Abstract: Training on a visual task leads to increased perceptual and neural responses to visual features that were attended during training as well as decreased responses to neglected distractor features. However, the time course of these attention-based modulations of neural sensitivity for visual features has not been investigated before. Here we measured event related potentials (ERP) in response to motion stimuli with different coherence levels before and after training on a speed discrimination task requiring object-based attentional selection of one of the two competing motion stimuli. We found that two peaks on the ERP waveform were modulated by the strength of the coherent motion signal; the response amplitude associated with motion directions that were neglected during training was smaller than the response amplitude associated with motion directions that were attended during training. The first peak of motion coherence-dependent modulation of the ERP responses was at 300 ms after stimulus onset and it was most pronounced over the occipitotemporal cortex. The second peak was around 500 ms and was focused over the parietal cortex. A control experiment suggests that the earlier motion coherence-related response modulation reflects the extraction of the coherent motion signal whereas the later peak might index accumulation and readout of motion signals by parietal decision mechanisms. These findings suggest that attention-based learning affects neural responses both at the sensory and decision processing stages.
Abstract: AIM: Hypothermia is often induced to reduce brain injury in newborns, following perinatal hypoxic-ischaemic events, and in adults following traumatic brain injury, stroke or cardiac arrest. We aimed to devise a method, based on diffusion-weighted MRI, to measure non-invasively the temperature of the cerebrospinal fluid in the lateral ventricles. METHODS: The well-known temperature dependence of the water diffusion constant was used for the estimation of temperature. We carried out diffusion MRI measurements on a 3T Philips Achieva Scanner involving phantoms (filled with water or artificial cerebrospinal fluid while slowly cooling from 41 to 32 degrees C) and healthy adult volunteers. RESULTS: The estimated temperature of water phantoms followed that measured using a mercury thermometer, but the estimates for artificial cerebrospinal fluid were 1.04 degrees C lower. After correcting for this systematic difference, the estimated temperature within the lateral ventricles of volunteers was 39.9 degrees C. Using diffusion directions less sensitive to cerebrospinal fluid flow, it was 37.7 degrees C, which was in agreement with the literature. CONCLUSION: Although further improvements are needed, measuring the temperature within the lateral ventricles using diffusion MRI is a viable method that may be useful for clinical applications. We introduced the method, identified sources of error and offered remedies for each.
Abstract: PURPOSE: To study how central visual motion integration and segmentation processes are influenced by the congruence or incongruence of peripheral contextual moving surrounds and to determine their clinical relevance. METHODS: Nine subjects participated in experiments 1 and 2 (12-second blocks containing 2-second static fixation and 10-second surface plaid movement) and 15 in experiment 3 (72-second blocks, with 12-second fixation, and 60-second motion). Observers reported whether they perceived nontransparent (corresponding to visual integration of motion cues into one surface) or transparent (segmentation of two surfaces from motion cues) plaid motion within a 5 degrees central circular region. Surround stimuli were 20 degrees transparent or nontransparent moving plaids. RESULTS: Contextual effects required the presence of both local and global ambiguity. If central local motion became unambiguous, then surrounds became ineffective. Under local and global ambiguity, transparent surrounds invariably induced central congruence while also strongly suppressing incongruent percepts. Nontransparent surrounds produced similar but less consistent congruent bias, especially for longer viewing times. In the latter case, however, suppression of incongruent central interpretations became barely detectable compared to the observed significant facilitation of congruent percepts. CONCLUSIONS: Local ambiguity is critical in contextual modulation, and the peripheral enhancement or suppression of central motion integration depends both on transparency bias and center-surround congruence. The importance of local ambiguity in contextual modulation is clinically relevant, because it implies that contextual effects will be stronger in disorders with impaired central vision, such as macular degeneration. Moreover, the increased efficacy of global context under conditions of increased local ambiguity may be useful in future rehabilitation approaches.
Abstract: Functional magnetic resonance imaging examinations became an integral part of epilepsy surgery workup. In the pediatric population these examinations are usually carried out in full anesthesia, however in some forms of epilepsy, e.g. electrical status epilepticus in sleep, anesthesia could jeopardize the success of the examination. Here, we show on the example of an eight-year-old polymicrogyric epileptic child, that clonazepam can help suppressing the epileptic activity during propofol anesthesia, so it could facilitate performing passive sensorimotor functional MRI in such a case. Furthermore, among the methodological issues addressed, this case provides evidence for a post-hemispherotomy like functional redistribution of sensorimotor activations to the unaffected hemisphere in a case of childhood epilepsy.
Abstract: When learning to master a visual task in a cluttered natural environment, it is important to optimize the processing of task-relevant information and to efficiently filter out distractors. However, the mechanisms that suppress task-irrelevant information are not well understood. Here we show that training leads to a selective increase in motion coherence detection thresholds for task-irrelevant motion directions that interfered with the processing of task-relevant directions during training. Furthermore, using functional magnetic resonance imaging we found that training attenuated neural responses associated with the task-irrelevant direction compared with the task-relevant direction in the visual cortical areas involved in processing of visual motion. The strongest suppression of functional magnetic resonance imaging responses to task-irrelevant motion information was observed in human area MT+. These findings reveal that perceptual learning leads to the suppression and efficient filtering of task-irrelevant visual information.
