The effect of masking in the human subject on the solving of a visual-spatial task

Images of two fragments of regular geometrical figures (square, triangle etc.) have been presented to 58 healthy persons successively with intervals of 20, 80, 120 and 380 ms. The subject must compare these fragments mentally, decide whether they form the standard figure and press a button by the right or left hand according to the instruction. At presentation of both fragments in one visual field, left or right, the number of correct responses is greater when they form the figure. The greater the interstimulus intervals, the greater the number of correct responses to stimuli forming and not forming the standard figure. At presentation of fragments in different visual fields, the number of correct decisions is the same, independently from forming the standard figure. The reaction time is shorter when exposing fragments forming the figure, independently from the way of their presentation; with prolongation of interstimulus intervals the reaction time decreases in all cases. The number of correct decisions is greater and the reaction time is shorter when the stimuli are presented in different visual fields.     

Asymmetry of the cerebral hemispheres during comparison of paired visual stimuli

In conditions of tachistoscopic presentation of visual stimuli, healthy (male and female) right-handed subjects carried out a paired comparison of the stimuli presented unilaterally and in the center of the visual field. In case of recognition of images of words and objects, the number of correct answers and motor reaction time usually did not significantly differ at two interstimuli intervals (1 and 10 s). In comparing images of faces, there also were no differences by the number of reactions, and the reaction time was less at the intervals of 1 s. The left hemisphere dominated at the identification of words and female faces, the right one--at the recognition of male faces. When the right visual field was stimulated images of various classes were recognized more differentially than at the stimulation of the left visual field. The male subjects had more prominent interhemispheric differences than the females. The increase of the interstimuli interval from 1 to 10 s brought to a weakening of the functional interhemispheric asymmetry and decreasing of the differences between the male and female subjects. The obtained data show that in the processes connected with short-time memory, functional interhemispheric asymmetry is basically formed at the initial stages of the information processing.     

Learning to differentiate microintervals of time using verbal feedback

Reaction time (RT) and the number of correct estimations of time microintervals (10 and 180 ms) between two visual stimuli were recorded in healthy subjects. It has been shown that 10 ms interval is better estimated when the stimuli are presented in the right visual field, i.e. when they are addressed directly to the left hemisphere. At the same time the number of correct estimations of 180 ms interval is greater and their RT is less when the stimuli are addressed directly to the right hemisphere. This points to different hemispheric mechanisms of time microintervals estimation. Study of the influence of different forms of verbal reinforcement on this learning has shown that after positive reinforcement (the word "good") the number of correct estimations is on average by 10% greater than after negative reinforcement (the word "error"). This may be connected with such processes as isolation and identification of erroneous reaction.     

Hemispheric lateralization of visuo-spatial function in man

Two parts of a geometrical figure are consecutively presented to healthy adult subjects in the left and right visual fields; the subjects have to compare them mentally and to decide whether these parts form a standard figure or not. Correctness of the reaction is controlled by a computer which lights up on the screen the words "good" or "error". The number of correct decisions of this visual-spatial task does not depend on the hemisphere to which information is addressed. The reaction time is substantially shorter if the information comes "directly" to the right hemisphere. Due to better training in the left hemisphere interhemispheric difference in reaction time gradually disappears in repeated tests. Training to mental "constructing" takes place only in the tests following positive feedback stimulus. Analysis of amplitude-temporal parameters of P300 wave shows that at correct decision of the visual-spatial task the level of activation in the right hemisphere is higher than in the left one.     

