Response Properties Of Cortical Neurons Research Articles

Neural processing in the primary auditory cortex following cholinergic lesions of the basal forebrain in ferrets

Cortical acetylcholine (ACh) release has been linked to various cognitive functions, including perceptual learning. We have previously shown that cortical cholinergic innervation is necessary for accurate sound localization in ferrets, as well as for their ability to adapt with training to altered spatial cues. To explore whether these behavioral deficits are associated with changes in the response properties of cortical neurons, we recorded neural activity in the primary auditory cortex (A1) of anesthetized ferrets in which cholinergic inputs had been reduced by making bilateral injections of the immunotoxin ME20.4-SAP in the nucleus basalis (NB) prior to training the animals. The pattern of spontaneous activity of A1 units recorded in the ferrets with cholinergic lesions (NB ACh–) was similar to that in controls, although the proportion of burst-type units was significantly lower. Depletion of ACh also resulted in more synchronous activity in A1. No changes in thresholds, frequency tuning or in the distribution of characteristic frequencies were found in these animals. When tested with normal acoustic inputs, the spatial sensitivity of A1 neurons in the NB ACh– ferrets and the distribution of their preferred interaural level differences also closely resembled those found in control animals, indicating that these properties had not been altered by sound localization training with one ear occluded. Simulating the animals’ previous experience with a virtual earplug in one ear reduced the contralateral preference of A1 units in both groups, but caused azimuth sensitivity to change in slightly different ways, which may reflect the modest adaptation observed in the NB ACh– group. These results show that while ACh is required for behavioral adaptation to altered spatial cues, it is not required for maintenance of the spectral and spatial response properties of A1 neurons.

The dynamical response properties of cortical neurons

Event Abstract Back to Event The dynamical response properties of cortical neurons Daniele Linaro1, Istvan Biro1 and Michele Giugliano1* 1 Universiteit Antwerpen, Department of Biomedical Sciences, Belgium Cortical neurons are often classified by current-frequency relationship. Such a static description is inadequate to interpret neuronal responses to time-varying stimuli. Theoretical studies suggested that single-cell dynamical response properties are necessary to interpret ensemble responses to fast input transients. Further, it was shown that input-noise linearizes and boosts the response bandwidth, and that the interplay between the barrage of noisy synaptic currents and the spike-initiation mechanisms determine the dynamical properties of the firing rate. To test these model predictions, we estimated the linear response properties of layer 5 pyramidal cells by injecting a superposition of a small-amplitude sinusoidal wave and a background noise. We characterized the evoked firing probability across many stimulation trials and a range of oscillation frequencies (1-1000 Hz), quantifying response amplitude and phase-shift while changing noise statistics. We found that neurons track unexpectedly fast transients, as their response amplitude has no attenuation up to 200 Hz. This cut-off frequency is higher than the limits set by passive membrane properties (approximately 50 Hz) and average firing rate (approximately 20 Hz) and is not affected by the rate of change of the input. Finally, above 200 Hz, the response amplitude decays as a power-law with an exponent that is independent of voltage fluctuations induced by the background noise. Acknowledgements Financial support from the University of Antwerp, the Flanders Research Foundation (12C9112N), and the European Commission (FP7-FETOPEN project "BRAINLEAP", contract m. 306502) is kindly acknowledged. References Köndgen, H., Geisler, C., Fusi, S. , Lüscher, H.-R. , Wang, X.J. and Giugliano, M. (2008) The dynamical response properties of neocortical neurons to temporally modulated noisy inputs in vitro. Cerebral Cortex 18(9):2086-97. Linaro D, Couto J, Giugliano M (2014) Command-line cellular electrophysiology for conventional and real-time closed-loop experiments. J Neurosci Methods. 2014 Jun 15;230:5-19. doi: 10.1016/j.jneumeth.2014.04.003. Robinson HPC, (2009), “Synaptic conductances and spike generation in cortical cells.” In: Dynamic clamp: from Principles to Applications, Springer. Testa-Silva G, Verhoog MB, Linaro D, de Kock CP, Baayen JC, Meredith RM, De Zeeuw CI, Giugliano M, Mansvelder HD (2014) High bandwidth synaptic communication and frequency tracking in human neocortex. PLoS Biol. 2014 Nov 25;12(11):e1002007. doi: 10.1371/journal.pbio.1002007. Keywords: dynamic-clamp, human cortical neurons, correlations, Noise, action potential initation Conference: Spike Initiation: Models & Experiments, Prague, Czechia, 22 Jul - 22 Jul, 2015. Presentation Type: Oral Presentation Topic: Neuroscience Citation: Linaro D, Biro I and Giugliano M (2015). The dynamical response properties of cortical neurons. Front. Neurosci. Conference Abstract: Spike Initiation: Models & Experiments. doi: 10.3389/conf.fnins.2015.90.00002 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 30 Jun 2015; Published Online: 06 Jul 2015. * Correspondence: Prof. Michele Giugliano, Universiteit Antwerpen, Department of Biomedical Sciences, Wilrijk, Flanders, 2610, Belgium, michele.giugliano@sissa.it Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Daniele Linaro Istvan Biro Michele Giugliano Google Daniele Linaro Istvan Biro Michele Giugliano Google Scholar Daniele Linaro Istvan Biro Michele Giugliano PubMed Daniele Linaro Istvan Biro Michele Giugliano Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Curtailing effect of awakening on visual responses of cortical neurons by cholinergic activation of inhibitory circuits.

