Is there a thalamic component to experience-dependent cortical plasticity?

Lateral geniculate neurons projecting to primary visual cortex show ocular dominance plasticity in adult mice

Experience-dependent plasticity in the mature visual system is widely considered to be cortical. Using chronic two-photon Ca²⁺ imaging of thalamic afferents in layer 1 of binocular visual cortex, we provide evidence against this tenet: the respective dorsal lateral geniculate nucleus (dLGN) cells showed pronounced ocular dominance (OD) shifts after monocular deprivation in adult mice. Most (86%), but not all, of dLGN cell boutons were monocular during normal visual experience. Following deprivation, initially deprived-eye-dominated boutons reduced or lost their visual responsiveness to that eye and frequently became responsive to the non-deprived eye. This cannot be explained by eye-specific cortical changes propagating to dLGN via cortico-thalamic feedback because the shift in dLGN responses was largely resistant to cortical inactivation using the GABA_A receptor agonist muscimol. Our data suggest that OD shifts observed in the binocular visual cortex of adult mice may at least partially reflect plasticity of eye-specific inputs onto dLGN neurons.

Visual BOLD Response in Late Blind Subjects with Argus II Retinal Prosthesis

Retinal prosthesis technologies require that the visual system downstream of the retinal circuitry be capable of transmitting and elaborating visual signals. We studied the capability of plastic remodeling in late blind subjects implanted with the Argus II Retinal Prosthesis with psychophysics and functional MRI (fMRI). After surgery, six out of seven retinitis pigmentosa (RP) blind subjects were able to detect high-contrast stimuli using the prosthetic implant. However, direction discrimination to contrast modulated stimuli remained at chance level in all of them. No subject showed any improvement of contrast sensitivity in either eye when not using the Argus II. Before the implant, the Blood Oxygenation Level Dependent (BOLD) activity in V1 and the lateral geniculate nucleus (LGN) was very weak or absent. Surprisingly, after prolonged use of Argus II, BOLD responses to visual input were enhanced. This is, to our knowledge, the first study tracking the neural changes of visual areas in patients after retinal implant, revealing a capacity to respond to restored visual input even after years of deprivation.

Thalamocortical Projections onto Behaviorally Relevant Neurons Exhibit Plasticity during Adult Motor Learning

Layer 5 neurons of the neocortex receive direct and relatively strong input from the thalamus. However, the intralaminar distribution of these inputs and their capacity for plasticity in adult animals are largely unknown. In slices of the primary motor cortex (M1), we simultaneously recorded from pairs of corticospinal neurons associated with control of distinct motor outputs: distal forelimb versus proximal forelimb. Activation of ChR2-expressing thalamocortical afferents in M1 before motor learning produced equivalent responses in monosynaptic excitation of neurons controlling the distal and proximal forelimb, suggesting balanced thalamic input at baseline. Following skilled grasp training, however, thalamocortical input shifted to bias activation of corticospinal neurons associated with control of the distal forelimb. This increase was associated with a cell-specific increase in mEPSC amplitude but not presynaptic release probability. These findings demonstrate distinct and highly segregated plasticity of thalamocortical projections during adult learning.

Increased cortical responses to forepaw stimuli immediately after peripheral deafferentation of hindpaw inputs

Both central and peripheral injuries of the nervous system induce dramatic reorganization of the primary somatosensory cortex. We recently showed that spinal cord injuries at thoracic level in anesthetized rats can immediately increase the responses evoked in the forepaw cortex by forepaw stimuli (above the level of the lesion), suggesting that the immediate cortical reorganization after deafferentation can extend across cortical representations of different paws. Here we show that a complete deafferentation of inputs from the hindpaw induced by injury or pharmacological block of the peripheral nerves in anesthetized rats also increases the responses evoked in the forepaw cortex by forepaw stimuli. This increase of cortical responses after peripheral deafferentation is not associated with gross alterations in the state of cortical spontaneous activity. The results of the present study, together with our previous works on spinal cord injury, suggest that the forepaw somatosensory cortex is critically involved in the reorganization that starts immediately after central or peripheral deafferentation of hindpaw inputs.