EEG coherence during unconscious visual set under conditions of increased motivation of subjects

Prestimulus EEG was recorded in the state of "operative rest" after the instruction and at the stages of formation, actualization, and extinction of unconscious visual set to perception of unequal circles. Two motivation conditions were used: (1) subjects were promised to be rewarded with a small money price for each correct response (a "general" rise of motivation) and (2) only correct assessments of stimuli of a certain kind were rewarded (a "selective" rise of motivation). In both conditions, additional motivation of subjects to the results of their performance led to an increase in EEG coherence most pronounced in the theta and alpha 1 frequency ranges in the left temporal area of the cortex. During the "general" rise of motivation the EEG coherence (as compared to the control group) was higher in a greater number of derivation pairs than during the "selective" rise. EEG coherence in "motivated" subjects was increased already at the stage of operative rest. Later on, at the set stages, no significant changes were revealed. Thus, the realized set formed by the verbal instruction, which increased motivation of subjects to the results of their performance, produced substantially more prominent changes in coherence of cortical potentials than the unconscious set formed during perception of visual stimuli.     

The discrimination of time microintervals by emotionally excitable personalities

Function of time microintervals estimation was studied in emotionally excitable subjects (23 persons) as well as learning of this function by means of positive and negative reinforcements--words "good" and "error"--and interhemispheric relations. In excitable subjects, discerning is disturbed of short pauses between the visual stimuli, especially of 10 ms pause, presented in the right visual field. Deficit of learning of the intervals discerning is noted by reaction time parameter. There is no advantage of the left hemisphere in discerning the 10 ms pause from 60 ms, which is observed in healthy people.     

Hemispheric lateralization of the visual-spatial function in chronic alcoholic intoxication in humans

A series of behavioural and electrophysiological parameters was recorded in subjects with chronic alcohol intoxication during solving of visual-spatial nonverbalized task. It is shown that in comparison with the healthy subjects, their reaction time (RT) of correct decisions was increased; it was more expressed when stimuli were presented in the left visual field, i.e., directly to the right hemisphere, and the number of correct reactions decreased at stimuli presentation directly to the left hemisphere. During repeated tests there were no changes in the number of correct reactions and RT value in the group with chronic alcohol intoxication. It is found that long-term taking of alcohol produces an increase of latency and decrease of the amplitude of the late positive wave P300, more pronounced in the right cerebral hemisphere.     

Unimanual Reaction Time during Comparison of Lateralized Verbal Stimuli: the Features of Interhemispheric Interactions Related to Subjects' Gender

Reaction time (RT) and number of correct responses to letter stimuli were measured in 49 male and 47 female subjects under conditions of crossed and uncrossed lateralization of a pair of stimuli and the hand that performed motor reaction. Gender differences were detected on the basis of RT in the hemispheric organization when a task was performed. Males reacted more rapidly to the stimuli presented in the right visual field, while females demonstrated no lateral effects. There was a significant difference in the case of the males' right hand between crossed and uncrossed lateralization of visual stimuli, which exceeded that in the females and in the case of the males' left hand. There were no gender differences in the number of correct responses. When motor response was performed by either hand, the number of correct responses was significantly higher when the stimuli were presented in the right visual field.     

Whole-Body Prepulse Inhibition Protocol to Test Sensorymotor Gating Mechanisms in Monkeys

Prepulse inhibition (PPI) is the decrease of startle reflex amplitude when a slight stimulus is previously generated. This paradigm may provide valuable information about sensorimotor gating functionality. Here we aimed at determining the inhibited and uninhibited startle response of capuchin monkeys (Sapajus spp.), and to evaluate the role of the superior colliculus in PPI. Capuchin monkeys were tested in a whole-body protocol, to determine the best startle amplitude and interstimuli interval. Additionally we tested two subjects with bilateral superior colliculus damage in this protocol. Results show that 115 dB auditory pulse has induced the best startle response. In contrast to reports in other species, no habituation to the auditory stimuli was observed here in capuchins. Also, startle reflex inhibition was optimal after 120 msec interstimuli interval. Finally, there was a downward tendency of percentage inhibition in superior colliculus-lesioned monkeys. Our data provides the possibility of further studies with whole-body protocol in capuchin monkeys and reinforces the importance of the superior colliculus in PPI.     

Visual processing levels revealed by response latencies to changes in different visual attributes.

