Cortical plasticity: from synapses to maps

Critical periods during sensory development

Recent studies have made progress in characterizing the determinants of critical periods for experience-dependent plasticity. They highlight the role of neurotrophins, NMDA receptors and GABAergic inhibition. In particular, genetic manipulation of a single molecule, brain-derived neurotrophic factor (BDNF), has been shown to alter the timing of the critical period of plasticity in mouse visual cortex, establishing a causal relation between neurotrophin action, the development of visual function, and the duration of the critical period.

Differential activation in somatosensory cortex for different discrimination tasks

Maps of the body surface in somatosensory cortex have been shown to be highly plastic, altering their configuration in response to changes in use of body parts. The current study investigated alterations in the functional organization of the human somatosensory cortex resulting from massed practice. Over a period of 4 weeks, subjects were given synchronous tactile stimulation of thumb (D1) and little finger (D5) for 1 hr/d. They had to identify the orientation of the stimuli. Neuroelectric source localization based on high-resolution EEG revealed that, when subjects received passive tactile stimulation of D1 or D5, the representations of the fingers in primary somatosensory cortex were closer together after training than before. There was also an apparently correlative tendency to anomalously mislocalize near-threshold tactile stimuli equally to the distant finger costimulated during training rather than preferentially to the finger nearest to the finger stimulated in a post-training test. However, when the stimulus discrimination had to be made, neuroelectric source imaging revealed that the digital representations of D1 and D5 were further apart after training than before. Thus, the same series of prolonged repetitive stimulations produced two different opposite effects on the spatial relationship of the cortical representations of the digits, suggesting that differential activation in the same region of somatosensory cortex is specific to different tasks.

Brain plasticity and stroke rehabilitation. The Willis lecture

Neuronal connections and cortical maps are continuously remodeled by our experience. Knowledge of the potential capabilityof the brain to compensate for lesions is a prerequisite for optimal stroke rehabilitation strategies. Experimental focal cortical lesions induce changes in adjacent cortex and in the contralateral hemisphere. Neuroimaging studies in stroke patients indicate altered poststroke activation patterns, which suggest some functional reorganization. To what extent functional imaging data correspond to outcome data needs to be evaluated. Reorganization may be the principle process responsible for recovery of function after stroke, but what are the limits, and to what extent can postischemic intervention facilitate such changes? Postoperative housing of animals in an enriched environment can significantly enhance functional outcome and can also interact with other interventions, including neocortical grafting. What role will neuronal progenitor cells play in future rehabilitation-stimulated in situ or as neural replacement? And what is the future for blocking neural growth inhibitory factors? Better knowledge of postischemic molecular and neurophysiological events, and close interaction between basic and applied research, will hopefully enable us to design rehabilitation strategies based on neurobiological principles in a not-too-distant future.

Cellular analog of differential classical conditioning in Aplysia: disruption by the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate

We previously showed that the associative enhancement of Aplysia siphon sensorimotor synapses in a cellular analog of classical conditioning is disrupted by infusing the Ca(2+) chelator 1, 2-bis(2-aminophenoxy)ethane-N,N-N',N'-tetraacetic acid into the postsynaptic motor neuron before training or by training in the presence of the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (APV). Our earlier experiments with APV used a nondifferential training protocol, in which different preparations were used for associative and nonassociative training. In the present experiments we extended our investigation of the role of NMDA receptor type potentiation in learning in Aplysia to differential conditioning. A cellular analog of differential conditioning was performed with a reduced preparation that consisted of the CNS plus two pedal nerves. A siphon motor neuron and two siphon sensory neurons, both of which were presynaptically connected to the motor neuron, were impaled with sharp microelectrodes. One sensorimotor synapse received paired stimulation with a conditioned stimulus (brief activation of a single sensory neuron) and an unconditioned stimulus (pedal nerve shock), whereas the other sensorimotor synapse received unpaired stimulation. Training in normal artificial seawater (ASW) resulted in significant differential enhancement of synapses that received the paired stimulation. Training in APV blocked this differential synaptic enhancement. A comparison of the present data with the data from earlier experiments that used nondifferential training is consistent with the possibility that differential training comprises competition between the presynaptic sensory neurons. Synaptic competition may contribute significantly to the associative effect of paired stimulation in the differential training paradigm.