Sensory experience shapes the development of the visual system's first synapse

Specific connectivity patterns among neurons create the basic architecture underlying parallel processing in our nervous system. Here we focus on the visual system's first synapse to examine the structural and functional consequences of sensory deprivation on the establishment of parallel circuits. Dark rearing reduces synaptic strength between cones and cone bipolar cells, a previously unappreciated effect of sensory deprivation. In contrast, rod bipolar cells, which utilize the same glutamate receptor to contact rods, are unaffected by dark rearing. Underlying the physiological changes, we find the localization of metabotropic glutamate receptors within cone bipolar, but not rod bipolar, cell dendrites is a light-dependent process. Furthermore, although cone bipolar cells share common cone partners, each bipolar cell type that we examined depends differentially on sensory input to achieve mature connectivity. Thus, visual experience differentially affects maturation of rod versus cone pathways and of cell types within the cone pathway.

Non-Hebbian long-term potentiation of inhibitory synapses in the thalamus

The thalamus integrates and transmits sensory information to the neocortex. The activity of thalamocortical relay (TC) cells is modulated by specific inhibitory circuits. Although this inhibition plays a crucial role in regulating thalamic activity, little is known about long-term changes in synaptic strength at these inhibitory synapses. Therefore, we studied long-term plasticity of inhibitory inputs to TC cells in the posterior medial nucleus of the thalamus by combining patch-clamp recordings with two-photon fluorescence microscopy in rat brain slices. We found that specific activity patterns in the postsynaptic TC cell induced inhibitory long-term potentiation (iLTP). This iLTP was non-Hebbian because it did not depend on the timing between presynaptic and postsynaptic activity, but it could be induced by postsynaptic burst activity alone. iLTP required postsynaptic dendritic Ca(2+) influx evoked by low-threshold Ca(2+) spikes. In contrast, tonic postsynaptic spiking from a depolarized membrane potential (-50 mV), which suppressed these low-threshold Ca(2+) spikes, induced no plasticity. The postsynaptic dendritic Ca(2+) increase triggered the synthesis of nitric oxide that retrogradely activated presynaptic guanylyl cyclase, resulting in the presynaptic expression of iLTP. The dependence of iLTP on the membrane potential and therefore on the postsynaptic discharge mode suggests that this form of iLTP might occur during sleep, when TC cells discharge in bursts. Therefore, iLTP might be involved in sleep state-dependent modulation of thalamic information processing and thalamic oscillations.

Environmental enrichment causes a global potentiation of neuronal responses across stimulus complexity and lamina of sensory cortex

Enriched social and physical housing produces many molecular, anatomical, electrophysiological and behavior benefits even in adult animals. Much less is known of its effects on cortical electrophysiology, especially in how sensory cortex encodes the altered environment, and extant studies have generally been restricted to neurons in input laminae in sensory cortex. To extend the understanding of how an enriched environment alters the way in which cortex views the world, we investigated enrichment-induced changes in neuronal encoding of sensory stimuli across all laminae of the rat barrel cortex receiving input from the face whisker tactile system. Animals were housed in Enriched (n = 13) or Isolated housing (n = 13) conditions for 8 weeks before extracellular recordings were obtained from barrel cortex in response to simple whisker deflections and whisker motions modeling movements seen in awake animals undertaking a variety of different tasks. Enrichment resulted in increases in neuronal responses to all stimuli, ranging from those modeling exploratory behavior through to discrimination behaviors. These increases were seen throughout the cortex from supragranular layers through to input Layer 4 and for some stimuli, in infragranular Layer 5. The observed enrichment-induced effect is consistent with the postulate that enrichment causes shift in cortical excitatory/inhibitory balance, and we demonstrate this is greatest in supragranular layers. However, we also report that the effects are non-selective for stimulus parameters across a range of stimuli except for one modeling the likely use of whiskers by the rats in the enriched housing.

Tone-detection training enhances spectral integration mediated by intracortical pathways in primary auditory cortex

Auditory-cued behavioral training can alter neural circuits in primary auditory cortex (A1), but the mechanisms and consequences of experience-dependent cortical plasticity are not fully understood. To address this issue, we trained adult rats to detect a 5 kHz target in order to receive a food reward. After 14 days training we identified three locations within A1: (i) the region representing the characteristic frequency (CF) 5 kHz, (ii) a nearby region with CF ∼10 kHz, and (iii) a more distant region with CF ∼20 kHz. In order to compare functional connectivity in A1 near to, vs. far from, the representation of the target frequency, we placed a 16-channel multiprobe in middle- (∼10 kHz) and high- (∼20 kHz) CF regions and obtained current-source density (CSD) profiles evoked by a range of tone stimuli (CF±1-3 octaves in quarter-octave steps). Our aim was to construct "CSD receptive fields" (CSD RFs) in order to determine the laminar and spectral profile of tone-evoked current sinks, and infer changes to thalamocortical and intracortical inputs. Behavioral training altered CSD RFs at the 10 kHz, but not 20 kHz, site relative to CSD RFs in untrained control animals. At the 10 kHz site, current sinks evoked by the target frequency were enhanced in layer 2/3, but the initial current sink in layer 4 was not altered. The results imply training-induced plasticity along intracortical pathways connecting the target representation with nearby cortical regions. Finally, we related behavioral performance (sensitivity index, d') to CSD responses in individual animals, and found a significant correlation between the development of d' over training and the amplitude of the target-evoked current sink in layer 2/3. The results suggest that plasticity along intracortical pathways is important for auditory learning.