Visual latencies, and their variation with stimulus attributes, can provide information about the level in the visual system at which different attributes of the image are analysed, and decisions about them made. A change in the colour, structure or movement of a visual stimulus brings about a highly reproducible transient constriction of the pupil that probably depends on visual cortical mechanisms. We measured this transient response to changes in several attributes of visual stimuli, and also measured manual reaction times to the same stimulus changes. Through analysis of latencies, we hoped to establish whether changes in different stimulus attributes were processed by mechanisms at the same or different levels in the visual pathway. Pupil responses to a change in spatial structure or colour are almost identical, but both are ca. 40 ms slower than those to a change in light flux, which are thought to depend largely on subcortical pathways. Manual reaction times to a change in spatial structure or colour, or to the onset of coherent movement, differ reliably, and all are longer than the reaction time to a change in light flux. On average, observers take 184 ms to detect a change in light flux, 6 ms more to detect the onset of a grating, 30 ms more to detect a change in colour, and 37 ms more to detect the onset of coherent motion. The pattern of latency variation for pupil responses and reaction times suggests that the mechanisms that trigger the responses lie at different levels in cortex. Given our present knowledge of visual cortical organization, the long reaction time to the change in motion is surprising. The range of reaction times across different stimuli is consistent with decisions about the onset of a grating being made in V1 and decisions about the change in colour or change in motion being made in V4.     

Hemispheric asymetry of the evoked electrical activity of the cerebral cortex to letter and non-verbal stimuli]

Average evoked potentials (AEP) were recorded in practically healthy subjects to "meaningless" figures and letters, presented to different halves of the visual field. Analysis of the amplitudes of AEP late components to verbal and non-verbal stimuli reveals hemispheric asymmetry. A higher amplitude of the late positive evoked response (P300) to a "direct" stimulation both by verbal and non-verbal stimuli (in the contralateral field of vision) is recorded in the left hemisphere than in the right one. Similar stimulation of the right hemisphere does not reveal sucha difference. In the left hemisphere the P300 wave is of a clearly greater amplitude to a "direct" stimulation (contralateral visual field) than to an "indirect" one (ipsilateral visual field), regardless of the nature of the stimulus. No such difference is observed in the right hemisphere. The magnitude of the late negative wave (component N200) to non-verbal stimuli is greater in the right hemisphere both in response to "direct" and "indirect" stimulations. No intrahemispheric difference has been found in the amplitude of late evoked responses of the cerebral cortex to verbal and non-verbal stimuli.     

Visual event-related potentials to moving stimuli: normative data

Visual cognitive responses (P300) to moving stimuli were tested in 36 subjects with the aim to find the normal range of P300 parameters. Concomitantly, the circadian intra-individual variability of the P300 was studied in a subgroup of 6 subjects. Visual stimuli consisted of either coherent (frequent stimulus) or non-coherent motion (random stimulus). The oddball paradigm was applied for recording cognitive responses. P300 to rare stimuli had an average latency of 447.3 +/- 46.6 ms and amplitude of 12.9 +/- 6.0 microV. The average reaction time was in the range from 322 to 611 ms and there was no correlation between the reaction time and P300 latency. We did not find any significant circadian changes of the P300 parameters in the 6 subjects tested four times during the same day. Cognitive (event-related) responses (P300) displayed distinctly greater inter-individual variability (S.D. of 50 ms) when compared with pattern-reversal and motion-onset VEPs (S.D. of 6.0 ms and 14 ms, respectively). For this reason, the clinical use of P300 elicited by this kind of visual stimuli seems to be rather restricted and the evaluation of its intra-individual changes is preferable.     

Dynamics of cortical evoked activity during human learning to discriminate microintervals of time using feedback

The dynamics of the cortical evoked activity in the process of learning of time microintervals (10, 60 and 180 ms) discrimination was studied in healthy adults. Feedback stimulus visually informing of the real correlations of the differentiated pauses facilitates the discrimination. The factor of the visual field does not affect the estimation of brief time intervals. At correct identifications, the P300 wave is recorded with a higher amplitude, than at errors. In the trial following the "nonconfirming" feedback stimulus, the standard and test stimuli evoke in the left hemisphere a greater P300 wave, than in the trial after the "confirming" stimulus. Feedback influence is retained in the long-term memory.     