The growth hormone axis and cognition: empirical results and integrated theory derived from giant transgenic mice

Sleep is required for the consolidation of memory for complex tasks, and elements of the growth-hormone (GH) axis may regulate sleep. The GH axis also up-regulates protein synthesis, which is required for memory consolidation. Transgenic rat GH mice (TRGHM) express plasma GH at levels 100-300 times normal and sleep 3.4 h longer (30%) than their normal siblings. Consequently, we hypothesized that they might show superior ability to learn a complex task (8-choice radial maze); 47% of the TRGHM learned the task before any normal mice. All 17 TRGHM learned the task, but 33% of the 18 normal mice learned little. TRGHM learned the task significantly faster than normal mice (p < 0.05) and made half as many errors in doing so, even when the normal nonlearners were excluded from the analysis. Whereas normal mice expressed a linear learning curve, TRGHM showed exponentially declining error rates. The contribution of the GH axis to cognition is conspicuously sparse in literature syntheses of knowledge concerning neuroendocrine mechanisms of learning and memory. This paper synthesizes the crucial role of major components of the GH axis in brain functioning into a holistic framework, integrating learning, sleep, free radicals, aging, and neurodegenerative diseases. TRGHM show both enhanced learning in youth and accelerated aging. Thus, they may provide a powerful new probe for use in gaining an understanding of aspects of central nervous system functioning, which is highly relevant to human health.

Neuronal responses in cat primary auditory cortex to electrical cochlear stimulation. III. Activation patterns in short- and long-term deafness

The effects of auditory deprivation on the spatial distribution of cortical response thresholds to electrical stimulation of the adult cat cochlea were evaluated. Threshold distributions for single- and multiple-unit responses from the middle cortical layers were obtained on the ectosylvian gyrus in three groups of animals: adult, acutely implanted animals ("acute group"); adult animals, 2 wk after deafening and implantation ("short-term group"); adult, neonatally deafened animals ("long-term group") implanted after 2-5 years of deafness. For all three groups, we observed similar patterns of circumscribed regions of low response thresholds in the region of primary auditory cortex (AI). A dorsal and a ventral region of low response thresholds were found separated by a narrow, anterior-posterior strip of elevated thresholds. The two low-threshold regions in the acute and the short-term group were arranged cochleotopically. This was reflected in a systematic shift of the cortical locations with minimum thresholds as a function of cochlear position of the radial and monopolar stimulation electrodes. By contrast, the long-term deafened animals maintained only weak or no signs of cochleotopicity. In some cases of this group, significant deviations from a simple tri-partition of the dorsoventral axis of AI was observed. Analysis of the spatial extent of the low-threshold regions revealed that the activated area in acute cases was significantly smaller than the long- and the short-term cases for both dorsal and ventral AI. There were no significant differences in the rostrocaudal extent of activation between long- and short-term deafening, although the total activated area in the short-term cases was larger than in long-term deafened animals. The width of the narrow high-threshold ridge that separated the dorsal and ventral low-threshold regions was the widest for the acute cases and the narrowest for the short-term deafened animals. The findings of relative large differences in cortical response distributions between the acute and short-term animals suggests that the effects observed in long-term deafened animals are not solely a consequence of loss of peripheral innervation density. The effects may reflect electrode-specific effects or reorganizational changes based on factors such as differences in excitatory and inhibitory balance.

Building neural representations of habits

Memories for habits and skills ("implicit or procedural memory") and memories for facts ("explicit or episodic memory") are built up in different brain systems and are vulnerable to different neurodegenerative disorders in humans. So that the striatum-based mechanisms underlying habit formation could be studied, chronic recordings from ensembles of striatal neurons were made with multiple tetrodes as rats learned a T-maze procedural task. Large and widely distributed changes in the neuronal activity patterns occurred in the sensorimotor striatum during behavioral acquisition, culminating in task-related activity emphasizing the beginning and end of the automatized procedure. The new ensemble patterns remained stable during weeks of subsequent performance of the same task. These results suggest that the encoding of action in the sensorimotor striatum undergoes dynamic reorganization as habit learning proceeds.

Loss of synaptic depression in mammalian anterior cingulate cortex after amputation

Two forms of activity-dependent long-term depression (LTD) in the CNS, as defined by their sensitivity to the blockade of NMDA receptors, are thought to be important in learning, memory, and development. Here, we report that NMDA receptor-independent LTD is the major form of long-term plasticity in the anterior cingulate cortex (ACC). Both L-type voltage-gated calcium channels and metabotropic glutamate receptors are required for inducing LTD. Amputation of a third hindpaw digit in an adult rat induced rapid expression of immediate early genes in the ACC bilaterally and caused a loss of LTD that persisted for at least 2 weeks. Our results suggest that synaptic LTD in the ACC may contribute to enhanced neuronal responses to subsequent somatosensory stimuli after amputation.

