallid bat. These neurons respond poorly to individual tones but are facilitated by combinations of tones with specific spectral and temporal intervals. Facilitation neurons show remarkable sensitivity to sub-millisecond differences in time delays between the two tones. Interestingly, the range of delays eliciting facilitation is not fixed but varies systematically with frequency difference between the two tones. Properties of facilitation strongly predict selectivity for the direction and rate of FM sweeps. Together with previous studies, there appear to be at least three mechanisms underlying FM rate and direction selectivity: sideband inhibition, duration tuning, and facilitation. Interestingly, similar mechanisms underlie direction and velocity tuning in the visual system, suggesting the evolution of similar computations across sensory systems to process dynamic sensory stimuli.

Cortical gamma generators suggest abnormal auditory circuitry in early-onset psychosis

Neurobiological theories of schizophrenia and related psychoses have increasingly emphasized impaired neuronal coordination (i.e., dysfunctional connectivity) as central to the pathophysiology. Although neuroimaging evidence has mostly corroborated these accounts, the basic mechanism(s) of reduced functional connectivity remains elusive. In this study, we examine the developmental trajectory and underlying mechanism(s) of dysfunctional connectivity by using gamma oscillatory power as an index of local and long-range circuit integrity. An early-onset psychosis group and a matched cohort of typically developing adolescents listened to monaurally presented click-trains, as whole-head magnetoencephalography data were acquired. Consistent with previous work, gamma-band power was significantly higher in right auditory cortices across groups and conditions. However, patients exhibited significantly reduced overall gamma power relative to controls, and showed a reduced ear-of-stimulation effect indicating that ipsi- versus contralateral presentation had less impact on hemispheric power. Gamma-frequency oscillations are thought to be dependent on gamma-aminobutyric acidergic interneuronal networks, thus these patients' impairment in generating and/or maintaining such activity may indicate that local circuit integrity is at least partially compromised early in the disease process. In addition, patients also showed abnormality in long-range networks (i.e., ear-of-stimulation effects) potentially suggesting that multiple stages along auditory pathways contribute to connectivity aberrations found in patients with psychosis.

Neuronal identification of acoustic signal periodicity

Acoustic signals transmit information by temporal characteristics and envelope periodicity as well as by their frequency content. Many animals can extract the frequency content of a signal by means of specialized organs such as the cochlea but for the detection and identification of higher-order periodicity, e.g., amplitude modulations, this type of organ is useless. In addition, many animals do not have a cochlea but still depend on a reliable identification of different frequencies in the vast variety of acoustic signals they perceive in their natural environment. Hence, neural mechanisms to decode periodicity information must exist. We present a detailed mathematical analysis of a recurrent and a feedforward model of neuronal periodicity extraction and discuss basic constraints for neuronal circuitry performing such a task in a biological system. Both the recurrent and the feedforward model perform well using neuronal parameters typical for the auditory system. Performance is limited mainly by the temporal precision of the connections between the neurons.

Children and adolescents with autism exhibit reduced MEG steady-state gamma responses

BACKGROUND

Recent neuroimaging studies of autism have indicated reduced functional connectivity during both cognitive tasks and rest. These data suggest long-range connectivity may be compromised in this disorder, and current neurological theories of autism contend disrupted inter-regional interactions may be an underlying mechanism explaining behavioral symptomatology. However, it is unclear whether deficient neuronal communication is attributable to fewer long-range tracts or more of a local deficit in neural circuitry. This study examines the integrity of local circuitry by focusing on gamma band activity in auditory cortices of children and adolescents with autism.

METHODS

Ten children and adolescents with autism and 10 matched controls participated. Both groups listened to 500 ms duration monaural click trains with a 25 ms inter-click interval, as magnetoencephalography was acquired from the contralateral hemisphere. To estimate 40 Hz spectral power density, we performed time-frequency decomposition of the single-trial magnetic steady-state response data using complex demodulation.

RESULTS

Children and adolescents with autism exhibited significantly reduced left hemispheric 40 Hz power from 200-500 ms post-stimulus onset. In contrast, no significant between group differences were observed for right hemispheric cortices.

CONCLUSIONS

The production and/or maintenance of left hemispheric gamma oscillations appeared abnormal in participants with autism. We interpret these data as indicating that in autism, particular brain regions may be unable to generate the high-frequency activity likely necessary for binding and other forms of inter-regional interactions. These findings augment connectivity theories of autism with novel evidence that aberrations in local circuitry could underlie putative deficiencies in long-range neural communication.

