Long-term changes in synaptic strength along specific intrinsic pathways in the cat visual cortex

Induction of excitatory brain state governs plastic functional changes in visual cortical topology

Adult visual plasticity underlying local remodeling of the cortical circuitry in vivo appears to be associated with a spatiotemporal pattern of strongly increased spontaneous and evoked activity of populations of cells. Here we review and discuss pioneering work by us and others about principles of plasticity in the adult visual cortex, starting with our study which showed that a confined lesion in the cat retina causes increased excitability in the affected region in the primary visual cortex accompanied by fine-tuned restructuring of neuronal function. The underlying remodeling processes was further visualized with voltage-sensitive dye (VSD) imaging that allowed a direct tracking of retinal lesion-induced reorganization across horizontal cortical circuitries. Nowadays, application of noninvasive stimulation methods pursues the idea further of increased cortical excitability along with decreased inhibition as key factors for the induction of adult cortical plasticity. We used high-frequency transcranial magnetic stimulation (TMS), for the first time in combination with VSD optical imaging, and provided evidence that TMS-amplified excitability across large pools of neurons forms the basis for noninvasively targeting reorganization of orientation maps in the visual cortex. Our review has been compiled on the basis of these four own studies, which we discuss in the context of historical developments in the field of visual cortical plasticity and the current state of the literature. Overall, we suggest markers of LTP-like cortical changes at mesoscopic population level as a main driving force for the induction of visual plasticity in the adult. Elevations in excitability that predispose towards cortical plasticity are most likely a common property of all cortical modalities. Thus, interventions that increase cortical excitability are a promising starting point to drive perceptual and potentially motor learning in therapeutic applications.

Lifelong changes of neurotransmitter receptor expression and debilitation of hippocampal synaptic plasticity following early postnatal blindness

In the weeks immediately after onset of sensory loss, extensive reorganization of both the cortex and hippocampus occurs. Two fundamental characteristics comprise widespread changes in the relative expression of GABA and glutamate receptors and debilitation of hippocampal synaptic plasticity. Here, we explored whether recovery from adaptive changes in the expression of plasticity-related neurotransmitter receptors and hippocampal synaptic plasticity occurs in the time-period of up to 12 months after onset of sensory loss. We compared receptor expression in CBA/J mice that develop hereditary blindness, with CBA/CaOlaHsd mice that have intact vision and no deficits in other sensory modalities throughout adulthood. GluN1-subunit expression was reduced and the GluN2A:GluN2B ratio was persistently altered in cortex and hippocampus. GABA-receptor expression was decreased and metabotropic glutamate receptor expression was altered. Hippocampal synaptic plasticity was persistently compromised in vivo. But although LTP in blind mice was chronically impaired throughout adulthood, a recovery of the early phase of LTP became apparent when the animals reached 12 months of age. These data show that cortical and hippocampal adaptation to early postnatal blindness progresses into advanced adulthood and is a process that compromises hippocampal function. A partial recovery of hippocampal synaptic plasticity emerges in advanced adulthood, however.

Interneuron Simplification and Loss of Structural Plasticity As Markers of Aging-Related Functional Decline

Changes in excitatory neuron and synapse structure have been recognized as a potential physical source of age-related cognitive decline. Despite the importance of inhibition to brain plasticity, little is known regarding aging-associated changes to inhibitory neurons. Here we test for age-related cellular and circuit changes to inhibitory neurons of mouse visual cortex. We find no substantial difference in inhibitory neuron number, inhibitory neuronal subtypes, or synapse numbers within the cerebral cortex of aged mice compared with younger adults. However, when comparing cortical interneuron morphological parameters, we find differences in complexity, suggesting that arbors are simplified in aged mice. In vivo two-photon microscopy has previously shown that in contrast to pyramidal neurons, inhibitory interneurons retain a capacity for dendritic remodeling in the adult. We find that this capacity diminishes with age and is accompanied by a shift in dynamics from balanced branch additions and retractions to progressive prevalence of retractions, culminating in a dendritic arbor that is both simpler and more stable. Recording of visually evoked potentials shows that aging-related interneuron dendritic arbor simplification and reduced dynamics go hand in hand with loss of induced stimulus-selective response potentiation (SRP), a paradigm for adult visual cortical plasticity. Chronic treatment with the antidepressant fluoxetine reversed deficits in interneuron structural dynamics and restored SRP in aged animals. Our results support a structural basis for age-related impairments in sensory perception, and suggest that declines in inhibitory neuron structural plasticity during aging contribute to reduced functional plasticity.SIGNIFICANCE STATEMENT Structural alterations in neuronal morphology and synaptic connections have been proposed as a potential physical basis for age-related decline in cognitive function. Little is known regarding aging-associated changes to inhibitory neurons, despite the importance of inhibitory circuitry to adult cortical plasticity and the reorganization of cortical maps. Here we show that brain aging goes hand in hand with progressive structural simplification and reduced plasticity of inhibitory neurons, and a parallel decline in sensory map plasticity. Fluoxetine treatment can attenuate the concurrent age-related declines in interneuron structural and functional plasticity, suggesting it could provide an important therapeutic approach for mitigating sensory and cognitive deficits associated with aging.

Adult plasticity and cortical reorganization after peripheral lesions

Following loss of input due to peripheral lesions, functional reorganization occurs in the deprived cortical region in adults. Over a period of hours to months, cells in the lesion projection zone (LPZ) begin to respond to novel stimuli. This reorganization is mediated by two processes: a reduction of inhibition in a gradient throughout the cortex and input remapping via sprouting of axonal arbors from cortical regions spatially adjacent to the LPZ, and strengthening of pre-existing subthreshold inputs. Together these inputs facilitate receptive field remapping of cells in the LPZ. Recent experiments have revealed time courses and potential interactions of the mechanisms associated with functional reorganization, suggesting that large scale reorganization in the adult may utilize plasticity mechanisms prominent during development.