Abstract: Activity in the human motion complex (hMT(+)/V5) is related to the perception of motion, be it either real surface motion or an illusion of motion such as apparent motion (AM) or motion aftereffect (MAE). It is a long-lasting debate whether illusory motion-related activations in hMT(+) represent the motion itself or attention to it. We have asked whether hMT(+) responses to MAEs are present when shifts in arousal are suppressed and attention is focused on concurrent motion versus nonmotion features. Significant enhancement of hMT(+) activity was observed during MAEs when attention was focused either on concurrent spatial angle or color features. This observation was confirmed by direct comparison of adapting (MAE inducing) versus nonadapting conditions. In contrast, this effect was diminished when subjects had to report on concomitant speed changes of superimposed AM. The same finding was observed for concomitant orthogonal real motion (RM), suggesting that selective attention to concurrent illusory or real motion was interfering with the saliency of MAE signals in hMT(+). We conclude that MAE-related changes in the global activity of hMT(+) are present provided selective attention is not focused on an interfering feature such as concurrent motion. Accordingly, there is a genuine MAE-related motion signal in hMT(+) that is neither explained by shifts in arousal nor by selective attention.
Abstract: A part of patients with the therapy resistant epilepsy can be cured by surgical interventions. The more concordant the presurgical evaluation data, the better prognosis the patient has postoperatively. In case of discordant examination data, multimodal evaluation or case-specific decision might be successful. We report on a five-year-old boy with bilateral (left-dominated) cortical dysplasia and therapy resistant epilepsy. The ictal EEG did not help to localize the seizure onset zone, semiology had little lateralization value; however, FDG-PET showed left hemispheric hypermetabolism. The child became almost seizure-free and showed improved development after left-sided hemispherotomy.
Abstract: OBJECTIVES: To quantify chromatic dysfunction in Best disease to reassess the classic categorization of macular chromatic damage and to investigate psychophysical and clinical correlations. METHODS: Color-contrast discrimination was measured using 2 different psychophysical strategies in age-matched control (n = 41) and patient (n = 34) eyes. The first strategy measured performance along 3 main confusion lines (testing cone function), and the second evaluated discrimination ellipses (modified Cambridge Color Test). The main outcome measures were chromatic discrimination variables (confusion line length, ellipse length, angle, and axis ratio) and visual acuity (VA). RESULTS: Significant loss of performance was seen in all color axes in our patients, and it increased monotonically with staging, becoming significant in Fishman stages 2 and 3. The classically assumed preferential type I red-green deficit was true only for stage 4. Substantial chromatic dysfunction occurred even with relatively preserved VA despite that negative correlations between all test variables and VA reached statistical significance. Partial correlation analysis showed that protan/deutan loss was related to VA independent of tritan loss. Statistically significant positive correlations were also found between lesion size and chromatic dysfunction. CONCLUSIONS: Chromatic discrimination is often impaired in Best disease, even when VA is still spared. Our quantitative psychophysical approach shows that the classic categorization as a type I red-green deficit is valid only for disease stage 4.
Abstract: PURPOSE: To use a new methodological approach, based on luminance noise, to assess without bias the relative damage of blue-yellow and red-green pathways in ocular hypertension and glaucoma and to correlate obtained measurements with clinical markers of disease progression. METHODS: A psychophysical procedure modified from Cambridge color test was used to assess color discrimination in three different groups: patients with primary open-angle glaucoma (n = 51 eyes), patients with ocular hypertension (n = 95 eyes), and control subjects (n = 46 eyes). Viewing conditions were such that the function of the macula was being tested, using a discrimination task under noisy conditions. Confusion vectors, and parameters obtained from discrimination ellipses were correlated with perimetric and clinical data taken from the same groups. RESULTS: The lengths of the major axis of chromatic discrimination ellipses and all confusion vectors were significantly different between the groups. These measures followed a significant gradient of worsening performance from the control to hypertensive and glaucoma groups, which was steeper for the tritan axis. There was a significant global positive correlation between test parameters and cup-to-disc ratio and a significant negative correlation with the perimetric mean deviation index. Ellipse length in patients with ocular hypertension correlated significantly with the duration of their hypertensive state. CONCLUSIONS: This psychophysical test can detect visual dysfunction in a significant subset of patients with long-term hypertension with preserved visual fields. Macular function is damaged earlier than previously believed, in both the blue-yellow and red-green pathways.
Abstract: This review first gives an overview on the concept of fractal geometry with definitions and explanations of the most fundamental properties of fractal structures and processes like self-similarity, power law scaling relationship, scale invariance, scaling range and fractal dimensions. Having laid down the grounds of the basics in terminology and mathematical formalism, the authors systematically introduce the concept and methods of monofractal time series analysis. They argue that fractal time series analysis cannot be done in a conscious, reliable manner without having a model capable of capturing the essential features of physiological signals with regard to their fractal analysis. They advocate the use of a simple, yet adequate, dichotomous model of fractional Gaussian noise (fGn) and fractional Brownian motion (fBm). They demonstrate the importance of incorporating a step of signal classification according to the fGn/fBm model prior to fractal analysis by showing that missing out on signal class can result in completely meaningless fractal estimates. Limitation and precision of various fractal tools are thoroughly described and discussed using results of numerical experiments on ideal monofractal signals. Steps of a reliable fractal analysis are explained. Finally, the main applications of fractal time series analysis in biomedical research are reviewed and critically evaluated.