Visual responsiveness of cortical neurons changes depending on the brain state. Neural circuit mechanism underlying this change is unclear. By applying the method of in vivo two-photon functional calcium imaging to transgenic rats in which GABAergic neurons express fluorescent protein, we analyzed changes in visual response properties of cortical neurons when animals became awakened from anesthesia. In the awake state, the magnitude and reliability of visual responses of GABAergic neurons increased whereas the decay of responses of excitatory neurons became faster. To test whether the basal forebrain (BF) cholinergic projection is involved in these changes, we analyzed effects of electrical and optogenetic activation of BF on visual responses of mouse cortical neurons with in vivo imaging and whole-cell recordings. Electrical BF stimulation in anesthetized animals induced the same direction of changes in visual responses of both groups of neurons as awakening. Optogenetic activation increased the frequency of visually evoked action potentials in GABAergic neurons but induced the delayed hyperpolarization that ceased the late generation of action potentials in excitatory neurons. Pharmacological analysis in slice preparations revealed that photoactivation-induced depolarization of layer 1 GABAergic neurons was blocked by a nicotinic receptor antagonist, whereas non-fast-spiking layer 2/3 GABAergic neurons was blocked only by the application of both nicotinic and muscarinic receptor antagonists. These results suggest that the effect of awakening is mediated mainly through nicotinic activation of layer 1 GABAergic neurons and mixed nicotinic/muscarinic activation of layer 2/3 non-fast-spiking GABAergic neurons, which together curtails the visual responses of excitatory neurons.

Unsupervised Learning of Generative and Discriminative Weights Encoding Elementary Image Components in a Predictive Coding Model of Cortical Function

A method is presented for learning the reciprocal feedforward and feedback connections required by the predictive coding model of cortical function. When this method is used, feedforward and feedback connections are learned simultaneously and independently in a biologically plausible manner. The performance of the proposed algorithm is evaluated by applying it to learning the elementary components of artificial and natural images. For artificial images, the bars problem is employed, and the proposed algorithm is shown to produce state-of-the-art performance on this task. For natural images, components resembling Gabor functions are learned in the first processing stage, and neurons responsive to corners are learned in the second processing stage. The properties of these learned representations are in good agreement with neurophysiological data from V1 and V2. The proposed algorithm demonstrates for the first time that a single computational theory can explain the formation of cortical RFs and also the response properties of cortical neurons once those RFs have been learned.