Diverse excitatory and inhibitory synaptic plasticity outcomes in complex horizontal circuits near a functional border of adult neocortex

The primary somatosensory cortex (SI) is topographically organized into a map of the body. This organization is dynamic, undergoing experience-dependent modifications throughout life. It has been hypothesized that excitatory and inhibitory synaptic plasticity of horizontal intracortical connections contributes to functional reorganization. However, very little is known about synaptic plasticity of these connections; particularly the characteristics of inhibitory synaptic plasticity, its relationship to excitatory synaptic plasticity, and their relationship to the functional organization of the cortex. To investigate this, we located the border between the forepaw and lower jaw representation of SI in vivo, and used whole cell-patch electrophysiology to record post-synaptic excitatory and inhibitory currents in complex horizontal connections in vitro. Connections that remained within the representation (continuous) and those that crossed from one representation to another (discontinuous) were stimulated differentially, allowing us to examine differences associated with the border. To induce synaptic plasticity, tetanic stimulation was applied to either continuous or discontinuous pathways. Tetanic stimulation induced diverse forms of excitatory and inhibitory synaptic plasticity, with LTP dominating for excitation and LTD dominating for inhibition. The border did not restrict plasticity in either case. In contrast, tetanization elicited LTP of monosynaptic inhibitory responses in continuous, but not discontinuous connections. These results demonstrate that continuous and discontinuous pathways are capable of diverse synaptic plasticity responses that are differentially inducible. Furthermore, continuous connections can undergo monosynaptic inhibitory LTP, independent of excitatory drive onto interneurons. Thus, coordinated excitatory and inhibitory synaptic plasticity of horizontal connections are capable of contributing to functional reorganization.

Structural basis for the role of inhibition in facilitating adult brain plasticity

While inhibition has been implicated in mediating plasticity in the adult brain, the mechanism remains unclear. Here we present a structural mechanism for the role of inhibition in experience-dependent plasticity. Using chronic in vivo two-photon microscopy in the mouse neocortex we show that experience drives structural remodeling of superficial layer 2/3 interneurons in an input- and circuit-specific manner, with up to 16% of branch tips remodeling. Visual deprivation initially induces dendritic branch retractions accompanied by loss of inhibitory inputs onto neighboring pyramidal cells. The resulting decrease in inhibitory tone, also achievable pharmacologically by the antidepressant fluoxetine, provides a permissive environment for further structural adaptation, including addition of new synapse bearing branch tips. Our findings suggest that therapeutic approaches that reduce inhibition, when combined with an instructive stimulus, could facilitate restructuring of mature circuits impaired by damage or disease, improving function and perhaps enhancing cognitive abilities.

Plasticity of horizontal connections at a functional border in adult rat somatosensory cortex

Horizontal connections in superficial cortical layers integrate information across sensory maps by connecting related functional columns. It has been hypothesized that these connections mediate cortical reorganization via synaptic plasticity. However, it is not known if the horizontal connections from discontinuous cortical regions can undergo plasticity in the adult. Here we located the border between two discontinuous cortical representations in vivo and used either pairing or low-frequency stimulation to induce synaptic plasticity in the horizontal connections surrounding this border in vitro. Individual neurons revealed significant and diverse forms of synaptic plasticity for horizontal connections within a continuous representation and discontinuous representations. Interestingly, both enhancement and depression were observed following both plasticity paradigms. Furthermore, plasticity was not restricted by the border's presence. Depolarization in the absence of synaptic stimulation also produced synaptic plasticity, but with different characteristics. These experiments suggest that plasticity of horizontal connections may mediate functional reorganization.

Increased motor cortical excitability after whole-hand electrical stimulation: a TMS study

OBJECTIVE

To examine the neuromodulatory effect of whole-hand mesh-glove (MG) stimulation on motor cortical pathways, we explored motor cortical excitability before and after suprathreshold whole-hand MG stimulation using transcranial magnetic stimulation (TMS).

METHODS

Twenty-eight healthy volunteers (14 controls) were studied at baseline, immediately post and 1h post-MG stimulation for 30 min. Motor thresholds (MTs), motor evoked potentials (MEPs) recruitment curve, short intracortical inhibition (SICI) and intracortical facilitation (ICF) after paired magnetic stimuli were evaluated.

RESULTS

After MG stimulation the MTs were significantly reduced and slope of MEP recruitment curve significantly increased; furthermore, the stimulation led to a sustained decrease of SICI and increase of ICF in the contralateral motor cortex. These effects lasted for at least 60 min and were stronger 1h post-stimulation compared with testing immediately after stimulation. A sham group did not show any differences before and after MG stimulation.

CONCLUSIONS

We provide a first demonstration that MG whole-hand stimulation induces increases in motor cortical excitability lasting at least 1h. Both the strength of the corticospinal projections and the inhibitory and facilitatory intracortical mechanisms are involved. Synaptic modifications such as long-term potentiation mechanisms may underlie this stimulation-induced cortical plasticity changes.

SIGNIFICANCE

Present results prove the MG stimulation to be a promising tool in neurorehabilitation.

Synapses of horizontal connections in adult rat somatosensory cortex have different properties depending on the source of their axons

In somatosensory cortex (S1) tactile stimulation activates specific regions. The borders between representations of different body parts constrain the spread of excitation and inhibition: connections that cross from one representation to another (cross-border, CB) are weaker than those remaining within the representation (noncross border, NCB). Thus, physiological properties of CB and NCB synapses onto layer 2/3 pyramidal neurons were compared using whole-cell recordings in layer 2/3 neurons close to the border between the forepaw and lower jaw representations. Electrical stimulation of CB and NCB connections was used to activate synaptic potentials. Properties of excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials (PSP) were determined using 3 methods: 1) minimal stimulation to elicit single-fiber responses; 2) stimulation in the presence of extracellular Sr(2+) to elicit asynchronous quantal responses; 3) short trains of stimulation at various frequencies to examine postsynaptic potential (PSP) dynamics. Both minimal and asynchronous quantal EPSPs were smaller when evoked by CB than NCB stimulation. However, the dynamics of EPSP and IPSP trains were not different between CB and NCB stimulation. These data suggest that individual excitatory synapses from connections that cross a border (CB) have smaller amplitudes than those that come from within a representation (NCB), and suggest a postsynaptic locus for the difference.

