Dynamics of tuning to the shape of a cross-like figure in striate neurons

Dynamics of tuning to the shape of cross-like figure flashed in receptive field was studied in 83 striate neurons by the method of temporal slices. Tuning was estimated by the total number of spikes in the response and by this number in successive fragments of the response with 20 ms steps. It was found that only in 11.7% cases neurons showed stable tuning to the same shape of the preferred figure (an angle between its lines), in other cases (88.3%) during response generation this tuning changes being one-phase (7.2%) or two-phase (27.0%), or undulatory (54.1%). Different dynamical reorganization of receptive field zones is discussed as a possible mechanism of the revealed effects as well as their correlation with previously described dynamics of tuning to orientation of a single bar and a cross in striate cells.     

Role of intracortical inhibition in orientation tuning dynamics in the cat striate cortex neurons

A suggestion about the leading role of GABA-induced intracortical inhibition in the dynamics of orientation tuning (OT) of the cat striate cortical neurons was tested in acute experiments before and during the local blockade of their inhibition by iontophoretic application of bicucculine. In the course of the investigation of these dynamics, with the use of a temporal scanning method, two types of neurons differing in the inhibition blockade-induced OT changes were found. In the neurons of the first type (57%), bicuculline induced the OT dynamics or enhanced it, if it pre-existed before the bicuculline application. In the neurons of the second type (43%), bicuculline strongly reduced or eliminated the dynamic shift of a preferred orientation. These results mean that under normal conditions the inhibition stabilizes and sharpens OT in some cells, while in other cells, in contrast, it causes the OT dynamics. The following mechanisms may underlie the observed effects: an elimination of the inhibition originating from lateral non-isoorientational excitatory inputs of a receptive field; an inhibition of these inputs via the adjacent interneurons activated by a powerful discharge of the examined neuron; a long-term afterhyperpolarization of the neuron, and the dynamics of the excitatory and inhibitory zones of the receptive field.Neirofiziologiya/Neurophysiology, Vol. 27, No. 2, pp. 100–109, March–April, 1995.     

Simulation of the role of intracortical inhibition as a source of sensitivity to cross-like figures

Simulation of receptive fields of striate neurons sensitive to cross-like figures under the blockade of intracortical inhibition was performed. It was shown that without the inhibition, a neuron with convergence of signals from two orientation detectors widens its tuning to a cross in such a way that the tuning becomes invariant to the shape and orientation of the cross. Detector of a cross whose orientation is based on the disinhibition mechanisms becomes a bar orientation detector under conditions of inhibition blockade. Another scheme of receptive field is advanced, in which the inhibitory zones mask tuning to cross-like figures, but the blockade of inhibition unveils such sensitivity. We specified the features of the receptive fields (shape, localization, and weight of their zones), which, being applied in simulation, imitate properties of the real striate neurons sensitive to shape and orientation of a cross-like figures.     

Simulation of mechanisms of tuning to incomplete cross-like figures in neurons of visual cortex

We performed a digital simulation of the receptive fields (RF) of cat cortical neurons in the area 17 that are able to detect cross-like figures with partly masked central or peripheral fragments. It was shown that the reciprocal interaction between the RF center and periphery may produce sharp, selective, and pronounced tuning to a cross shape and orientation due to blocking the end-stopping inhibition in the RF by its side-disinhibitory zone. Under conditions of cooperative interaction between the RF center and periphery the sensitivity index (cross/bar response ratio) was typically lower than in the reciprocal model. Features of the model that are critical for sensitivity to cross in cases of small or incomplete figures (the shape, localization, and weight of the RF zones) are specified.     

Dynamics of tuning to orientation of cross-like figures in neurons from the cat visual cortex

Dynamics of tuning to orientation of flashing light bar and to orientation of cross-like figure was studied by a temporal slices method in 87 neurons of the cat primary visual cortex. Tuning was plotted by spikes number in the entire response and in its successive fragments with a step of 20 ms. It was found that successive dynamic shift of preferred orientation of a bar was typical for 87% units, white such shift of preferred orientation of a cross was met in 75% of cases. Comparison of tuning dynamics for bar and cross allowed to separate units into three groups: the first one (58.6% of cases) with larger dynamic shift of a bar preferred orientation then of a cross (74.9 +/- 5.8 degrees [symbol: see text] 29.8 +/- 4.1 degrees, correspondingly, p < 0.00001), the second group (21.5%) with opposite effect (24.2 +/- 5.2 degrees and 69.2 +/- 10.0 degrees, p < 0.0002) and the third group (19.8%) without significant shift of preferred orientation of bar and cross and without difference in their dynamics. Possible mechanisms of the preferred orientation dynamics and its difference for bar and cross are discussed.     

