A model study of the dynamic mechanisms of the double orientation tuning of the visual cortex neurons]

The dynamics of double orientation tuning of neurons in the visual cortex of cat was studied by means of computer simulation. It was possible to test the role of shape, relative localization and weight of the inhibitory and excitatory zones of a receptive field and the dynamics of these characteristics. It was shown that selective and acute double orientation tuning can be achieved only through changes in the weight of zones in the receptive fields with the end-stopping and lateral inhibitory zones whereas only the presence of the end-stopping inhibitory zone in the receptive field is sufficient during changes in zone localization and area.     

Double orientation tuning of cat visual cortical neurons

Orientation tuning of 148 primary visual cortical neurons was studied in acute experiments on unanesthetized, curarized cats by analysis of their spike responses to flashes in a receptive field of a bar of light of optimal size. Orientation tuning of 88 neurons (59%) was found to be bimodal: Besides the principal preferred orientation there was a second, making an angle with the first. The second tuning maximum in some cases (64%) was exhibited only with a change in stimulus intensity or background brightness. Analysis of orientation tuning by the time-slice method, i.e., on the basis of individual cuts of the spike trace, showed double tuning to be present in 69% of cases only at certain moments after the beginning of stimulation. The results of analysis of the model showed that the double orientation tuning effect may be the result of the specific configuration of the receptive field, the use of a stimulus longer than the receptive field, the presence of a series of alternating excitatory and inhibitory zones in the receptive field, and also of end inhibitory zones on the narrow ends of the field. The unequal change in zones of the receptive fields in time explains the appearance of double orientation tuning in individual fragments of the spike trace. The functional role of double, "cross-wise" tuning in some primary visual cortical neurons and their role in the detection of the features of visual patterns are discussed.     

Effect of Nembutal narcosis on dynamics of orientation tuning of cat visual cortex neurons

The dynamics of orientation tuning (OT) were investigated in acute experiments on immobilized locally anesthetized cats during response development in 40 neurons of the primary visual cortex before and after Nembutal injection. The range of OT scanning decreased in 53.8% of neurons after Nembutal administration (on the average, by 53.4±5.1°; P<0.001); the phenomenon disappeared completely in some neurons. After Nembutal anesthesia, scanning in 20.5% of units either increased or started up in cases of its absence. The scanning range remained constant in 25.6% of neurons. The mentioned changes in the scanning range were consistently more accentuated in cells for which the preferred orientation, as estimated by standard criteria, was shifted under narcosis than in cells invariant to general anesthesia. In the latter group, units with an unchanged scanning range occurred four times more often at all stages of the experiment as against the group of unstable neurons.Translated from Neirofiziologiya, Vol. 25, No. 2, pp. 141–146, March–April, 1993.     

Bicuculline and orientation tuning of neurons of the visual cortex

Orientation tuning (OT) of 68 visual cortex neurons (field 17) was studied in cats under conditions of a GABA-ergic inhibition blockade by microiontophoretic bicuculline applications; the neuronal responses were evoked by flashing light strips. All characteristics of orientational detection in most neurons got worse after the applications. The OT became wider in 76.3% of cases: its mean value increased from 52.7±2.8° to 85.2±4.6°. In 63.6% of cases OT selectivity decreased by one-third, and in 68.5% of neurons the detection quality decreased by 60%, on average. The threshold dose of bicuculline causing the OT extension was injected by the phoretic current of 31.0±4.5 nA, and the optimum effect was reached at 67.1±6.0 nA. The background activity and the response magnitude increased under the bicuculline influence 3.0 and 4.4 times, respectively, compared with the control. A few minutes after the iontophoresis termination, the frequency of neuronal discharges and OT characteristics returned to their initial values. We conclude that the local blocking of intracortical inhibition, which causes disinhibition of afferent inputs from the neighboring cells with different (compared with the recorded cell) preferred orientations, considerably worsens orientational specificity of visual cortex neurons, or even results in a complete loss of such specificity. These data are consistent with the concept that intracortical inhibition plays a leading role in the formation and sharpening of OT in the visual cortex neurons.Neirofiziologiya/Neurophysiology, Vol. 27, No. 1, pp. 54–62, January–February, 1995.     