Strengthening of lateral activation in adult rat visual cortex after retinal lesions captured with voltage-sensitive dye imaging in vivo

Sensory deprivation caused by peripheral injury can trigger functional cortical reorganization across the initially silenced cortical area. It is proposed that intracortical connectivity enables recovery of function within such a lesion projection zone (LPZ), thus substituting lost subcortical input. Here, we investigated retinal lesion-induced changes in the function of lateral connections in the primary visual cortex of the adult rat. Using voltage-sensitive dye recordings, we visualized in millisecond-time resolution spreading synaptic activity across the LPZ. Shortly after lesion, the majority of neurons within the LPZ were subthresholdly activated by delayed propagation of activity that originated from unaffected cortical regions. With longer recovery time, latencies within the LPZ gradually decreased, and activation reached suprathreshold levels. Targeted electrode recordings confirmed that receptive fields of intra-LPZ neurons were displaced to the retinal lesion border while displaying normal orientation and direction selectivity. These results corroborate the view that cortical horizontal connections have a central role in functional reorganization, as revealed here by progressive facilitation of synaptic activity and the traveling wave of excitation that propagates horizontally into the deprived cortical region.

Circuitry, architecture and functional dynamics of visual cortex

A fundamental understanding of the mechanisms of cortical processing requires an examination of the relationships of cortical circuitry, cortical functional architecture and receptive field properties. Ultimately, this kind of analysis can be used to explore the neurobiological basis of psychophysics and perception. At the outset our studies were intended to account for the then known receptive field properties of cortical cells in terms of their underlying circuitry but, surprisingly, a good part of the cortical circuit appeared to be in violation of the principles of cortical architecture. This led us to explore the possibility of new, more complex properties of cortical cells. It has become increasingly possible to relate the responsive specificity of cortical cells and the circuitry underlying this specificity to the perceptual capabilities of the visual system by performing analogous experiments on single cells and in human psychophysics.

Cortical reorganization consistent with spike timing–but not correlation-dependent plasticity

The receptive fields of neurons in primary visual cortex that are inactivated by retinal damage are known to 'shift' to nondamaged retinal locations, seemingly due to the plasticity of intracortical connections. We have observed in cats that these shifts occur in a pattern that is highly convergent, even among receptive fields that are separated by large distances before inactivation. Here we show, using a computational model of primary visual cortex, that the observed convergent shifts are inconsistent with the common assumption that the underlying intracortical connection plasticity is dependent on the temporal correlation of pre- and postsynaptic action potentials. The shifts are, however, consistent with the hypothesis that this plasticity is dependent on the temporal order of pre- and postsynaptic action potentials. This convergent reorganization seems to require increased neuronal gain, revealing a mechanism that networks may use to selectively facilitate the didactic transfer of neuronal response properties.

A homeostatic rule for inhibitory synapses promotes temporal sharpening and cortical reorganization

Experience with transient stimuli leads to stronger neural responses that also rise and fall more sharply in time. This sharpening enhances the processing of transients and may be especially relevant for speech perception. We consider a learning rule for inhibitory connections that promotes this sharpening effect by adjusting these connections to maintain a target homeostatic level of activity in excitatory neurons. We analyze this rule in a recurrent network model of excitatory and inhibitory units. Strengthening inhibitory-->excitatory connections along with excitatory-->excitatory connections is required to obtain a sharpening effect. Using the homeostatic rule, we show that repeated presentations of a transient signal will "teach" the network to respond to the signal with both higher amplitude and shorter duration. The model also captures reorganization of receptive fields in the sensory hand area after amputation or peripheral nerve resection.

Dynamics and specificity of cortical map reorganization after retinal lesions

Neurons in the mature visual cortex deprived of their normal retinotopic inputs by matched binocular retinal lesions are initially silenced but become reactivated with time when the "blind" cortical lesion projection zone (LPZ) is filled in by new suprathreshold visual responses. In an attempt to gain further insight into the dynamics of this process, we investigated in detail the spatiotemporal pattern of single-cell properties and recording probability during cortical reorganization up to 12 months after retinal lesions. In the early phases of filling in, a transient peak of hyperactivity moves from the border of the normal cortex into the LPZ and forms the leading edge of a functional reconnection process. In the course of this process hyperactive cells inside the LPZ develop ectopic receptive fields that are initially enlarged and regain orientation specificity. During the proceeding recovery, hyperactivity and receptive field size normalize, while the quality of orientation tuning remains reduced at longer distances inside the LPZ at all stages of recovery up to 1 year. Within the adult anatomical framework of cortical connectivity, the maximal lateral distance of reconnection is limited, and the probability to encounter spiking cells decreases with increasing distance inside the LPZ. However, this recording probability was significantly increased after 1 year.

Intracortical mechanism of stimulus-timing-dependent plasticity in visual cortical orientation tuning

Visual stimuli are known to induce various changes in the receptive field properties of adult cortical neurons, but the underlying mechanisms are not well understood. Repetitive pairing of stimuli at two orientations can induce a shift in cortical orientation tuning, with the direction and magnitude of the shift depending on the temporal order and interval between the pair. Although the temporal specificity of the effect on the order of tens of milliseconds strongly suggests spike-timing-dependent synaptic plasticity (STDP) as the underlying mechanism, it remains unclear whether the modification occurs within the cortex or at earlier stages of the visual pathway. In the present study, we examined the involvement of an intracortical mechanism in this functional modification. First, we measured interocular transfer of the shift induced by monocular conditioning. We found complete transfer of the effect at both the physiological and psychophysical levels, indicating that the modification occurs largely in the cortex. Second, we analyzed the spike timing of cortical neurons during conditioning and found it commensurate with the requirement of STDP. Finally, we compared the measured shift in orientation tuning with the prediction of a model circuit that exhibits STDP at intracortical connections. This model can account for not only the temporal specificity of the effect but also the dependence of the shift on both orientations in the conditioning pair. These results indicate that modification of intracortical connections is a key mechanism in the stimulus-timing-dependent plasticity in orientation tuning.

