Biomedical subjects

Alexander Sokolov

Publications and source records attributed to Alexander Sokolov.

15 recordsLinked to original sources

PubMed2006

Visual motion detection in hierarchical spatial frames of reference.

Neurophysiological and neuroimaging work has uncovered modulatory influence of long-range lateral connections from outside of the classical receptive field on neuronal and behavioral responses to localized targets. We report two psychophysical experiments investigating visual detection of real and apparent motion in central vision with and without remote and immediate stationary references. At a particular temporal frequency (0.1-12.8 Hz), participants adjusted the amplitude of either triangle-wave (real) or square-wave (stroboscopic/apparent) oscillatory motion of a vertical bar along a straight, horizontal trajectory for the first impression of the target's stationarity/nonstationarity (the displacement threshold). In the relative motion conditions, a stationary reference bar was positioned 23' apart from the target; in the absolute motion conditions, the bar was absent. The thresholds were measured with a dimly-lit uniform background (13 x 13 degrees ) and either in the darkness (experiment 1) or moving-background conditions (experiment 2). For both real and apparent motion, varying the observation conditions yields three sensitivity levels: irrespective of the background, the lowest thresholds occur in the presence of an immediate reference, followed by the moderately increased thresholds obtained with a dimly-lit background alone. The equally high thresholds occur in the darkness and moving-background conditions without any visible stationary references. The results suggest that the spatial frames of reference for visual motion detection are hierarchically nested, yet independent. The findings provide support for the view that absolute motion perception should be considered relative, extending neurophysiological evidence for the existence of long-range lateral connections across the visual field.

Adult↗

PubMed2006

Visual navigation in adolescents with early periventricular lesions: knowing where, but not getting there.

Visual navigation in familiar and unfamiliar surroundings is an essential ingredient of adaptive daily life behavior. Recent brain imaging work helps to recognize that establishing connectivity between brain regions is of importance for successful navigation. Here, we ask whether the ability to navigate is impaired in adolescents who were born premature and suffer congenital bilateral periventricular brain damage that might affect the pathways interconnecting subcortical structures with cortex. Performance on a set of visual labyrinth tasks was significantly worse in patients with periventricular leukomalacia (PVL) as compared with premature-born controls without lesions and term-born adolescents. The ability for visual navigation inversely relates to the severity of motor disability, leg-dominated bilateral spastic cerebral palsy. This agrees with the view that navigation ability substantially improves with practice and might be compromised in individuals with restrictions in active spatial exploration. Visual navigation is negatively linked to the volumetric extent of lesions over the right parietal and frontal periventricular regions. Whereas impairments of visual processing of point-light biological motion are associated in patients with PVL with bilateral parietal periventricular lesions, navigation ability is specifically linked to the frontal lesions in the right hemisphere. We suggest that more anterior periventricular lesions impair the interrelations between the right hippocampus and cortical areas leading to disintegration of neural networks engaged in visual navigation. For the first time, we show that the severity of right frontal periventricular damage and leg-dominated motor disorders can serve as independent predictors of the visual navigation disability.

Adolescent↗

PubMed2006

Periventricular leukomalacia specifically affects cortical MEG response to biological motion.

OBJECTIVE: Periventricular leukomalacia (PVL) underlies most of the neurological morbidity including visual-perceptual deficits in survivors of premature birth. However, it is unknown whether and, if so, how PVL affects functional cortical activity. METHODS: Here, we assessed changes in the magnetoencephalographic (MEG) response to visual displays depicting human locomotion in adolescents who were born premature with magnetic resonance imaging signs of PVL. RESULTS: Dynamics of MEG activity parallel behavioral deficits. Early (140-170 milliseconds) brain activation over the right parietal cortex was weaker in patients compared with term-born controls. INTERPRETATION: This is the first evidence for stimulus-specific modulation of cortical activity by periventricular lesions providing new insights into the functional pathology of PVL.

Adolescent↗

PubMed2005

Biological motion processing in adolescents with early periventricular brain damage.

