Biomedical subjects

Stephan A Brandt

Publications and source records attributed to Stephan A Brandt.

13 recordsLinked to original sources

PubMed2006

What determines sustained visual attention? The impact of distracter positions, task difficulty and visual fields compared.

We quantified the interference of distracter stimuli on sustained visuo-spatial attention as a function of the distribution of attended positions in the visual fields (bilateral/unilateral, left/right, upper/lower), distracter positions (peripheral, between attended positions, between fixation and attended positions) and task difficulty. Compared to distinct distracter positions, bilateral field and lower field presentation had much stronger influence on the performance. Additionally, interactive effects between task difficulty and distracter position were found. This result was at variance with the previous models of visuo-spatial attention, which attached much more importance to the role of distracter positions compared to visual field effects. In directly comparing the impact of the abovementioned factors, the converse finding is evident-visual field effects, in particular bilateral presentations have a much stronger importance. Moreover, metaphoric concepts of attention like the "zoom lens" are not compatible with these results. The findings are discussed in the light of alternative models of sustained visuo-spatial attention. The visual system architecture and top-down mechanisms are considered.

Adult↗

PubMed2006

Alpha-band electroencephalographic activity over occipital cortex indexes visuospatial attention bias and predicts visual target detection.

Covertly directing visual attention toward a spatial location in the absence of visual stimulation enhances future visual processing at the attended position. The neuronal correlates of these attention shifts involve modulation of neuronal "baseline" activity in early visual areas, presumably through top-down control from higher-order attentional systems. We used electroencephalography to study the largely unknown relationship between these neuronal modulations and behavioral outcome in an attention orienting paradigm. Covert visuospatial attention shifts to either a left or right peripheral position in the absence of visual stimulation resulted in differential modulations of oscillatory alpha-band (8-14 Hz) activity over left versus right posterior sites. These changes were driven by varying degrees of alpha-decreases being maximal contralateral to the attended position. When expressed as a lateralization index, these alpha-changes differed significantly between attention conditions, with negative values (alpha_right < alpha_left) indexing leftward and more positive values (alpha_left < or = alpha_right) indexing rightward attention. Moreover, this index appeared deterministic for processing of forthcoming visual targets. Collapsed over trials, there was an advantage for left target processing in accordance with an overall negative bias in alpha-index values. Across trials, left targets were detected most rapidly when preceded by negative index values. Detection of right targets was fastest in trials with most positive values. Our data indicate that collateral modulations of posterior alpha-activity, the momentary bias of visuospatial attention, and imminent visual processing are linked. They suggest that the momentary direction of attention, predicting spatial biases in imminent visual processing, can be estimated from a lateralization index of posterior alpha-activity.

Adult↗

PubMed2005

fMRI localizer technique: efficient acquisition and functional properties of single retinotopic positions in the human visual cortex.

Current fMRI retinotopic mapping procedures often use checkerboard stimuli consisting of expanding rings and rotating wedges to measure the topography within human visual areas. Efficient procedures are well described in the literature. For many experimental paradigms, e.g., visuo-spatial attention paradigms, the identification of task-relevant positions is the only mandatory prerequisite. To define these specific "regions-of-interest" (ROIs), spatially defined localizers are used. A precise evaluation of localizer techniques in regard to efficient scanning time, optimal BOLD (blood oxygenic level dependent) response, as well as quantification of the resulting ROIs within each visual area (size, overlap, surround effects) has not been studied to date. Here, we suggest a mapping procedure designed to quantify spatial and functional properties of single positions at close proximity in multiple human visual areas. During a passive viewing task, various stimuli (e.g., checkerboards or colored objects) subtending 1.4 degrees of visual angle were presented at one out of four positions in a randomized block design. We measured the degree of overlap between positions at different hierarchical levels of the visual system (V1-V4v) and quantified modulatory effects on a specific position by stimulation at neighboring (1.7 degrees spacing) or distant positions (5.1 degrees or 8.5 degrees spacing). Within each visual area, "mexican-hat" distributions of local signal intensity changes, which describe a particular combination of facilitatory and suppressive effects, were found. Cubic fitting revealed the most localized tuning effect in V1, which gradually decreased throughout the higher visual areas. Colored objects were most efficient in localizing circumscribed retinotopic positions in both early and higher areas.

Adult↗

PubMed2005

Interactions between task difficulty and hemispheric distribution of attended locations: implications for the splitting attention debate.

Whether attention can be split between multiple regions in space simultaneously is an ongoing controversy in attention research. We argue that the debate could be resolved if the distribution of target locations over hemifields and task difficulty are both considered. This premise was tested in five experiments in which 48 subjects compared the identity of two out of four stimuli. In an easy task, within each hemifield, performance (reaction times and error rates) was better for adjacent targets than for separated ones, but across hemifields, performance for separated and adjacent stimuli was similar. In difficult tasks, performance was always better when the stimuli were presented across the hemifields indicating a bilateral field advantage. Moreover, the difference between adjacent and separate conditions within one hemifield diminished with increasing task difficulty. We propose a modified model of visuo-spatial attention, which permits the hemispheres to maintain and control simultaneous attentional foci.

Adult↗

PubMed2005

The predictive value of the leveling off of within session performance for procedural memory consolidation.

In addition to performance gains accrued concurrently with a given training experience (within-session gains) robust, delayed (between-session) performance gains may slowly evolve in the absence of any additional practice in a variety of tasks. The latter is regarded as a behavioral manifestation of skill memory consolidation. It is not known, however, how much practice is necessary for the triggering of these consolidation effects. Here, using an enumeration task, we show that the triggering of delayed gains could be robustly predicted from each individual's performance curve. Delayed performance gains evolved consistently only when practice continued to the point at which within-session performance leveled off (saturation). No delayed gains were found when training was stopped before this individually determined point. Our results support the notion that the triggering of consolidation processes depends on the saturation of a distinct, early phase of learning rather than on the absolute number of task repetitions, and suggest the possibility that training and rehabilitation protocols could be optimized on an individual basis.