Synaptic basis for developmental plasticity in somatosensory cortex

Sensory experience drives plasticity of the body map in developing and adult somatosensory cortex, but the synaptic mechanisms underlying such plasticity are not well understood. Recently, several mechanisms that are likely to contribute to map plasticity have been directly observed in response to altered experience in vivo. These mechanisms include long-term potentiation and long-term depression at specific excitatory synapses, competition between lemniscal (barrel) and non-lemniscal (septal) processing streams, and regulation of the number of inhibitory synapses.

Glutamate levels and transport in cat (Felis catus) area 17 during cortical reorganization following binocular retinal lesions

Glutamate is known to play a crucial role in the topographic reorganization of visual cortex after the induction of binocular central retinal lesions. In this study we investigated the possible involvement of the glial high-affinity Na+/K+-dependent glutamate transporters in cortical plasticity using western blotting and intracortical microdialysis. Basal extracellular glutamate levels and the re-uptake activity for glutamate have been determined by comparing the extracellular glutamate concentration before and during the blockage of glutamate removal from the synaptic cleft with the potent transporter inhibitor l-trans-pyrrolidine-3,4-dicarboxylic acid. In cats with central retinal lesions we observed increased basal extracellular glutamate concentrations together with a decreased re-uptake activity in non-deprived, peripheral area 17, compared with the sensory-deprived, central cortex of the same animal as well as the topographically matching regions of area 17 in normal subjects. Western blotting experiments revealed a parallel decrease in the expression level of the glial glutamate transporter proteins GLT-1 and GLAST in non-deprived cortex compared with sensory-deprived cortex of lesion cats and the corresponding regions of area 17 of normal subjects. This study shows that partial sensory deprivation of the visual cortex affects the removal of glutamate from the synaptic cleft and implicates a role for glial-neuronal interactions in adult brain plasticity.

Spatio-temporal plasticity of cortical receptive fields in response to repetitive visual stimulation in the adult cat

Many psychophysical experiments on perceptual learning in humans show increases of performance that are most probably based on functions of early visual cortical areas. Long-term plasticity of the primary visual cortex has so far been shown in vivo with the use of visual stimuli paired with electrical or pharmacological stimulation at the cellular level. Here, we report that plasticity in the adult visual cortex can be achieved by repetitive visual stimulation. First, spatial receptive field profiles of single units (n=38) in area 17 or 18 of the anesthetized cat were determined with optimally oriented flashing light bars. Then a conditioning protocol was applied to induce associative synaptic plasticity. The receptive field center and an unresponsive region just outside the excitatory receptive field were synchronously stimulated ('costimulation', repetition rate 1 Hz; for 10-75 min). After costimulation the receptive field and its adjacent regions were mapped again. We observed specific increases of the receptive field size, changes of the receptive field subfield structure as well as shifts in response latency. In 37% of the cells the receptive field size increased specifically towards the stimulated side but not towards the non-stimulated opposite side of the receptive field. In addition, changes in the relative strength and size of the on and off subfield regions were observed. These specific alterations were dependent on the level of neuronal activity during costimulation. During recovery, the new responses dropped down to 120% of the preconditioning value on average within 103 min; however, the decay times significantly depended on the response magnitude after costimulation. In the temporal domain, the latency of new responses appeared to be strongly influenced by the latency of the response during costimulation.Twenty-nine percent of the units displayed no receptive field enlargement, most likely because the activity during costimulation was significantly lower than in the cases with enlarged receptive fields. An unspecific receptive field enlargement towards both the stimulated and non-stimulated side was observed in 34% of the tested cells. In contrast to the cells with specifically enlarged receptive fields, the unspecific increase of receptive field size was always accompanied by a strong increase of the general activity level. We conclude that the receptive field changes presumably took place by strengthening of synaptic inputs at the recorded cells in a Hebbian way as previously shown in the visual cortex in vitro and in vivo. The observed receptive field changes may be related to preattentive perceptual learning and could represent a basis of the 'filling in' of cortical scotomas obtained with specific training procedures in human patients suffering from visual cortex lesions.

Local circuit properties underlying cortical reorganization

Peripheral denervation has been shown to cause reorganization of the deafferented somatotopic region in primary somatosensory cortex (S1). However, the basic mechanisms that underlie reorganization are not well understood. In the experiments described in this paper, a novel in vivo/in vitro preparation of adult rat S1 was used to determine changes in local circuit properties associated with the denervation-induced plasticity of the cortical representation in rat S1. In the present studies, deafferentation of rat S1 was induced by cutting the radial and median nerves in the forelimb of adult rats, resulting in a rapid shift of the location of the forepaw/lower jaw border; the amount of the shift increased over the times assayed, through 28 days after denervation. The locations of both borders (i.e., original and reorganized) were marked with vital dyes, and slices from the marked region were used for whole-cell recording. Responses were evoked using electrical stimulation of supragranular S1 and recorded in supragranular neurons close to either the original or reorganized border. For each neuron, postsynaptic potentials (PSPs) were evoked by stimulation of fibers that crossed the border site (CB stim) and by equivalent stimulation that did not cross (NCB stim). Monosynaptic inhibitory postsynaptic potentials (IPSPs) were also examined after blocking excitatory transmission with 15 microM CNQX plus 100 microM DL-APV. The amplitudes of PSPs and IPSPs were compared between CB and NCB stimulation to quantify effects of the border sites on excitation and inhibition. Previous results using this preparation in the normal (i.e., without induced plasticity) rat S1 demonstrated that at a normal border both PSPs and IPSPs were smaller when evoked with CB stimulation than with NCB stimulation. For most durations of denervation, a similar bias (i.e., smaller responses with CB stimulation) for PSPs and IPSPs was observed at the site of the novel reorganized border, while no such bias was observed at the suppressed original border site. Thus changes in local circuit properties (excitation and inhibition) can reflect larger-scale changes in cortical organization. However, specific dissociations between these local circuit properties and the presence of the novel border at certain durations of denervation were also observed, suggesting that there are several intracortical processes contributing to cortical reorganization over time and that excitation and inhibition may contribute differentially to them.