Corticofugal modulation of acoustically induced Fos expression in the rat auditory pathway

To investigate the corticofugal modulation of acoustic information ascending through the auditory pathway of the rat, immunohistochemical techniques were used to study the functional expression of Fos protein in neurons. With auditory stimulation at different frequencies, Fos expression in the medial geniculate body (MGB), inferior colliculus (IC), superior olivary complex, and cochlear nucleus was examined, and the extent of Fos expression on the two sides was compared. Strikingly, we found densely Fos-labeled neurons in all divisions of the MGB after both presentation of an auditory stimulus and administration of a gamma-aminobutyric acid type A (GABA(A)) antagonist (bicuculline methobromide; BIM) to the auditory cortex. The location of Fos-labeled neurons in the ventral division (MGv) after acoustic stimulation at different frequencies was in agreement with the known tonotopic organization. That no Fos-labeled neurons were found in the MGv with acoustic stimuli alone suggests that the transmission of ascending thalamocortical information is critically governed by corticofugal modulation. The dorsal (DCIC) and external cortices (ECIC) of the IC ipsilateral to the BIM-injected cortex showed a significantly higher number of Fos-labeled neurons than the contralateral IC. However, no difference in the number of Fos-labeled neurons was found between the central nucleus of the IC on either side, indicating that direct corticofugal modulation occurs only in the ECIC and DCIC. Further investigations are needed to assess the functional implications of the morphological differences observed between the descending corticofugal projections to the thalamus and the IC.

Mechanisms underlying the temporal precision of sound coding at the inner hair cell ribbon synapse

Our auditory system is capable of perceiving the azimuthal location of a low frequency sound source with a precision of a few degrees. This requires the auditory system to detect time differences in sound arrival between the two ears down to tens of microseconds. The detection of these interaural time differences relies on network computation by auditory brainstem neurons sharpening the temporal precision of the afferent signals. Nevertheless, the system requires the hair cell synapse to encode sound with the highest possible temporal acuity. In mammals, each auditory nerve fibre receives input from only one inner hair cell (IHC) synapse. Hence, this single synapse determines the temporal precision of the fibre. As if this was not enough of a challenge, the auditory system is also capable of maintaining such high temporal fidelity with acoustic signals that vary greatly in their intensity. Recent research has started to uncover the cellular basis of sound coding. Functional and structural descriptions of synaptic vesicle pools and estimates for the number of Ca(2+) channels at the ribbon synapse have been obtained, as have insights into how the receptor potential couples to the release of synaptic vesicles. Here, we review current concepts about the mechanisms that control the timing of transmitter release in inner hair cells of the cochlea.

Tonality and nonlinear resonance

We outline a theory of tonality that predicts tonal stability, attraction, and categorization based on the principles of nonlinear resonance. Perception of tonality is the natural consequence of neural resonance, arising from central auditory nonlinearities.

Changes in the latency of mouse inferior colliculus neuron responses depending on the position and direction of movement of spectral contrast

Changes in the latency of mouse (Mus musculus) inferior colliculus neuron responses in the presence of wideband sound signals with spectral notches and noise bands with regularly varying central notch/band frequencies were studied. Relationships between the latency and the magnitude of the response on the one hand and the central notch/band frequency on the other were obtained (latency and spike count functions). Crossing of the margins of the excitatory areas of the responses of the frequency receptive fields of neurons by spectral notches/noise bands could lead to displacement of latency functions (and corresponding displacements in spike count functions). Direction-dependent shifts in latency and spike count functions were more characteristic of primary-like and V-shaped neurons. The most interesting feature of the directional sensitivity of inhibition-dependent neurons was the selective decrease in the latency and selective synchronization of the initial spike response (with a corresponding increase in the spike count). The dynamic properties of inhibition-dependent neurons can be explained on the basis of their selective sensitivity to the position of the spectral contrast in the frequency receptive field, which is associated with disinhibition, and by the nature of the distribution of the excitatory and inhibitory inputs. The extents of these effects depended on the spectral shape of the signals and the widths of the spectral notches.

