Showing papers on "Summation published in 2009"

Encoding and decoding bursts by NMDA spikes in basal dendrites of layer 5 pyramidal neurons.

Abstract: Bursts of action potentials are important information-bearing signals in the brain, although the neuronal specializations underlying burst generation and detection are only partially understood. In apical dendrites of neocortical pyramidal neurons, calcium spikes are known to contribute to burst generation, but a comparable understanding of basal dendritic mechanisms is lacking. Here we show that NMDA spikes in basal dendrites mediate both detection and generation of bursts through a postsynaptic mechanism. High-frequency inputs to basal dendrites markedly facilitated NMDA spike initiation compared with low-frequency activation or single inputs. Unlike conventional temporal summation effects based on voltage, however, NMDA spike facilitation depended mainly on residual glutamate bound to NMDA receptors from previous activations. Once triggered by an input burst, we found that NMDA spikes in turn reliably trigger output bursts under in vivo-like stimulus conditions. Through their unique biophysical properties, NMDA spikes are thus ideally suited to promote the propagation of bursts through the cortical network.

An arithmetic rule for spatial summation of excitatory and inhibitory inputs in pyramidal neurons

Abstract: Dendritic integration of excitatory and inhibitory inputs is critical for neuronal computation, but the underlying rules remain to be elucidated. Based on realistic modeling and experiments in rat hippocampal slices, we derived a simple arithmetic rule for spatial summation of concurrent excitatory glutamatergic inputs (E) and inhibitory GABAergic inputs (I). The somatic response can be well approximated as the sum of the excitatory postsynaptic potential (EPSP), the inhibitory postsynaptic potential (IPSP), and a nonlinear term proportional to their product (k*EPSP*IPSP), where the coefficient k reflects the strength of shunting effect. The k value shows a pronounced asymmetry in its dependence on E and I locations. For I on the dendritic trunk, k decays rapidly with E-I distance for proximal Es, but remains largely constant for distal Es, indicating a uniformly high shunting efficacy for all distal Es. For I on an oblique branch, the shunting effect is restricted mainly within the branch, with the same proximal/distal asymmetry. This asymmetry can be largely attributed to cable properties of the dendrite. Further modeling studies showed that this rule also applies to the integration of multiple coincident Es and Is. Thus, this arithmetic rule offers a simple analytical tool for studying E-I integration in pyramidal neurons that incorporates the location specificity of GABAergic shunting inhibition.

What is the source of the EEG

Abstract: Neurons in the human cortex generally process their information by means of electrical signals and thus enable the electrical recording of their activity, the electroencephalogram (EEG). Due to their unique orientation with their long apical dendrites perpendicular to the cortical surface, large cortical pyramidal neurons in deep cortical layers play a major role in the generation of the EEG. Specific and non-specific thalamic nuclei, as well as distant cortical areas, terminate on these apical dendrites and form myriads of excitatory and inhibitory afferents. The release of excitatory and inhibitory neurotransmitters by these fibers activates specific postsynaptic receptors and generates excitatory and inhibitory postsynaptic potentials, respectively. By electrotonic spread of postsynaptic potentials along the apical dendrites and equivalent capacitive currents, they become electrical dipoles. Positive or negative deflections are generated by both excitatory and inhibitory afferents, depending on the loc...

Reliability of temporal summation and diffuse noxious inhibitory control

Abstract: BACKGROUND: The test-retest reliability of temporal summation (TS) and diffuse noxious inhibitory control (DNIC) has not been reported to date. Establishing such reliability would support the possibility of future experimental studies examining factors affecting TS and DNIC. Similarly, the use of manual algometry to induce TS, or an occlusion cuff to induce DNIC of TS to mechanical stimuli, has not been reported to date. Such devices may offer a simpler method than current techniques for inducing TS and DNIC, affording assessment at more anatomical locations and in more varied research settings.

