Search PubMed⌕ Search

Biomedical subjects

Arno Villringer

Publications and source records attributed to Arno Villringer.

52 records · Page 3Linked to original sources
PubMed2003

An fMRI study of simple ethical decision-making.

Recent functional neuroimaging studies suggest that ventromedial prefrontal cortex (vmPFC), left posterior superior temporal sulcus (pSTS) and posterior cingulate cortex are engaged during moral decision-making on complex dilemmatic or salient emotional stimuli. In this fMRI study we investigated which of these brain regions are activated during simple ethical decision-making about unambiguous scenarios not containing direct bodily harm or violence. Simple moral decisions compared to semantic decisions resulted in activation of left pSTS and middle temporal gyrus, bilateral temporal poles, left lateral PFC and bilateral vmPFC. These results suggest that pSTS and vmPFC are a common neuronal substrate of decision-making about complex ethical dilemmas, processing material evocative of moral emotions and simple ethical decision-making about scenarios devoid of violence and direct bodily harm.

Adult↗
PubMed2003

A physiological correlate of the "Zoom Lens" of visual attention.

Attending a certain region in space enhances activity in visual areas retinotopically mapped to this region; stimuli presented in this region are preferentially processed. The zoom lens model of visual attention proposes that the attended region can be adjusted in size and predicts a tradeoff between its size and processing efficiency because of limited processing capacities. By means of event-related functional magnetic resonance imaging, we analyzed neural activity in multiple visual areas as a function of the size of an attended visual field region, which was defined by a spatial cue stimulus. After cueing, a target object, defined by a specific feature conjunction, had to be identified among objects within the cued region. Neural activity preceding the objects in multiple retinotopic visual areas correlated with the size of the attended region, as did subjects' performance. While the extent of activated retinotopic visual cortex increased with the size of the attended region, the level of neural activity in a given subregion decreased. These findings are consistent with the physiological predictions of the zoom lens model. Size-related modulations of neural activity were pronounced in early visual areas. We relate this finding to the small receptive field of these areas, whereby only neuronal units with receptive fields covering the attended region received a top-down bias. This preactivation of neuronal units may then have gated selective processing of the features of the object that appeared at the attended location, thus enabling feature integration and object identification.

Adult↗
PubMed2003

Early setting of grammatical processing in the bilingual brain.

The existence of a "critical period" for language acquisition is controversial. Bilingual subjects with variable age of acquisition (AOA) and proficiency level (PL) constitute a suitable model to study this issue. We used functional magnetic resonance imaging to investigate the effects of AOA and PL on neural correlates of grammatical and semantic judgments in Italian-German bilinguals who learned the second language at different ages and had different proficiency levels. While the pattern of brain activity for semantic judgment was largely dependent on PL, AOA mainly affected the cortical representation of grammatical processes. These findings support the view that both AOA and PL affect the neural substrates of second language processing, with a differential effect on grammar and semantics.

Adult↗
PubMed2003

Are VEP correlated fast optical signals detectable in the human adult by non-invasive nearinfrared spectroscopy (NIRS)?

The potential of near infrared spectroscopy (NIRS) to detect vascular changes in cerebral cortical tissue elicited by functional stimulation has been established (1). The vascular response is considered the result of neuronal activity in the investigated area and forms the basis of imaging techniques such as BOLD-contrast fMRI and PET. In the animal optical methods have been shown to detect optical changes in the illuminated tissue which exactly follow the time course of electrical potential changes, thus optical techniques can potentially assess both the 'fast' neuronal and the 'slow' vascular response. Lately a group of investigators has reported data showing that the 'fast optical signal' is detectable in the adult human in response to a visual stimulus ("optical VEP" termed EROS, i.e. event related optical signal). We failed to reproduce these results, with an almost identical instrumentation and experimental protocol. The negative result is reported in this paper. To check the expected magnitude of such changes we performed a simulation based on data reported in the animal and a human head model of optical properties. The result indicates that changes in optical properties to be expected in a non-invasive approach in the human adult are about three orders of magnitude smaller than those reported previously by the group of Gratton and co-workers (2). Also they are so small that they are below the noise level of the presently available NIRS monitors.

Adult↗
PubMed2003

Correlates of alpha rhythm in functional magnetic resonance imaging and near infrared spectroscopy.

