The influence of cortical feature maps on the encoding of the orientation of a short line

The inhomogeneous distribution of the receptive fields of cortical neurons influences the cortical representation of the orientation of short lines seen in visual images. We construct a model of the response of populations of neurons in the human primary visual cortex by combining realistic response properties of individual neurons and cortical maps of orientation and location preferences. The encoding error, which characterizes the difference between the parameters of a visual stimulus and their cortical representation, is calculated using Fisher information as the square root of the variance of a statistically efficient estimator. The error of encoding orientation varies considerably with the location and orientation of the short line stimulus as modulated by the underlying orientation preference map. The average encoding error depends only weakly on the structure of the orientation preference map and is much smaller than the human error of estimating orientation measured psychophysically. From this comparison we conclude that the actual mechanism of orientation perception does not make efficient use of all the information available in the neuronal responses and that it is the decoding of visual information from neuronal responses that limits psychophysical performance. Action Editor: Terrence Sejnowski     

The meaning of the Weber-Fechner law and description of scenes: III. Description of the visual space

The Weber-Fechner fraction was measured in experiments where a subject was presented with a pair of lines, the length of one (reference) line being constant and the length of the other (test) one varying. The subject had to say whether the longer line was above or below the shorter one. The subject was trained to distinguish the positions of lines in areas limited in the number of reference lines located at different degrees of eccentricity. The fraction curve for each area proved to be of the same shape as the curve obtained for the entire range and reflecting the Weber-Fechner law. The fraction was originally maximum and only slightly decreased afterwards. On the basis of these data, a neural construction is suggested that serves for describing the visual space and explains the shape of the curve reflecting the Weber-Fechner law.     

The meaning of the Weber-Fechner law: IV. The psychometric curve and interhemispheric and intrahemispheric interactions

A subject was presented two horizontal lines. One of them was an objective reference line (Ro), whose length was unchanged. There were a total of five Ro lines. The other line was a test line (T), whose length varied. The subject had to say whether the longer line was above or below the shorter one. The results were treated using a model of neuronal constructions consisting of a number of similar constructions fulfilling different functions along the route from inputting information to obtaining Weber’s ΔL/L ratio. Psychometric curves and the reference stimulus (Rs) as their derivative were determined at the initial stage. In this case, Rs≠Ro. At the final stage of simulation, Rs = Ro. The Weber-Fechner fraction was calculated using the standard formula (T - Ro)/Ro. The same initial data introduced to the model yielded a curve entirely coinciding with the curve calculated experimentally. At the final stage of stimulation, Rs and Ro were equal to each other. Their inequality at the initial stages was necessary for calculating the mutual positions of the elements of a scene. It was concluded that the model constructions are similar to real neuronal constructions of the visual brain.     

The interaction of 2,3,4-trihydroxybenzylhydrazine with dopa decarboxylase from pig kidney

The spectral modifications induced by addition of 2,3,4,-trihydroxybenzylhydrazine (Ro 4-5127) to Dopa decarboxylase indicate the binding of this compound to the coenzyme binding site and allow the titration of the enzyme: 1.07 moles of Ro 4-5127 bind 1 mole of enzyme. Inhibition data indicate that Ro 4-5127 behaves as a pseudoirreversible inhibitor of Dopa decarboxylase and that 100% inhibition is not reached at a 1:1 inhibitor/ enzyme molar ratio, as expected from spectral data, but at a molar ratio of about 6. On this basis it would be possible to suggest, beside the coenzyme binding site, the existence on the enzyme molecule of other sites where the compound could bind without affecting its spectral feature. The interaction of Ro 4-5127 with Dopa decarboxylase shows that “in vivo” Ro 4-5127 behaves as a more powerful inhibitor of Dopa decarboxylase than its precursor trihydroxybenzylhydrazine seryl derivative (Ro 4-4602) indicating that the strong “in vivo” inhibition exerted by Ro 4-4602 might reflect the effective interaction of Dopa decarboxylase with Ro 4-5127.     

The central performance drop in texture segmentation: a simulation based on a spatial filter model

 This article presents a computational model of early visual information processing that attempts to account for the central performance drop (CPD) in texture segmentation. CPD is the finding that detection performance on short stimulus displays of line textures using orientation differences to set off the target is not maximal at the foveal center but in parafoveal areas. A comparison between a simulation and psychophysical experimental data supported the assumption that the CPD may be explained by properties of spatial frequency channels whose band-pass filter characteristics are not constant over the retina but differ with eccentricity in a defined manner. The model provided satisfactory predictions of experimental data based on densely or widely spaced line elements in texture fields. It is concluded that preattentive texture analysis might be performed by a relatively small number of simple spatial filters. Received: 14 November 1996 / Accepted in revised form: 3 June 1997     