The developing brain is traditionally viewed to possess a great compensatory potential. Here we ask whether visual processing of point-light displays depicting human walking is compromised in adolescents who were born premature (between 27 and 33 gestation weeks) and suffer early bilateral damage to periventricular brain regions. Combining psychophysics with volumetric analysis of structural magnetic resonance imaging (MRI), we found that even relatively small periventricular parieto-occipital lesions may result in long-lasting breakdown of visual processing of point-light displays. Analysis of receiver operating characteristic (ROC) curves revealed higher susceptibility of the patients' perceptual system to camouflage of a point-light figure. The lack of difference in sensitivity between former preterms with normal MRI scan and term-born controls indicates that perceptual deficiencies in patients with periventricular leukomalacia (PVL) are not simply due to premature birth. Most importantly, sensitivity in patients even with mild PVL was lower than in both control groups. Display 180 degrees inversion in the image plane, which is known to impair processing of point-light displays, resulted in a substantial reduction of sensitivity so that it no longer differed between the groups. Despite the social and ecological significance of human locomotion, the capacity of the brain to detect biological motion is substantially modulated by periventricular lesions even if they occur very early in life. The findings point to specific restrictions on the brain's spontaneous compensatory plasticity in perceptual development.

Adolescent↗

PubMed2005

Attentional modulation of cortical neuromagnetic gamma response to biological movement.

Processing of biological motion represented solely by a set of lights on the joints of a human body is traditionally viewed as largely independent of attention. Here, by manipulating attention-related task demands, we assess changes in the neuromagnetic cortical response to a point-light walker. Irrespective of task demands, biological motion evokes an increase in oscillatory gamma activity over the left parieto-occipital region at 80 ms post-stimulus. Only an attended walker, however, yielded further peaks over the right parietal (120 ms) and temporal (155 ms) cortices. By contrast, the magnetoencephalographic (MEG) response to an ignored walker is restricted to the left parieto-occipital region. In addition, peaks in oscillatory activity occur in response to both attended (canonical and scrambled) configurations at 180-200 ms from stimulus onset over the right fronto-temporal regions, most likely reflecting maintenance of the target configuration in working memory. For the first time, we demonstrate that the time course and topographic dynamics of oscillatory gamma activity in response to biological movement undergoes top-down influences and can be profoundly modulated by the withdrawal of attention.

Adaptation, Physiological↗

PubMed2005

Perceived dynamics of static images enables emotional attribution.

Perception of intentions and dispositions of others is an essential ingredient of adaptive daily-life social behaviour. Dynamics of moving images leads to veridical perception of social attributes. Anecdotal observations in art, science, and popular culture indicate that dynamic imbalance can be revealed in static images. Here, we ask whether perceived dynamics of abstract figures is related to emotional attribution. Participants first estimated instability of geometric shapes rotated in 15 degrees steps in the image plane, and then rated the intensity of basic emotions that can be ascribed to the figures. We found no substantial link between the deviation of the figures from the vertical orientation and perceived instability. Irrespective of shape, a strong positive correlation was found between negative emotions and perceived instability. By contrast, positive emotions were inversely linked with deviation of the figure from vertical orientation. The work demonstrates for the first time that dynamics conveyed by static images enables specific emotional attributions, and agrees well with the assumption that neural networks for production of movements and understanding the dispositions of others are intimately linked. The findings are also of importance for exploring the ability to reveal social properties through dynamics in normal and abnormal development, for example in patients with early brain injury or autistic spectrum disorders.

Adult↗

PubMed2004

Recruitment of periventricular parietal regions in processing cluttered point-light biological motion.

Recent findings point to the existence of a cortical-subcortical parietal network that drives attention-related integration of features and elements. Here we ask whether the functioning of this network might be modulated by early periventricular lesions. To this end, a cohort of adolescents who were born premature with different severity of bilateral periventricular leukomalacia (PVL) and two groups of matched peers (term-born adolescents and former preterms with normal MRI scan) were shown a set of impoverished point-light stimuli. Observers had to detect a point-light walker embedded in an array of distracters mimicking the motion of the target's dots. Patients exhibited higher susceptibility to distortions caused by distracters. In patients only, sensitivity to the point-light figure highly correlates not just with performance on additionally administered feature integration tasks but also on visual attention-demanding IQ tasks. Moreover, the sensitivity index, as well as the values of both IQ factors, decreases with an increase in the volumetric PVL extent in the parieto-occipital region. No relationship was found between these variables and the lesion extent in the frontal or temporal periventricular regions. The data suggest that visual integration and attention in processing cluttered point-light displays are intimately connected. Most importantly, periventricular parieto-occipital regions might be part of a distributed network recruited in deployment of the posterior attentional system. The functioning of this system seems to be vulnerable to bilateral periventricular damage even if it occurs very early in brain development.

Adolescent↗

PubMed2004

Reciprocal modulation of neuromagnetic induced gamma activity by attention in the human visual and auditory cortex.