Functionally independent components of early event-related potentials in a visual spatial attention task.

Spatial visual attention modulates the first negative-going deflection in the human averaged event-related potential (ERP) in response to visual target and non-target stimuli (the N1 complex). Here we demonstrate a decomposition of N1 into functionally independent subcomponents with functionally distinct relations to task and stimulus conditions. ERPs were collected from 20 subjects in response to visual target and non-target stimuli presented at five attended and non-attended screen locations. Independent component analysis, a new method for blind source separation, was trained simultaneously on 500 ms grand average responses from all 25 stimulus-attention conditions and decomposed the non-target N1 complexes into five spatially fixed, temporally independent and physiologically plausible components. Activity of an early, laterally symmetrical component pair (N1aR and N1aL) was evoked by the left and right visual field stimuli, respectively. Component N1aR peaked ca. 9 ms earlier than N1aL. Central stimuli evoked both components with the same peak latency difference, producing a bilateral scalp distribution. The amplitudes of these components were no reliably augmented by spatial attention. Stimuli in the right visual field evoked activity in a spatio-temporally overlapping bilateral component (N1b) that peaked at ca. 180 ms and was strongly enhanced by attention. Stimuli presented at unattended locations evoked a fourth component (P2a) peaking near 240 ms. A fifth component (P3f) was evoked only by targets presented in either visual field. The distinct response patterns of these components across the array of stimulus and attention conditions suggest that they reflect activity in functionally independent brain systems involved in processing attended and unattended visuospatial events.     

Relative meaning of lines and corners in geometric figures for their recognition by humans

Neurons tuned to line-crossings (corners, crosses, Y-like and three-ray star-like figures) of different shape and orientation rather than to a single bar were found in the area 17 of the cat visual cortex. We studied the relative role of lines and corners of 2D and 3D geometrical figures for their recognition by humans. Probability of figure recognition during its tachistoscopic presentation was compared for the whole (control) and partly masked figures. Sides or corners of the figures were masked to varying degrees and probabilities of correct response were compared. The recognition probability successively decreases with increasing extent of figure masking. This decrease is significantly more pronounced for the figures without corners than for the figures without part of the lines. The relatively greater significance of the corners than sides of geometrical figures for human visual recognition and some possible neuronal mechanisms of this effect are discussed.     

Subconscious Visual Cues during Movement Execution Allow Correct Online Choice Reactions

Part of the sensory information is processed by our central nervous system without conscious perception. Subconscious processing has been shown to be capable of triggering motor reactions. In the present study, we asked the question whether visual information, which is not consciously perceived, could influence decision-making in a choice reaction task. Ten healthy subjects (28±5 years) executed two different experimental protocols. In the Motor reaction protocol, a visual target cue was shown on a computer screen. Depending on the displayed cue, subjects had to either complete a reaching movement (go-condition) or had to abort the movement (stop-condition). The cue was presented with different display durations (20–160 ms). In the second Verbalization protocol, subjects verbalized what they experienced on the screen. Again, the cue was presented with different display durations. This second protocol tested for conscious perception of the visual cue. The results of this study show that subjects achieved significantly more correct responses in the Motor reaction protocol than in the Verbalization protocol. This difference was only observed at the very short display durations of the visual cue. Since correct responses in the Verbalization protocol required conscious perception of the visual information, our findings imply that the subjects performed correct motor responses to visual cues, which they were not conscious about. It is therefore concluded that humans may reach decisions based on subconscious visual information in a choice reaction task.     