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.     

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.     

Origin of orientation tuning in the visual cortex.

The results of recent experiments have thrown new light on the neuronal connections underlying orientation-selective responses in the primary visual cortex of adult animals. The pattern of afferent input from the lateral geniculate nucleus to the cortex appears to be specific for orientation, while intracortical inhibitory connections appear to be non-specific in this respect. Experimental and theoretical studies have suggested that the development of cortical cell orientation tuning is an activity-dependent process.     

Adaptation-induced plasticity of orientation tuning in adult visual cortex

A key emergent property of the primary visual cortex (V1) is the orientation selectivity of its neurons. The extent to which adult visual cortical neurons can exhibit changes in orientation selectivity is unknown. Here we use single-unit recording and intrinsic signal imaging in V1 of adult cats to demonstrate systematic repulsive shifts in orientation preference following short-term exposure (adaptation) to one stimulus orientation. In contrast to the common view of adaptation as a passive process by which responses around the adapting orientation are reduced, we show that changes in orientation tuning also occur due to response increases at orientations away from the adapting stimulus. Adaptation-induced orientation plasticity is thus an active time-dependent process that involves network interactions and includes both response depression and enhancement.     

A geometric model for initial orientation errors in pigeon navigation

All mobile animals respond to gradients in signals in their environment, such as light, sound, odours and magnetic and electric fields, but it remains controversial how they might use these signals to navigate over long distances. The Earth's surface is essentially two-dimensional, so two stimuli are needed to act as coordinates for navigation. However, no environmental fields are known to be simple enough to act as perpendicular coordinates on a two-dimensional grid. Here, we propose a model for navigation in which we assume that an animal has a simplified ‘cognitive map’ in which environmental stimuli act as perpendicular coordinates. We then investigate how systematic deviation of the contour lines of the environmental signals from a simple orthogonal arrangement can cause errors in position determination and lead to systematic patterns of directional errors in initial homing directions taken by pigeons. The model reproduces patterns of initial orientation errors seen in previously collected data from homing pigeons, predicts that errors should increase with distance from the loft, and provides a basis for efforts to identify further sources of orientation errors made by homing pigeons.     

Dynamics of orientational tuning of cat visual cortical neurons

Orientational tuning of primary visual cortical unit activity was investigated in acute experiments on cats immobilized by a muscle relaxant, by the time slices method. Poststimulus histograms of responses of a neuron to presentation of a flashing bar of light in the center of its receptive field, with different orientations, were plotted; graphs of orientational tuning with respect to mean discharge frequency in consecutive time cuts of the responses with a 10 or 20 msec step were then plotted. Orientational tuning in all cortical neurons studied exhibited considerable dynamic changes in sharpness and preferred orientation. In two thirds of cells an effect of scanning a certain part of the range of orientations was observed, in the form of a successive shift of the maximum of the orientational tuning curve from some preferred orientations to others was discovered. The possible functional significance of spike discharges of visual cortical neurons is discussed.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 5, pp. 451–459, September–October, 1981.     

A spherical model for orientation and spatial-frequency tuning in a cortical hypercolumn