Thalamocortical inputs show post-critical-period plasticity

Experience-dependent plasticity in the adult brain has clinical potential for functional rehabilitation following central and peripheral nerve injuries. Here, plasticity induced by unilateral infraorbital (IO) nerve resection in 4-week-old rats was mapped using MRI and synaptic mechanisms were elucidated by slice electrophysiology. Functional MRI demonstrates a cortical potentiation compared to thalamus 2 weeks after IO nerve resection. Tracing thalamocortical (TC) projections with manganese-enhanced MRI revealed circuit changes in the spared layer 4 (L4) barrel cortex. Brain slice electrophysiology revealed TC input strengthening onto L4 stellate cells due to an increase in postsynaptic strength and the number of functional synapses. This work shows that the TC input is a site for robust plasticity after the end of the previously defined critical period for this input. Thus, TC inputs may represent a major site for adult plasticity in contrast to the consensus that adult plasticity mainly occurs at cortico-cortical connections.

Rapid plasticity of visual responses in the adult lateral geniculate nucleus

Compared to the developing visual system, where neuronal plasticity has been well characterized at multiple levels, little is known about plasticity in the adult, particularly within subcortical structures. We made intraocular injections of 2-amino-4-phosphonobutyric acid (APB) in adult cats to block visual responses in On-center retinal ganglion cells and examined the consequences on visual responses in the lateral geniculate nucleus (LGN) of the thalamus. In contrast to current views of retinogeniculate organization, which hold that On-center LGN neurons should become silent with APB, we find that ∼50% of On-center neurons rapidly develop Off-center responses. The time course of these emergent responses and the actions of APB in the retina indicate the plasticity occurs within the LGN. These results suggest there is greater divergence of retinogeniculate connections than previously recognized and that functionally silent, nonspecific retinal inputs can serve as a substrate for rapid plasticity in the adult.

Targeted mini-strokes produce changes in interhemispheric sensory signal processing that are indicative of disinhibition within minutes

Most processing of sensation involves the cortical hemisphere opposite (contralateral) to the stimulated limb. Stroke patients can exhibit changes in the interhemispheric balance of sensory signal processing. It is unclear whether these changes are the result of poststroke rewiring and experience, or whether they could result from the immediate effect of circuit loss. We evaluated the effect of mini-strokes over short timescales (<2 h) where cortical rewiring is unlikely by monitoring sensory-evoked activity throughout much of both cortical hemispheres using voltage-sensitive dye imaging. Blockade of a single pial arteriole within the C57BL6J mouse forelimb somatosensory cortex reduced the response evoked by stimulation of the limb contralateral to the stroke. However, after stroke, the ipsilateral (uncrossed) forelimb response within the unaffected hemisphere was spared and became independent of the contralateral forelimb cortex. Within the unaffected hemisphere, mini-strokes in the opposite hemisphere significantly enhanced sensory responses produced by stimulation of either contralateral or ipsilateral pathways within 30-50 min of stroke onset. Stroke-induced enhancement of responses within the spared hemisphere was not reproduced by inhibition of either cortex or thalamus using pharmacological agents in nonischemic animals. I/LnJ acallosal mice showed similar rapid interhemispheric redistribution of sensory processing after stroke, suggesting that subcortical connections and not transcallosal projections were mediating the novel activation patterns. Thalamic inactivation before stroke prevented the bilateral rearrangement of sensory responses. These findings suggest that acute stroke, and not merely loss of activity, activates unique pathways that can rapidly redistribute function within the spared cortical hemisphere.