We used simultaneous electroencephalogram-functional magnetic resonance imaging (EEG-fMRI) and EEG-near infrared spectroscopy (NIRS) to investigate whether changes of the posterior EEG alpha rhythm are correlated with changes in local cerebral blood oxygenation. Cross-correlation analysis of slowly fluctuating, spontaneous rhythms in the EEG and the fMRI signal revealed an inverse relationship between alpha activity and the fMRI-blood oxygen level dependent signal in the occipital cortex. The NIRS-EEG measurements demonstrated a positive cross-correlation in occipital cortex between alpha activity and concentration changes of deoxygenated hemoglobin, which peaked at a relative shift of about 8 s. Our data suggest that alpha activity in the occipital cortex is associated with metabolic deactivation. Mapping of spontaneously synchronizing distributed neuronal networks is thus shown to be feasible.

Adult↗
PubMed2003

The functional neuroanatomy of visual conjunction search: a parametric fMRI study.

Visual conjunction search is proposed to be a multicomponent process which involves scaling and successive shifts of attention in space as well as object identification. Here, we first mapped brain areas sustaining the proposed attentional subprocesses and then tested whether their activity was modulated by search load, i.e., the number of shifts, as predicted by serial search models. Search load was manipulated indirectly by precueing a varying number of locations at which relevant objects were shown. Multiple subregions within the intraparietal sulcus (IPS) and the prefrontal cortex were activated after cueing. Activity in the right posterior IPS was modulated by the distance of attention shifts and in the left posterior IPS by "zooming out" to cover a large region of the visual field. More anterior subregions of the left IPS responded to object identification irrespective of the need for serial scanning. Corresponding regions in the right IPS were modulated parametrically with respect to search load, along with the right temporoparietal junction. These results support a functional segregation of subregions of the IPS. The posterior regions participate in large-scale shifts and scaling of the attentional focus and the anterior regions in object identification and rapid serial shifts during search. The sustained activation in the frontal eye fields after cueing suggests a role in maintaining attention in the periphery. Together with the findings in early visual areas from this experiment (Müller et al., 2003) the current observations are best accounted for by hybrid models of visual conjunction search, where parallel processing in visual and temporoparietal regions and serial scanning controlled by the right IPS cooperate.

Adult↗
PubMed2003

Evidence for a rostral-to-caudal somatotopic organization in human primary somatosensory cortex with mirror-reversal in areas 3b and 1.

Medial-to-lateral somatotopy is a well-established feature of the human primary somatosensory cortex (SI); however, it is unknown whether, similarly to non-human primates, a rostral-to-caudal somatotopic arrangement exists as well. Therefore, in this functional magnetic resonance imaging (fMRI) study on eight healthy human subjects, five circumscribed skin areas sequentially located on the third finger and the palm of the hand were stimulated with innocuous electrical pulses. Within area 3b of contralateral SI, successive cortical representation sites ordered in a rostral-to-caudal fashion were seen in the group analysis and in six individual subjects. The fingertip was located most rostrally, whereas the proximal parts of the finger as well as the distal palm were represented at more caudal locations. Within area 1, the group analysis revealed a similar pattern of discrete representations. However, in contrast to area 3b, the fingertip was located most caudally, whereas the more proximal parts of the finger were found to be represented rostrally within area 1. Thus, the representation pattern of area 1 appeared as a 'mirror image' of that of area 3b. In comparison to the representations of the finger and the distal palm, the proximal palm was found to be represented at a more medial position of the postcentral gyrus.

Adult↗
PubMed2003

Beyond the visible--imaging the human brain with light.

Optical approaches to investigate cerebral function and metabolism have long been applied in invasive studies. From the neuron cultured to the exposed cortex in the human during neurosurgical procedures, high spatial resolution can be reached and several processes such as membrane potential, cell swelling, metabolism of mitochondrial chromophores, and vascular response can be monitored, depending on the respective preparation. The authors focus on an extension of optical methods to the noninvasive application in the human. Starting with the pioneering work of Jöbsis 25 years ago, near-infrared spectroscopy (NIRS) has been used to investigate functional activation of the human cerebral cortex. Recently, several groups have started to use imaging systems that allow the generation of images of a larger area of the subject's head and, thereby, the production of maps of cortical oxygenation changes. Such images have a much lower spatial resolution compared with the invasively obtained optical images. The noninvasive NIRS images, however, can be obtained in undemanding set-ups that can be easily combined with other functional methods, in particular EEG. Moreover, NIRS is applicable to bedside use. The authors briefly review some of the abundant literature on intrinsic optical signals and the NIRS imaging studies of the past few years. The weaknesses and strengths of the approach are critically discussed. The authors conclude that NIRS imaging has two major advantages: it can address issues concerning neurovascular coupling in the human adult and can extend functional imaging approaches to the investigation of the diseased brain.

Brain↗
PubMed2003

Assessment of diffusion and perfusion deficits in patients with small subcortical ischemia.