Altered quantitative sensory testing outcome in subjects with opioid therapy

Abstract: Preclinical studies have suggested that opioid exposure may induce a paradoxical decrease in the nociceptive threshold, commonly referred as opioid-induced hyperalgesia (OIH). While OIH may have implications in acute and chronic pain management, its clinical features remain unclear. Using an office-based quantitative sensory testing (QST) method, we compared pain threshold, pain tolerance, and the degree of temporal summation of the second pain in response to thermal stimulation among three groups of subjects: those with neither pain nor opioid therapy (group 1), with chronic pain but without opioid therapy (group 2), and with both chronic pain and opioid therapy (group 3). We also examined the possible correlation between QST responses to thermal stimulation and opioid dose, opioid treatment duration, opioid analgesic type, pain duration, or gender in group 3 subjects. As compared with both group 1 (n=41) and group 2 (n=41) subjects, group 3 subjects (n=58) displayed a decreased heat pain threshold and exacerbated temporal summation of the second pain to thermal stimulation. In contrast, there were no differences in cold or warm sensation among three groups. Among clinical factors, daily opioid dose consistently correlated with the decreased heat pain threshold and exacerbated temporal summation of the second pain in group 3 subjects. These results indicate that decreased heat pain threshold and exacerbated temporal summation of the second pain may be characteristic QST changes in subjects with opioid therapy. The data suggest that QST may be a useful tool in the clinical assessment of OIH.

Spatial characteristics of center-surround antagonism in younger and older adults.

Abstract: Sensitivity to motion direction is affected by stimulus size, contrast (D. Tadin & J. S. Lappin, 2005; D. Tadin, J. S. Lappin, L. A. Gilroy, & R. Blake, 2003), and observer age (L. R. Betts, C. P. Taylor, A. B. Sekuler, & P. J. Bennett, 2005). Here, we investigated the effect of spatial frequency on motion discrimination and how sensitivity changes in older adulthood. We measured stimulus duration thresholds for younger (18-30 years) and older (60-75 years) observers using drifting Gabor gratings that differed in size, spatial frequency, and contrast. A simple model characterized age differences in the threshold-vs.-size functions. The model parameter fits were consistent with an age-related decrease in the strength of spatial suppression. The model also provided good fits to the thresholds when plotted as a function of the total stimulus contrast energy, which suggests that age-related changes in summation and suppression can be modeled as reduced sensitivity to contrast energy. The summation parameter scaled according to stimulus spatial frequency in younger observers only. Suppression strength decreased as a function of spatial frequency in both age groups. Our findings provide additional evidence for age-related changes in summation and suppression mechanisms and suggest that the total contrast energy in the stimulus plays an important role in determining sensitivity to motion direction in both younger and older adults.

Neuronal convergence in early contrast vision: binocular summation is followed by response nonlinearity and area summation

Abstract: We assessed summation of contrast across eyes and area at detection threshold ( C t). Stimuli were sine-wave gratings (2.5 c/deg) spatially modulated by cosine- and anticosine-phase raised plaids (0.5 c/deg components oriented at ±45°). When presented dichoptically the signal regions were interdigitated across eyes but produced a smooth continuous grating following their linear binocular sum. The average summation ratio ( C t1/([ C t1+2]) for this stimulus pair was 1.64 (4.3 dB). This was only slightly less than the binocular summation found for the same patch type presented to both eyes, and the area summation found for the two different patch types presented to the same eye. We considered 192 model architectures containing each of the following four elements in all possible orders: (i) linear summation or a MAX operator across eyes, (ii) linear summation or a MAX operator across area, (iii) linear or accelerating contrast transduction, and (iv) additive Gaussian, stochastic noise. Formal equivalences reduced this to 62 different models. The most successful four-element model was: linear summation across eyes followed by nonlinear contrast transduction, linear summation across area, and late noise. Model performance was enhanced when additional nonlinearities were placed before binocular summation and after area summation. The implications for models of probability summation and uncertainty are discussed.

Filling-In, Spatial Summation, and Radiation of Pain: Evidence for a Neural Population Code in the Nociceptive System

Abstract: The receptive field organization of nociceptive neurons suggests that noxious information may be encoded by population-based mechanisms. Electrophysiological evidence of population coding mechanism...