Signal but not noise changes with perceptual learning

Perceptual discrimination improves with practice. This 'perceptual learning' is often specific to the stimuli presented during training, indicating that practice may alter the response characteristics of cortical sensory neurons. Although much is known about how learning modifies cortical circuits, it remains unclear how these changes relate to behaviour. Different theories assume that practice improves discrimination by enhancing the signal, diminishing internal noise or both. Here, to distinguish among these alternatives, we fashioned sets of faces and textures whose signal strength could be varied, and we trained observers to identify these patterns embedded in noise. Performance increased by up to 400% across several sessions over several days. Comparisons of human performance to that of an ideal discriminator showed that learning increased the efficiency with which observers encoded task-relevant information. Observer response consistency, measured by a double-pass technique in which identical stimuli are shown twice in each experimental session, did not change during training, showing that learning had no effect on internal noise. These results indicate that perceptual learning may enhance signal strength, and provide important constraints for theories of learning.

Altered taste responses in adult NST after neonatal chorda tympani denervation

Anatomic and behavioral changes have been observed in the taste system after peripheral deafferentation, but their physiological consequences remain unknown. Interestingly, a recent behavioral study suggested that peripheral denervation could induce central plasticity. After neonatal chorda tympani (CT) transection, adult rats demonstrated a marked preference for a normally avoided salt, NH(4)Cl. In the present study, taste responses were recorded from the nucleus of the solitary tract (NST) in similarly CT-denervated rats to investigate a physiological basis for this behavioral phenomenon. We hypothesized that alterations in functional connectivity of remaining afferent nerves might underlie the behavioral change. Specifically, if NST neurons formerly activated by sodium-selective CT fibers were instead driven by more broadly tuned glossopharyngeal (GL) afferents, neural coding of salt responses would be altered. Such a change should be accompanied by a shift in orotopic representation and increased NH(4)Cl responses. This hypothesis was not supported. After CT denervation, orotopy was unaltered, NH(4)Cl responsiveness declined, and no other changes occurred that could simply explain the behavioral effects. Indeed, the most pronounced consequence of CT denervation was a 68% reduction in NaCl responses, supporting previous evidence for a critical role of this nerve in coding sodium salts. In addition, we found "reorganizational" changes similar to, albeit smaller than, those observed in other sensory systems after deafferentation. There was a trend for increased responses elicited by stimulation of receptor subpopulations innervated by the GL and greater superficial petrosal nerves. In addition, the spontaneous rate of nasoincisor duct-responsive cells increased significantly. This effect on spontaneous rate is opposite to that produced by CT anesthesia, suggesting that acute versus chronic denervation may affect central taste neurons differently. In conclusion, the taste system at the medullary level seems more resistant to large-scale plasticity than other sensory systems, but nevertheless reacts to lost afferent input. Because the most robust plastic changes have been documented at cortical levels in other sensory pathways, the substrate for the behavioral effect of neonatal CT transection may be located more centrally in the gustatory system.

Improved auditory spatial acuity in visually deprived ferrets

We have examined the effects on auditory spatial acuity in the horizontal plane of depriving ferrets of patterned visual cues by binocular eyelid suture in infancy or for a comparable period in adulthood. Minimum audible angles (MAAs) were measured for 500-, 100- and 40-ms broadband noise bursts at the midline and at 45 degrees to one side. A logistic regression analysis revealed no consistent difference between the midline MAAs of normal and infant lid-sutured ferrets. However, the lateral field MAAs of the infant-deprived group were significantly smaller and showed less inter-subject variability than those of normal-sighted ferrets. The animals deprived in adulthood were tested in the lateral field only, firstly 6 months after binocular eyelid suture and again after a further 10 months. For the first test, the MAAs achieved by these animals with 500- and 100-ms noise bursts were significantly smaller than the normal values and no different from those of the infant-deprived group. A significant improvement in performance at the two shortest stimulus durations (100 and 40 ms) was observed when the adult-deprived animals were re-tested. Their second-test MAAs did not differ from those of the infant-deprived group at any of the three stimulus durations used, and both groups achieved significantly better scores than the normal-sighted control animals. These results show that prolonged visual deprivation in both juvenile and adult ferrets can lead to a significant improvement in auditory spatial acuity in the lateral sound field. This is consistent with reports that congenitally blind humans can localize peripheral sounds more accurately than normal controls.

