Temporal properties of shape processing by event-related MEG adaptation.

Object recognition is a fundamental mechanism of visual processing and requires the extraction of shape information. Early visual areas have been linked to the analysis of local image features, while higher visual areas of the ventral visual pathway rather mediate the perception and recognition of global shapes. Investigations of the spatiotemporal characteristics of shape analysis in the human visual cortex by rapid event-related fMRI adaptation in combination with a region of interest analysis suggested a transient manner of contour integration and shape processing in early visual areas compared to sustained processing in higher visual areas. fMRI adaptation (or repetition suppression) paradigms offer the possibility to enhance the restricted spatial resolution of conventional fMRI by focusing on decreased responses for repeated stimulus presentation. However, improving our understanding of complex neuronal mechanisms in the human brain requires the investigation not only at high spatial but also temporal resolution. A limitation of fMRI adaptation can be found in its poor temporal resolution which EEG- and MEG-techniques can overcome, though at a lower spatial resolution. The present study aimed to investigate temporal characteristics of shape processing in the human brain by adapting the principles of fMRI adaption in a MEG study. In parallel to an earlier fMRI study, the two stimuli of a trial were presented at varied interstimulus intervals. Additional analyses by means of a dipole analysis and co-registration of MEG and fMRI data were conducted. Adaptation was observed for the short as well as the longer interstimulus interval. Interestingly, the latency of the adaptation effects varied with the interstimulus interval. The findings support a late onset of adaption that possibly underlies global discrimination processes and recognition in higher areas of the ventral visual pathway. Further, the present results indicate a useful extension of adaptation paradigms and 'region of interest'-analyses from fMRI to MEG at a high temporal resolution.

Cortical response to social interaction is affected by gender.

The ability of humans to predict and explain other people's actions is of immense value for adaptive behavior and nonverbal communication. Gender differences are often evident in the comprehension of social signals, but the underlying neurobiological basis for these differences is unclear. Combining visual psychophysics with an analysis of neuromagnetic activity, we assessed gender effects on the induced oscillatory response to visual social interaction revealed by motion. A robust difference in the induced gamma response was found between females and males over the left prefrontal cortex, a region implicated in perceptual decision making. The induced gamma neuromagnetic response peaked earlier in females than in males. Moreover, it appears that females anticipate social interaction predicting others' actions ahead of their realization, whereas males require accumulation of more sensory evidence for proper social decisions. The findings reflect gender-dependent modes in cortical processing of visually acquired social information. Contrary to popular wisdom, the outcome of this study indicates that gender effects are not evident in the neural circuitry underpinning visual social perception, but in the regions engaged in perceptual decision making.

Social interaction revealed by motion: dynamics of neuromagnetic gamma activity.

Perception of dispositions of others revealed by movement is an essential ingredient of adaptive daily-life social behavior. Brain imaging points to several brain regions involved in visual processing of social interaction represented by motion of geometric shapes. However, temporal interrelations among these regions remain unknown. Keeping in mind that successful visual social perception depends on intact communication throughout the brain, we focus here on analysis of the induced gamma neuromagnetic response to social interaction revealed by motion. A peak of induced gamma activity of 62 Hz was found at 1 s from the stimulus onset over the right parieto-temporal junction. Two further enhancements in gamma response of lower frequency of 44 Hz occurred at 1.4 s over the medial prefrontal and posterior temporal cortices in the right hemisphere. Subsequent boosts of 44 Hz were found at 1.6 s over the left temporal and right posterior temporal cortices. For the first time, the findings identify the cortical network engaged in visual processing of social interaction revealed by motion and help to better understand proper functioning of the social brain circuitry.

Recognition of affective prosody in brain-damaged patients and healthy controls: a neurophysiological study using EEG and whole-head MEG.