Synaptic physiology of the flow of information in the cat's visual cortex in vivo

Each stage of the striate cortical circuit extracts novel information about the visual environment. We asked if this analytic process reflected laminar variations in synaptic physiology by making whole-cell recording with dye-filled electrodes from the cat's visual cortex and thalamus; the stimuli were flashed spots. Thalamic afferents terminate in layer 4, which contains two types of cell, simple and complex, distinguished by the spatial structure of the receptive field. Previously, we had found that the postsynaptic and spike responses of simple cells reliably followed the time course of flash-evoked thalamic activity. Here we report that complex cells in layer 4 (or cells intermediate between simple and complex) similarly reprised thalamic activity (response/trial, 99 +/- 1.9 %; response duration 159 +/- 57 ms; latency 25 +/- 4 ms; average +/- standard deviation; n = 7). Thus, all cells in layer 4 share a common synaptic physiology that allows secure integration of thalamic input. By contrast, at the second cortical stage (layer 2+3), where layer 4 directs its output, postsynaptic responses did not track simple patterns of antecedent activity. Typical responses to the static stimulus were intermittent and brief (response/trial, 31 +/- 40 %; response duration 72 +/- 60 ms, latency 39 +/- 7 ms; n = 11). Only richer stimuli like those including motion evoked reliable responses. All told, the second level of cortical processing differs markedly from the first. At that later stage, ascending information seems strongly gated by connections between cortical neurons. Inputs must be combined in newly specified patterns to influence intracortical stages of processing.

Cortical plasticity revealed by circumscribed retinal lesions or artificial scotomas

We review the work of others in which the effects of circumscribed, topographically corresponding binocular retinal lesions on the topographic organization of the visual cortex revealed that there is a substantial degree of topographical plasticity in the primary visual cortices of adult cats and macaque monkeys. Despite the evidence indicating that the reorganization of the topographic map in primary visual cortices of adult cats and macaques related to the input from one eye could be suppressed for a long time by inputs related to the other eye, we observed a substantial degree of topographical plasticity in the primary visual cortices of adult cats in which we have made circumscribed monocular retinal lesions. Overall, in both binocularly and monocularly lesioned adult animals, most cells recorded in the cortical projection zone of the retinal lesion (LPZ), several hours, several weeks or several months after placement of the lesions exhibited 'ectopic' excitatory visual receptive fields (RFs) which were displaced to the normal retina in the immediate vicinity of the lesion. The presence of ectopic RFs in cells recorded in the cortical LPZ, combined with the presence of normal cortical representation of the part of the retina in the vicinity of the lesion, indicate a clear expansion of the cortical representation of the part of the retina surrounding the lesion. When stimulated via the ectopic RFs, cortical cells exhibited normal orientation tuning and in the case of animals with monocular lesions, the orientation tuning of binocular cells when stimulated via ectopic RFs appeared to be very similar to that when the cells were stimulated via the RFs in the normal, unlesioned eye. In both binocularly and monocularly lesioned animals, the responses evoked by optimal visual stimuli from the ectopic RFs were substantially weaker than those evoked from their normal counterparts. Similarly, upper velocity limits were significantly lower when visual stimuli were presented via the ectopic RFs. In contrast to cats in which the retinal lesions were made in adulthood, in cats lesioned monocularly in adolescence (8-11 weeks postnatal), both the peak discharge rates and upper velocity limits of responses to photic stimuli presented via the ectopic RFs were very similar to those to stimuli presented via the normal eye. The intracortical mechanism(s) underlying the long-term cortical plasticity revealed by retinal lesions are likely to be closely linked to the mechanism(s) underlying the short-term reversible enlargement of cortical receptive fields observed with artificial scotomas. Furthermore, a similar putative intracortical mechanism(s) appears to underlie psychophysical phenomena observed in studies of retinal scotomas in humans. Overall, the research reviewed here strongly challenges the view that receptive fields of neurons in mammalian visual cortices are 'hard-wired'.

Mechanisms underlying rapid experience-dependent plasticity in the human visual cortex

Visual deprivation induces a rapid increase in visual cortex excitability that may result in better consolidation of spatial memory in animals and in lower visual recognition thresholds in humans. gamma-Aminobutyric acid (GABA)ergic, N-methyl-d-aspartate (NMDA), and cholinergic receptors are thought to be involved in visual cortex plasticity in animal studies. Here, we used a pharmacological approach and found that lorazepam (which enhances GABA(A) receptor function by acting as a positive allosteric modulator), dextrometorphan (NMDA receptor antagonist), and scopolamine (muscarinic receptor antagonist) blocked rapid plastic changes associated with light deprivation. These findings suggest the involvement of GABA, NMDA, and cholinergic receptors in rapid experience-dependent plasticity in the human visual cortex.

Viewpoint dependency in visual object recognition does not necessarily imply viewer-centered representation

The nature of visual object representation in the brain is the subject of a prolonged debate. One set of theories asserts that objects are represented by their structural description and the representation is "object-centered." Theories from the other side of the debate suggest that humans store multiple "snapshots" for each object, depicting it as seen under various conditions, and the representation is therefore "viewer-centered." The principal tool that has been used to support and criticize each of these hypotheses is subjects' performance in recognizing objects under novel viewing conditions. For example, if subjects take more time in recognizing an object from an unfamiliar viewpoint, it is common to claim that the representation of that object is viewpoint-dependent and therefore viewer-centered. It is suggested here, however, that performance cost in recognition of objects under novel conditions may be misleading when studying the nature of object representation. Specifically, it is argued that viewpoint-dependent performance is not necessarily an indication of viewer-centered representation. An account for the neural basis of perceptual priming is first provided. In light of this account, it is conceivable that viewpoint dependency reflects the utilization of neural paths with different levels of sensitivity en route to the same representation, rather than the existence of viewpoint-specific representations. New experimental paradigms are required to study the validity of the viewer-centered approach.