Preceding Inhibition Silences Layer 6 Neurons in Auditory Cortex

A canonical feedforward circuit is proposed to underlie sensory cortical responses with balanced excitation and inhibition in layer 4 (L4). However, in another input layer, L6, sensory responses and the underlying synaptic circuits remain largely unclear. Here, cell-attached recordings in rat primary auditory cortex revealed that for the majority of L6 excitatory neurons, tonal stimuli did not drive spike responses, but suppressed spontaneous firings. Whole-cell recordings further revealed that the silencing resulted from tone-evoked strong inhibition arriving earlier than excitation. This pattern of inputs can be attributed to a parallel feedforward circuit with both excitatory and inhibitory inputs disynaptically relayed. In contrast, in the other neurons directly driven by thalamic input, stimuli evoked excitation preceding relatively weak inhibition, resulting in robust spike responses. Thus, the dichotomy of L6 response properties arises from two distinct patterns of excitatory-inhibitory interplay. The parallel circuit module generating preceding inhibition may provide a gating mechanism for conditional corticothalamic feedback.

Investigating the effect of Cortical Discharge Variability on the accuracy of population decoders

Estimation of the response properties of cortical neurons from within a recorded population is an essential component in a cortically-controlled brain machine interface application. The response properties of interest typically include precise spike timing, mean firing rate and any inherent correlation in the activity of the recorded ensemble. These response properties are essential for the operation of a neural decoder that translates the observed cortical activity to command signals for controlling robotic arms. In this work we investigate the effect of cortical discharge variability on the decoding performance. Specifically cortical discharge variability was induced in two types of cortical network models. The first one is a probabilistic model in which the activity was modeled as an inhomogeneous Poisson process with firing probability that depends on the neuron's own firing history and those of other neurons connected to it through time-varying synaptic couplings. The second is a biophysical leaky integrate and fire model. Neurons in both models were cosine-tuned to movement direction with random tuning widths.

The role of thalamic inputs in surround receptive fields of barrel neurons.

Controversy exists regarding the relative roles of thalamic versus intracortical inputs in shaping the response properties of cortical neurons. In the whisker-barrel system, this controversy centers on the mechanisms determining the receptive fields of layer IV (barrel) neurons. Whereas principal whisker-evoked responses are determined by thalamic inputs, the mechanisms responsible for adjacent whisker (AW) responses are in dispute. Here, we took advantage of the fact that lesions of the spinal trigeminal nucleus interpolaris (SpVi) significantly reduce the receptive field size of neurons in the ventroposterior thalamus. We reasoned that if AW responses are established by these thalamic inputs, brainstem lesions would significantly reduce the receptive field sizes of barrel neurons. We obtained extracellular single unit recordings from barrel neurons in response to whisker deflections from control rats and from rats that sustained SpVi lesions. After SpVi lesions, the receptive field of both excitatory and inhibitory barrel neurons decreased significantly in size, whereas offset/onset response ratios increased. Response magnitude decreased only for inhibitory neurons. All of these findings are consistent with the hypothesis that AW responses are determined primarily by direct thalamic inputs and not by intracortical interactions.

Spatial organization of frequency response areas and rate/level functions in the developing AI.

The current study was conducted to extend our understanding of changes in spatial organization and response properties of cortical neurons in the developing mammalian forebrain. Extracellular multiunit responses to tones were recorded from a dense array of penetrations covering entire isofrequency contours in the primary auditory cortex (AI) of pentobarbital anesthetized kittens. Ages ranged from postnatal day 14 (P14), shortly after acquisition of normal auditory response thresholds, through postnatal day 111 (P111), when the kittens were largely mature. Spatial organization of the AI was tonotopically ordered by P14. The tonotopic gradient decreased with chronological maturation. At P14 the gradient was about 3.5 kHz/mm. By P111 it had declined to about 2.5 kHz/mm, so that the cortical region encompassing a fixed 3- to 15-kHz frequency range enlarged along its posterior-anterior dimension. Response properties of developing AI neurons changed in both frequency selectivity and intensity selectivity. The mean frequency tuning bandwidth increased with age. Initially, tuning bandwidths were narrow throughout the entire AI. With progressive maturation, broader bandwidths were observed in areas dorsal and ventral to a central region in which neurons remained narrowly tuned. The resulting spatial organization of tuning bandwidth was similar to that reported in adult cats. The majority of recording sites manifested nonmonotonic rate/level functions at all ages. However, the proportion of sites with monotonic rate/level functions increased with age. No spatial organization of rate/level functions (monotonic and nonmonotonic) was observed through P111. The relatively late development of bandwidth tuning in the AI compared with the early presence of tonotopic organization suggests that different developmental processes are responsible for structuring these two dimensions of acoustic selectivity.

