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E Nelle

Publications and source records attributed to E Nelle.

4 recordsLinked to original sources
PubMed1998

The influence of corticofugal feedback on the temporal structure of visual responses of cat thalamic relay cells.

1. Visually driven single-unit activity was recorded in the dorsal lateral geniculate nucleus (dLGN) of the anaesthetized cat while inactivating or stimulating the corticofugal feedback from area 17/18 by means of cortical cooling or application of GABA (inactivation), or application of glutamate or quisqualate (Glu, Quis; stimulation) to layer VI. 2. Manipulations of the corticofugal feedback primarily affected the multimodal interspike interval pattern previously reported to be present in the tonic component of visual responses elicited by spot-like stimuli. 3. Sixty-three per cent of all neurons could be influenced, and temporally localized interspike interval distributions were measured which commonly consisted of one fundamental interval peak (leftmost peak) and integer multiples thereof (higher order peaks). During blockade of the corticofugal feedback, interspike intervals were redistributed into the higher order peaks in about 70 % of the cases, accompanied by a reduced mean firing rate. During stimulation the reverse effect occurred in 69 % of cases. 4. Increased synchronization of the EEG (increased power in the delta-wave range, 1-4 Hz) had an effect similar to cortex inactivation. The specificity of corticofugal effects was verified by consideration of these EEG effects and by dLGN double recordings with one dLGN cell topographically matched with the cortical inactivation/activation site and the second cell outside this area. Clear effects due to manipulation of the corticofugal feedback were found only for the matched dLGN site. 5. In addition we observed that the peaks of the interval distributions were narrower during active corticofugal feedback, such that the temporal dispersion of the signal transmission to the cortex was reduced. 6. The mechanisms underlying this effect were further analysed in a biophysically realistic model demonstrating that the timing of the spikes in the dLGN is improved as soon as the cortical feedback is active. The high degree of convergence/divergence between neurons along the closed feedback loop thereby leads to a temporal averaging effect which reduces the interval dispersion and also introduces synchronization between dLGN cells. 7. Such a mechanism may thus counteract the deterioration of spike timing accuracy which would otherwise occur as a consequence of synaptic noise and other uncorrelated sources of activity at a given neuron.

Action Potentials↗
PubMed1998

A possible basic cortical microcircuit called "cascaded inhibition." Results from cortical network models and recording experiments from striate simple cells.

The robust behavior, the degree of response linearity, and the aspect of contrast gain control in visual cortical simple cells are (amongst others) the result of the interplay between excitatory and inhibitory afferent and intracortical connections. The goal of this study was to suggest a simple intracortical connection pattern, which could also play a role in other cortical substructures, in order to generically obtain these desired effects within large physiological parameter ranges. To this end we explored the degree of linearity of spatial summation in visual simple cells experimentally and in different models based on half-wave rectifying cells ("push-pull models"). Visual cortical push-pull connection schemes originated from antagonistic motor-control models. Thus, this model class is widely applicable but normally requires a rather specific design. On the other hand we showed that a more generic version of a push-pull model, the so-called cascaded inhibitory intracortical connection scheme, which we implemented in a biologically realistic simulation, naturally explains much of the experimental data. We investigated the influence of the afferent and intracortical connection structure on the measured linearity of spatial summation in simple cells. The analysis made use of the relative modulation measure, which is easy to apply but is limited to moving sinusoidal grating stimuli. We introduced two basic push-pull models, where the order of threshold nonlinearity and linear summation is reversed. Very little difference is observed with the relative modulation measure for these models. Alterative models, like half-wave squaring models, were also briefly discussed. Of all model parameters, the ratio of excitation to inhibition in the simple cell exerts the most crucial influence on the relative modulation. Linearity deteriorates as soon as excitatory and inhibitory inputs are imbalanced and the relative modulation drops. This prediction was tested experimentally by extracellular recordings from cat area 17 simple cells and we found that about 62% showed a significant deviation from linear behavior. The problem that individual basic push-pull models are hard to distinguish experimentally led us to suggest a different solution. In order to generically account for the observed behavior (e.g., imbalance of excitation versus inhibition), we suggested a rather generic version of a push-pull model where it no longer mattered about (the hard-to-distinguish) fine differences in connectivity. Thus, we introduced a new class of biophysically realistic models ("cascaded inhibition"). This model class requires very little connection specificity and is therefore highly robust against parameter variations. Up to 25 cells are connected to each target cell. Thereby a highly interconnected network is generated, which also leads to disinhibition at some parts of an individual receptive field. We showed that the performance of these models simulates the degree of linearity and its variability in recal simple cells with comparatively high accuracy. This behavior can be explained by the self-regulating properties of a cascaded inhibitory connection scheme by which the balance between excitation and inhibition at a given cell is improved by the joint network effects. The virtues and the generic design of this connection pattern, therefore, allow to speculate that it is used also in other parts of the cortex.

Animals↗
PubMed1996

Corticofugal feedback improves the timing of retino-geniculate signal transmission.

The temporal accuracy of visually induced spike patterns can be expected to deteriorate with increasing distance from the retina due to synaptic noise and other sources of activity unrelated to the retinal signal. Here we report the opposite effect: the interspike interval distributions of relay cells of cat lateral geniculate nucleus (LGN) are sharper than those of their retinal afferents. A biophysically realistical simulation of the retino-cortical pathway, including excitatory cortico-geniculate feedback, produced a similar effect. Without the feedback the mean firing rate in the LGN dropped and interval peaks broadened by an average of 25%. The same effect was found during reversible experimental inactivation of the visual cortex in cat. This indicates that cortico-geniculate feedback could be employed to improve the temporal accuracy of signal transmission.

Afferent Pathways↗