Sequence dependence of post-tetanic potentiation after sequential heterosynaptic stimulation in the rat auditory cortex

1. To investigate the mechanisms for the coding stimulus sequence in the auditory cortex (AC), post-tetanic potentiation (PTP) was recorded after sequentially combined heterosynaptic stimulation was applied in rat AC slices. 2. Brief tetanic stimulation (TS) was applied at two sites on AC slices at intervals of 0.5-10 s. PTP of field potentials was induced by the earlier TS, rather than the later TS. PTP was followed by sequence-dependent long-term potentiation (LTP). 3. Using Ca(2+) imaging in the slices loaded with rhod-2, a Ca(2+) indicator, a sequence-dependent distribution of PTP was found in AC slices. 4. The sequence-dependent PTP in excitatory postsynaptic potentials (EPSPs) was observed in supragranular pyramidal neurons. 5. The sequence dependence of PTP was not significantly affected by 1 microM bicuculline, an antagonist of GABA(A) receptors, or 100 microM 2-hydroxysaclofen, an antagonist of GABA(B) receptors. 6. Depolarization and firing recorded in pyramidal neurons during the later TS were less vigorous than when the slices were incubated in the control medium. However, this suppression of the responses during the later TS was not observed in the presence of 50 microM atropine, an antagonist of muscarinic receptors. 7. PTP was induced by the earlier and later TS in the presence of 50 microM atropine, so that the sequence dependence of PTP was abolished. Pirenzepine (50 microM), an antagonist of muscarinic M1 receptors, but not methoctramine (30 microM), an antagonist of M2 receptors, eliminated the sequence dependence of PTP. 8. These findings suggest that the sequence dependence of PTP in AC might have a role in the temporal processing of auditory information on the scale of seconds.

Intracortical pathway involving dysgranular cortex conveys hindlimb inputs to S-I forelimb-stump representation of neonatally amputated rats

Reorganization of the primary somatosensory cortex (S-I) forelimb-stump representation of rats that sustained neonatal forelimb removal is characterized by the expression of hindlimb inputs that are revealed when cortical GABA receptors are pharmacologically blocked. Recent work has shown that the majority of these inputs are transmitted from the S-I hindlimb representation to the forelimb-stump field via an, as yet, unidentified pathway between these regions. In this study, we tested the possibility that hindlimb inputs to the S-I forelimb-stump representation of neonatally amputated rats are conveyed through an intracortical pathway between the S-I hindlimb and forelimb-stump representations that involves the intervening dysgranular cortex by transiently inactivating this area and evaluating the effect on hindlimb expression in the S-I forelimb-stump representation during GABA receptor blockade. Of 332 S-I forelimb-stump recording sites from six neonatally amputated rats, 68.3% expressed hindlimb inputs during GABA receptor blockade. Inactivation of dysgranular cortex with cobalt chloride (CoCl(2)) resulted in a significant decrease in the number of hindlimb responsive sites (9.5%, P < 0.001 vs. cortex during GABA receptor blockade before CoCl(2) treatment). Results were also compiled from S-I forelimb recording sites from three normal rats: 14.1% of 136 sites were responsive to the hindlimb during GABA receptor blockade, and all of these responses were abolished during inactivation of dysgranular cortex with CoCl(2) (P < 0.05). These results indicate that the S-I hindlimb representation transmits inputs to the forelimb-stump field of neonatally amputated rats through a polysynaptic intracortical pathway involving dysgranular cortex. Furthermore the findings from normal rats suggest that this pathway might reflect the amplification of a neuronal circuit normally present between the two representations.

Visual responses of neurons in the middle temporal area of new world monkeys after lesions of striate cortex

In primates, lesions of striate cortex (V1) result in scotomas in which only rudimentary visual abilities remain. These aspects of vision that survive V1 lesions have been attributed to direct thalamic pathways to extrastriate areas, including the middle temporal area (MT). However, studies in New World monkeys and humans have questioned this interpretation, suggesting that remnants of V1 are responsible for both the activation of MT and residual vision. We studied the visual responses of neurons in area MT in New World marmoset monkeys in the weeks after lesions of V1. The extent of the scotoma in each case was estimated by mapping the receptive fields of cells located near the lesion border and by histological reconstruction. Two response types were observed among the cells located in the part of MT that corresponds, in visuotopic coordinates, to the lesioned part of V1. Many neurons (62%) had receptive fields that were displaced relative to their expected location, so that they represented the visual field immediately surrounding the scotoma. This may be a consequence of a process analogous to the reorganization of the V1 map after retinal lesions. However, another 20% of the cells had receptive fields centered inside the scotoma. Most of these neurons were strongly direction-selective, similar to normal MT cells. These results show that MT cells differ in their responses to lesioning of V1 and that only a subpopulation of MT neurons can be reasonably linked to residual vision and blindsight.

Visual responses of neurons in the middle temporal area of new world monkeys after lesions of striate cortex

In primates, lesions of striate cortex (V1) result in scotomas in which only rudimentary visual abilities remain. These aspects of vision that survive V1 lesions have been attributed to direct thalamic pathways to extrastriate areas, including the middle temporal area (MT). However, studies in New World monkeys and humans have questioned this interpretation, suggesting that remnants of V1 are responsible for both the activation of MT and residual vision. We studied the visual responses of neurons in area MT in New World marmoset monkeys in the weeks after lesions of V1. The extent of the scotoma in each case was estimated by mapping the receptive fields of cells located near the lesion border and by histological reconstruction. Two response types were observed among the cells located in the part of MT that corresponds, in visuotopic coordinates, to the lesioned part of V1. Many neurons (62%) had receptive fields that were displaced relative to their expected location, so that they represented the visual field immediately surrounding the scotoma. This may be a consequence of a process analogous to the reorganization of the V1 map after retinal lesions. However, another 20% of the cells had receptive fields centered inside the scotoma. Most of these neurons were strongly direction-selective, similar to normal MT cells. These results show that MT cells differ in their responses to lesioning of V1 and that only a subpopulation of MT neurons can be reasonably linked to residual vision and blindsight.

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.

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.