Two directions of plasticity in the sensory-deprived adult cortex

Damage or deprivation of a localized region of the skin surface has been shown to induce a selective expansion of adjacent skin surface representations in the adult somatosensory cortex. Here, we use repeated optical imaging in conjunction with single unit recordings to assess the plasticity of a single whisker's functional representation in the adult rat. We observed a large-scale expansion of a single whisker's functional representation following innocuous removal of all neighboring whiskers. Surprisingly, the same manipulation can also induce a large-scale contraction of the representation if the animal is removed from its home cage and given a brief opportunity to use its whiskers for active exploration of a different environment. Both the expansion and contraction reverse upon regrowth of the deprived whiskers. Thus, allowing the animal to use its deprived receptor organ in active exploration can determine the direction of plasticity in the adult cortex.

A multielectrode implant device for the cerebral cortex

A new class of brain implant technology was developed that allows the simultaneous recording of voltage signals from many individual neurons in the cerebral cortex during cognitive tasks. The device allows recording from 49 independent positions spanning a 2 x 2-mm region of neural tissue. The recording electrodes are positioned in a square grid with 350 microm spacing, and each microelectrode can be precisely independently vertically positioned using a hydraulic microdrive. The device utilizes ultrafine, sharp iridium microelectrodes that minimize mechanical disturbance of the region near the electrode tip and produce low noise neuronal recordings. The total weight of this device is less than 20 g, and the device is reusable. The implant device has been used for transdural recordings in primary somatosensory and auditory cortices of marmosets, owl monkeys, and rats. On a typical day, one-third of the microelectrodes yield well-discriminated single neuron action potential waveforms. Additional array electrodes yield lower amplitude driven multiunit activity. The average signal-to-noise ratio of discriminated action potential waveforms 6 months after implantation was greater than 9. Simple design alternatives are discussed that can increase the number of electrodes in the array and the depths at which dense array recordings can be achieved.

Sensory loss by selected whisker removal produces immediate disinhibition in the somatosensory cortex of behaving rats

This study used extracellular unit recordings in behaving animals to evaluate thalamocortical response transformations in the rat whisker-barrel system. Based on previous acute studies using controlled whisker stimulation, we hypothesized that in a cortical barrel adjacent (non-principal) whiskers exert a net inhibitory effect. In contrast, in thalamic barreloid neurons, the effects of neighboring whiskers should be net facilitatory. We evaluated these hypotheses by recording unit activity at 21 sites in 17 animals trained to explore a wire mesh screen with their whiskers. In the middle of the recording session, selected vibrissae were clipped close to the skin surface. The absence of whiskers surrounding the principal whisker was associated with a mean 20% increase in cortical activity and, conversely, a 37% decrease in the thalamic activity. Removal of the principal whisker resulted in a 50% decrease in cortical unit firing. Findings are consistent with the idea that, in the behaving animal, each barrel uses multi-whisker thalamic inputs and local inhibitory circuitry to sharpen the receptive field properties of its constituent neurons. Cortical disinhibition as a consequence of selective whisker removal is likely to be an important factor underlying altered receptive field properties in sensory-deprived animals.

An in vitro electrophysiological and Co2+-uptake study on the effect of infraorbital nerve transection on the cortical and thalamic neuronal activity

Changes of neuronal membrane characteristics in somatosensory barrel cortex and barreloid thalamus were investigated in rats following unilateral transection of the infraorbital nerve. Kainate induced Co2+-uptake method and image analysis were used to assess the Ca2+ permeability of non-NMDA (N-methyl-D-aspartate) glutamate receptors. Changes in some biophysical parameters of the affected cortical neurons were also investigated by intracellular recording in slice experiments. The altered neuronal activity was measured on days 1, 5 and 14 after surgery. Kainate induced Co2+ uptake increased markedly reflecting enhanced Ca2+ permeability of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/KAIN)-type receptors. Changes were more pronounced in the cortex than in the thalamus and peaked on the first day following nerve transection. After that, parameters gradually returned to the normal level. However, a small enhancement was still detectable in the cortex at the end of the 2-week-long observation period. In parallel with the increased Co2+-uptake, moderate membrane potential changes, stronger spiking activity and enhanced excitability were characteristic for cortical neurons. The observed alterations in neuronal characteristics underlie the reorganization and regeneration processes following injuries or surgeries. We can conclude that immediate change of the receptive field in the barrel cortex following unilateral nerve transection is based on changes in biophysical parameters of the neurons. Altered peripheral activation evokes changes in the neuronal activity, thus providing opportunity for a quick synaptic rearrangement. AMPA/KAIN-type glutamate receptors have a decisive role in the regulation of these processes. This kind of synaptic plasticity is more significant in the cortex than in the thalamus.