Differentiation of the bimodal stimuli in a frog's retina

In work electric activity of frog's retina was investigated by silent substitution technique. Electroretinogram was recorded as a response to abrupt exchange of the referent stimulus-line with fixed values of luminance and orientation to test lines with varied luminance and orientations. As a result of the analysis it has been allocated two types of responses of a retina. The response to onset-offset of a stimulus-line was similar to the response at homogeneous illumination of a retina (ERG), and was characterized by both the high amplitude of b-wave (hundreds mkV) and significant asymmetry of b- and d-waves. Whereas the same waves in response to substitution of the same stimuli were more symmetric and had on ten times smaller amplitudes. Such activity of frog's retina was referred as pattern electroretinogram (PERG) recorded in a high vertebrate's retina as response to stimuli whose contrast was temporally modulated. The analysis of interaction of luminance and line orientation channels in retina was carried out on the base of construction V-shaped functions of stimuli differentiation. It has shown, that activities of both channels are linearly summarized in PERG. It means independent and parallel functioning of these mechanisms. However, it takes the short subdivision of luminance, namely, when luminance of test line not far from luminance of referent line. At the same time, from the moment of the double prevalence of test line in relation to referent line, growth of PERG amplitude has nonlinearly form. Such two-stage changing of PERG amplitude speaks presence in a retina of a frog of two mechanisms of coding of luminance. One mechanism coding light intensity by power of the discharge, it forms the information on an absolute level of light in the environment. Its activity is caused basically, by receptors and cells of external plexiform layer and is submitted by b-wave of electroretinogram. Other mechanism submitted in PERG, is based on the vector code of stimulus, it forms the information on spatial and time differentiation of a light in the visual field and is connected, basically, with cells of internal plexiform layer of frog's retina.     

Color and luminance contrasts attract independent attention

Paying attention can improve vision in many ways, including some very basic functions such as contrast discrimination, a task that probably reflects very early levels of visual processing. Electrophysiological, psychophysical, and imaging studies on humans as well as single recordings in monkey show that attention can modulate the neuronal response at an early stage of visual processing, probably by acting on the response gain. Here, we measure incremental contrast thresholds for luminance and color stimuli to derive the contrast response of early neural mechanisms and their modulation by attention. We show that, for both cases, attention improves contrast discrimination, probably by multiplicatively increasing the gain of the neuronal response to contrast. However, the effects of attention are highly specific to the visual modality: concurrent attention to a competing luminance, but not chromatic pattern, greatly impedes luminance contrast discrimination; and attending to a competing chromatic, but not luminance, task impedes color contrast discrimination. Thus, the effects of attention are highly modality specific, implying separate attentional resources for different fundamental visual attributes at early stages of visual processing.     

Formation of plastic connections of the visual system during ontogenesis of the rabbit

Analysis of evoked potentials and unit activity in the visual cortical projection area of rabbits revealed a definite succession of forming of interneuronal connections in ontogeny. In early postnatal period, the neuronal reactions were characterized by stable responses with one excitatory phase corresponding to initially negative surface evoked potential. Similarity of reactions of neurones situated in the same vertical column was observed and explained by functioning of a system of rigid connections of the thalamic relay nuclei afferents with cortical pyramidal neurones. Beginning from the third week of postnatal life of rabbits the neuronal reactions assumed a distinctly expressed phasic character, and variability of responses was seen along the vertical line. The changes revealed correlated with formation of a system of interneurones providing a possibility of plastic neuronal interaction. A study of the influence of preliminary cortical stimulation of the associative areas showed that intercentral cooperation mediated by cortical interneurones providing a systemic analysis of visual information began to form from the third week of postnatal life and reached the definitive level at later stages of development.     

Neuronal processing delays are compensated in the sensorimotor branch of the visual system

Moving objects change their position until signals from the photoreceptors arrive in the visual cortex. Nonetheless, motor responses to moving objects are accurate and do not lag behind the real-world position. The questions are how and where neural delays are compensated for. It was suggested that compensation is achieved within the visual system by extrapolating the position of moving objects. A visual illusion supports this idea: when a briefly flashed object is presented in the same position as a moving object, it appears to lag behind. However, moving objects do not appear ahead of their final or reversal points. We investigated a situation where participants localized the final position of a moving stimulus. Visual perception and short-term memory of the final target position were accurate, but reaching movements were directed toward future positions of the target beyond the vanishing point. Our results show that neuronal latencies are not compensated for at early stages of visual processing, but at a late stage when retinotopic information is transformed into egocentric space used for motor responses. The sensorimotor system extrapolates the position of moving targets to allow for precise localization of moving targets despite neuronal latencies.     