Dynamic regulation of cpg15 during activity-dependent synaptic development in the mammalian visual system

During visual system development, neural activity regulates structural changes in connectivity including axonal branching and dendritic growth. Here we have examined a role for the candidate plasticity gene 15 (cpg15), which encodes an activity-regulated molecule that can promote dendritic growth, in this process. We report that cpg15 is expressed in the cat visual system at relatively high levels in the lateral geniculate nucleus (LGN) but at very low levels in its synaptic target, layer 4 of the visual cortex. Prenatally, when cpg15 mRNA in the LGN is most abundant, expression is insensitive to action potential blockade by tetrodotoxin. Postnatally, activity regulation of cpg15 emerges in the LGN coincident with development of ocular dominance columns in the visual cortex. cpg15 can be detected in layers 2/3 and 5/6 of visual cortex postnatally, and expression in layers 2/3 is activity-regulated during known periods of activity-dependent plasticity for these layers. Localization and regulation of cpg15 expression in the visual system are consistent with a presynaptic role for CPG15 in shaping dendritic arbors of target neurons during activity-dependent synaptic rearrangements, both in development and adulthood.

Dynamic regulation of cpg15 during activity-dependent synaptic development in the mammalian visual system

During visual system development, neural activity regulates structural changes in connectivity including axonal branching and dendritic growth. Here we have examined a role for the candidate plasticity gene 15 (cpg15), which encodes an activity-regulated molecule that can promote dendritic growth, in this process. We report that cpg15 is expressed in the cat visual system at relatively high levels in the lateral geniculate nucleus (LGN) but at very low levels in its synaptic target, layer 4 of the visual cortex. Prenatally, when cpg15 mRNA in the LGN is most abundant, expression is insensitive to action potential blockade by tetrodotoxin. Postnatally, activity regulation of cpg15 emerges in the LGN coincident with development of ocular dominance columns in the visual cortex. cpg15 can be detected in layers 2/3 and 5/6 of visual cortex postnatally, and expression in layers 2/3 is activity-regulated during known periods of activity-dependent plasticity for these layers. Localization and regulation of cpg15 expression in the visual system are consistent with a presynaptic role for CPG15 in shaping dendritic arbors of target neurons during activity-dependent synaptic rearrangements, both in development and adulthood.

The role of dendritic filtering in associative long-term synaptic plasticity

Several forms of synaptic plasticity in the neocortex and hippocampus depend on the temporal coincidence of presynaptic activity and postsynaptic trains of action potentials (APs). This requirement is consistent with the Hebbian, or correlational, type of cellular learning rule used in many studies of associative synaptic plasticity. Recent experimental evidence suggests that APs initiated in the axosomatic area are actively back-propagated to the dendritic arborization of neocortical and pyramidal cells. High-frequency trains of postsynaptic APs that are used as conditioning stimuli for the induction of Hebbian-like plasticity in both neocortical and hippocampal pyramidal cells display attenuation of the dendritic AP amplitude during the train. This attenuation has been shown to be modulated by neurotransmitters and by electrical activity. We suggest here that both spike train attenuation in the dendrite and its modulation by neurotransmitters and electrical activity may have important functional consequences on the magnitude and/or the sign of the synaptic plasticity induced by a Hebbian pairing procedure.

Long-term potentiation of Ca2+ signal in the rat auditory cortex

The Ca2+ signal in supragranular layers of the rat auditory cortex (AC) was studied in slice preparations using rhod-2, a Ca2+ indicator. White matter stimulation elicited an increase in the Ca2+ signal, which was maximal in the image taken 34 ms after stimulation. This peak time was the same as that of the Ca2+ signal in pyramidal neurons injected with rhod-2. The intensity of the Ca2+ signal was proportional to the amplitude of the field potentials in supragranular layers. The Ca2+ signal was inhibited almost completely by 200 microM Ni2+ , but only slightly by 50 microM D-2-amino-5-phosphonovalerate (APV), an NMDA-receptor antagonist. Tetanic stimulation of the white matter or supragranular layers elicited long-term potentiation (LTP) of the Ca2+ signal in AC slices, but the potentiation was not clear in slices of the visual cortex (VC). The induction of LTP of the field potentials in AC slices was blocked by 50 microM APV or 50 microM Ni2+. These results indicate that Ca2+ influx through Ni2+ -sensitive Ca2+ channels in pyramidal neurons is potentiated by tetanic stimulation in parallel with LTP of neural activities and might be important for the induction of LTP in AC slices.

Source of inappropriate receptive fields in cortical somatotopic maps from rats that sustained neonatal forelimb removal

Previously this laboratory demonstrated that forelimb removal at birth in rats results in the invasion of the cuneate nucleus by sciatic nerve axons and the development of cuneothalamic cells with receptive fields that include both the forelimb-stump and the hindlimb. However, unit-cluster recordings from primary somatosensory cortex (SI) of these animals revealed few sites in the forelimb-stump representation where responses to hindlimb stimulation also could be recorded. Recently we reported that hindlimb inputs to the SI forelimb-stump representation are suppressed functionally in neonatally amputated rats and that GABAergic inhibition is involved in this process. The present study was undertaken to assess the role that intracortical projections from the SI hindlimb representation may play in the functional reorganization of the SI forelimb-stump field in these animals. The SI forelimb-stump representation was mapped during gamma-aminobutyric acid (GABA)-receptor blockade, both before and after electrolytic destruction of the SI hindlimb representation. Analysis of eight amputated rats showed that 75.8% of 264 stump recording sites possessed hindlimb receptive fields before destruction of the SI hindlimb. After the lesions, significantly fewer sites (13.2% of 197) were responsive to hindlimb stimulation (P < 0.0001). Electrolytic destruction of the SI lower-jaw representation in four additional control rats with neonatal forelimb amputation did not significantly reduce the percentage of hindlimb-responsive sites in the SI stump field during GABA-receptor blockade (P = 0.98). Similar results were obtained from three manipulated rats in which the SI hindlimb representation was silenced temporarily with a local cobalt chloride injection. Analysis of response latencies to sciatic nerve stimulation in the hindlimb and forelimb-stump representations suggested that the intracortical pathway(s) mediating the hindlimb responses in the forelimb-stump field may be polysynaptic. The mean latency to sciatic nerve stimulation at responsive sites in the GABA-receptor blocked SI stump representation of neonatally amputated rats was significantly longer than that for recording sites in the hindlimb representation [26.3 +/- 8.1 (SD) ms vs. 10.8 +/- 2.4 ms, respectively, P < 0.0001]. These results suggest that hindlimb input to the SI forelimb-stump representation detected in GABA-blocked cortices of neonatally forelimb amputated rats originates primarily from the SI hindlimb representation.