A passive oddball experiment was used in which stimuli were emotional exclamations differing in their affective tone. In both electroencephalography (EEG) and magnetoencephalography (MEG), deviants elicited an N300 component, sometimes accompanied by a slow wave. Both components had a symmetrical distribution, but the former was more posterior than the latter. The same responses to prosodic stimuli were significant in 6 of 27 patients with severe disorders of consciousness (persistent vegetative state and minimally conscious state) and in all 3 of the examined locked-in patients, indicating that the procedure can be applied for testing neurological patients. The occurrence of significant responses depended on the presence or absence of a lesion to the right temporal lobe. Obviously, the N300 depends on the activity of the right temporal cortex but does not originate there. We suggest that the component is related not to the recognition of affective prosody as such, but to the following detection of affective mismatch due to violations of emotional context of stimulation.

Task- and performance-related modulation of domain-specific auditory short-term memory representations in the gamma-band.

The short-term retention of information has been related to oscillatory activity in the gamma-band. In recent auditory spatial short-term memory studies we have found stimulus-specific components of parieto-occipital gamma-band activity (GBA) which might reflect the activation of local networks tuned to task-relevant stimulus features. The present magnetoencephalography study (N=22) tested this interpretation by assessing whether the topography of stimulus-specific GBA depends on task demands. Sample sounds were characterized by both a variable interaural time delay and a variable central frequency. In separate task blocks, either the lateralization or the frequency of the same stimuli had to be maintained. Statistical probability mapping of differences in oscillatory responses to the retention of sample sounds replicated the contralateral posterior topography for GBA components distinguishing between medial and lateral sounds in the spatial memory task. In contrast, lower- and higher-frequency stimuli were accompanied by frontal GBA components in the frequency task. Memory for lateralization versus frequency selectively enhanced oscillatory activity for these posterior versus frontal components, directly demonstrating their modulation by task demands. Incorrect "non-match" responses were negatively correlated with delay-phase GBA to the relevant feature, whereas incorrect "match" responses correlated positively with GBA to the irrelevant feature. In summary, the topography of stimulus-specific GBA to identical stimuli reflected the selective representation of task-relevant features. Task performance was predicted by both enhanced stimulus-specific GBA for the task-relevant stimulus attribute and reduced gamma-band representations of the task-irrelevant stimulus feature. Stimulus-specific GBA may reflect the memory representation that is used in subsequent recognition.

Time course of early audiovisual interactions during speech and nonspeech central auditory processing: a magnetoencephalography study.

Cross-modal fusion phenomena suggest specific interactions of auditory and visual sensory information both within the speech and nonspeech domains. Using whole-head magnetoencephalography, this study recorded M50 and M100 fields evoked by ambiguous acoustic stimuli that were visually disambiguated to perceived /ta/ or /pa/ syllables. As in natural speech, visual motion onset preceded the acoustic signal by 150 msec. Control conditions included visual and acoustic nonspeech signals as well as visual-only and acoustic-only stimuli. (a) Both speech and nonspeech motion yielded a consistent attenuation of the auditory M50 field, suggesting a visually induced "preparatory baseline shift" at the level of the auditory cortex. (b) Within the temporal domain of the auditory M100 field, visual speech and nonspeech motion gave rise to different response patterns (nonspeech: M100 attenuation; visual /pa/: left-hemisphere M100 enhancement; /ta/: no effect). (c) These interactions could be further decomposed using a six-dipole model. One of these three pairs of dipoles (V270) was fitted to motion-induced activity at a latency of 270 msec after motion onset, that is, the time domain of the auditory M100 field, and could be attributed to the posterior insula. This dipole source responded to nonspeech motion and visual /pa/, but was found suppressed in the case of visual /ta/. Such a nonlinear interaction might reflect the operation of a binary distinction between the marked phonological feature "labial" versus its underspecified competitor "coronal." Thus, visual processing seems to be shaped by linguistic data structures even prior to its fusion with auditory information channel.

Temporal dynamics of stimulus-specific gamma-band activity components during auditory short-term memory.

