1,720,964 research outputs found
Dynamic synaptic modification threshold: computational model of experience-dependent plasticity in adult rat barrel cortex
No full textPrevious electrophysiological experiments have documented the response of neurons in the adult rat somatic sensory ('barrel') cortex to whisker movement after normal experience and after periods of experience with all but two whiskers trimmed close to the face (whisker 'pairing'). To better understand how the barrel cortex adapts to changes in the flow of sensory activity, we have developed a computational model of a single representative barrel cell based on the Bienenstock, Cooper, and Munro (BCM) theory of synaptic plasticity. The hallmark of the BCM theory is the dynamic synaptic modification threshold, θ(M), which dictates whether a neuron's activity at any given instant will lead to strengthening or weakening of the synapses impinging on it. The threshold θ(M) is proportional to the neuron'sactivity averaged over some recent past. Whisker pairing was simulated by setting input activities of the cell to the noise level, except for two inputs that represented untrimmed whiskers. Initially low levels of cell activity, resulting from whisker trimming, led to low values for θ(M). As certain synaptic weights potentiated, due to the activity of the paired inputs, the values of θ(M) increased and after some time their mean reached an asymptotic value. This saturation of θ(M) led to the depression of some inputs that were originally potentiated. The changes in cell response generated by the model replicated those observed in in vivo experiments. Previously, the BCM theory has explained salient features of developmental experience-dependent plasticity in kitten visual cortex. Our results suggest that the idea of a dynamic synaptic modification threshold, θ(M), is general enough to explain plasticity in different species, in different sensory systems, and at different stages of brain maturity
An innocuous bias in whisker use in adult rats modifies receptive fields of barrel cortex neurons
No full textThe effect of innocuously biasing the flow of sensory activity from the whiskers for periods of 3-30 d in awake, behaving adult rats on the receptive field organization of rat SI barrel cortex neurons was studied. One pair of adjacent whiskers, D2 and either D1 or D3, remained intact unilaterally (whisker pairing), all others being trimmed throughout the period of altered sensation. Receptive fields of single cells in the contralateral D2 barrel were analyzed under urethane anesthesia by peristimulus time histogram (PSTH) and latency histogram analysis after 3, 7-10, and 30 d of pairing and compared with controls, testing all whiskers cut to the same length. Response magnitudes to surround receptive field in-row whiskers D1 and D3 were not significantly different for control animals. The same was found for surround in-arc whiskers C2 and E2. However, after 3 d of whisker pairing a profound bias occurred in response to the paired D-row surround whisker relative to the opposite trimmed surround D-row whisker and to the C2 and E2 whiskers. This bias increased with the duration of pairing, regardless of which surround whisker (D1 or D3) was paired with D2. For all three periods of pairing the mean response to the paired surround whisker was increased relative to controls, but peaked at 7-10 d. Response to the principal center- receptive (D2) whisker was increased for the 3 and 7-10 d groups and then decreased at 30 d. Responses to trimmed arc surround whiskers (C2 and E2) were decreased in proportion to the duration of changed experience. Analysis of PSTH data showed that earliest discharges (5-10 msec poststimulus) to the D2 whisker increased progressively in magnitude with duration of pairing. For the paired surround whisker similar early discharges newly appeared after 30 d of pairing. At 3 and 7-10 d of pairing, increases in response to paired whiskers and decreases to cut surround whiskers were confined to late portions of the PSTH (10-100 msec poststimulus). Changes at 3-10 d can be attributed to alterations in intracortical synaptic relay between barrels. Longer-term changes in response to both paired whisker inputs (30 d) largely appear to reflect increases in thalamocortical synaptic efficacy. Our findings suggest that novel innocuous somatosensory experiences produce changes in the receptive field configuration of cortical cells that are consistent with Hebbian theories of experience-dependent potentiation and weakening of synaptic efficacy within SI neocortical circuitry, for correlated and uncorrelated sensory inputs, respectively
Laminar comparison of somatosensory cortical plasticity