Differential effects of iontophoretic in vivo application of the GABA(A)-antagonists bicuculline and gabazine in sensory cortex

We have compared the effects of microiontophoretic application of the GABA(A)-receptor antagonists bicuculline (BIC) and gabazine (SR95531) on responses to pure tones and to sinusoidally amplitude-modulated (AM) tones in cells recorded extracellularly from primary auditory cortex (AI) of Mongolian gerbils. Besides similar effects in increasing spontaneous and stimulus-evoked activity and their duration, both drugs elicited differential effects on spectral tuning and synchronized responses to AM tones. In contrast to gabazine, iontophoresis of the less potent GABA(A)-antagonist BIC often resulted in substantial broadening of frequency tuning for pure tones and an elimination of synchronized responses to AM tones, particularly with high ejecting currents. BIC-induced effects which could not be replicated by application of gabazine were presumably due to the well-documented, non-GABAergic side-effects of BIC on calcium-dependent potassium channels. Our results thus provide strong evidence that GABA(A)-mediated inhibition in AI does not sharpen frequency tuning for pure tones, but rather contributes to the processing of fast temporal modulations of sound envelopes. They also demonstrate that BIC can have effects on neuronal response selectivity which are not due to blockade of GABAergic inhibition. The results have profound implications for microiontophoretic studies of the role of intracortical inhibition in sensory cortex.

Enhancement of Spike-Timing Precision by Autaptic Transmission in Neocortical Inhibitory Interneurons

In vivo studies suggest that precise firing of neurons is important for correct sensory representation. Principal neocortical neurons fire imprecisely when repeatedly activated by fixed sensory stimuli or current depolarizations. Here we show that in contrast to pyramidal neurons, firing in neocortical GABAergic fast-spiking (FS) interneurons is quite precise. FS interneurons are self-innervated by powerful GABAergic autaptic connections reliably activated after each spike, suggesting that autapses strongly regulate FS-cell spike timing. Indeed, blockade of autaptic transmission degraded temporal precision in multiple ways. Under these conditions, realistic dynamic-clamp hyperpolarizing autapses restored precision of spike timing, even in the presence of synaptic noise. Furthermore, firing precision was increased in pyramidal neurons by artificial GABAergic autaptic conductances, suggesting that tightly coupled synaptic feedback inhibition regulates spike timing in principal cells. Thus, well-timed inhibition, whether autaptic or synaptic, facilitates precise spike timing and promotes synchronized cortical network oscillations relevant to several behaviors.

Interaural delay-dependent changes in the binaural interaction component of the guinea pig brainstem responses

Auditory brainstem responses to monaural and binaural clicks with 23 different interaural time differences (ITDs) were recorded from ten guinea pigs without anesthesia. Binaural interaction component was obtained by subtracting the sum of the appropriately time-shifted left and right monaural responses from the binaural one. With increasing ITD, the most prominent peak of the binaural difference potential so obtained shifted to longer latencies and its amplitude gradually decreased. The way these changes depended on binaural delay was basically similar to that previously observed in a cat study [P. Ungan, S. Yagcioglu, B. Ozmen. Interaural delay-dependent changes in the binaural difference potential in cat auditory brainstem response: implications about the origin of the binaural interaction component. Hear. Res. 106 (1997) 66-82]. The data were successfully simulated by the model suggested in that report. We therefore concluded that the same model, which was based on the difference between the mean onset latencies of the ipsilateral excitation and contralateral inhibition in a typical neuron in the lateral superior olive, their standard deviations, and the duration of the contralateral inhibition, should also be valid for the binaural interaction in the guinea pig brainstem. The results, which were discussed in connection with sound lateralization models, supported a model based on population coding, where the lateral position of a sound source is coded by the ratio of the discharge intensity in the left and right lateral superior olives, rather than the models based on coincidence detection.