Postsynaptic actin and neuronal plasticity

In the adult brain, actin is concentrated in dendritic spines where it can produce rapid changes in their shape. Through various synaptic junction proteins, this postsynaptic actin is linked to neurotransmitter receptors, influencing their function and, in turn, being influenced by them. Thus, the actin cytoskeleton is emerging as a key mediator between signal transmission and anatomical plasticity at excitatory synapses.

Scanning patients with tasks they can perform

We present an overview of the types of imaging experiments that can be performed on psychologically impaired patients. The critical observation from such studies is a differential pattern of activation in the patients and normals. Underactivity is interpretable only when the patients make normal responses. In this context, a failure to activate a component region of the normal system implies that this region was not necessary for task performance. Overactivity indicates either cognitive or neuronal reorganisation. Neuronal reorganisation is indicated only if the patient performs the task using the same set of cognitive operations as normal subjects. Cognitive reorganisation can be demonstrated if the same activation pattern is elicited by normals when they are co-erced into using the same cognitive implementation as the patient. We conclude that the interpretation of neuroimaging studies of psychologically impaired patients depends on intact task performance and a detailed task analysis. When these criteria are met, patient studies can be used to identify: (1) necessary and sufficient brain systems, (2) dysfunction at sites distant to damage, (3) peri-damage activation, and (4) compensation either at a neuronal level when pre-existing cognitive strategies are re-instantiated using duplicated neuronal systems (degeneracy), or at a cognitive level when alternative cognitive strategies (and their corresponding brain systems) are adopted.

Recruitment of the auditory cortex in congenitally deaf cats by long-term cochlear electrostimulation

In congenitally deaf cats, the central auditory system is deprived of acoustic input because of degeneration of the organ of Corti before the onset of hearing. Primary auditory afferents survive and can be stimulated electrically. By means of an intracochlear implant and an accompanying sound processor, congenitally deaf kittens were exposed to sounds and conditioned to respond to tones. After months of exposure to meaningful stimuli, the cortical activity in chronically implanted cats produced field potentials of higher amplitudes, expanded in area, developed long latency responses indicative of intracortical information processing, and showed more synaptic efficacy than in naïve, unstimulated deaf cats. The activity established by auditory experience resembles activity in hearing animals.

Distinct functional types of associative long-term potentiation in neocortical and hippocampal pyramidal neurons

The response of a neuron to a time-varying stimulus is influenced by both short- and long-term synaptic plasticity. Both these forms of plasticity produce changes in synaptic efficacy of similar magnitude on very different time scales. A full understanding of the functional role of each form of plasticity relies on understanding how they interact. Here we examine how long-term potentiation (LTP) and short-term plasticity (STP) interact in two different cell types that exhibit NMDA-dependent LTP: neocortical L-II/III and hippocampal CA1 pyramidal cells. STP was examined using both paired pulses and trains of pulses before and after the induction of LTP. In both cell types, the same pairing protocol was used to induce LTP in the presence of an unpaired control pathway. Pairing produced a robust increase in the amplitude of the first EPSP both in the neocortex and hippocampus. However, although in CA1 neurons the same degree of potentiation was maintained throughout the duration of a brief stimulus train, in L-II/III neurons relatively less potentiation was seen in the later EPSPs of the train. Paired-pulse analyses revealed that a uniform potentiation is observed at intervals >100 msec, but at shorter intervals there is a preferential enhancement of the first pulse. Thus, in the cortex LTP may preferentially amplify stimulus onset. These results suggest that there are distinct forms of associative LTP and that the different forms may reflect the underlying computations taking place in different areas.