No full textDuring tactile learning there is a transformation in the way the primary somatosensory cortex integrates, represents, and distributes information from the skin. To define this transformation, the site of earliest modification has been identified in rat somatosensory cortex after a change in sensory experience. Afferent activity was manipulated by clipping all except two whiskers on one side of the snout ("whisker pairing"), and the receptive fields of neurons at different cortical depths were mapped 24 hours later. Neurons in layer IV, the target of the primary thalamic pathway, were unaltered, whereas neurons located above and below layer IV showed significant changes. These changes were similar to those that occur in layer IV after longer periods of whisker pairing. The findings support the hypothesis that the layers of cortex contribute differently to plasticity. Neurons in the supragranular and infragranular layers respond rapidly to changes in sensory experience and may contribute to subsequent modification in layer IV
Somatic sensory responses in the rostral sector of the posterior group (POm) and in the ventral posterior medial nucleus (VPM) of the rat thalamus
No full textThe rodent barrel field cortex integrates somatosensory information from two separate thalamic nuclei, the ventral posterior medial nucleus (VPM) and the rostral sector of the posterior complex (POm). This paper compares the sensory responses of POm and VPM cells in urethane‐anesthetized rats as a first step in determining how cortex integrates multiple sensory pathways. A complete representation of the contralateral body surface was identified in POm. Trigeminal receptive fields (RFs) of POm and VPM cells were mapped by computer‐controlled displacement of individual whiskers; responses were quantified by using peristimulus time histograms. Average RF size was similar in POm (5.1 whiskers) and VPM (4.4 whiskers), but evoked responses in the two nuclei differed significantly according to all other measures. VPM cells were maximally responsive to one single whisker‐the „center RF.” Stimulating this whisker evoked, on average, a response of 1.4 spikes/stimulus at a latency of 7 ms; surrounding whiskers evoked responses of < 1 spike/stimulus at latencies of > 8 ms. In contrast, POm cells were nearly equally responsive to several whiskers. Quantitative criteria allowed us to designate a single whisker as the „center RF” and stimulating this whisker evoked, on average, a response of 0.5 spikes/stimulus at a latency of 19 ms. VPM cells, but not POm cells, were able to „follow” repeated whisker deflection at > 5 Hz. We conclude that, when a single whisker is deflected, VPM activates the related cortical barrel‐column at short latency— before the onset of activity in POm. The timing of activation could allow POm cells to modulate the spread of activity between cortical columns. Copyright © 1992 Wiley‐Liss, Inc
Computational study of experience-dependent plasticity in adult rat cortical barrel-column
No full textWe model experience-dependent plasticity in the adult rat S1 cortical representation of the whiskers (the barrel cortex) which has been produced by trimmmg all whiskers on one side of the snout except two. This manipulation alters the pattern of afferent sensory activity while avoiding any direct nerve damage. Our simplified model circuitry represents multiple cortical layers and inhibitory neurons within each layer of a barrel-column. Utilizing a computational model we show that the evolution of the response bias in the barrel-column towards spared whiskers is consistent with synaptic modifications that follow the rules of the Bienenstock, Cooper and Munro (BCM) theory. The BCM theory postulates that a neuron possesses a dynamic synaptic modification threshold, θM, which dictates whether the neuron's activity at any given instant will lead to strengthening or weakening of the synapses impinging on it. However, the major prediction of our model is the explanation of the delay in response potentiation in the layer-IV neurons through a masking effect produced by the thresholded monotonically increasing inhibition expressed by either the logarithmic function, h(x) = μ log(1 + x), or by the power function, h(x) = μx0.8-0.9, where μ is a constant. Furthermore, simulated removal of the supragranular layers (layers II/III) reduces plasticity of neurons in the remaining layers (IV-VI) and points to the role of noise in synaptic plasticity
Contribution of supragranular layers to sensory processing and plasticity in adult rat barrel cortex