A 14-day period of hindpaw sensory deprivation enhances the responsiveness of rat cortical neurons

Hypodynamia-hypokinesia (HH) is a model of hindpaw sensory deprivation. It is obtained by unloading of the hindquarters during 14 days. In this situation, the feet are not in contact with the ground and as a consequence, the cutaneous receptors are not activated; the sensory input to the primary somatosensory cortex (SmI) is thus reduced. In a previous study, we have shown that HH induced a cortical reorganisation of the hindlimb representation. The understanding of the mechanisms involved in cortical map plasticity requires a close examination of the changes in response properties of cortical neurons during HH. The aim of the present study was thus to study the characteristics of neurons recorded from granular and infragranular layers in hindlimb representation of SmI. A total of 289 cortical neurons were recorded (158 from control rats and 131 from HH rats) in pentobarbital-anaesthetized rats. Cutaneous threshold, cutaneous receptive fields, spontaneous activity (discharge rate and instantaneous frequency) and activity evoked by air-jet stimulation (response latency and duration, amplitude) were analysed. The present study suggests that activity-dependent changes occur in the cortex. The duration of the spike waveform presented two populations of spikes: thin-spike cells (<1 ms, supposed to be inhibitory interneurons) and regular cells (>1 ms). Thin-spike cells were less frequently encountered in HH than in control rats. The analysis of regular cells revealed that after HH (1) spontaneous activity was unchanged and (2) cortical somatosensory neurons were more responsive: the cutaneous threshold was reduced and the response magnitude increased. Taken together, these results suggest a down-regulation of GABAergic function.

Sensory input directs spatial and temporal plasticity in primary auditory cortex.

The cortical representation of the sensory environment is continuously modified by experience. Changes in spatial (receptive field) and temporal response properties of cortical neurons underlie many forms of natural learning. The scale and direction of these changes appear to be determined by specific features of the behavioral tasks that evoke cortical plasticity. The neural mechanisms responsible for this differential plasticity remain unclear partly because important sensory and cognitive parameters differ among these tasks. In this report, we demonstrate that differential sensory experience directs differential plasticity using a single paradigm that eliminates the task-specific variables that have confounded direct comparison of previous studies. Electrical activation of the basal forebrain (BF) was used to gate cortical plasticity mechanisms. The auditory stimulus paired with BF stimulation was systematically varied to determine how several basic features of the sensory input direct plasticity in primary auditory cortex (A1) of adult rats. The distributed cortical response was reconstructed from a dense sampling of A1 neurons after 4 wk of BF-sound pairing. We have previously used this method to show that when a tone is paired with BF activation, the region of the cortical map responding to that tone frequency is specifically expanded. In this report, we demonstrate that receptive-field size is determined by features of the stimulus paired with BF activation. Specifically, receptive fields were narrowed or broadened as a systematic function of both carrier-frequency variability and the temporal modulation rate of paired acoustic stimuli. For example, the mean bandwidth of A1 neurons was increased (+60%) after pairing BF stimulation with a rapid train of tones and decreased (-25%) after pairing unmodulated tones of different frequencies. These effects are consistent with previous reports of receptive-field plasticity evoked by natural learning. The maximum cortical following rate and minimum response latency were also modified as a function of stimulus modulation rate and carrier-frequency variability. The cortical response to a rapid train of tones was nearly doubled if BF stimulation was paired with rapid trains of random carrier frequency, while no following rate plasticity was observed if a single carrier frequency was used. Finally, we observed significant increases in response strength and total area of functionally defined A1 following BF activation paired with certain classes of stimuli and not others. These results indicate that the degree and direction of cortical plasticity of temporal and receptive-field selectivity are specified by the structure and schedule of inputs that co-occur with basal forebrain activation and suggest that the rules of cortical plasticity do not operate on each elemental stimulus feature independently of others.