Optimum spatiotemporal receptive fields for vision in dim light

Abstract: Many nocturnal insects depend on vision for daily life and have evolved different strategies to improve their visual capabilities in dim light. Neural summation of visual signals is one strategy to improve visual performance, and this is likely to be especially important for insects with apposition compound eyes. Here we develop a model to determine the optimum spatiotemporal sampling of natural scenes at gradually decreasing light levels. Image anisotropy has a strong influence on the receptive field properties predicted to be optimal at low light intensities. Spatial summation between visual channels is predicted to extend more strongly in the direction with higher correlations between the input signals. Increased spatiotemporal summation increases signal-to-noise ratio at low frequencies but sacrifices signal-to-noise ratio at higher frequencies. These results, while obtained from a model of the insect visual system, are likely to apply to visual systems in general.

Spatial Summation Properties for Magnocellular and Parvocellular Pathways in Glaucoma

Abstract: METHODS. Contrast discrimination thresholds were measured for six different stimulus array sizes, using steady- and pulsedpedestal paradigms designed to measure function of the M and P pathways, respectively. This study involved 15 participants with glaucoma and 17 approximately age-matched controls. All participants completed trials foveally and at 12.5° eccentricity. The peripheral stimulus location for each glaucoma participant was within a quadrant of early visual field loss, and locations were matched for the control group. RESULTS. The glaucoma group demonstrated significantly elevated thresholds compared with the control group (F(1,30) 16.29; P 0.001). Thresholds were also significantly dependent on the stimulus sizes, testing location, and pedestal condition. Data obtained for the steady-pedestal paradigm were fit with an exponential decay function, whereas the pulsed-pedestal data were fit with a linear function, demonstrating different spatial summation properties for the M and P pathways, consistent with previous studies using this methodology. Analyses of the curve parameters obtained from the curve fits indicated no significant difference in the shape of the curves between glaucoma and control participants. CONCLUSIONS. Although spatial summation properties are different for presumed M- and P-mediated pathways, the underlying spatial summation properties associated with these pathways are similar in the control and glaucoma participants in this study, centrally and midperipherally. (Invest Ophthalmol Vis Sci. 2009;50:1221–1226) DOI:10.1167/iovs.08-2517

Dynamics of glutamatergic synapses in the medial vestibular nucleus of the mouse.

Abstract: During sinusoidal rotation or translation, primary vestibular afferents modulate their discharge rates at the frequency of motion, effectively transmitting frequency-modulated (FM) signals. This study indicates a possible role for excitatory synapses in the processing of FM signals by vestibular brainstem pathways. Inputs to medial vestibular neurons were activated with FM pulse trains, while inhibitory transmission was blocked. The relationship between the presynaptic pulse rate and the postsynaptic membrane potential was found to be linear within a range of pulse rates. Short-term plasticity was a factor contributing to sensitivity at higher modulating frequencies. The amount of low-pass filtering was correlated with excitatory postsynaptic potential (EPSP) shape, which affected temporal summation during the train. Although the NMDA component of glutamatergic transmission affected EPSP shape, it made only a minor contribution to the dynamics of synaptic transmission. Most responses showed low-pass filtering over the entire 1-16 Hz range. Overall, excitatory synapses in the medial vestibular nucleus contribute a low-pass filter to central vestibular processing and complement the high-pass filtering that is introduced both by peripheral vestibular dynamics and by the intrinsic dynamics of secondary vestibular neurons.

Stimulus duration and the oxcillatory potentials of the human electroretinogram.

Abstract: The effect of duration of stimulus light on the threshold of oscillatory and the slow (a- and b-wave) potentials of the human ERG were assessed on light adaptation to light stimuli. On weak light adaptation induced by stimuli given at 2 min intervals there was a temporal summation of the oscillatory potentials over a range of at least 2 log units (4 msec-400msec). The threshold of the a-wave and b-wave was dependent on duration up to a critical duration of about 40 msec. On strong light adaptation by repetitive light stimulation with 15 sec intervals the oscillations were determined by intensity alone over a range of 2 log units. The a-wave revealed a similar behaviour, whereas the b-wave integrated stimuli in the temporal domain up to about 10 msec. Of main importance; the results provide evidence which is suggestive of temporal integration and discrimination being independently governed by the neurons (probably at the inner plexiform layer), which generate the oscillatory potentials.