Recently we have demonstrated that during auditory short-term memory maintenance, gamma-band activity (GBA) components can be identified which are specific to the retained stimulus. These activations peaked in the middle of the delay phase between sample and test stimuli, and their magnitude during the final part of this period correlated with performance. However, using a constant delay duration did not allow to answer the question whether stimulus-specific GBA components represented responses to sample sounds or anticipatory activations preceding test stimuli. Here we addressed this unresolved issue by investigating the temporal dynamics of stimulus-specific GBA during two delay durations. Magnetoencephalogram was recorded in 18 adults during an auditory spatial short-term memory task involving lateralized sample stimuli presented with two different interaural time delays. Subjects had to decide whether test stimuli presented after retention phases of 800 or 1200 ms had the same lateralization as sample sounds. Statistical probability mapping served to identify oscillatory activations differentiating between the two sample sounds. We found stimulus-specific GBA components over posterior cortex peaking about 400 ms prior to the onset of test stimuli regardless of delay duration. Their magnitude correlated with task performance. In summary, stimulus-specific GBA components with a predictive power for short-term memory performance were observed in anticipation of test stimuli. They may reflect the preparatory activation of memory representations or the shifting of attention to the specific expected location of the test stimulus.

Improvement of microcirculation after percutaneous transluminal angioplasty in the lower limb with prostaglandin E1.

OBJECTIVE: The aim of the study was to investigate prospectively the microcirculation after angioplasty and its improvement with additional Prostaglandin E1 (PGE1) therapy assessed by transcutaneous pressure of oxygen. PATIENTS AND METHODS: 45 patients with intermittent claudication eligible for angioplasty were enrolled in a prospective randomised controlled clinical trial. Patients received either intra-arterial bolus of 40 microg PGE1 in addition to angioplasty or a 40 microg PGE1 intravenous infusion. Control group received no trial medication. Additional 15 patients undergoing intra-arterial angiography were also investigated. tcpO(2) values were recorded distal to the PTA region before, during the intervention, 24h, 2 and 4 weeks after intervention. Clinical endpoint was the change of tcpO(2) values 4 weeks after intervention. RESULTS: During the 4 week follow-up tcpO(2) values decreased in patients treated with angioplasty. At the same time tcpO(2) increased significantly in those patients additionally treated with intra-arterial PGE1 bolus injection as well as with intravenous PGE1 infusion. CONCLUSIONS: Impaired microcirculation after angioplasty can be improved with additional intravenous as well as intra-arterial PGE1 administration.

Direct contrasts between experimental conditions may yield more focal oscillatory activations than comparing pre- versus post-stimulus responses.

Contrasting electro- or magnetoencephalographic oscillatory responses to sensory stimuli with a pre-stimulation baseline commonly yields spectrally broad and topographically distributed activations. In contrast, comparisons between closely matched task conditions usually result in more focal differences. In the present study, we reanalyzed an existing set of MEG data recorded during stimulation with virtual Kanizsa figures and no-triangle control stimuli to contrast results yielded by the two approaches. Statistical analysis showed that visual stimulation compared to baseline gave rise to spectral amplitude reductions in lower frequencies including alpha and beta and amplitude enhancements in gamma frequencies above 55 Hz. These changes reached significance by about 100 ms post-stimulus onset, were topographically widespread over posterior cortex, and did not differ between stimuli. A second, more focal component over ventral occipital cortex peaked at about 300 ms in the gamma range at approximately 70 Hz. It was more pronounced for the Kanizsa triangle than for the no-triangle stimulus. A third gamma component over lateral occipito-temporal cortex showed an amplitude increase at around 450 ms for virtual figures and a concomitant decrease for the nongestalt-like control stimulus, and no overall task-related activity. Our findings illustrate that direct comparisons between conditions yield effects with a more focal spectral and topographical distribution than comparisons with a pre-stimulus baseline. Moreover, they exemplify that contrasts between conditions may reveal additional activations not captured by comparisons with a pre-stimulus baseline.

Behavioral relevance of gamma-band activity for short-term memory-based auditory decision-making.