Hebbian synapses in cortical and hippocampal pathways

Use-dependent alterations in synaptic efficacy are believed to form the basis for such complex brain functions as learning and memory and significantly contribute to the development of neuronal networks. The algorithm of synapse modification proposed by Hebb as early as 1949 is the coincident activation of pre- and postsynaptic neurons. The present review considers the evolution of experimental protocols in which postsynaptic cell depolarization through the recording microelectrode was used to reveal the manifestation of Hebb-type plasticity in the synaptic inputs of the neocortex and hippocampus. Special attention is focused on the inhibitory control of the Hebb-type plasticity. Disinhibition within the local neuronal circuits is considered to be an important factor in Hebbian plasticity, contributing to such phenomena as priming, primed burst potentiation, hippocampal theta-rhythm and cortical arousal. The role of various transmitters (acetylcholine, norepinephrine, gamma-amino-butyric acid) in disinhibition is discussed with a special emphasis on the brain noradrenergic system. Possible mechanisms of Hebbian synapse modification and their modulation by memory enhancing substances are considered. It is suggested that along with their involvement in disinhibition processes these substances may control Hebb-type plasticity through intracellular second messenger systems.

Evolutionary neurocontrollers for autonomous mobile robots

In this article we describe a methodology for evolving neurocontrollers of autonomous mobile robots without human intervention. The presentation, which spans from technological and methodological issues to several experimental results on evolution of physical mobile robots, covers both previous and recent work in the attempt to provide a unified picture within which the reader can compare the effects of systematic variations on the experimental settings. After describing some key principles for building mobile robots and tools suitable for experiments in adaptive robotics, we give an overview of different approaches to evolutionary robotics and present our methodology. We start reviewing two basic experiments showing that different environments can shape very different behaviours and neural mechanisms under very similar selection criteria. We then address the issue of incremental evolution in two different experiments from the perspective of changing environments and robot morphologies. Finally, we investigate the possibility of evolving plastic neurocontrollers and analyse an evolved neurocontroller that relies on fast and continuously changing synapses characterized by dynamic stability. We conclude by reviewing the implications of this methodology for engineering, biology, cognitive science and artificial life, and point at future directions of research.

Strengthening of horizontal cortical connections following skill learning

Learning a new motor skill requires an alteration in the spatiotemporal pattern of muscle activation. Motor areas of cerebral neocortex are thought to be involved in this type of learning, possibly by functional reorganization of cortical connections. Here we show that skill learning is accompanied by changes in the strength of connections within adult rat primary motor cortex (M1). Rats were trained for three or five days in a skilled reaching task with one forelimb, after which slices of motor cortex were examined to determine the effect of training on the strength of horizontal intracortical connections in layer II/III. The amplitude of field potentials in the forelimb region contralateral to the trained limb was significantly increased relative to the opposite 'untrained' hemisphere. No differences were seen in the hindlimb region. Moreover, the amount of long-term potentiation (LTP) that could be induced in trained M1 was less than in controls, suggesting that the effect of training was at least partly due to LTP-like mechanisms. These data represent the first direct evidence that plasticity of intracortical connections is associated with learning a new motor skill.

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.

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.

Toward a biophysically plausible bidirectional Hebbian rule

Although the commonly used quadratic Hebbian-anti-Hebbian rules lead to successful models of plasticity and learning, they are inconsistent with neurophysiology. Other rules, more physiologically plausible, fail to specify the biological mechanism of bidirectionality and the biological mechanism that prevents synapses from changing from excitatory to inhibitory, and vice versa. We developed a synaptic bidirectional Hebbian rule that does not suffer from these problems. This rule was compared with physiological homosynaptic conditions in the hippocampus, with the results indicating the consistency of this rule with long-term potentiation (LTP) and long-term depression (LTD) phenomenologies. The phenomenologies considered included the reversible dynamics of LTP and LTD and the effects of N-methyl-D-aspartate blockers and phosphatase inhibitors.

Adult cortical dynamics

There are many influences on our perception of local features. What we see is not strictly a reflection of the physical characteristics of a scene but instead is highly dependent on the processes by which our brain attempts to interpret the scene. As a result, our percepts are shaped by the context within which local features are presented, by our previous visual experiences, operating over a wide range of time scales, and by our expectation of what is before us. The substrate for these influences is likely to be found in the lateral interactions operating within individual areas of the cerebral cortex and in the feedback from higher to lower order cortical areas. Even at early stages in the visual pathway, cells are far more flexible in their functional properties than previously thought. It had long been assumed that cells in primary visual cortex had fixed properties, passing along the product of a stereotyped operation to the next stage in the visual pathway. Any plasticity dependent on visual experience was thought to be restricted to a period early in the life of the animal, the critical period. Furthermore, the assembly of contours and surfaces into unified percepts was assumed to take place at high levels in the visual pathway, whereas the receptive fields of cells in primary visual cortex represented very small windows on the visual scene. These concepts of spatial integration and plasticity have been radically modified in the past few years. The emerging view is that even at the earliest stages in the cortical processing of visual information, cells are highly mutable in their functional properties and are capable of integrating information over a much larger part of visual space than originally believed.

Repetitive optical stimulation elicits fast receptive field changes in mature visual cortex

Fast associative cellular plasticity of visual cortical cells has been shown in vitro with electrical stimulation as well as in vivo by pairing of natural stimuli with artificial depolarization. Here we experimentally changed receptive field properties of single cells in the mature cat visual cortex with purely natural associative stimulation in vivo. Single cortical cells expanded their receptive fields within minutes into unresponsive regions and changed their functional receptive field structure for hours after associative co-stimulation of active and primarily unresponsive regions. The effects are interpreted as strengthening of subthreshold synaptic inputs. Repetitive stimulation with the same visual pattern can modify single cell properties in vivo within minutes and may be related to preattentive fast perceptual learning.