For attentional control of behavior, the brain permanently resolves a competition between the impressions supplied by different senses. Here, using a dual-modality temporal order detection task, we studied attentional modulation of oscillatory neuromagnetic activity in the human cerebral cortex. On each trial, after simultaneous exposure to visual and auditory noise, subjects were presented with an asynchronous pair of a visual and an auditory stimulus. Either of the two stimuli could occur first equally often, their order was not cued. Subjects had to determine the leading stimulus in a pair and attentively monitor it to respond upon its offset. With the attended visual or auditory stimuli, spectral power analysis revealed marked enhancements of induced gamma activity within 250 ms post-stimulus onset over the modality-specific cortices (occipital at 64 Hz, right temporal at 53 Hz). When unattended, however, the stimuli led to a significantly decreased (beneath baseline) gamma response in these cortical regions. The gamma decreases occurred at lower frequencies ( approximately 30 Hz) than did the gamma increases. An increase in the gamma power and frequency for the attended modality and their decrease for the unattended modality suggest that attentional regulation of multisensory processing involves reciprocal changes in synchronization of respective cortical networks. We assume that the gamma decrease reflects an active suppression of the task-irrelevant sensory input. This suppression occurs at lower frequencies, suggesting an involvement of larger scale cell assemblies.

Acoustic Stimulation↗

PubMed2004

Dissociable cortical processing of recognizable and non-recognizable biological movement: analysing gamma MEG activity.

Recognition of biological motion is one of the essential ingredients of human evolutionary survival. When biological motion is revealed solely by a set of light dots on the joints of an invisible human figure, the perceptual system reliably distinguishes it from similar configurations. Here, we assessed the changes in neuromagnetic cortical responses during visual perception of biological motion. Healthy humans saw a randomized set of stimuli consisting of a point-light canonical walker and a scrambled configuration in which the spatial positions of dots were randomly rearranged on the screen. In separate runs, configurations were presented either within an upright or inverted (180 degrees ) orientation in the image plane. Participants performed a one-back repetition task lifting a forefinger in response to the second of two consecutive identical stimuli of each type. Both recognizable upright and non-recognizable inverted walkers evoke enhancements in oscillatory gamma brain activity (25-30 Hz) over the left occipital cortices as early as 100 ms from stimulus onset. Only a recognizable upright walker, however, yields further consecutive peaks over the parietal (130 ms) and right temporal (170 ms) lobes. Scrambled displays do not elicit any increases in the gamma response. The stimulus-specific time course and topographic dynamics of cortical oscillatory activity indicate that the brain rapidly dissociates spatial coherence and meaning revealed through biological movement.

Adult↗

PubMed2003

Timing accuracy in motion extrapolation: reverse effects of target size and visible extent of motion at low and high speeds.

By varying target size, speed, and extent of visible motion we examined the timing accuracy in motion extrapolation. Small or large targets (0.2 or 0.8 deg) moved at either 2.5, 5, or 10 deg s(-1) across a horizontal path (2.5 or 10 deg) and then vanished behind an occluder. Observers responded when they judged that the target had reached a randomly specified position between 0 and 12 deg. With higher speeds, the timing accuracy (the reverse of absolute error) was better for small than for large targets, and for long than for short visible extents. With low speed, these effects were reversed. In addition, while long visible extents yielded a greater accuracy at high than at low speeds, for short extents the accuracy was much better with the low speed. The findings suggest that, when extrapolating motion with targets and visible extents of different sizes, the visual system implements different scaling algorithms depending on target speed. At higher speeds, processing of visible and occluded motion is likely to share a common scaling mechanism based on velocity transposition. Reverse effects for target size and extent of visible motion at low and high speeds converge with the assumption of two distinct speed-tuned motion-processing mechanisms in human vision.

Humans↗

PubMed2003

Prior knowledge about display inversion in biological motion perception.

Display inversion severely impedes veridical perception of point-light biological motion (Pavlova and Sokolov, 2000 Perception & Psychophysics 62 889-899; Sumi, 1984 Perception 13 283-286). Here, by using a spontaneous-recognition paradigm, we ask whether prior information about display orientation improves biological motion perception. Participants were shown a set of 180 degrees inverted point-light stimuli depicting a human walker and quadrupeds (dogs). In experiment 1, one group of observers was not aware of the orientation of stimuli, whereas the other group was told beforehand that stimuli will be presented upside down. In experiment 2, independent groups of participants informed about stimulus orientation saw the same set of stimuli, in each of which either a moving or a static background line was inserted. The findings indicate that information about display inversion is insufficient for reliable recognition of inverted point-light biological motion. Instead, prior information facilitates display recognition only when it is complemented by additional contextual elements. It appears that visual impressions from inverted point-light stimuli remain impenetrable with respect to one's knowledge about display orientation. The origins of orientation specificity in biological motion perception are discussed in relation to the recent neuroimaging data obtained with point-light stimuli and fragmented Mooney faces.