Disinhibition as a mechanism of tuning of cross-like figures in the visual cortex neurons

A discrete neural net was used for simulation of cross-sensitivity in 40% of neurones of the cat visual cortex' area 17th. It is based on disinhibition of the end-stopping inhibition in receptive field from the side-disinhibitory zone. Highly selective or invariant sensitivity of the simulated neurone in respect to shape and orientation of a cross-like figure was observed under changes of location, size and weight of the receptive field zones. The disinhibitory mechanism seems to be critically involved in the selection of the second-order features of the images in the primary visual cortex.     

Sensitivity of striate neurons to Y-like figures: experiment and simulation

Under stimulation of the receptive fields (RF) of neurons in the cat area 17 by flashing Y-like figures of different shape and orientation, the sensitivity to these figures was revealed in 72% of the studied cells, while 62% of units were sensitive to cross-like figures as well. Tuning to Y-like figures was typically selective to their shape and orientation, but in some cases it was invariant to these features. Response magnitudes to single bar, Y-like figure and cross were positively correlated. Simulation showed that the disinhibition might be a sufficient mechanism for effective detection of Y-like figures in a classical receptive field.     

Temporal course of disinhibition of the visual cortex neurons and their sensitivity to cross-like figures

In acute experiments with narcotized and paralyzed cats, we studied responses of 74 striate neurons to cross-like figure under synchronous and asynchronous presentation of its lines. The aim of the study was to characterize the temporal course of interaction between three RF zones: main excitatory, end-inhibitory, and side disinhibitory ones. Previously we have found that this interaction is responsible for sensitivity to a cross in near 3/4 of cat striate cells with such sensitivity. In neurons with sensitivity to a cross, we found two types of temporal interaction between zones of RF. In the 1st type cells (14/23), the response significantly increased if the disinhibitory and the main excitatory zones of RF were stimulated simultaneously. Neurons of the 2nd type (9/23) revealed opposite temporal function: synchronous activation of RF zones evoked a minimal response. Simulation shows that the 1st type of behavior is connected with disinhibitory mechanism, while that of the 2nd type--with combination of this mechanism with convergence of orientation detectors of the previous functional level.     

Disinhibitory zone of the striate neuron receptive field and its sensitivity to the cross-like shape]

Sensitivity of 84 neurones to presentation of cross-form figures was studied in cats as well as the lever length function prior to and after stimulation of the RF disinhibitory zone. Responses to lever length of the cross-sensitive cells were augmented in simultaneous stimulation of the RF. The data obtained suggest involvement of the end inhibition and its blocking by a side-disinhibitory zone of the RF.     

Selective sensitivity of the cat striate neurons to cruciform and angular figures of various orientations

Selectivity and invariability of tuning were studied in 51 neurons of the primary visual cortex (area 17); cruciform and angular figures (CF and AF, respectively) of different configurations and orientations were presented in their receptive fields. Twenty-three neurons, or 45% of the studied cells, demonstrated selective sensitivity to these figures. Their responses considerably (2.38±0.36 times, on average) increased, as compared with those evoked by presentation of a single bar of preferred orientation. In the examined group, 2 cells demonstrated sensitivity both to the CF and AF. A wide range of detector properties related to the CF and AF analysis was found in the analyzed neuronal population. Detectors of configuration of these figures are described. Selective sensitivity to the angle between branches of these figures was observed in 17 neurons, and responses of 2 neurons among them showed invariability to orientation of these figures. Four cells were selective for orientation and were insensitive to configuration, and 4 other cells showed no specific sensitivity to either of these properties, but were sensitive to the appearance of a CF itself in their receptive field (these cells were regarded as invariant detectors of crossing nodes). Data inconsistent with the hierarchic principle of detection of the above properties are presented. Possible mechanisms and functional significance of selective sensitivity of striate neurons to the CF and AF are discussed.Neirofiziologiya/Neurophysiology, Vol. 27, No. 5/6, pp. 403–412, September–December, 1995.     