Imaging near-surface inhomogeneities using weighted multipath summation

Abstract: Imaging of shallow subsurface objects in 3D space can be performed by a spatial summation of seismic waves diffracted by these objects. The summation can be implemented using either a fixed velocity or, alternatively, a recently introduced multipath summation approach. The multipath summation does not require any explicit information on velocities since it is performed for all possible velocity values within a wide specified range. However, the spatial resolution of the images obtained by the multipath summation is usually inferior to those obtained using the fixed velocity approach. In order to improve the image quality, we propose a new approach to the multipath summation performing the summation with image dependent weights. The ‘optimal’ weights can be obtained from asymptotics of the corresponding path integral (which is a continuous analogue of the multipath summation). Application of the weighted multipath summation to synthetic and real data demonstrates that the quality of the imaging can be greatly improved as compared to the unweighted summation and sometimes even to the fixed velocity approach. Moreover, the weighted multipath summation can produce reliable estimates of the summation parameter (velocity above the target object), which in some cases can be used to further improve the image quality.

Auditory temporal summation in pinnipeds.

Abstract: In addition to improving the understanding of auditory processing in pinnipeds, direct measures of temporal summation are relevant to the selection of signal parameters when conducting audiometric research, assessing the effects of signal duration on communication ranges, and evaluating the potential auditory impacts of anthropogenic signals. In the present study, individuals from three pinniped species were tested to determine how signal duration influenced pure‐tone hearing thresholds. The psychophysical method of constant stimuli was used to obtain aerial thresholds for each subject at nine different signal durations ranging from 25 to 500 ms. Parameter estimates derived for a California sea lion (Zalophus californianus) from an exponential model of temporal summation yielded time constants (τ) of 176, 98, and 141 ms at frequencies of 2.5, 5, and 10 kHz, respectively. Preliminary results with a northern elephant seal (Mirounga angustirostris) at 5 kHz (this study), and a harbor seal (Phoca vitulina) at...

Heterosynaptic potentiation of cholinergic excitatory postsynaptic responses of command neurons in the common snail

Abstract: Short-term heterosynaptic potentiation of cholinergic excitatory postsynaptic currents and potentials in defensive behavior command neurons was found to be evoked by stimulation of the visceral sac in the common snail. It is suggested that a mechanism increasing the choline resistance of the postsynaptic zones of command neuron membranes is involved in potentiating the excitatory postsynaptic responses to sensory stimulation.

Spatial summation and visual adaptation in the inner part of the frog retina.

Abstract: The spatial summation of the proximal negative response (PNR) of frog retina was analyzed during different states of adaptation induced by repetitive light stimulation with either 15 sec or 1 min interstimulus interval. The spatial summation area of the PNR was found to be wider during relatively more dark adapted conditions compared to relatively more light adapted ones. This finding indicates that lateral mechanisms in the inner network of the retina seem to be activated by adapting stimuli. Secondly, the results suggest that the activity of the amacrines, as reflected by the PNR, may initiate the series of neuronal events which underlie the oscillatory potentials (OPs) of the electroretinogram (ERG).

RAPID COMMUNICATION Area CA3 Interneurons Receive Two Spatially Segregated Mossy Fiber Inputs

Abstract: Area CA3 receives two extrinsic excitatory inputs, the mossy fibers (MF), and the perforant path (PP). Interneurons with somata in str. lacunosum moleculare (L-M) of CA3 modulate the influ- ence of the MF and PP on pyramidal cell activity by providing strong feed-forward inhibitory influence to pyramidal cells. Here we report that L-M interneurons receive two separate MF inputs, one to the dorsal dendrites from the suprapyramidal blade of the dentate gyrus (MFSDG), and a second to ventral dendrites from the str. lucidum (MFSL). Responses elicited from MFSDG and MFSL stimulation sites have strong paired-pulse facilitation, similar DCG-IV sensitivity, amplitude, and decay kinetics but target spatially segregated domains on the inter- neuron dendrites. These data demonstrate that certain interneuron sub- types are entrained by two convergent MF inputs to spatially separated regions of the dendritic tree. This anatomical arrangement could make these interneurons considerably more responsive to the excitatory drive from dentate granule cells. Furthermore, temporal summation is linear or slightly sublinear between PP and MFSL but supralinear between PP and MFSDG. This specific boosting of the excitatory drive to interneur- ons from the SDG location may indicate that L-M interneurons could be specifically involved in the processing of the associational component of the recognition memory. V C 2009 Wiley-Liss, Inc.