Spike-timing precision underlies the coding efficiency of auditory receptor neurons

Sensory systems must translate incoming signals quickly and reliably so that an animal can act successfully in its environment. Even at the level of receptor neurons, however, functional aspects of the sensory encoding process are not yet fully understood. Specifically, this concerns the question how stimulus features and neural response characteristics lead to an efficient transmission of sensory information. To address this issue, we have recorded and analyzed spike trains from grasshopper auditory receptors, while systematically varying the stimulus statistics. The stimulus variations profoundly influenced the efficiency of neural encoding. This influence was largely attributable to the presence of specific stimulus features that triggered remarkably precise spikes whose trial-to-trial timing variability was as low as 0.15 ms--one order of magnitude shorter than typical stimulus time scales. Precise spikes decreased the noise entropy of the spike trains, thereby increasing the rate of information transmission. In contrast, the total spike train entropy, which quantifies the variety of different spike train patterns, hardly changed when stimulus conditions were altered, as long as the neural firing rate remained the same. This finding shows that stimulus distributions that were transmitted with high information rates did not invoke additional response patterns, but instead displayed exceptional temporal precision in their neural representation. The acoustic stimuli that led to the highest information rates and smallest spike-time jitter feature pronounced sound-pressure deflections lasting for 2-3 ms. These upstrokes are reminiscent of salient structures found in natural grasshopper communication signals, suggesting that precise spikes selectively encode particularly important aspects of the natural stimulus environment.

Sound lateralization in Parkinson's disease

The symptoms primarily associated with Parkinson's disease (PD) are of a motor and cognitive nature, but sensory deficits may also be involved. Previous studies have reported disturbed spatial perception in visual and tactile tasks. We have investigated whether PD patients show deficits in auditory spatial perception. For this purpose, we employed a simple task involving left/right judgments about dichotic stimuli presented with various interaural time differences (ITD). The acuity of sound lateralization was significantly reduced in PD: the just noticeable difference (JND) in interaural time seen in PD patients was about twice that seen for age-matched healthy controls. We propose that this deficit may be related to a potential role of the basal ganglia in spatial hearing functions, as has been suggested by neurophysiological and neuroanatomical studies on animals.

Disentangling sub-millisecond processes within an auditory transduction chain

Every sensation begins with the conversion of a sensory stimulus into the response of a receptor neuron. Typically, this involves a sequence of multiple biophysical processes that cannot all be monitored directly. In this work, we present an approach that is based on analyzing different stimuli that cause the same final output, here defined as the probability of the receptor neuron to fire a single action potential. Comparing such iso-response stimuli within the framework of nonlinear cascade models allows us to extract the characteristics of individual signal-processing steps with a temporal resolution much finer than the trial-to-trial variability of the measured output spike times. Applied to insect auditory receptor cells, the technique reveals the sub-millisecond dynamics of the eardrum vibration and of the electrical potential and yields a quantitative four-step cascade model. The model accounts for the tuning properties of this class of neurons and explains their high temporal resolution under natural stimulation. Owing to its simplicity and generality, the presented method is readily applicable to other nonlinear cascades and a large variety of signal-processing systems.

Comparing auditory stimuli that give the same neural response within the framework of a computational model, the authors extract intermediary signal-processing steps with sub- millisecond temporal resolution

Reliability and precision of neural spike timing: simulation of spectrally broadband synaptic inputs

Spectrally broadband stimulation of neurons has been an effective method for studying their dynamic responses to simulated synaptic inputs. Previous studies with such stimulation were mostly based upon the direct intracellular injection of noisy current waveforms. In the present study we analyze and compare the firing output of various identified molluscan neurons to aperiodic, broadband current signals using three types of stimulus paradigms: 1. direct injection in current clamp mode, 2. conductance injection using electrotonic coupling of the input waveform to the neuron, and 3. conductance injection using a simulated chemical excitatory connection. The current waveforms were presented in 15 successive trials and the trial-to-trial variations of the spike responses were analyzed using peri-stimulus spike density functions. Comparing the responses of the neurons to the same type of input waveforms, we found that conductance injection resulted in more reliable and precise spike responses than direct current injection. The statistical parameters of the response spike trains depended on the spectral distribution of the input. The reliability increased with increasing cutoff frequency, while the temporal jitter of spikes changed in the opposite direction. Neurons with endogenous bursting displayed lower reproducibility in their responses to noisy waveforms when injected directly; however, they fired far more reliably and precisely when receiving the same waveforms as conductance inputs. The results show that molluscan neurons are capable of accurately reproducing their responses to synaptic inputs. Conductance injection provides an enhanced experimental technique for assessing the neurons' spike timing reliability and it should be preferred over direct current injection of noisy waveforms.