听神经元的频率调谐

频率作为声音的一个重要参数,在听敏感神经元对声音进行分析和编码过程中扮演重要角色。一般用频率调谐曲线来表示听敏感神经元的频率调谐特性,并用Qn(10,30,50)值表达频率调谐曲线的尖锐程度,Qn值越大,频率调谐曲线也越尖锐,神经元的频率调谐能力越好,对频率的分辨能力越高。从听觉外周到中枢,听敏感神经元的频率调谐逐级锐化,而这种锐化主要是由听中枢的多种抑制性神经递质的作用而产生的,其中起主要作用的是GABA能和甘氨酸能神经递质。此外,离皮层调控,双侧下丘间的联合投射以及弱噪声前掩蔽等因素也会影响听敏感神经元的频率调谐特性。     

Responses of rabbit visual cortical neurons to intracortical electrical stimulation

Responses of rabbit visual cortical neurons to single and repetitive intracortical electrical stimulation were investigated. The stimulating electrode was located 0.7–1.2 mm away from the recording electrode. Response thresholds to single stimulation were as a rule 150–180 µA, whereas to series of stimuli they were 30–60 µA. The latent period to the first spike averaged 5–15 msec but the probability of the initial discharge was very low, namely 3–6%. With an increase in current intensity the duration of the initial inhibitory pause was increased in half of the neurons responding to it, whereas in the rest it was unchanged. After presentation of series of stimuli spontaneous activity was enhanced for a short time (4–6 sec). In about half of the cells the same kinds of discharge dynamics were observed in response to repetitive stimulation (frequency 0.25 Hz) as in responses to light, but more neurons with sensitization of discharge and fewer "habituating" neurons took part in responses to electrical stimulation. It is postulated that stimulation of a given point of the visual cortex evokes excitation of a local neuron hypercolumn and inhibition of neighboring cell columns.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 15, No. 4, pp. 412–419, July–August, 1983.     

Development of adrenergic neurons in uiuo: inhibition by ganglionic blockade

T he N ormal biochemical maturation of postsynaptic adrenergic neurons in mouse and rat superior cervical ganglion depends upon an intact preganglionic innervation (B lack , H endry and I versen , 1971a, 1972; T hoenen , S aner and K eitler , 1972). In recent studies tyrosine hydroxylase, the rate-limiting enzyme in norepinephrine biosynthesis (L evitt , S pector , S joerdsma and U denfriend , 1965), with localization to adrenergic neurons in the ganglion (B lack , H endry and I versen , 1971b), was used to monitor maturation of these cells. The developmental increase in tyrosine hydroxylase activity occurred simultaneously with the appearance of ganglionic synapses and was prevented by transection of the preganglionic nerve trunk (B lack , H endry and I versen , 1971a). These observations suggest that presynaptic cholinergic nerve terminals regulate the biochemical development of postsynaptic neurons in the superior cervical ganglion. The mechanism(s) by which presynaptic cholinergic terminals regulate postsynaptic development has not been elucidated. Such trans-synaptic regulation may be dependent on normal impulse transmission and/or may involve other unidentified, trophic factors. The results presented in the present communication suggest that normal development of ganglionic tyrosine hydroxylase activity is dependent on depolarization of postsynaptic adrenergic neurons.     

A geometric model of orientation tuning dynamics in visual neurons]

A geometrical model for the dynamics of orientation tuning of visual neurones was proposed, which makes it possible to study the dynamics of configuration, localization, and weight of excitatory and inhibitory subzones of the receptive field. The model reproduces typical patterns of orientation tuning dynamics, observed in neurophysiological experiments on cat visual cortex neurones. The parameters of the model (size and mutual position of excitatory and inhibitory zones of the receptive field, their weight, and dynamics type) were estimated that correspond to the main types of orientation tuning dynamics in natural conditions. It is shown that selective and acute tuning of neurones can be formed and/or sharpened by intracotrical inhibition, while the dynamics of preferred orientation is due to changes in the geometry of the inhibitory subzone of the receptive field.     

Attention to stimulus features shifts spectral tuning of V4 neurons during natural vision

Previous neurophysiological studies suggest that attention can alter the baseline or gain of neurons in extrastriate visual areas but that it cannot change tuning. This suggests that neurons in visual cortex function as labeled lines whose meaning does not depend on task demands. To test this common assumption, we used a system identification approach to measure spatial frequency and orientation tuning in area V4 during two attentionally demanding visual search tasks, one that required fixation and one that allowed free viewing during search. We found that spatial attention modulates response baseline and gain but does not alter tuning, consistent with previous reports. In contrast, feature-based attention often shifts neuronal tuning. These tuning shifts are inconsistent with the labeled-line model and tend to enhance responses to stimulus features that distinguish the search target. Our data suggest that V4 neurons behave as matched filters that are dynamically tuned to optimize visual search.     