Dissociable Perceptual Effects of Visual Adaptation

Neurons in the visual cortex are responsive to the presentation of oriented and curved line segments, which are thought to act as primitives for the visual processing of shapes and objects. Prolonged adaptation to such stimuli gives rise to two related perceptual effects: a slow change in the appearance of the adapting stimulus (perceptual drift), and the distortion of subsequently presented test stimuli (adaptational aftereffects). Here we used a psychophysical nulling technique to dissociate and quantify these two classical observations in order to examine their underlying mechanisms and their relationship to one another. In agreement with previous work, we found that during adaptation horizontal and vertical straight lines serve as attractors for perceived orientation and curvature. However, the rate of perceptual drift for different stimuli was not predictive of the corresponding aftereffect magnitudes, indicating that the two perceptual effects are governed by distinct neural processes. Finally, the rate of perceptual drift for curved line segments did not depend on the spatial scale of the stimulus, suggesting that its mechanisms lie outside strictly retinotopic processing stages. These findings provide new evidence that the visual system relies on statistically salient intrinsic reference stimuli for the processing of visual patterns, and point to perceptual drift as an experimental window for studying the mechanisms of visual perception.     

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.     

计算方法在蛋白质相互作用研究中的应用

计算方法在蛋白质相互作用研究的各个阶段扮演了一个重要的角色。对此,作者将从以下几个方面对计算方法在蛋白质相互作用及相互作用网络研究中的应用做一个概述：蛋白质相互作用数据库及其发展;数据挖掘方法在蛋白质相互作用数据收集和整合中的应用;高通量方法实验结果的验证;根据蛋白质相互作用网络预测和推断未知蛋白质的功能;蛋白质相互作用的预测。     

The meaning of the Weber-Fechner law and description of scenes in terms of neural networks

An attempt was made to determine to which functions of the visual cortex the Weber-Fechner law pertains. Pairs of lines were presented in different halves of the visual field. One of them was a reference line with an unchanged length, and the other (the test line) was of variable length. Psychometric curves reflecting the probability of the answer that the test line was longer than the reference one were plotted. A neuronal scheme for calculating the subjective reference stimulus, which differed from the objective reference stimulus, was proposed. The central zone of the visual field was determined. Two psychometric curves were obtained in the zone of each hemisphere. One of them was based on the results of tests where the test stimulus was presented above the reference stimulus and the other, on those where the test stimulus was below the reference one. The mutual positions of the curves were asymmetric in the hemispheres. One psychometric curve was obtained outside the central zone in each hemisphere. There was no dependence on the positions of the test and reference stimuli. The obtained data, according to which ΔL = const in the central zone and ΔL/L = const outside it, served as the basis for postulating the existence of three neuronal mechanisms. One of them is responsible for the interaction of neural networks between the hemispheres and serves for describing the scene, estimating the perspective, and determining the relative distances between objects. The second and third mechanisms are responsible for the interactions within the left and right hemispheres and serve only for describing the scene. Only the mechanism of describing the scene operates outside the central zone. It is assumed that the three postulated mechanisms, together with the mechanism of image recognition, create the visual image of the world perceived by the brain.     

Molecular mechanisms involved in the adenosine A and A receptor-induced neuronal differentiation in neuroblastoma cells and striatal primary cultures

Adenosine A1 receptors (A1Rs) and adenosine A(2A) receptors (A(2A)Rs) are the major mediators of the neuromodulatory actions of adenosine in the brain. In the striatum A1Rs and A(2A)Rs are mainly co-localized in the GABAergic striatopallidal neurons. In this paper we show that agonist-induced stimulation of A1Rs and A(2A)Rs induces neurite outgrowth processes in the human neuroblastoma cell line SH-SY5Y and also in primary cultures of striatal neuronal precursor cells. The kinetics of adenosine-mediated neuritogenesis was faster than that triggered by retinoic acid. The triggering of the expression of TrkB neurotrophin receptor and the increase of cell number in the G1 phase by the activation of adenosine receptors suggest that adenosine may participate in early steps of neuronal differentiation. Furthermore, protein kinase C (PKC) and extracellular regulated kinase-1/2 (ERK-1/2) are involved in the A1R- and A(2A)R-mediated effects. Inhibition of protein kinase A (PKA) activity results in a total inhibition of neurite outgrowth induced by A(2A)R agonists but not by A1R agonists. PKA activation is therefore necessary for A(2A)R-mediated neuritogenesis. Co-stimulation does not lead to synergistic effects thus indicating that the neuritogenic effects of adenosine are mediated by either A1 or A(2A) receptors depending upon the concentration of the nucleoside. These results are relevant to understand the mechanisms by which adenosine receptors modulate neuronal differentiation and open new perspectives for considering the use of adenosine agonists as therapeutic agents in diseases requiring neuronal repair.     