Oscillatory activity in the gamma-band range has been established as a correlate of cognitive processes, including perception, attention and memory. Only a few studies, however, have provided evidence for an association between gamma-band activity (GBA) and measures of behavioral performance. Here we focused on the comparison between sample and test stimuli S1 and S2 during an auditory spatial short-term memory task. Applying statistical probability mapping to magnetoencephalographic recordings from 28 human subjects, we identified GBA components distinguishing nonidentical from identical S1-S2 pairs. This activity was found at frequencies between 65 and 90 Hz and was localized over posterior cortical regions contralateral to the hemifield in which the stimuli were presented. The 10 best task performers showed higher amplitudes of this GBA component than the 10 worst performers. This group difference was most pronounced between about 150 and 300 ms after stimulus onset. Apparently the decision about whether test stimuli matched the stored representation of previously presented sample sounds relied partly on the oscillatory activation of networks representing differences between both stimuli. This result could be replicated by reanalyzing the combined data from two previous studies assessing short-term memory for sound duration and sound lateralization, respectively. Similarly to our main study, GBA amplitudes to nonmatching vs. matching S1-S2 pairs were higher in good performers than poor performers. The present findings demonstrate the behavioral relevance of GBA.

Selective attention increases the dependency of cortical responses on visual motion coherence in man.

Attention improves visual discrimination and consequently allows to discern stimuli with low signal-to-noise ratios that otherwise would remain undetected. We used magnetoencephalography (MEG) to test whether neuromagnetic responses recorded from occipito-temporal cortex, reflecting the size of visual motion signals embedded in noise (motion coherence), would mirror the perceptual changes induced by attention. Attention directed to a given hemifield increased and decreased the coherence modulation of the MEG response over contralateral and ipsilateral visual cortex, respectively, indicating a change in the neuronal signal-to-noise ratio at the population level.

Distinct gamma-band components reflect the short-term memory maintenance of different sound lateralization angles.

Oscillatory activity in human electro- or magnetoencephalogram has been related to cortical stimulus representations and their modulation by cognitive processes. Whereas previous work has focused on gamma-band activity (GBA) during attention or maintenance of representations, there is little evidence for GBA reflecting individual stimulus representations. The present study aimed at identifying stimulus-specific GBA components during auditory spatial short-term memory. A total of 28 adults were assigned to 1 of 2 groups who were presented with only right- or left-lateralized sounds, respectively. In each group, 2 sample stimuli were used which differed in their lateralization angles (15 degrees or 45 degrees) with respect to the midsagittal plane. Statistical probability mapping served to identify spectral amplitude differences between 15 degrees versus 45 degrees stimuli. Distinct GBA components were found for each sample stimulus in different sensors over parieto-occipital cortex contralateral to the side of stimulation peaking during the middle 200-300 ms of the delay phase. The differentiation between "preferred" and "nonpreferred" stimuli during the final 100 ms of the delay phase correlated with task performance. These findings suggest that the observed GBA components reflect the activity of distinct networks tuned to spatial sound features which contribute to the maintenance of task-relevant information in short-term memory.

Increases in language lateralization in normal children as observed using magnetoencephalography.

Previous functional magnetic resonance imaging (fMRI) studies investigating hemispheric dominance for language have shown that hemispheric specialization increases with age. We employed magnetoencephalography (MEG) to investigate these effects as a function of normal development. In sum, 22 healthy children aged 7-16 years were investigated using two language tasks: a verb-generation (VG) task and a vowel-identification (VI) task. Significant hemispheric differences were found for both tasks in cerebral language areas using oscillatory MEG spectral analyses, confirming the MEG's ability to detect hemispheric specialization for language in children. Additionally, a significant increase of this lateralization as a function of age was observed for both tasks. As performance in the VI task showed no correlation with age, this increase seems to be unrelated to performance. These results confirm an increase in hemispheric specialization as a function of normal brain maturation.

Cerebrocortical beta activity in overweight humans responds to insulin detemir.