Experience-induced plasticity of thalamocortical axons in both juveniles and adults

We examined the effect of sensory deprivation on thalamocortical (TC) projections to the rat primary somatosensory cortex at different postnatal ages ranging from P0 to P96. Rats had their whiskers clipped off with one or two vibrissae spared. TC axons innervating barrel cortex were specifically labeled by injecting virus expressing fluorescent proteins into the corresponding primary (VPM) and/or secondary (POm) thalamic nuclei. The density of VPM axons in deprived columns was ≈34% lower relative to spared columns with a concomitant decrease in bouton density, suggesting a deprivation-induced retraction of VPM axons. Axonal changes were reversible upon regrowth of the clipped whiskers and independent of age at deprivation, indicating the absence of a critical period for anatomical plasticity. The POm projection was not obviously altered by sensory deprivation. We suggest that retraction and regrowth of TC axons substantially contribute to long-term deprivation-dependent functional plasticity.

Environmental enrichment differentially modifies specific components of sensory-evoked activity in rat barrel cortex as revealed by simultaneous electrophysiological recordings and optical imaging in vivo

Environmental enrichment of laboratory animals leads to multi-faceted changes to physiology, health and disease prognosis. An important and under-appreciated factor in enhancing cognition through environmental manipulation may be improved basic sensory function. Previous studies have highlighted changes in cortical sensory map plasticity but have used techniques such as electrophysiology, which suffer from poor spatial resolution, or optical imaging of intrinsic signals, which suffers from low temporal resolution. The current study attempts to overcome these limitations by combining voltage-sensitive dye imaging with somatosensory-evoked potential (SEP) recordings: the specific aim was to investigate sensory function in barrel cortex using multi-frequency whisker stimulation under urethane anaesthesia. Three groups of rats were used that each experienced a different level of behavioural or environmental enrichment. We found that enrichment increased all SEP response components subsequent to the initial thalamocortical input, but only when evoked by single stimuli; the thalamocortical component remained unchanged across all animal groups. The optical signal exhibited no changes in amplitude or latency between groups, resembling the thalamocortical component of the SEP response. Permanent and extensive changes to housing conditions conferred no further enhancement to sensory function above that produced by the milder enrichment of regular handling and behavioural testing, a finding with implications for improvements in animal welfare through practical changes to animal husbandry.

EXPERIENCE-DEPENDENT IMMEDIATE EARLY GENE EXPRESSION IN THE ADULT CENTRAL NERVOUS SYSTEM: EVIDENCE FROM ENRICHED-ENVIRONMENT STUDIES

Here I discuss evidence from our group's work that implicates the immediate early genes NGFI-A and arc as possible regulators of neuronal plasticity. The enriched environment (EE) paradigm has been demonstrated to induce neural plasticity in both developing and mature mammals. Others and we have recently demonstrated that adult rats placed within an enriched environment underwent central nervous system-wide increases in the expression levels for the IEGs NGFI-A and arc. The relationships between the altered expression profiles for both genes in response to an EE exposure, and their putative role in orchestrating network restructuring in response to enhanced environmental complexity are discussed.

Long-term plasticity in mouse sensorimotor circuits after rhythmic whisker stimulation

Mice actively explore their environment by rhythmically sweeping their whiskers. As a consequence, neuronal activity in somatosensory pathways is modulated by the frequency of whisker movement. The potential role of rhythmic neuronal activity for the integration and consolidation of sensory signals, however, remains unexplored. Here, we show that a brief period of rhythmic whisker stimulation in anesthetized mice resulted in a frequency-specific long-lasting increase in the amplitude of somatosensory-evoked potentials in the contralateral primary somatosensory (barrel) cortex. Mapping of evoked potentials and intracortical recordings revealed that, in addition to potentiation in layers IV and II/III of the barrel cortex, rhythmic whisker stimulation induced a decrease of somatosensory-evoked responses in the supragranular layers of the motor cortex. To assess whether rhythmic sensory input-based plasticity might arise in natural settings, we exposed mice to environmental enrichment. We found that it resulted in somatosensory-evoked responses of increased amplitude, highlighting the influence of previous sensory experience in shaping sensory responses. Importantly, environmental enrichment-induced plasticity occluded further potentiation by rhythmic stimulation, indicating that both phenomena share common mechanisms. Overall, our results suggest that natural, rhythmic patterns of whisker activity can modify the cerebral processing of sensory information, providing a possible mechanism for learning during sensory perception.

Experience-dependent plasticity mechanisms for neural rehabilitation in somatosensory cortex

Functional rehabilitation of the cortex following peripheral or central nervous system damage is likely to be improved by a combination of behavioural training and natural or therapeutically enhanced synaptic plasticity mechanisms. Experience-dependent plasticity studies in the somatosensory cortex have begun to reveal those synaptic plasticity mechanisms that are driven by sensory experience and might therefore be active during behavioural training. In this review the anatomical pathways, synaptic plasticity mechanisms and structural plasticity substrates involved in cortical plasticity are explored, focusing on work in the somatosensory cortex and the barrel cortex in particular.