Sensory experience modifies the short-term dynamics of neocortical synapses

Many representations of sensory stimuli in the neocortex are arranged as topographic maps. These cortical maps are not fixed, but show experience-dependent plasticity. For instance, sensory deprivation causes the cortical area representing the deprived sensory input to shrink, and neighbouring spared representations to enlarge, in somatosensory, auditory or visual cortex. In adolescent and adult animals, changes in cortical maps are most noticeable in the supragranular layers at the junction of deprived and spared cortex. However, the cellular mechanisms of this experience-dependent plasticity are unclear. Long-term potentiation and depression have been implicated, but have not been proven to be necessary or sufficient for cortical map reorganization. Short-term synaptic dynamics have not been considered. We developed a brain slice preparation involving rat whisker barrel cortex in vitro. Here we report that sensory deprivation alters short-term synaptic dynamics in both vertical and horizontal excitatory pathways within the supragranular cortex. Moreover, modifications of horizontal pathways amplify changes in the vertical inputs. Our findings help to explain the functional cortical reorganization that follows persistent changes of sensory experience.

Facial nerve injury produces a latent somatosensory input through recruitment of the motor cortex in the rat

Short-latency effects of unilateral facial nerve transection were studied on neuronal activation evoked in the primary motor cortex (MI) on both sides by vibrissa stimulation in adult rats. In the controls, unilateral trigeminal stimulation evoked activity in the whisker representation of both the contralateral somatosensory cortex (SI) and MI, but never in the ipsilateral MI. Unilateral transection of the facial motoric nerve facilitated evoked responses in the contralateral MI, and induced further neuronal activation (gross potentials and unit activity) in the MI ipsilateral to the stimulation. Since these changes appeared rapidly and could be mimicked by picrotoxin application onto the SI contralateral to the stimulation, they are considered to be based on the disinhibition of preexisting associative and commissural connections, which are unmasked by facial nerve transection.

Distribution of tactile learning and its neural basis

The brain's sensory processing systems are modified during perceptual learning. To learn more about the spatial organization of learning-related modifications, we trained rats to utilize the sensory signal from a single intact whisker to carry out a behavioral task. Once a rat had mastered the task, we clipped its "trained" whisker and attached a "prosthetic" one to a different whisker stub. We then tested the rat to determine how quickly it could relearn the task by using the new whisker. We observed that rats were immediately able to use the prosthetic whisker if it were attached to the stub of the trained whisker but not if it were attached to a different stub. Indeed, the greater the distance between the trained and prosthetic whisker, the more trials were needed to relearn the task. We hypothesized that this "transfer" of learning between whiskers might depend on how much the representations of individual whiskers overlap in primary somatosensory cortex. Testing this hypothesis by using 100-electrode cortical recordings, we found that the overlap between the cortical response patterns of two whiskers accounted well for the transfer of learning between them: The correlation between the electrophysiological and behavioral data was very high (r = 0.98). These findings suggest that a topographically distributed memory trace for sensory-perceptual learning may reside in primary sensory cortex.

Topographic organization of human visual areas in the absence of input from primary cortex

Recently, there has been evidence for considerable plasticity in primary sensory areas of adult cortex. In this study, we asked to what extent topographical maps in human extrastriate areas reorganize after damage to a portion of primary visual (striate) cortex, V1. Functional magnetic resonance imaging signals were measured in a subject (G.Y.) with a large calcarine lesion that includes most of primary visual cortex but spares the foveal representation. When foveal stimulation was present, intact cortex in the lesioned occipital lobe exhibited conventional retinotopic organization. Several visual areas could be identified (V1, V2, V3, V3 accessory, and V4 ventral). However, when stimuli were restricted to the blind portion of the visual field, responses were found primarily in dorsal extrastriate areas. Furthermore, cortex that had formerly shown normal topography now represented only the visual field around the lower vertical meridian. Several possible sources for this reorganized activity are considered, including transcallosal connections, direct subcortical projections to extrastriate cortex, and residual inputs from V1 near the margin of the lesion. A scheme is described to explain how the reorganized signals could occur based on changes in the local neural connections.

Selective disorders of reading?

Over the past few decades, refined cognitive architectures with highly specific components have been proposed to explain apparently selective disorders of reading, resulting from brain disease or injury, in previously literate adults. Recent analysis of the more general linguistic and cognitive abilities supported by neural systems damaged in the various forms of alexia favours a rather different view of reading and the kinds of models sufficient to account for its acquisition, skilled performance and disruption.