BACKGROUND AND PURPOSE: Using perfusion- and diffusion-weighted MR imaging in acute ischemic stroke of the middle cerebral artery (MCA), previous studies have shown a typical pathophysiologic pattern that is characterized by a perfusion deficit larger than the diffusion lesion (mismatch), with the final lesion usually comprising the initial diffusion lesion (core) plus parts of the initial mismatch area. Little is known about underlying pathophysiology in small ischemic stroke. In this study, we used perfusion- and diffusion-weighted MR imaging to investigate the underlying pathophysiology of small subcortical ischemia. METHODS: Six consecutive patients (age range, 42-76 years) with small subcortical ischemia were examined by using a 1.5-T MR system 2-5, 22-55, and 144-392 hours after the onset of symptoms. T2-weighted, diffusion-weighted imaging at b=0 s/mm2 and b=1000 s/mm2, and bolus-track perfusion-weighted imaging were performed. Lesion sizes were determined on the basis of T2-weighted findings as well as those of apparent diffusion coefficient (ADC) maps and CBF. RESULTS: In every patient, the initial CBF lesion was smaller than the initial ADC lesion. Both the CBF lesion and the ADC lesion increased in size from first to second examination. In all instances, however, the CBF lesion remained smaller than the ADC lesion. The CBF lesion observed during the acute phase and the one seen on the following days were both smaller than the final T2 lesion. CONCLUSION: Our data suggest that in contrast to previous findings in MCA ischemia in small subcortical infarcts tissue damage may spread beyond the area of the initial perfusion disturbance. In light of the small number of patients, further studies will have to address the relevance of this observation.

Adult↗
PubMed2002

Event-related fMRI of the somatosensory system using electrical finger stimulation.

Cortical signal intensity changes due to brief (1 s) innocuous electrical stimuli applied to the second and fifth finger of the right hand were measured by means of fMRI at 1.5 T. The activation pattern in this event-related fMRI approach closely resembled that obtained in recent block-design studies. Activations were found in contralateral primary (SI) and bilaterally in secondary (SII) somato-sensory cortex as well as in posterior parietal cortex, insula, and supplementary motor area (SMA). In SI, the somatotopic organization of the hand area is demonstrated, more clearly to be seen in area 3b than in area 1 and 2. In conclusion, the feasibility to employ event-related somatosensory stimulation paradigms in fMRI studies is demonstrated.

Adult↗
PubMed2002

Visual feature and conjunction searches of equal difficulty engage only partially overlapping frontoparietal networks.

According to a classical view of visual object recognition, targets are detected "pre-attentively" if they carry unique features, whereas attention has to be deployed serially to object locations for feature binding if the targets can be distinguished from distracters only in terms of their feature conjunctions. Consistent with this view, recent reports suggest a contribution of the posterior parietal cortex (PPC; one major region controlling spatial attention) to conjunction search as opposed to feature search. However, PPC engagement in conjunction search might also reflect feature-based attention or the difficulty of target selection. The present fMRI study compared regions and amplitudes of cortical activity reflecting the attention mechanisms of a conjunction and a feature search of equal difficulty performed during maintenance of fixation. Attention-related activity was assessed by comparing each hard feature and conjunction search with an easy feature search. Hard feature and conjunction search activated overlapping regions in multiple PPC areas and in the frontal eye field (FEF). Most consistent PPC overlaps were located in the anterior and posterior intraparietal sulcus (IPS). The response amplitude of posterior IPS did not differ between both search tasks. However, the IPS junction with the transverse occipital sulcus and the FEF responded at a higher amplitude during conjunction search. Moreover, regions of the prefrontal cortex and the PPC were activated only during either hard feature or conjunction search. These findings suggest that equally difficult visual searches for features and conjunctions are controlled by overlapping frontoparietal networks, but also that both search types involve specific mechanisms.

Adult↗
PubMed2002

Habituation of the visually evoked potential and its vascular response: implications for neurovascular coupling in the healthy adult.