Adolescent↗

PubMed2003

Perception and production of biological movement in patients with early periventricular brain lesions.

Recent neuroimaging and psychophysical findings suggest that perception and production of human body motion share a common representational network. In the present study, we address the issue of whether early disorders in production of biological movement correspond to impairment in biological motion perception. By using the simultaneous masking paradigm, we examined visual sensitivity to biological motion in adolescents (aged 13-16 years) who were born very preterm (at 27-33 gestational weeks). In a confidence rating procedure, the presence of a point-light walking figure embedded in a moving mask was judged. The participants differed in their locomotion ability, ranging from normal to a complete walking disability exhibiting signs of leg-dominated bilateral spastic cerebral palsy (BS-CP) caused by periventricular leukomalacia (PVL). Irrespective of an ability to produce movement, patients with a similar extent of PVL in the parieto-occipital complex exhibit nearly the same sensitivity to biological motion. Sensitivity correlates negatively with the extent of PVL over the parieto-occipital complex, whereas neither the severity of motor disorder nor the severity of pyramidal tract affection relate significantly to the sensitivity index. The data suggest that perception of biological motion is not substantially affected by an observer's early restrictions in body movement. Instead, the findings favour the assumption that the common network for perception and production of biological motion might be inherent for the brain. Motor experience per se does not appear to be necessary for the visual analysis of human movement.

Adolescent↗

PubMed2002

Transcatheter patch correction of secundum atrial septal defects.

The efficacy and safety of the transcatheter patch (TP) correction of a secundum atrial septal defect (ASD) was studied acutely and on short-term follow-up in 20 patients, successfully implanted with the device. TPs are made of polyurethane foam and require temporary balloon catheter immobilization on the atrial septum for 48 hours. Eighteen patients were not suitable for disk-device repair. The patient median age and ASD diameter were 37 years and 26 mm, respectively. Eighteen patients had immediate effective ASD occlusion; 2 patients had significant residual shunts. Premature leaks of the supportive balloons were responsible for the residual shunts. One of the patients with residual shunt received a second patch 6 months later with full occlusion. All patients with implants were doing well up to 24 months after implantation. Existing symptoms improved although residual shunts remained; septal anatomy was normalized, with the patched area becoming progressively indistinguishable from the rest of the septum. In conclusion, TP occlusion of secundum ASD is feasible and effective even for defects unsuitable for disk-device repair. The method appears safe acutely and on short-term follow-up, with symptomatic improvement and normalization of septal anatomy.

Adolescent↗

PubMed2002

Biological motion shown backwards: the apparent-facing effect.

We examined how showing a film backwards (reverse transformation) affects the visual perception of biological motion. Adults and 6-year-old children saw first a point-light quadruped moving normally as if on a treadmill, and then saw the same display in reverse transformation. For other groups the order of presentation was the opposite. Irrespective of the presentation mode (normal or reverse) and of the facing of the point-light figure (rightward or leftward), a pronounced apparent-facing effect was observed: the perceptual identification of a display was mainly determined by the apparent direction of locomotion. The findings suggest that in interpreting impoverished point-light biological-motion stimuli the visual system may neglect distortions caused by showing a film backwards. This property appears to be robust across perceptual development. Possible explanations of the apparent-facing effect are discussed.

Adult↗

PubMed2002

Response scale transfer for visual speed.

By reversing the presentation order and frequency of stimuli between two series of trials, we studied how the category scale for visual speed is transferred across stimulus contexts. Participants judged five stimulus speeds, using three categories (slow, moderate, and fast). In Experiment 1, mainly frequent speeds (either low or high) occurred on the initial trials. This manipulation produced divergent preshift ratings for identical stimuli. Although subsequent reversal of stimulus context resulted in a reversal of scales, the adjustment was incomplete: The postshift ratings did not match the comparable preshift ones. In Experiment 2, mainly infrequent speeds were presented initially. Now the preshift ratings coincided, but higher postshift ratings occurred with the frequent high-speed rather than with the frequent low-speed stimuli. We conclude that with transfer into a new context, the spontaneous adjustment of response scale is determined (1) by the preshift primacy and the postshift frequency effects and (2) by the preshift frequency effect extended to the postshift trials.

Adult↗