[Study of visual perception under conditions of extreme light exposures].

Neurophysiological processes of transduction and analysis of information concerning brightness of photostimuli and peculiarities of functioning of visual system under exposure to extra-bright light effects or extreme contrasting of visual objects are studied poorly and present significant interest for unraveling actual mechanisms of visual perception under extreme conditions. Conducting analysis of visual system in maximal broad range both in terms of adequate perception and reflection of light stimulus intensity and adaptive mechanisms to extra-strong illumination regimes is as well very important. The to-date obtained data indicate that under the extreme light effects pathology of the peripheral portion of visual analysator can be induced. The presented review does not pretend to comprise all present information concerning all achievements and results over this problem. Its goal concludes in maximal approaching to understanding the mechanisms of damaging and following recover of visual function under extreme effects of affecting light factors.

Influence of motivational centers of the hypothalamus on the functioning of the visual area of cerebral cortex.

It has been demonstrated in chronic experiments on awake rabbits that a solitary stimulation of the middle hypothalamus, its ventromedial and lateral nuclei (VMN and LN), exerts a phasic effect on a formation of the primary response of the visual cortex evoked by a test light flash. In the initial period of their action, the hypothalamo-cortical responses (1-43 msec for the VMN and 1-10 msec for the LN) completely inhibit the formation of the response of the visual cortex to the light stimulus, while in the second period (43-130 msec for the VMN and 10-150 msec for the LN, respectively), selectively and highly significantly facilitate the formation of the positive phase of the primary response. In the process the negative component is suppressed, and more significantly and longer with stimulation of the VMN than of the LN (140 and 50 msec, respectively). The data obtained make it possible to hypothesize the existence of a highly organized apparatus of phasic hypothalamic (both from the VMN and the LN) control of the function of the visual cortex which is realized both by means of a facilitatory axosomatic mechanism at the level of the dendrites of the basal neurons of layer IV of the cortex and by means of a suppressant mechanism at the level of the apical dendrites of the surface layers of the cortex.

[The effect of the hypothalamic motivational centers on the function of the cortical visual area of the hemispheres]. / Vliianie motivatsionnykh tsentrov gipotalamusa na funktsiiu zritel'noi oblasti kory bol'shikh polusharii.

Chronic experiments were carried out in waking rabbits. A single stimulation of the middle hypothalamus (the ventromedial, VMN, and the lateral, LN, nuclei) was shown to induce the phasic action on a formation of the primary response (PR) of the visual cortex (VC) to the testing light flash. At the first phase (1-43 ms for the VMN and 1-10 ms for the LN) the hypothalamic-cortical responses completely inhibited the formation of the VC response to the light stimulus. At the second phase (43-130 ms for the VMN and 10-150 ms for the LN) they selectively and rather significantly facilitated the formation of the positive PR component the negative one being suppressed. This suppression was stronger and longer during the stimulation of the VMN, than that of the LN (140 and 50 ms correspondingly). The data obtained suggested the existence of a highly organized apparatus of the phasic (both by the VMN and LN) control of the VC function being realized both by means of facilitatory axo-somatic mechanisms at the level of the basal dendrites of the layer IV cortical neurons, and by means of the inhibitory mechanisms at the level of the apical dendrites of the superficial cortical layers.

Role of the posterolateral nucleus of the thalamus in mechanism of information transmission to the visual cortex.

It has been demonstrated in chronic experiments on wakeful rabbits that the posterolateral nucleus of the thalamus exerts tonic and phasic facilitatory influences on the formation of responses of the visual cortex elicited by a light flash. The tonic influences were expressed in an increase in the amplitude parameters of the responses of the visual cortex in conditions of the formation in the posterolateral nucleus of the thalamus of a focus of heightened excitability (anode polarization), and their perceptible diminution with potassium depression in this nucleus. The phasic influences were expressed in the formation in the visual cortex of thalamocortical responses elicited by pulse stimulation of the posterolateral nucleus of the thalamus, which displayed a close interaction with the responses of the cortex elicited by the light flash. This interaction consisted in a noticeable facilitation of the responses of the visual cortex against the background of a conditioning pulse stimulation of the posterolateral nucleus. The facilitatory effect of the phasic influences was more significant than the effect of the tonic influence.

[The role of the posterior lateral thalamic nucleus in the rabbit in the mechanisms of information transmission in the visual cortex]. / Rol' zadnego lateral'nogo iadra talamusa krolika v mekhanizme peredachi informatsii v zritel'nuiu koru.