In the human, visually evoked potentials (VEP) and cerebral oxygenation changes, as measured by near-infrared spectroscopy (NIRS), are assessed to elucidate the relation between electrophysiological and vascular responses to a checkerboard stimulus reversing at 3 Hz. Habituation of either response is analysed on two time scales. Within the 1-min stimulation period we find a decrease in P100N135-component amplitude, closely coupled to a decrease in the amplitude of the oxygenation parameters (concentration changes in oxygenated and deoxygenated haemoglobin, [oxy-Hb] and [deoxy-Hb]). The N75P100-component amplitude exhibits a different behaviour along the 1-min stimulation period. An initial increase is overridden by an overall decrease, the latter not reaching statistical significance. The analysis across the 13 successive stimulation blocks separated by resting periods of equal duration yields a trend for an decrease in the VEP-components' amplitude, not reflected in the vascular response. When calculating a ratio between the amplitude of the P100N135-component and the concentration changes in the haemoglobins a "coupling index" of a 0.2 microM decrease in [deoxy-Hb] and an increase of 0.6 microM in [oxy-Hb] is found per 1 microV increase in VEP-component amplitude. The ratio is the same irrespective of its assessment from the difference stimulation/ rest or from the habituation effect, i.e., the difference between the amplitudes at the beginning and towards the end of the stimulation period. Although supporting the notion that the coupling between neuronal activation and the vascular response exhibits linear aspects, our findings cannot be taken as a proof of such a linearity across all brain regions and activation types. On the contrary, tentatively calculating a coupling index for the data assessed in the visual system, we intend to stress the necessity to assess both neuronal and vascular response to allow for a comparison between different systems and conditions in whom neurovascular coupling is expected to be altered (Miller et al, 2001; Mechelli et al, 2001).

Adult↗
PubMed2002

Human vestibular cortex as identified with caloric stimulation in functional magnetic resonance imaging.

Anatomic and electrophysiological studies in monkeys have yielded a detailed map of cortex areas receiving vestibular afferents. In contrast, comparatively little is known about the cortical representation of the human vestibular system. In this study we applied caloric stimulation and fMRI to further characterize human cortical vestibular areas and to test for hemispheric dominance of vestibular information processing. For caloric vestibular stimulation we used cold nitrogen to avoid susceptibility artifacts induced by water calorics. Right and left side vestibular stimulation was repetitively performed inducing a nystagmus for at least 90 s after the end of the stimulation in all subjects. Only the first 60 s of this nystagmus period was included for statistical analysis and compared with the baseline condition. Activation maps revealed a cortical network with right hemispheric dominance, which in all subjects comprised the temporoparietal junction extending into the posterior insula and, furthermore, the anterior insula, pre- and postcentral gyrus, areas in the parietal lobe, the ventrolateral portion of the occipital lobe, and the inferior frontal gyrus extending into the inferior part of the precentral sulcus. In conclusion, caloric stimulation in fMRI reveals a widespread cortical network involved in vestibular signal processing corresponding to the findings from animal experiments and previous functional imaging studies in humans. Furthermore, this study demonstrates a strong right hemispheric dominance of vestibular cortex areas regardless of the stimulated side, consistent with the current view of a rightward asymmetrical cortical network for spatial orientation.

Adult↗
PubMed2002

Noninvasive monitoring of cerebral blood flow by a dye bolus method: separation of brain from skin and skull signals.

Tracking a bolus of contrast agent traveling through the cerebral vasculature provides a measure of the blood flow velocity in the respective cerebral tissue. This principle has been the basis for the first approaches in functional magnetic resonance (MR) imaging and is of great value for investigating patients with vascular disease, especially stroke. While bolus measurements are a standard procedure in MR imaging, optical bolus tracking is as yet not established. Here we study optical absorption changes induced by a bolus of the dye indocyanine-green with near infrared spectroscopy in healthy volunteers. The aim is to assess the latency and shape of the change in absorption. Since application in the adult human critically depends on differentiation between extra- and intracerebral vascular compartments we focus on an approach for such a separation. To do this frequency-domain and multidistance measurements are analyzed by a Monte Carlo based model for photon transport in tissue. Based on measurements of both the photon's mean time of flight (phase) and the intensity, our results allow differentiation between an upper (skin and skull) and a lower layer (brain). The bolus in the deeper tissue layers has a peak of about 10 s width, while the change in absorption in the upper layers shows a much longer recovery time. This is in qualitative agreement with MR imaging results using a gadolinium bolus. This result is promising with respect to the potential of bedside monitoring of mean transit time (MTT) changes in patients with stroke or related vascular disease.

Adult↗
PubMed2002

Diffusion-weighted imaging in acute stroke--a tool of uncertain value?

The concept of a mismatch between the lesion volume in diffusion- and perfusion-weighted magnetic resonance imaging (MRI) indicating 'tissue at risk of infarction' is based on the assumption that tissue with diffusion slowing in diffusion-weighted MRI (DWI) or decreased values of the apparent diffusion coefficient represents irreversibly damaged tissue. Recent experimental as well as clinical studies, however, have shown that tissue with diffusion slowing may well normalize if the hypoperfusion is moderate or transient. We will interpret these findings in the light of experimental data and suggest a way for the interpretation of different time courses of lesion development in DWI within a clinical MRI protocol. MR stroke imaging delivers important information in acute stroke, particularly in defining the 'tissue at risk of infarction'.

Brain Ischemia↗