BACKGROUND: Insulin stimulates cerebrocortical beta and theta activity in lean humans. This effect is reduced in obese individuals indicating cerebrocortical insulin resistance. In the present study we tested whether insulin detemir is a suitable tool to restore the cerebral insulin response in overweight humans. This approach is based on studies in mice where we could recently demonstrate increased brain tissue concentrations of insulin and increased insulin signaling in the hypothalamus and cerebral cortex following peripheral injection of insulin detemir. METHODOLOGY/PRINCIPAL FINDINGS: We studied activity of the cerebral cortex using magnetoencephalography in 12 lean and 34 overweight non-diabetic humans during a 2-step hyperinsulinemic euglycemic clamp (each step 90 min) with human insulin (HI) and saline infusion (S). In 10 overweight subjects we additionally performed the euglycemic clamp with insulin detemir (D). While human insulin administration did not change cerebrocortical activity relative to saline (p = 0.90) in overweight subjects, beta activity increased during D administration (basal 59+/-3 fT, 1(st) step 62+/-3 fT, 2(nd) step 66+/-5, p = 0.001, D vs. HI). As under this condition glucose infusion rates were lower with D than with HI (p = 0.003), it can be excluded that the cerebral effect is the consequence of a systemic effect. The total effect of insulin detemir on beta activity was not different from the human insulin effect in lean subjects (p = 0.78). CONCLUSIONS/SIGNIFICANCE: Despite cerebrocortical resistance to human insulin, insulin detemir increased beta activity in overweight human subjects similarly as human insulin in lean subjects. These data suggest that the decreased cerebral beta activity response in overweight subjects can be restored by insulin detemir.

Gamma oscillations underlying the visual motion aftereffect.

After having been exposed to strong visual motion in one direction, a subsequently presented stationary visual scene seems to move in the opposite direction. This motion aftereffect (MAE) is usually ascribed to short-term functional changes in cortical areas involved in visual motion analysis akin to adaptation. Using magnetoencephalography (MEG), we show increased global field activity due to the MAE which could mostly be explained by a dipole located near the putative location of human area MT+. We further demonstrate that the induced MAE is accompanied by a significant increase in gamma-band activity (GBA) recorded from parietooccipital cortex contralateral to the visual motion stimulus. This gamma oscillation most likely reflects an increase in neuronal response coherence due to decreased inhibition of a group of neurons with similar preferred direction, namely the direction opposite to the adapted one. A second focal GBA response was picked up by the most posterior sensors ipsilateral to the side of the stimulus, reflecting the size of the MAE, whose source could not be reliably located.

The involvement of ipsilateral temporoparietal cortex in tactile pattern working memory as reflected in beta event-related desynchronization.

Cortical oscillatory activity in various frequency bands has been shown to reflect working memory processes operating on visual and auditory stimulus information. Here we use magnetoencephalography to investigate cortical oscillatory activity related to working memory for tactile patterns. Right-handed subjects made same-different judgements on two dot patterns sequentially applied with a 3-s delay to the right middle fingertip. Spectral analysis revealed beta desynchronization (17+/-2.5 Hz) at contralateral postcentral and ipsilateral temporoparietal regions preceding and during the presentation of both tactile stimuli as well as during the early and late delay periods. Whereas contralateral beta desynchronization preceding tactile stimulation may reflect anticipation of incoming stimuli, ipsilateral beta desynchronization may underlie working memory maintenance of tactile patterns. The later hypothesis is supported by a significant positive correlation between subjects' performance and the amplitude of ipsilateral beta desynchronization 800 ms to 500 ms before the onset of the second pattern stimulus. Thus, our results suggest that ipsilateral temporoparietal cortex contributes to the maintenance of tactile pattern information in working memory.

Alpha synchronization during auditory spatial short-term memory.

Recent studies have suggested an active role of cortical alpha oscillations for cognitive functions including short-term memory. We used magnetoencephalography to assess alpha activity during an auditory spatial delayed matching-to-sample task compared with a nonmemory control condition. In the memory task, participants had to memorize the lateralization angle of a noise stimulus S1 and compare it with another lateralized sound S2 presented after an 800-ms delay phase. Whereas alpha desynchronization following S1 was observed over superior temporal areas under both conditions, only the memory task was accompanied by posterior parietal alpha synchronization during the subsequent delay period. The findings are consistent with the notion of alpha activity reflecting active inhibition of interfering processes during memory maintenance of spatial sounds.

Oscillatory MEG response to human locomotion is modulated by periventricular lesions.