Real-time visualization of neural synchrony for identifying coordinated cell assemblies

We introduce a synchrony map that translates the fine temporal organization of multi-unit responses in the visual cortex into an easily interpreted spatial display. We test the synchrony map on microelectrode array recordings in Area 17 of anesthetized and paralyzed cats. We first examine the synchrony map using averaged data and probability calculations to demonstrate orientation-dependent changes in synchrony. We then demonstrate how the synchrony map can be implemented for real-time visualization of synchrony among neural assemblies.

Variability of spike trains and the processing of temporal patterns of acoustic signals-problems, constraints, and solutions

Object recognition and classification by sensory pathways is rooted in spike trains provided by sensory neurons. Nervous systems had to evolve mechanisms to extract information about relevant object properties, and to separate these from spurious features. In this review, problems caused by spike train variability and counterstrategies are exemplified for the processing of acoustic signals in orthopteran insects. Due to size limitations of their nervous system we expect to find solutions that are stripped to the computational basics. A key feature of auditory systems is temporal resolution, which is likely limited by spike train variability. Basic strategies to reduce such variability are to integrate over time, or to average across several neurons. The first strategy is constrained by its possible interference with temporal resolution. Grasshoppers do not seem to explore temporal integration much, in spite of the repetitive structure of their songs, which invites for 'multiple looks' at the signal. The benefits of averaging across neurons depend on uncorrelated responses, a factor that may be crucial for the performance and evolution of small nervous systems. In spite of spike train variability the temporal information necessary for the recognition of conspecifics is preserved to a remarkable degree in the auditory pathway.

Disruption in the inhibitory architecture of the cell minicolumn: implications for autism

The modular arrangement of the neocortex is based on the cell minicolumn: a self-contained ecosystem of neurons and their afferent, efferent, and interneuronal connections. The authors' preliminary studies indicate that minicolumns in the brains of autistic patients are narrower, with an altered internal organization. More specifically, their minicolumns reveal less peripheral neuropil space and increased spacing among their constituent cells. The peripheral neuropil space of the minicolumn is the conduit, among other things, for inhibitory local circuit projections. A defect in these GABAergic fibers may correlate with the increased prevalence of seizures among autistic patients. This article expands on our initial findings by arguing for the specificity of GABAergic inhibition in the neocortex as being focused around its mini- and macrocolumnar organization. The authors conclude that GABAergic interneurons are vital to proper minicolumnar differentiation and signal processing (e.g., filtering capacity of the neocortex), thus providing a putative correlate to autistic symptomatology.

Differential arithmetic of shunting inhibition for voltage and spike rate in neocortical pyramidal cells

The main inhibitory neurotransmitter in the mammalian forebrain is gamma-amino butyric acid (GABA), which acts through A and B type receptors. GABAA receptors mediate inhibition via an increase in membrane conductance (shunting) and/or membrane potential hyperpolarization. Shunting inhibition is thought to decrease the gain between neural input and output, and thus to act as a divisor, but may do so only below the spike threshold. To investigate the role of shunting inhibition in neocortical neurons, whole-cell patch-clamp recordings were obtained from layer V pyramidal cells of somatosensory cortex in juvenile rats. Sub- and suprathreshold voltage responses were elicited by somatic step current injections and a shunting conductance was generated via a dynamic clamp. Increasing the dynamic clamp shunting conductance led to a parallel shift of the current-discharge curves and a reduced slope of the current-voltage relationship, i.e. a decrease of neural gain. Selective activation of GABAAA receptors with the competitive agonist isoguvacine or rises of endogenous GABA with the specific reuptake blocker nipecotic acid led to a proportional decrease of subthreshold membrane voltage, but a constant offset of discharge rates, thus acting like a shunting conductance. Similarly, isoguvacine and nipecotic acid decreased the gain of excitatory postsynaptic potentials. In all three experimental conditions, shunting inhibition divisively affected subthreshold voltages, while the time-averaged suprathreshold membrane potential was offset by a constant amount. I conclude that shunting inhibition in pyramidal cells has a dual impact on neural output: it is divisive for subthreshold voltages but subtractive for spike frequencies.