A theory is presented of the way in which the hypercolumns in primary visual cortex (V1) are organized to detect important features of visual images, namely local orientation and spatial-frequency. Given the existence in V1 of dual maps for these features, both organized around orientation pinwheels, we constructed a model of a hypercolumn in which orientation and spatial-frequency preferences are represented by the two angular coordinates of a sphere. The two poles of this sphere are taken to correspond, respectively, to high and low spatial-frequency preferences. In Part I of the paper, we use mean-field methods to derive exact solutions for localized activity states on the sphere. We show how cortical amplification through recurrent interactions generates a sharply tuned, contrast-invariant population response to both local orientation and local spatial frequency, even in the case of a weakly biased input from the lateral geniculate nucleus (LGN). A major prediction of our model is that this response is non-separable with respect to the local orientation and spatial frequency of a stimulus. That is, orientation tuning is weaker around the pinwheels, and there is a shift in spatial-frequency tuning towards that of the closest pinwheel at non-optimal orientations. In Part II of the paper, we demonstrate that a simple feed-forward model of spatial-frequency preference, unlike that for orientation preference, does not generate a faithful representation when amplified by recurrent interactions in V1. We then introduce the idea that cortico-geniculate feedback modulates LGN activity to generate a faithful representation, thus providing a new functional interpretation of the role of this feedback pathway. Using linear filter theory, we show that if the feedback from a cortical cell is taken to be approximately equal to the reciprocal of the corresponding feed-forward receptive field (in the two-dimensional Fourier domain), then the mismatch between the feed-forward and cortical frequency representations is eliminated. We therefore predict that cortico-geniculate feedback connections innervate the LGN in a pattern determined by the orientation and spatial-frequency biases of feed-forward receptive fields. Finally, we show how recurrent cortical interactions can generate cross-orientation suppression.     

Theoretical analysis of reverse-time correlation for idealized orientation tuning dynamics

A theoretical analysis is presented of a reverse-time correlation method used in experimentally investigating orientation tuning dynamics of neurons in the primary visual cortex. An exact mathematical characterization of the method is developed, and its connection with the Volterra–Wiener nonlinear systems theory is described. Various mathematical consequences and possible physiological implications of this analysis are illustrated using exactly solvable idealized models of orientation tuning.     

reflection of stimulus orientation in reactions of visual cortex neurons

A mathematical model of neuronal target structure with spatial anisotropy of lateral inhibition is discussed. The positions of the neuronal target to oriented sensory stimuli are investigated by computer simulation. It is suggested that visual stimuli orientation is coded in the late phase of dynamic responses of cortical neurons. This idea is in agreement with the data obtained in experiments on guinea pig visual cortex neurons.     

The emergence of contrast-invariant orientation tuning in simple cells of cat visual cortex

Simple cells in primary visual cortex exhibit contrast-invariant orientation tuning, in seeming contradiction to feed-forward models that rely on lateral geniculate nucleus (LGN) input alone. Contrast invariance has therefore been thought to depend on the presence of intracortical lateral inhibition. In vivo intracellular recordings instead suggest that contrast invariance can be explained by three properties of the excitatory pathway. (1) Depolarizations evoked by orthogonal stimuli are determined by the amount of excitation a cell receives from the LGN, relative to the excitation it receives from other cortical cells. (2) Depolarizations evoked by preferred stimuli saturate at lower contrasts than the spike output of LGN relay cells. (3) Visual stimuli evoke contrast-dependent changes in trial-to-trial variability, which lead to contrast-dependent changes in the relationship between membrane potential and spike rate. Thus, high-contrast, orthogonally oriented stimuli that evoke significant depolarizations evoke few spikes. Together these mechanisms, without lateral inhibition, can account for contrast-invariant stimulus selectivity.     