Role of intracortical inhibition in forming homo- and heterosensory interaction in sensorimotor cortex neurons in kittens

Homo- and heterosensory interaction were investigated in sensorimotor cortex neurons before and after picrotoxin application to anesthetized and immobilized kittens belonging to three age groups (12–30 days, 31–47 days, and 2–4 months old). Only slight inhibition of response to presentation of a second stimulus was observed in a small proportion of cells in the youngest age group at test intervals of 100, 200, and 300 msec. Picrotoxin application only produced the effect of raised background activity. Numbers of neurons with partially or fully inhibited response to test stimuli (especially spaced at 100 msec intervals) rose in the middle and older age groups. The dynamics of heterosensory interaction and how this is affected by picrotoxin application gradually approximated to that observed with adult animals. The subject of the development of inhibitory mechanisms and how they contribute to the organization of homo- and heterosensory interaction during early postnatal ontogenesis is considered in the light of the results obtained.A. A. Ukhtomsii Institute of Physiology, Leningrad. A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 20, No. 2, pp. 234–243, March–April, 1988.     

The dynamics of intracortical interaction during thinking activities

A new method is described of the brain mapping, based on determination of the probability of appearance of isofrequency components in the EEG derivations allowing to evaluate functional interaction of the brain structures in the process of psychic activity. The process of mental construction of visual image from separate elements includes three stages. At the stage of image search the focus of activity is in the occipital cortical area; in the stage of construction it moves to the frontal cortical areas; completion of the task and verbalization of the image are accompanied by joining of the cortical connections in common system. Alongside with the main focus of activity secondary focuses in the temporal cortex are also revealed during the search of the visual image. The topography of interaction at the frequencies of alpha-range in mainly determined by the stage of image construction. In case of prevalence of the image and abstract thinking shift is marked of the activity focuses at the frequencies of theta-range respectively to the right and left hemispheres.     

Responses of neurons in cat visual cortex with sensitivity to bar and cross-like figure

In the cat primary visual cortex (area 17) the response magnitude and latency were studied in 280 neurons sensitive to bar or cross-like-figure. Under natural conditions half of the studied 195 cells preferred bar (first group) or cross (second group). In the first group responses to both figures were near equal, while in the second one cross evoked much stronger response. Response latencies with the optimal bar in the first group were shorter than in the second group and longer to a cross than to a bar while in the second group they were considerably shorter to a cross than to bar. Under local blockage of GABA-ergic inhibition by microiontophoretic application of bicuculline about one-fourth of 85 neurons generated greater responses and were bar-sensitive irrespective to presence or absence of inhibition. Other neurons were cross-sensitive at least in one of the conditions (with and/or without of inhibition). They responses grew under bicuculline action relatively more than in the first group. Significance of the data obtained for tuning to image features and temporal succession of their detection is discussed.     

Mode-specific inhibition of sodium-calcium exchange during protein phosphatase blockade

The effects of the protein phosphatase inhibitors calyculin A and okadaic acid on Na(+)/Ca(2+) exchange activity were examined in transfected Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger. Incubating the cells for 5-10 min with 100 nM calyculin A reduced exchange-mediated (45)Ca(2+) uptake or Ba(2+) influx by 50-75%. Half-maximal inhibition of (45)Ca(2+) uptake was observed at 15 nM calyculin A. The nonselective protein kinase inhibitors K252a and staurosporine provided partial protection against the effects of calyculin A. Okadaic acid, another protein phosphatase inhibitor, nearly completely blocked exchange-mediated Ba(2+) influx. Chinese hamster ovary cells expressing a mutant exchanger in which 420 out of 520 amino acid residues were deleted from the central hydrophilic domain of the exchanger remained sensitive to the inhibitory effects of calyculin A and okadaic acid. Surprisingly, Na(o)(+)-dependent Ca(2+) efflux appeared to be only modestly inhibited, if at all, by calyculin A or okadaic acid. We conclude that protein hyperphosphorylation during protein phosphatase blockade selectively inhibits the Ca(2+) influx mode of Na(+)/Ca(2+) exchange, probably by an indirect mechanism that does not involve phosphorylation of the exchanger itself.