Spectral Features Control Temporal Plasticity in Auditory Cortex

Cortical responses are adjusted and optimized throughout life to meet changing behavioral demands and to compensate for peripheral damage. The cholinergic nucleus basalis (NB) gates cortical plasticity and focuses learning on behaviorally meaningful stimuli. By systematically varying the acoustic parameters of the sound paired with NB activation, we have previously shown that tone frequency and amplitude modulation rate alter the topography and selectivity of frequency tuning in primary auditory cortex. This result suggests that network-level rules operate in the cortex to guide reorganization based on specific features of the sensory input associated with NB activity. This report summarizes recent evidence that temporal response properties of cortical neurons are influenced by the spectral characteristics of sounds associated with cholinergic modulation. For example, repeated pairing of a spectrally complex (ripple) stimulus decreased the minimum response latency for the ripple, but lengthened the minimum latency for tones. Pairing a rapid train of tones with NB activation only increased the maximum following rate of cortical neurons when the carrier frequency of each train was randomly varied. These results suggest that spectral and temporal parameters of acoustic experiences interact to shape spectrotemporal selectivity in the cortex. Additional experiments with more complex stimuli are needed to clarify how the cortex learns natural sounds such as speech.

Bicuculline application affects discharge patterns, rate–intensity functions, and frequency tuning characteristics of bat auditory cortical neurons

This study examined the effect of bicuculline application on the auditory response properties in the auditory cortex of the big brown bat, Eptesicus fuscus. All auditory cortical neurons studied discharged either 1–2 or 3–7 impulses to 4 ms sound stimuli. Cortical neurons with high best frequencies tended to have high minimum thresholds. Bicuculline application increased the number of impulses and shortened the response latencies of all cortical neurons as well as changing the discharge patterns of half of the cortical neurons studied. Bicuculline application raised the rate–intensity functions but lowered the latency–intensity functions to varying degrees. Threshold–frequency tuning curves (FTCs) were either V-shaped, upper threshold or double-peaked. Threshold–FTCs and impulse–FTCs were mirror-images of each other. Bicuculline application expanded and raised the impulse–FTCs but lowered the threshold–FTCs, resulting in significantly decreased Q n values. Threshold–FTCs of cortical neurons determined within an orthogonally inserted electrode were very similar and expanded FTCs during bicuculline application were also very similar. Possible mechanisms for the contribution of GABAergic inhibition to shaping these response properties of cortical neurons are discussed.

A functional angle on some after-effects in cortical vision.

The question of how our brains and those of other animals code sensory information is of fundamental importance to neuroscience research. Visual illusions offer valuable insight into the mechanisms of perceptual coding. One such illusion, the tilt after-effect (TAE), has been studied extensively since the 1930s, yet a full explanation of the effect has remained elusive. Here, we put forward an explanation of the TAE in terms of a functional role for adaptation in the visual cortex. The proposed model accounts not only for the phenomenology of the TAE, but also for spatial interactions in perceived tilt and the effects of adaptation on the perception of direction of motion and colour. We discuss the implications of the model for understanding the effects of adaptation and surround stimulation on the response properties of cortical neurons.

Rapid Regulation of Brain-Derived Neurotrophic Factor mRNA within Eye-Specific Circuits during Ocular Dominance Column Formation