Veridical processing of biological movement is of immense value for adaptive behavior and social communication. Here we ask whether and, if so, how oscillatory cortical magnetoencephalographic (MEG) response to biological motion is modulated by early damage to periventricular regions that might affect the pathways interconnecting subcortical structures with cortex and cortico-cortical connectivity. The visual sensitivity to biological motion was lower in adolescents with periventricular leukomalacia (PVL). In controls, the evoked oscillatory MEG response (26.5 Hz) to biological motion peaked at a latency of 170 ms over the right temporo-parietal cortex. This increase was absent in PVL patients. By contrast, peaks in the oscillatory response to biological motion of lower frequency (23.5 Hz) were found in PVL patients later, at a latency of 290 ms over the left temporal region. The findings provide the first evidence for modulation of oscillatory cortical activity by periventricular lesions. The data suggest that PVL affects the brain connectivity with the right temporo-parietal cortex leading to disintegration of the neural network engaged in biological motion processing.

Transcutaneous oximetry compared to ankle-brachial-index measurement in the evaluation of percutaneous transluminal angioplasty.

OBJECTIVE: To investigate transcutaneous oximetry as parameter of the microcirculation is correlated to ankle-brachial-index as parameter of the macrocirculation after peripheral angioplasty procedures. DESIGN: Prospective study. MATERIALS AND METHODS: 60 patients suffering from intermittent claudication were scheduled for angioplasty treatment. 45 patients were considered as eligible for angioplasty after angiographic evaluation, 15 patients underwent angiography only. Transcutaneous oximetry measurements were performed before the procedure, at the end of intervention, 24h as well as 2 and 4 weeks after percutaneous transluminal angioplasty. Ankle-brachial-indices were obtained before intervention, 24h as well as 2 and 4 weeks later. RESULTS: Ankle-brachial-indices increased significantly at 24h after angioplasty in patients being treated with angioplasty. Transcutaneous oximetry values dropped significantly at the end of the procedure and returned close to the baseline levels at 2 and 4 weeks after angioplasty. Ankle-brachial-indices and transcutaneous oximetry were positively correlated before (r=0.3833, p=0.009) as well as 4 weeks after angioplasty (r=0.4596, p=0.001). Immediately after radiological interventions, ankle-brachial-indices and transcutaneous oximetry are not positively correlated. In patients undergoing angiography only, transcutaneous oximetry levels drop significantly immediately after angiography and remain at decreased levels even at 4 weeks after intervention. CONCLUSION: Transcutaneous oximetry as parameter of the microcirculation is positively correlated with ankle-brachial-index as parameter of the macrocirculation before and at 4 weeks after angioplasty. Intraarterial angiography leads to a sudden decrease in skin microcirculation without affecting macrocirculation. As indicated by a lack of recovery in transcutaneous oximetry levels after 4 weeks, angiography alone results in a prolonged impaired microcirculation which may reflect endothelial dysfunction caused by contrast material. The recovery of transcutaneous oximetry levels following angioplasty is counterbalanced by the adverse effects of the contrast material. Ankle-brachial-indices remains the most favourable parameter in evaluating the success of angioplasty procedures while transcutaneous oximetry serves as an indirect method in assessing endothelial dysfunction caused by contrast material.

Prefrontal gamma-band activity distinguishes between sound durations.

The present study used magnetoencephalography to assess the cortical representation of brief sound durations during a short-term memory task. Twelve subjects were instructed to memorize sounds S1 with durations of either 100 or 200 ms during an 800-ms delay phase. Subsequently, they had to judge whether the duration of a probe sound S2 matched the memorized stimulus. Statistical probability mapping of oscillatory signals revealed several components of gamma-band activity (GBA) over prefrontal cortex. A first component with a center frequency of 40 Hz responded more strongly to longer than shorter sounds during the encoding of S1. During the subsequent delay phase, shorter and longer durations were associated with topographically and spectrally distinct GBA enhancements at 71 and 80 Hz, respectively. S2 was again associated with stronger oscillatory activation for longer than shorter sounds at approximately 72 Hz. Non matching compared with matching S1-S2 pairs elicited an additional approximately 66 Hz GBA component peaking at approximately 200 ms after the offset of S2. The analysis of magnetoencephalographic GBA thus served to identify prefrontal network components underlying the representation of different sound durations during the various phases of a delayed matching-to-sample task.