Termination of lesion-induced plasticity in the mouse barrel cortex in the absence of oligodendrocytes

Termination of developmental plasticity occurs at specific points in development, and the mechanisms responsible for it are not well understood. One hypothesis that has been proposed is that oligodendrocytes (OLs) play an important role. Consistent with this, we found that OLs appeared in the mouse somatosensory cortex at the end of the critical period for whisker lesion-induced barrel structural plasticity. To test this hypothesis, we used two mouse lines with defective OL differentiation: Olig1-deficient and jimpy. In Olig1-deficient mice, although OLs were totally absent, the termination of lesion-induced plasticity was not delayed. The timing was normal even when the cytoarchitectonic barrel formation was temporarily blocked by pharmacological treatment in Olig1-deficient mice. Furthermore, the termination was not delayed in jimpy mice. These results demonstrate that, even though OLs appear at the end of the critical period, OLs are not intrinsically necessary for the termination of lesion-induced plasticity. Our findings underscore a mechanistic distinction between the termination of thalamocortical axonal plasticity in the barrel cortex and that in the visual cortex, in which OL-derived Nogo-A/B was recently suggested to be essential.

The excitatory thalamo-"cortical" projection within the song control system of zebra finches is formed by calbindin-expressing neurons

The learning and production of vocalizations in songbirds are controlled by a system of interconnected brain nuclei organized into a direct vocal motor pathway and an anterior forebrain (pallium-basal ganglia-thalamo-pallial) loop. Here we show that the thalamo-pallial ("thalamo-cortical") projection (from the medial part of the dorsolateral thalamic nucleus to the lateral magnocellular nucleus of the anterior nidopallium--DLM to LMAN) within the anterior forebrain loop is composed of cells positive for the calcium-binding protein calbindin. We show that the vast majority of cells within DLM express calbindin, based both on immunocytochemistry (ICC) for calbindin protein and in situ hybridization for calb mRNA. Using a combination of tract-tracing and ICC we show that the neurons that participate in the DLM-to-LMAN projection are calbindin-positive. We also demonstrate that DLM is devoid of cells expressing mRNA for the GABAergic marker zGAD65. This observation confirms that the calbindin-expressing cells in DLM are not GABAergic, in accordance with previous electrophysiological data indicating that the DLM-to-LMAN projection is excitatory. Furthermore, we use ICC to determine the trajectory of the fibers within the DLM-to-LMAN projection, and to demonstrate a sex difference in calbindin expression levels in the fibers of the DLM-to-LMAN projection. Our findings provide a clear-cut neurochemical signature for a critical projection in the songbird vocal control pathways that enable song learning.

Enhanced short-latency responses in the ventral posterior medial (VPM) thalamic nucleus following whisker trimming in the adult rat

This study examined the effects of whisker trimming on the functional organization of the adult somatosensory thalamus. In vivo extracellular unit recordings were made on ventral posterior medial (VPM) thalamic neurons in urethane anaesthetised adult rats. Neuronal response properties to controlled whisker deflection were recorded in untrimmed control animals and in animals where one row of whiskers had been trimmed for a median of 18 days. Trimming significantly increased short-latency responses to stimulation of the centre receptive field whisker (mean increase of 36%, p<.001). Longer latency responses to surround receptive field whiskers were unaffected. Spontaneous firing was significantly decreased in trimmed animals. A condition-test paradigm indicated that thalamic inhibition was reduced following whisker trimming, although this effect failed to reach statistical significance. These results demonstrate a capacity of the adult somatosensory thalamus to undergo functional reorganization in response to non-traumatic and innocuous whisker trimming.

