Lateral inhibition as a possible mechanism for evaluating stimulus intensity.

This article presents results obtained by simulation modeling of neural networks which show that lateral inhibitory connections between neurons can in principle, when the parameters of these connections are selected appropriately, support the existence in the CNS of detectors for the intensity of an acting signal. A condition for the operation of the proposed detection mechanism is the physiologically demonstrated fact of an inverse proportional relationship between the latency of neuron responses to a threshold stimulus and the intensity of this stimulus.

[Network of plastic neurons capable of forming conditioned reflexes ("membrane" model of learning)]. / Set' plasticheskikh neironov, sposobnaia k formirovaniiu uslovnykh refleksov ("membrannaia" model' obucheniia).

Simple net neuronal model was suggested which was able to form the conditioning due to changes of the neuron excitability. The model was based on the following main concepts: (a) the conditioning formation should result in reduction of the firing threshold in the same neurons where the conditioning and reinforcement stimuli were converged, (b) neuron threshold may have only two possible states: initial and final ones, these were identical for all cells, the threshold may be changed only once from the initial value to the final one, (c) isomorphous relation may be introduced between some pair of arbitrary stimuli and some subset of the net neurons; any two pairs differing at least in one stimulus have unlike subsets of the convergent neurons. Stochastically organized neuronal net was used for analysis of the model. Considerable information capacity of the net gives the opportunity to consider that the conditioning formation is possible on the basis of the nervous cells. The efficienty of the model turn out to be comparable with the well known models where the conditioning formation was due to the modification of the synapses.

[Lateral inhibition as a mechanism for stimulus intensity estimation]. / Lateral'noe tormozhenie kak vozmozhnyi mekhanizm otsenki intensivnosti stimula.

Results of imitational modeling of neural network are presented. Results show that lateral inhibitory connections between neurons with certain parameters are necessary for estimation of stimulus intensity in the central nervous system. An essential characteristic of suggested mechanism of intensity detection is known from physiology fact of negative dependence of response latency from the stimulus intensity.

[Changes in the excitability of command neurons during the initial period of conditioned reflex formation in grape snails]. / Izmeneniia vozbudimosti komandnogo neirona v nachal'nyi period formirovaniia uslovnogo refleksa u vinogradnoi ulitki.

The procedure of elaboration of conditioned defensive pneumostomum closure was reproduced in neurophysiological experiments on half-intact preparation of the snail. Activity of identified command neurones involved in the reinforcing unconditioned reflex, was recorded intracellularly. Conditioned and unconditioned stimuli converged on these neurones. Excitability of the explored command neurones increased during the first stages of conditioning. Appearance and consequent intensification of the initially absent spike response to the conditioned stimulus was essentially due to this excitability change. Command neurones action potentials evoked by the conditioned stimulus elicited some effector reactions which were the first signs of conditioning. The conclusion is made that the excitability changes in certain nerve cells are essential for determining the initial phase of conditioning.

[Command neurons in the unconditioned reflex arc of the grape snail]. / Komandnye neirony v duge bezuslovnogo refleksa vinogradnoi ulitki.

Independent intracellular activation of any one of the five identified neurones elicits a behavioral act of pneumostome closing. Similar closing is evoked by adequate stimulation of different modalities. Comparison of unconditioned reflex properties and characteristics of responses of the studied cells leads to the conclusion that neurones under discussion are command elements for the unconditioned reflex. Command neurones respond to all sensory stimuli which can evoke reflex closing. There is a lot of synchronous EPSPs in their spontaneous activity, but no action potentials. A spike discharge of command neurones evoked by an adequate stimulus always precedes the effector movements but there is no action potentials during the pneumostome closing. Conclusion is made that the functional role of command neurones in the unconditioned reflex arc consists in evaluating the sensory input and in triggering motor programmes.