Maps versus clusters: different representations of auditory space in the midbrain and forebrain

The auditory system determines the location of stimuli based on the evaluation of specific cues. The analysis begins in the tonotopic pathway, where these cues are processed in parallel, frequency-specific channels. This frequency-specific information is processed further in the midbrain and in the forebrain by specialized, space-processing pathways that integrate information across frequency channels, creating high-order neurons tuned to specific locations in space. Remarkably, the results of this integrative step are represented very differently in the midbrain and forebrain: in the midbrain, space is represented in maps, whereas, in the forebrain, space is represented in clusters of similarly tuned neurons. We propose that these different representations reflect the different roles that these two brain areas have in guiding behavior.

Where the abstract feature maps of the brain might come from

Three types of neuronal organization can be called 'brain maps': sets of feature-sensitive cells, ordered projections between neuronal layers and ordered maps of abstract features. The latter are most intriguing as they reflect the central properties of an organism's experiences and environment. It is proposed that such feature maps are learned in a process that involves parallel input to neurons in a brain area and adaptation of neurons in the neighborhood of the cells that respond most strongly to this input. This article presents a new mathematical formulation for such adaptation and relates it to physiological functions.

Transcranial magnetic stimulation can measure and modulate learning and memory

The potential uses for Transcranial Magnetic Stimulation (TMS) in the study of learning and memory range from a method to map the topography and intensity of motor output maps during visuomotor learning to inducing reversible lesions that allow for the precise temporal and spatial dissection of the brain processes underlying learning and remembering. Single-pulse TMS appears to be adequate to examine motor output maps but repetitive TMS (rTMS) appears necessary to affect most cognitive processes in measurable ways. The results we have reviewed in this article indicate that rTMS may have a potential clinical application in patients with epilepsy in whom it is important to identify the lateralization of verbal memory. Single-pulse TMS can help identify changes in motor output maps during training, that may indicate improved or diminished learning and memory processes following a stroke or other neurological insult. Other evidence indicates that rTMS may even have the capability of facilitating various aspects of memory performance. From a research perspective. rTMS has demonstrated site- and time-specific effects primarily in interfering with explicit retrieval of episodic information from long-term memory. rTMS may also be able to modulate retrieval from semantic memory as evidenced by response-time and accuracy changes after rTMS. All these findings suggest that the use of transcranial magnetic stimulation in the study of learning and memory will increase in the future and that it is already a valuable tool in the cognitive neuroscientists' belt.

CRE-mediated gene transcription in neocortical neuronal plasticity during the developmental critical period

Neuronal activity-dependent processes are believed to mediate the formation of synaptic connections during neocortical development, but the underlying intracellular mechanisms are not known. In the visual system, altering the pattern of visually driven neuronal activity by monocular deprivation induces cortical synaptic rearrangement during a postnatal developmental window, the critical period. Here, using transgenic mice carrying a CRE-lacZ reporter, we demonstrate that a calcium- and cAMP-regulated signaling pathway is activated following monocular deprivation. We find that monocular deprivation leads to an induction of CRE-mediated lacZ expression in the visual cortex preceding the onset of physiologic plasticity, and this induction is dramatically downregulated following the end of the critical period. These results suggest that CRE-dependent coordinate regulation of a network of genes may control physiologic plasticity during postnatal neocortical development.

Single-cell correlates of a representational boundary in rat somatosensory cortex

In primary somatosensory cortex (S1), the transition from one representation to the next is typically abrupt when assayed physiologically. However, the extent of anatomical projections to and within the cortex do not strictly respect these physiologically defined transitions. Physiological properties, such as synaptic strengths or intracortical inhibition, have been hypothesized to account for the functionally defined precision of these representational borders. Because these representational borders can be translocated across the cortex by manipulations or behaviors that change the activity patterns of inputs to the cortex, understanding the physiological mechanisms that delimit representations is also an important starting point for understanding cortical plasticity. A novel in vivo and in vitro preparation has been developed to examine the cellular and synaptic mechanisms that underlie representational borders in the rat. In vivo, a short segment of the border between the forepaw-lower jaw representations in rat S1 was mapped using standard electrophysiological methods and was visibly marked using iontophoresis of pontamine sky blue dye. Slices were then obtained from this marked region and maintained in vitro. Intracellularly recorded responses to electrical stimulation of supragranular cortex were obtained from single neurons near the border in response to stimulation within the representational zone or across the border. Both excitatory and inhibitory responses were smaller when evoked by stimuli that activated projections that crossed borders, as compared with stimuli to projections that did not. These findings indicate that intracortical network properties are contributing to the expressions of representational discontinuities in the cortex.