The neurotrophin brain-derived neurotrophic factor (BDNF) has emerged as a candidate retrograde signaling molecule for geniculocortical axons during the formation of ocular dominance columns. Here we examined whether neuronal activity can regulate BDNF mRNA in eye-specific circuits in the developing cat visual system. Dark-rearing throughout the critical period for ocular dominance column formation decreases levels of BDNF mRNA within primary visual cortex, whereas short-term (2 d) binocular blockade of retinal activity with tetrodotoxin (TTX) downregulates BDNF mRNA within the lateral geniculate nucleus (LGN) and visual cortical areas. Brief (6 hr to 2 d) monocular TTX blockade during the critical period and also in adulthood causes downregulation in appropriate eye-specific laminae in the LGN and ocular dominance columns within primary visual cortex. Monocular TTX blockade at postnatal day 23 also downregulates BDNF mRNA in a periodic fashion, consistent with recent observations that ocular dominance columns can be detected at these early ages by physiological methods. In contrast, 10 d monocular TTX during the critical period does not cause a lasting decrease in BDNF mRNA expression in columns pertaining to the treated eye, consistent with the nearly complete shift in physiological response properties of cortical neurons in favor of the unmanipulated eye known to result from long-term monocular deprivation. These observations demonstrate that BDNF mRNA levels can provide an accurate "molecular readout" of the activity levels of cortical neurons and are consistent with a highly local action of BDNF in strengthening and maintaining active synapses during ocular dominance column formation.

Acute reorganization of the forepaw representation in the rat SI cortex after focal cortical injury: neuroprotective effects of piracetam treatment.

Immediate postlesion reorganization of the somatosensory cortical representation was examined in adult rats. Response properties of small clusters of neurons were recorded in the area of the primary somatosensory cortex (SI) devoted to the contralateral forepaw representation. Electrophysiological maps were elaborated on the basis of the sensory 'submodality' (cutaneous or noncutaneous) and the location of the peripheral receptive fields (RFs) of layer IV neurons. Recordings were made prior to, and from 1 to 12 h after, induction of a focal neurovascular lesion to the SI cortex that initially destroyed a part (8.5%) of the cutaneous representation. Moreover, the influence of an anti-ischaemic substance (piracetam) on lesion-induced changes was analysed. The main observations were: (i) a gradual outward expansion of the area of the functional lesion, which was smaller in the piracetam-treated (PT) rats than in the control, placebo-treated (PL) rats; (ii) a substantial remodelling of the spared representational zones, both in cortical sectors adjoining the site of injury and those remote from the site; (iii) a significant postlesion increase in the size of cutaneous RFs in the PT rats, but not in the PL rats; (iv) a better preservation of RF submodality and topographic organization in the PT maps than in the PL maps; and (v) a decrease in neuronal responsiveness to cutaneous stimulation which was less pronounced in the PT than in the PL rats. Our results can be ascribed to a rapid change in the balance of excitatory and inhibitory connections which leads to unmasking of subthreshold inputs converging onto cortical neurons. Our findings also indicate that acute piracetam treatment exerts a protective function on the physiological response properties of cortical neurons after focal injury.

Contrast dependence of contextual effects in primate visual cortex.

The responses of neurons in the visual cortex to stimuli presented within their receptive fields can be markedly modulated by stimuli presented in surrounding regions that do not themselves evoke responses. This modulation depends on the relative orientation and direction of motion of the centre and surround stimuli, and it has been suggested that local cortical circuits linking cells with similar stimulus selectivities underlie these phenomena. However, the functional relevance and nature of these integrative processes remain unclear. Here we investigate how such integration depends on the relative activity levels of neurons at different points across the cortex by varying the relative contrast of stimuli over the receptive field and surrounding regions. We show that simply altering the balance of the excitation driving centre and surround regions can dramatically change the sign and stimulus selectivity of these contextual effects. Thus, the way that single neurons integrate information across the visual field depends not only on the precise form of stimuli at different locations, but also crucially on their relative contrasts. We suggest that these effects reflect a complex gain-control mechanism that regulates cortical neuron responsiveness, which permits dynamic modification of response properties of cortical neurons.