The effect of high-frequency stimulation of the midbrain reticular formation on the interaction of neocortical neurons]

The influence was studied of midbrain reticular formation (NRT) stimulation at the rate of 75-100 Hz by the current of 33-400 mkA upon the neuronal interaction in the visual and somatosensory regions of rabbits neocortex. Histograms of cross- and autocorrelation of impulse trains were plotted. NRT stimulation resulted (as compared with calm wakefulness) in an increase of the number of pairs of neurons with correlated activity and in an increase of the probability of neurons discharges 100-400 ms one after another. Mechanisms of cellular interaction did not change. Comparison with the previously obtained data allowed to conclude that NRT activation can induce certain changes in neuronal interaction observed during pseudoconditioning and early phases of conditioning.     

Neuronal oscillations and multisensory interaction in primary auditory cortex

Recent anatomical, physiological, and neuroimaging findings indicate multisensory convergence at early, putatively unisensory stages of cortical processing. The objective of this study was to confirm somatosensory-auditory interaction in A1 and to define both its physiological mechanisms and its consequences for auditory information processing. Laminar current source density and multiunit activity sampled during multielectrode penetrations of primary auditory area A1 in awake macaques revealed clear somatosensory-auditory interactions, with a novel mechanism: somatosensory inputs appear to reset the phase of ongoing neuronal oscillations, so that accompanying auditory inputs arrive during an ideal, high-excitability phase, and produce amplified neuronal responses. In contrast, responses to auditory inputs arriving during the opposing low-excitability phase tend to be suppressed. Our findings underscore the instrumental role of neuronal oscillations in cortical operations. The timing and laminar profile of the multisensory interactions in A1 indicate that nonspecific thalamic systems may play a key role in the effect.     

The ubiquitin-like protein Plic-1 enhances the membrane insertion of GABAA receptors by increasing their stability within the endoplasmic reticulum

Gamma-aminobutyric acid receptors (GABA(A)R) are the major sites of fast inhibitory neurotransmission in the brain, and a critical determinant for the efficacy of neuronal inhibition is the number of these receptors that are expressed on the neuronal cell surface. GABA(A)Rs are heteropentamers that can be constructed from seven subunit classes with multiple members; alpha, beta, gamma(1-3), delta, epsilon(1-3), theta, and pi. Receptor assembly occurs within the endoplasmic reticulum, and it is evident that transport-competent combinations exiting this organelle can access the cell surface, whereas unassembled subunits are ubiquitinated and subject to proteasomal degradation. In a previous report the ubiquitin-like protein Plic-1 was shown to directly interact with GABA(A)Rs and promote their accumulation at the cell surface. In this study we explore the mechanisms by which Plic-1 regulates the membrane trafficking of GABA(A)Rs. Using both recombinant and neuronal preparations it was apparent that Plic-1 increased the stability of endoplasmic reticulum resident GABA(A)Rs together with an increase in the abundance of poly-ubiquitinated receptor subunits. Furthermore, Plic-1 elevated cell surface expression levels by selectively increasing their rates of membrane insertion. Thus, Plic-1 may play a significant role in regulating the strength of synaptic inhibition by increasing the stability of GABA(A)Rs within the secretory pathway and thereby promoting their insertion into the neuronal plasma membrane.     

Stereoscopic subsystems for position in depth and for motion in depth.

We describe psychophysical evidence that the human visual system contains information-processing channels for motion in depth in addition to those for position in depth. These motion-in-depth channels include some that are selectively sensitive to the relative velocities of the left and right retinal images. We propose that the visual pathway contains stereoscopic (cyclopean) motion filters that respond to only a narrow range of the directions of motion in depth. Turning to the single-neuron level we report that, in addition to neurons turned to position to depth, cat visual cortex contains neurons that emphasize information about the direction of motion at the expense of positional information. We describe psychophysical evidence for the existence of channels that are sensitive to change size, and are separate from the channels both for motion and for flicker. These changing-size channels respond independently of whether the stimulus is a bright square on a dark ground or a dark square on a bright ground. At the physiological level we report single neurons in cat visual cortex that respond selectively to increasing or to decreasing size independently of the sign of stimulus contrast. Adaptation to a changing-size stimulus produces two separable after-effects: an illusion of changing size, and an illusion of motion in depth. These after-effects have different decay time constants. We propose a psychophysical model in which changing-size filters feed a motion-in-depth stage, and suppose that the motion-in-depth after-effect is due to activity at the motion-in-depth stage, while the changing-size after-effect is due to to activity at the changing-size and more peripheral stages. The motion-in-depth after-effect can be cancelled either by a changing-size test stimulus or by relative motion of the left and right retinal images. Opposition of these two cues can also cancel the impression of motion in depth produced by the adapting stimulus. These findings link the stereoscopic (cyclopean) motion filters and the changing-size filters: both feed the same motion-in-depth stage.