From another angle: Differences in cortical coding between fine and coarse discrimination of orientation

We measured the information available for orientation discrimination from metric distances for 24 cells in area 17 of cats that were paralyzed and anesthetized with Propofol and N(2)O. The metric distance information confirms fundamental coding differences for discrimination between fine (<10 degrees ) and coarse (>10 degrees ) orientation differences. The information for discriminating larger orientation differences is contained mainly in the firing rate, with minor enhancements from the coarse (30-70 ms) temporal structure in the firing rate. Both precise spike timing (9.2 ms) and intervals (6.8 ms) sustained over the stimulus presentation provide information for fine discrimination of orientation, where almost no reliable information is provided by the spike count. We compare and confirm the results (using the same data set) to vector distances based on classification theory. The results support a dynamic spiking mechanism where coordinated activity could provide fast and reliable information about detailed angle and/or direction information in the region of the preferred orientation.

Novel schemes for hearing and orientation in insects

Severe size constraints are imposed on the hearing organs of insects, yet they perform sophisticated tasks of auditory processing. Recent research has shown how flies acoustically locate targets in space, how mosquitoes afford highly sensitive ears, and how crickets avoid deafening themselves with their songs. These findings unveil the exquisite analytical capabilities of highly specialized microscale auditory systems.

Responses of inferior colliculus neurons to harmonic and mistuned complex tones

Responses of inferior colliculus neurons to simplified stimuli that may engage mechanisms that contribute to auditory scene analysis were obtained. The stimuli were harmonic complex tones, which are heard by human listeners as single sounds, and the same tones with one component 'mistuned', which are heard as two separate sounds. The temporal discharge pattern elicited by a harmonic complex tone usually resembled the same neuron's response to a pure tone. In contrast, tones with a mistuned component elicited responses with distinctive, stereotypical temporal patterns that were not obviously related to the stimulus waveform. For a particular stimulus configuration, the discharge pattern was similar across neurons with different pure-tone frequency selectivity. A computational model that compared response envelopes across multiple narrow bands successfully reproduced the stereotypical response patterns elicited by different stimulus configurations. The results suggest that mistuning created a temporally synchronous distributed representation of the mistuned component that could be identified by higher auditory centers in the presence of the ongoing response produced by the remaining components; this kind of representation might facilitate the identification of individual sound sources in complex acoustic environments.

Evolution and development of time coding systems

Precise temporal coding is a hallmark of both the electrosensory and auditory systems. Selective pressures to improve accuracy or encode more rapid changes have produced a suite of convergent physiological and morphological features that contribute to temporal coding. Comparative studies of temporal coding can also point to shared computational strategies, and suggest how selection might act to improve coding.

Expression of GABA(A) receptor subunits in rat brainstem auditory pathways: cochlear nuclei, superior olivary complex and nucleus of the lateral lemniscus

Inhibition by GABA is important for auditory processing, but any adaptations of the ionotropic type A receptors are unknown. Here we describe, using in situ hybridization, the subunit expression patterns of GABA(A) receptors in the rat cochlear nucleus, superior olivary complex, and dorsal and ventral nuclei of the lateral lemniscus. All neurons express the beta3 and gamma2L subunit messenger RNAs, but use different alpha subunits. In the dorsal cochlear nucleus, fusiform (pyramidal) and giant cells express alpha1, alpha3, beta3 and gamma2L. Dorsal cochlear nucleus interneurons, particularly vertical or tuberculoventral cells and cartwheel cells, express alpha3, beta3 and gamma2L. In the ventral cochlear nucleus, octopus cells express alpha1, beta3, gamma2L and delta. Spherical cells express alpha1, alpha3, alpha5, beta3 and gamma2L. In the superior olivary complex, the expression profile is alpha3, alpha5, beta3 and gamma2L. Both dorsal and ventral cochlear nucleus granule cells express alpha1, alpha6, beta3 and gamma2L; unlike their cerebellar granule cell counterparts, they do not express beta2, gamma2S or the delta subunit genes. The delta subunit's absence from cochlear nucleus granule cells may mean that tonic inhibition mediated by extrasynaptic GABA(A) receptors is less important for this cell type. In both the dorsal and ventral nuclei of the lateral lemniscus, alpha1, beta3 and gamma2L are the main subunit messenger RNAs; the ventral nucleus also expresses the delta subunit. We have mapped, using in situ hybridization, the subunit expression patterns of the GABA(A) receptor in the auditory brainstem nuclei. In contrast to many brain regions, the beta2 subunit gene and gamma2S splice forms are not highly expressed in auditory brainstem nuclei. GABA(A) receptors containing beta3 and gamma2L may be particularly well suited to auditory processing, possibly because of the unique phosphorylation profile of this subunit combination.