In chronic alert rabbits, both transient and sustained influence of the nucleus lateralis posterior upon visual cortical responses to the light flashes, were shown. The transient influences were revealed by an increase of the responses amplitude in the nucleus lateralis posterior and by a decrease of the amplitude during the K depression. The sustained influences took part in forming of thalamo-cortical responses in visual cortex. This reciprocity was particularly obvious during pulse stimulation of the nucleus lateralis posterior. The sustained influences were more significant than the transient ones.

Evoked electrical responses of nucleus lateralis posterior of the rabbit thalamus and their dependence upon the functional state of the cortex and reticular formation.

The electrical responses of the thalamic pulvinar (of its analogue, the nucleus lateralis posterior (nLP)) to light stimuli of various intensity were recorded in awake rabbits, and their interrelationships with the responses of the retina, superior colliculi, and visual cortex were analyzed. It is postulated that a major role in the afferent supply of the rabbit nLP belongs to the optic track and the superior colliculi. The presence was demonstrated of a highly organized analyzer of the reticular control of nLP function that is of a facilitatory character. The most important role in reticular influence belongs to its adrenoresponsive mechanism. An inhibitory influence was demonstrated for the visual and sensorimotor regions of the cerebral cortex on the nLP neuronal apparatus producing responses to light stimuli. The influence of the visual cortex are the most pronounced and stable.

[Oscillatory evoked potentials in the rabbit visual system during experimental retinal pathology induced by administration of monoiodoacetic acid]. / Ostsilliatornye vyzvannye potentsialy v zritel'noi sisteme krolika pri éksperimental'noi patologii setchatki, sozdannoi vvedeniem monoïoduksusnoi kisloty.

ERG and EP of the visual cortex (VC) and superior colliculus (SC) were investigated under experimental dystrophy of the retina induced by intravenous injection of monoiodoacetic acid in different doses. The relative resistance of the VC to the derangement of retinal function was established. It was manifested by less marked suppression of its EP as compared with ERG. Taking into consideration the selective projection to the SC of the rod system that was damaged to a greater degree under pathology in question, it is suggested that functional suppression of the SC might facilitate the conduction of visual information through the main retino-geniculo-cortical canal. This might be one of the factors of VC resistance to the impairment of retinal function. This assumption was confirmed in experiments with SC electrocoagulation in which one could observe the facilitation of EP formation of the contralateral VC.

[Characteristics of the evoked electrical reactions of the nucleus lateralis posterior in the rabbit thalamus and their dependence on cortical and reticular formation function]. / Kharakteristika vyzvannykh élektricheskikh reaktsii nucleus lateralis posterior talamusa krolika i ikh zavisimost' ot funktsional'nogo sostoianiia kory i retikuliarnoi formatsii.

The electrical responses of n. lateralis posterior of the thalamus (nLP) to light stimuli of different intensity were recorded in alert rabbits, and their interrelationships with the reactions of retina, superior colliculi and visual cortex were analysed. The retina rather than superior colliculi appeared to play a major role in afferent supply of nLP in rabbit. The organized mechanism of facilitating reticular control of the nLP function was shown, with the adrenoreactive mechanism playing the most important role in the reticular influence. The inhibitory influence of visual and sensomotor cortical areas upon neuronal mechanism of nLP forming the responses to photostimuli, was revealed. The effects of visual cortex were the most expressive and stable.

[The relationship between oscillatory potentials of the electroretinogpam and components of the visual cortex evoked response]. / O sootnoshenii ostsilliatornykh potentsialov élektroretinogrammy i komponentov vyzvannogo otvety zritel'noi kory

In the electroretingram of rabbit, the oscillatory potentials (OPs) were shown to appear in response to a wide range of the light stimuli intensities (0.023--120 joules). The minimum time required for the OP onset was 10 msec. The OP duration was independent of the stimulus intensity and ranged between 6--7 msec. The number of OPs, their amplitudes, latency, and peak latency varied along with the intensity of the stimuli. OP1 occurred during developing of "a" wave, while OP2, OP3, OP4--at the ascending phase of "b" wave of the electroretinogram. The latency and the peak latency of the OP1 were always shorter than the respective parameters of the first component of evoked potential in the visual cortex. A definite time correlation between the formation of OP1 and OP2 and the first two components of the evoked potential was revealed. Interrelationship between OP3, OP4, and later components of evoked potentials in the visual cortex was much more complicated.