No full textIn mature rat primary somatic sensory cortical area (SI) barrel field cortex, the thalamic-recipient granular layer IV neurons project especially densely to layers I, II, III, and IV. A prior study showed that cells in the supragranular layers are the fastest to change their response properties to novel changes in sensory inputs. Here we examine the effect of removing supragranular circuitry on the responsiveness and synaptic plasticity of cells in the remaining layers. To remove the layer II + III (supragranular) neurons from the circuitry of barrel field cortex, N-methyl-D-aspartate (NMDA) was applied to the exposed dura over the barrel cortex, which destroys those neurons by excitotoxicity without detectable damage to blood vessels or axons of passage. Fifteen days after NMDA treatment, the first responsive cells encountered were 400-430 μm below the pial surface. In separate cases triphenyltetrazolium chloride (TTC), a vital dye taken up by living cells, was absent from the lesion area. Cytochrome oxidase (CO) activity was absent in the first few tangential sections through the barrel field in all cases before arriving at the CO-dense barrel domains. These findings indicate that the lesions were quite consistent from animal to animal. Controls consisted of applying vehicle without NMDA under similar conditions. Responses of D2 barrel cells were assessed for spontaneous activity and level of response to stimulation of the principal D2 whisker and four surround whiskers D1, D3, C2, and E2. In two additional groups of animals treated in the same way, sensory plasticity was assessed by trimming all whiskers except D2 and either D1 or D3 (called Dpaired) for 7 days before recording cortical responses. Such whisker pairing normally potentiates D2 barrel cell responses to stimulation of the two intact whiskers (D2 + Dpaired). After NMDA lesions, cortical cells still responded to all whiskers tested. Cells in lesioned cortex showed reduced response amplitude compared with sham-operated controls to all D-row whiskers. In-arc surround whisker (C2 or E2) responses were normal. Spontaneous activity did not change significantly in any remaining layer at the time tested. Modal latencies to stimulation of principal D2 or surround D1 or D3 whiskers showed no significant change after lesioning. These findings indicate that there is a reasonable preservation of the response properties of layer IV, V, VI neurons after removal of layer II-III neurons in this way. Whisker pairing plasticity in layer IV-VI D2 barrel column neurons occurred in both lesioned and sham animals but was reduced significantly in lesioned animals compared with controls. The response bias generated by whisker trimming (Dpaired/Dcut + Dpaired ratio) was less pronounced in NMDA-lesioned than sham-lesioned animals. Proportionately fewer neurons in layer IV (52 vs. 64%) and in the infragranular layers (55 vs. 68%) exhibited a clear response bias to paired whiskers. We conclude that receptive-field plasticity can occur in layers IV-VI of barrel cortex in the absence of the supragranular layer circuitry. However, layer I-III circuitry does play a role in normal receptive-field generation and is required for the full expression of whisker pairing plasticity in granular and infragranular layer cells
Somatic sensory responses in the rostral sector of the posterior group (POm) and in the ventral posterior medial nucleus (VPM) of the rat thalamus: Dependence on the barrel field cortex
No full textThe projection from the whiskers of the rat to the S‐I (barrel) cortex is segregated into two separate pathways—a lemniscal pathway relayed by the ventral posterior medial nucleus (VPM) to cortical barrels, and a paralemniscal pathway relayed by the rostral sector of the posterior complex (POm) to the matrix between, above, and below barrels. Before investigating how the barrel cortex integrates these sensory pathways, it is important to learn more about the influence of the various inputs to the two thalamic nuclei. Based on the greater density of descending versus ascending projections to POm, it seemed likely that corticofugal inputs play an important role in the sensory activity of POm. To test this, the responses of POm and VPM cells to sensory stimuli were measured before, during, and after suppression of the S‐I cortex. S‐I was suppressed by application of magnesium or by cooling; the status of the barrel cortex was assessed continuously by an electrocorticogram. All VPM cells (n = 8) responded vigorously to whisker movement even when the barrel cortex was profoundly depressed. In contrast, all POm cells (n = 9) failed to respond to whisker movement once the barrel cortex became depressed, typically about 25 minutes after the start of cortical cooling or magnesium application. POm cells regained responsiveness about 30 minutes after the cessation of cortical cooling or the washoff of magnesium. These findings indicate that the transmission of sensory information through the lemniscal pathway occurs independently of the state of cortex, whereas transmission through the paralemniscal pathway depends upon the state of the cortex itself. © 1992 Wiley‐Liss, Inc. Copyright © 1992 Wiley‐Liss, Inc
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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