Sensory response properties of cortical neurons in the anterior ectosylvian sulcus of cats: intracellular recording and labeling

Visual and auditory sensory responses of cortical neurons in the caudal half of the anterior ectosylvian sulcus (AES) of cats were examined under α-chloralose anesthesia, using intracellular recording and labeling techniques. Stable intracellular recordings were obtained from 155 neurons, and 141 neurons exhibited responses to sensory stimuli. Of 141 sensory neurons, 74 (52%) were bimodal neurons that responded to both visual and auditory stimuli, and 67 (48%) were unimodal showing sensory responses only to visual (25) or auditory stimulation (42). Forty-five neurons (35 pyramidal neurons, 5 non-pyramidal neurons, 5 not classified) responsive to sensory stimuli were labeled with biocytin. The percentage of bimodal neurons of the biocytin-labeled neurons was 40% ( 4 10 ) in layer II, 50% ( 10 20 ) in layer III–IV, 70% ( 7 10 ) in layer V and 60%/) ( 3 5 ) in layer VI. Thus the convergence of visual and auditory inputs on single neurons was most intense in layer V. Auditory response latencies were in a narrow range from 10 to 40 ms, whereas visual response latencies were in a wide range from 15 to 100 ms. Late visual responses (> 60 ms) were more commonly elicited in bimodal neurons than in visual unimodal neurons. Visual responses in layer 11 were all elicited over 40 ms, whereas early visual responses within 40 ms were observed in the other cortical layers. A subgroup of neurons ( 22 141 ) had a propensity to exhibit a burst discharge, a train of three to seven action potentials on a deploarizing envelope in response to sensory stimuli. Their specific distribution in cortical tissue was suggested by the result that six out of nine biocytin-labeled neurons (seven pyramidal neurons, two non-pyramidal neurons) showing burst discharges to sensory stimuli were observed in layer V. These results are considered to signify some aspects of intracortical organization related to the cross-modal integration of sensory inputs.

Mechanisms underlying experience-dependent potentiation and depression of vibrissae responses in barrel cortex

Plasticity was studied in the barrel cortex of rats and mice. Vibrissae deprivation in adult and adolescent animals caused changes in the response properties of cortical neurons to stimulation of spared and deprived vibrissae. Plasticity involved both potentiation of spared vibrissae responses and depression of deprived vibrissae responses in layer II III of the cortex. Vibrissae response potentiation was found to require αCaMKII in adult cortex. Vibrissae response depression was not found to occur in adult cortex but was found in adolescent animals for the principal vibrissa. Preliminary results suggested that vibrissae response depression exhibits both hetero- and homosynaptic components.

A selection model for motion processing in area MT of primates

A computational model for motion processing in area MT is presented that is based on the observed response properties of cortical neurons and is consistent with the visual perception of partially occluded and transparent moving stimuli. In contrast to models of motion processing that assume spatial continuity and fail to compute the correct velocity for these visual stimuli, our model produces a distributed segmentation of the image into disjoint patches that represent distinct objects moving with common velocities. A key element in the model is the selection of regions of the visual field where the velocity estimates are most reliable. The processing units in the motion model that perform the selection have nonclassical receptive fields similar to those observed in area MT (Allman et al., 1985). The psychophysical responses of the model to coherently moving random dots and transparent plaid gratings are similar to those observed in primates.

Synchronous high-frequency oscillations in cat area 18.

The present study extends knowledge of the basic properties of correlated oscillatory activity patterns in the visual cortex of anaesthetized cats. Recordings with multiple electrodes were performed in area 18 and the correlations of multi-unit activity in the frequency range 35-80 Hz were determined using the coherence function. Statistical analysis revealed that the multi-unit correlations depended on the cortical distance between the recording sites, the orientation selectivity of the neurons and their cortical layer. On average, correlations dropped to chance level within several millimetres and were higher in lower than in upper cortical layers. Similar results were found by analysing the correlations of oscillatory patterns in local field potentials recorded from the same electrodes. Correlations of neurons with similar orientation preferences were higher than those of neurons with different orientation preferences. Comparison to a matched sample from area 17 showed that the correlations in areas 18 and 17 depended on similar properties of the neurons. The dependences of correlated oscillations resembled the known pattern and specificity of intra-areal fibre connections, suggesting that the correlations were intracortically established. Since correlations were specifically and not randomly related to the response properties of cortical neurons and were prominent in a visual area other than area 17, the findings suggest that correlated oscillatory activity provides a potential neural code supporting sensory information processing.