Gustatory terminal field organization and developmental plasticity in the nucleus of the solitary tract revealed through triple-fluorescence labeling

Early dietary sodium restriction has profound influences on the organization of the gustatory brainstem. However, the anatomical relationships among multiple gustatory nerve inputs have not been examined. Through the use of triple-fluorescence labeling and confocal laser microscopy, terminal fields of the greater superficial petrosal (GSP), chorda tympani (CT), and glossopharyngeal (IX) nerves were visualized concurrently in the nucleus of the solitary tract (NTS) of developmentally sodium-restricted and control rats. Dietary sodium restriction during pre- and postnatal development resulted in a twofold increase in the volume of both the CT and the IX nerve terminal fields but did not affect the volume of the GSP terminal field. In controls, these nerve terminal fields overlapped considerably. The dietary manipulation significantly increased the overlapping zones among terminal fields, resulting in an extension of CT and IX fields past their normal boundaries. The differences in terminal field volumes were exaggerated when expressed relative to the respective NTS volumes. Furthermore, increased terminal field volumes could not be attributed to an increase in the number of afferents because ganglion cell counts did not differ between groups. Taken together, selective increases in terminal field volume and ensuing overlap among terminal fields suggest an increased convergence of these gustatory nerve terminals onto neurons in the NTS. The genesis of such convergence is likely related to disruption of cellular and molecular mechanisms during the development of individual terminal fields, the consequences of which have implications for corresponding functional and behavioral alterations.

Age-related decrease of the chorda tympani nerve terminal field in the nucleus of the solitary tract is prevented by dietary sodium restriction during development

Institution of a low-NaCl diet beginning at embryonic day 3 and continued throughout pre- and postnatal development has widespread effects on the neuroanatomical organization of the first gustatory relay in the nucleus of the solitary tract. To determine when these effects are expressed postnatally, the terminal field of the chorda tympani nerve was compared between sodium-restricted and sodium-replete rats at postnatal days 15-17, postnatal days 25-27, postnatal days 35-37, and adults. Total terminal fields were significantly larger in postnatal days 35-37 and adult sodium-restricted rats compared with aged-matched controls. The group-related differences appear related more to a remodeling of the terminal field in the dorsal zone of the terminal field in controls. Specifically, the terminal field volume in the dorsal zone in controls decreased dramatically from postnatal days 25-27 to postnatal days 35-37 and then again from postnatal days 35-37 to adulthood. In contrast, the fields did not change during development in sodium-restricted rats. These findings suggest that remodeling of the chorda tympani field occurs in controls at about the developmental period of taste response maturation. The lack of remodeling in sodium-restricted rats may be explained by a corresponding lack of functional response development to sodium salts. These results also illustrate the specificity and extent of how early dietary manipulations shape the developing brainstem.

A Comparison of Experience-Dependent Plasticity in the Visual and Somatosensory Systems

In the visual and somatosensory systems, maturation of neuronal circuits continues for days to weeks after sensory stimulation occurs. Deprivation of sensory input at various stages of development can induce physiological, and often structural, changes that modify the circuitry of these sensory systems. Recent studies also reveal a surprising degree of plasticity in the mature visual and somatosensory pathways. Here, we compare and contrast the effects of sensory experience on the connectivity and function of these pathways and discuss what is known to date concerning the structural, physiological, and molecular mechanisms underlying their plasticity.

The origin of cortical surround receptive fields studied in the barrel cortex

The neocortex is thought to be organized into functional columns of neurons, each of which processes an element of a larger representation. In the barrel cortex, the thalamic input to the column preferentially terminates in a barrel. To study the extent and nature of functional connections between columns, we measured the degree to which whisker responses are relayed between columns in the barrel cortex. Inactivating a single barrel by iontophoresis of the GABA(A) agonist muscimol abolished the representation of that barrel's whisker in neighboring barrels. Reactivating a single barrel by iontophoresis of the GABA(A) antagonist bicuculline while the rest of the cortex was blocked by muscimol led to single whisker receptive fields. Under the same conditions, septal cells tended to exhibit multiwhisker receptive fields. These studies demonstrate that the surround receptive fields of barrel cells are generated by intracortical transmission and that many septal cells derive a component of their surround receptive field from the thalamus.

The origin of cortical surround receptive fields studied in the barrel cortex

The neocortex is thought to be organized into functional columns of neurons, each of which processes an element of a larger representation. In the barrel cortex, the thalamic input to the column preferentially terminates in a barrel. To study the extent and nature of functional connections between columns, we measured the degree to which whisker responses are relayed between columns in the barrel cortex. Inactivating a single barrel by iontophoresis of the GABA(A) agonist muscimol abolished the representation of that barrel's whisker in neighboring barrels. Reactivating a single barrel by iontophoresis of the GABA(A) antagonist bicuculline while the rest of the cortex was blocked by muscimol led to single whisker receptive fields. Under the same conditions, septal cells tended to exhibit multiwhisker receptive fields. These studies demonstrate that the surround receptive fields of barrel cells are generated by intracortical transmission and that many septal cells derive a component of their surround receptive field from the thalamus.