Neuromagnetic evidence that the P100 component of the pattern reversal visual evoked response originates in the bottom of the calcarine fissure

Visual evoked magnetic fields due to pattern reversal stimuli were measured in 5 normal subjects using a helmet-shaped 66 channel magnetoencephalography system linked to MRI. The magnetic topography of the prominent 100 ms response (P100m) evoked by full-field visual showed a double-dipole pattern in the occipital areas of all subjects. Right or left half-field stimuli and upper or lower quadrant-field stimuli evoked a single-dipole pattern in the contralateral occipital area. The P100m sources were when localized using a current dipole model and superimposed on MRI images of each subject. The visual cortex was morphologically variable among the subjects, but the P100m dipoles were all localized at the lateral bottom of the calcarine fissure. Moreover, these P100m dipoles had similar orientations for both half- or quadrant-field stimuli. These results suggest that the P100m is located in a smaller part of the striate cortex than previously reported.     

Temporal and topographic characteristics of evoked potentials in the conflict of two consecutive visual stimuli in a working memory task

Behavioral reactions and brain mechanisms involved in processing two matching or mismatching (conflicting) visual stimuli were studied in healthy subjects (mean age 22.57 ± 0.46 years). Line orientations (vertical, horizontal, or 45°) were used as stimuli and were presented with an interval of 1500–1800 ms. The reaction time was shown to increase in the case of a conflict of two orientations as compared with matching orientations. The reaction time depended on the orientation of the reference stimulus and was minimal when a vertical line was used as a reference. An increase in N2 negativity (time window 200–280 ms) in the frontal and parietal cortical areas was identified as an informative indicator of a conflict between the current orientation and the orientation stored in working memory. The dipole sources of N2 were localized to the prefrontal cortex (middle frontal gyrus, frontal pole, and pars orbitalis). The N2 amplitude was found to depend on the orientation of the first stimulus in a pair, being higher in the case of a 45° orientation. The visual areas were shown to play a role in detecting a conflict of two consecutive signals because the early sensory components increased in amplitude. The results implicate cortical structures, including the sensory-specific visual, parietal, and prefrontal areas, in comparing consecutive visual signals and detecting their conflict.     

EEG spectral characteristics at different stages of the unconscious visual set in two motivation conditions

Multichannel EEG were recorded in young healthy subjects in two series of experiments during formation, actualization, and extinction of the visual unconscious set to the perception of unequal circles under conditions of increased motivation of subjects to the result of their performance. In the first series of experiments, subjects were promised to be rewarded (a small money price) for each correct response (the "general" rise of motivation). In the second series, subjects were promised to be rewarded for correct responses only in cases when one of the circles was larger than the other one (the "directed" rise of motivation). The dynamics of the EEG spectral power derived under these two conditions was compared with similar indices obtained earlier during formation of the same set without any special motivation of subjects (control). In all experimental conditions, before the presentation of the stimuli the EEG power in the alpha range was higher in subjects with the stable set. The dynamics of changes in the alpha power at set stages was principally similar in all conditions. In all the experimental conditions, in subjects with unstable set the EEG power in the delta range was highest at the stage of set actualization. The most pronounced generalized changes in the EEG power in the theta-range during the "general" rise of motivation in subjects with stable and unstable forms of set and greater variability of the reaction time to the probe stimulus suggest that the task performance under these conditions required greater tension than under conditions of the "directed" rise of motivation.     

Lateralization of visuo-spatial function during masking in healthy subjects and in patients with chronic alcoholism

In healthy subjects and patients with chronic alcoholism, the ability to construct in mind geometric figures in conditions of masking was studied by means of recording the number of correct decisions, the reaction time and evoked cortical electrical activity. It has been found that solution of visual-spatial task of mentally constructing geometric figures is accomplished better if a figure-standard can be formed from the presented fragments. During solution of the visual-spatial task in norm the reaction time is shorter if the information comes "directly" to the right hemisphere. Such interhemispheric differences are not observed at chronic alcoholic intoxication. In patients with chronic alcoholism the late components of EPs N200 and P300 developed with a longer latency and lesser amplitude than in healthy subjects. The greatest depression of cortical activity is observed in the right hemisphere.