TrkB受体依赖的PV神经元调控小鼠视觉方位辨别能力的机制

神经营养因子-酪氨酸受体激酶B （tyrosine receptor kinase B,TrkB）信号通路在调控初级视皮层（primary visual cortex,V1）兴奋与抑制平衡上发挥着重要的作用,以往的研究揭示了其通过增加兴奋性传递效率来调控皮层兴奋性水平的机制,却并未阐明TrkB受体如何通过抑制系统来调控兴奋与抑制平衡,进而影响视觉皮层功能。为了探讨TrkB信号通路如何特异性地调控最主要的抑制性神经元——PV神经元进而对小鼠视觉皮层功能产生影响,本研究通过病毒特异性地降低V1区的PV神经元上TrkB受体的表达水平,并通过在体多通道电生理手段记录初级视皮层抑制性与兴奋性神经元功能变化,通过行为学实验测试小鼠的方位辨别能力改变。结果表明,初级视觉皮层中的PV抑制性神经元上的TrkB受体表达减少会显著增加兴奋性神经元的反应强度,减弱抑制性神经元与兴奋性神经元的方位辨别能力,增加二者的信噪比,但是小鼠个体水平的方位辨别能力出现下降。这些结果说明,TrkB信号通路并非单纯通过增加靶向PV神经元的兴奋性传递来调控PV神经元的功能,其对神经元信噪比的影响也并非由于抑制系统的增强所致。     

Feedforward origins of response variability underlying contrast invariant orientation tuning in cat visual cortex

Contrast invariant orientation tuning in simple cells of the visual cortex depends critically on contrast dependent trial-to-trial variability in their membrane potential responses. This observation raises the question of whether this variability originates from within the cortical circuit or the feedforward inputs from the lateral geniculate nucleus (LGN). To distinguish between these two sources of variability, we first measured membrane potential responses while inactivating the surrounding cortex, and found that response variability was nearly unaffected. We then studied variability in the LGN, including contrast dependence, and the trial-to-trial correlation in responses between nearby neurons. Variability decreased significantly with contrast, whereas correlation changed little. When these experimentally measured parameters of variability were applied to a feedforward model of simple cells that included realistic mechanisms of synaptic integration, contrast-dependent, orientation independent variability emerged in the membrane potential responses. Analogous mechanisms might contribute to the stimulus dependence and propagation of variability throughout the neocortex.     

Effects of sombrevine on orientational tuning of visual cortex neurons in the cat

Changed orientational tuning (OT) in 58 visual cortex units was investigated during acute experiments on immobilized cats under light short-lasting sombrevine-induced anesthesia. A 47.6±5.6° alteration in the preferred orientation of 60% of cells occurred following sombrevine injection but no change occurred at any stage of anesthesia in the remainder. The latter group showed a preference for horizontal and vertical orientations, less pronounced in the former category. "Stable" neurons also displayed less acute tuning and more selective detection in comparison with "unstable" units. Breadth of orientational tuning consistently changed by an average of 65.2±6.7° in 55% of neurons, while tuning deteriorated in 31% and sharpened in 24% of cells. No regular change in tuning band occurred in the remainder. Background firing rate and evoked spike activity declined by 58% and 35%, respectively under anesthesia in 2/3 of the cells tested. Tuning bandwidth of unit firing rate had generally recovered within 20–40 min after administering the anesthetic (i.e., as the anesthesia wore off).Higher Nervous Activity and Neurophysiology Research Institute. Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 21, No. 6, pp. 812–820, November–December, 1989.     

Size as a parameter for tuning visual attention]

The ability of visual attention to tune to the stimulus size (when this size could not be described by spatial frequencies) was studies. Sinusoidal gratings with frequencies of 1.5, 3, and 6 cycle/degree were used as test stimuli. All these stimuli consisted of 3 periods, consequently, they had different sizes: 2 x 2, 1 x 1, and 0.5 x 0.5 degrees. Three reference stimuli had the same sizes but were constructed as a superposition of all the test frequencies. The reference stimulus of suprathreshold contrast was displayed for 400 ms to the left or to the right of a fixation point at a distance of 3 degrees. After that, the test stimulus of threshold contrast was for 100 ms displayed symmetrically to the fixation point on the other side. Subjects were instructed that the sizes of the reference and test stimuli were the same. It was found that the probability of test detection decreased with increase in the difference between the sizes of the reference and test stimuli. Since in our experiments the spatial frequency could not be used for tuning visual attention, the obtained results suggest that there are specialized mechanisms in the visual system for estimation of the general image size.