Adult↗

PubMed2003

Differential contribution of early visual areas to the perceptual process of contour processing.

We investigated contour processing and figure-ground detection within human retinotopic areas using event-related functional magnetic resonance imaging (fMRI) in 6 healthy and naïve subjects. A figure (6 degrees side length) was created by a 2nd-order texture contour. An independent and demanding foveal letter-discrimination task prevented subjects from noticing this more peripheral contour stimulus. The contour subdivided our stimulus into a figure and a ground. Using localizers and retinotopic mapping stimuli we were able to subdivide each early visual area into 3 eccentricity regions corresponding to 1) the central figure, 2) the area along the contour, and 3) the background. In these subregions we investigated the hemodynamic responses to our stimuli and compared responses with or without the contour defining the figure. No contour-related blood oxygenation level-dependent modulation in early visual areas V1, V3, VP, and MT+ was found. Significant signal modulation in the contour subregions of V2v, V2d, V3a, and LO occurred. This activation pattern was different from comparable studies, which might be attributable to the letter-discrimination task reducing confounding attentional modulation. In V3a, but not in any other retinotopic area, signal modulation corresponding to the central figure could be detected. Such contextual modulation will be discussed in light of the recurrent processing hypothesis and the role of visual awareness.

Adult↗

PubMed2003

Parietal activation during visual search in the absence of multiple distractors.

Search for a target object embedded in a visual scene involves the posterior parietal cortex. This region is thought to play a role in visual attention by counteracting the effects of distractors on targets or by inhibiting distractors. Using fMRI, we investigated whether the parietal cortex is also engaged in visual search without distractors. Cortical activation was compared between two 'single object' search tasks differing only in difficulty. Activation differences between both tasks were found in the anterior and inferior part of the intraparietal sulcus, but in neither its posterior part nor the frontal eye fields. Thus a subset of parietal regions participates in the control of visual search even in the absence of distractors.

Adult↗

PubMed2003

Comparison of motor effects following subcortical electrical stimulation through electrodes in the globus pallidus internus and cortical transcranial magnetic stimulation.

Current concepts of transcranial magnetic stimulation (TMS) over the primary motor cortex are still under debate as to whether inhibitory motor effects are exclusively of cortical origin. To further elucidate a potential subcortical influence on motor effects, we combined TMS and unilateral subcortical electrical stimulation (SES) of the corticospinal tract. SES was performed through implanted depth electrodes in eight patients treated with deep brain stimulation (DBS) for severe dystonia. Chronaxie, conduction velocity (CV) of the stimulated fibres and poststimulus time histograms of single motor unit recordings were calculated to provide evidence of an activation of large diameter myelinated fibres by SES. Excitatory and inhibitory motor effects recorded bilaterally from the first dorsal interosseus muscle were measured after SES and focal TMS of the motor cortex. This allowed us to compare motor effects of subcortical (direct) and cortical (mainly indirect) activation of corticospinal neurons. SES activated a fast conducting monosynaptic pathway to the alpha motoneuron. Motor responses elicited by SES had significantly shorter onset latency and shorter duration of the contralateral silent period compared to TMS induced motor effects. Spinal excitability as assessed by H-reflex was significantly reduced during the silent period after SES. No ipsilateral motor effects could be elicited by SES while TMS was followed by an ipsilateral inhibition. The results suggest that SES activated the corticospinal neurons at the level of the internal capsule. Comparison of SES and TMS induced motor effects reveals that the first part of the TMS induced contralateral silent period should be of spinal origin while its later part is due to cortical inhibitory mechanisms. Furthermore, the present results suggest that the ipsilateral inhibition is predominantly mediated via transcallosal pathways.

Adolescent↗

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

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↗

PubMed2002

In vivo study indicating loss of intracortical inhibition in tumor-associated epilepsy.

Transcranial magnetic stimulation was performed in 2 patients with focal motor seizures in the right hand caused by a circumscribed tumor process affecting the left precentral gyrus. In both cases, paired-pulse transcranial magnetic stimulation showed a loss of intracortical inhibition for interstimulus intervals of 2 to 4msec that was replaced by an enormous facilitation in the lesioned hand motor cortex. The uniform impairment of inhibitory mechanisms in epileptogenic tumors with different histologies suggests a common, nonspecific cause of tumor-related epileptogenesis.

Adenocarcinoma↗

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

Changes in visual cortex excitability in blind subjects as demonstrated by transcranial magnetic stimulation.

Any attempt to restore visual functions in blind subjects with pregeniculate lesions provokes the question of the extent to which deafferented visual cortex is still able to generate conscious visual experience. As a simple approach to assessing activation of the visual cortex, subjects can be asked to report conscious subjective light sensations (phosphenes) elicited by focal transcranial magnetic stimulation (TMS) over the occiput. We hypothesized that such induction of phosphenes can be used as an indicator of residual function of the visual cortex and studied 35 registered blind subjects after partial or complete long-term (>10 years) deafferentation of the visual cortex due to pregeniculate lesions. TMS was applied over the visual cortex in 10 blind subjects with some residual vision (visual acuity <20/400; Group 1), 15 blind subjects with very poor residual vision (only perception of movement or light; Group 2), 10 blind subjects without any residual vision (Group 3) and 10 healthy controls. A stimulation mapping procedure was performed on a 1 x 1 cm skull surface grid with 130 stimulation points overlying the occipital skull. We analysed the occurrence of phosphenes at each stimulation point with regard to frequency and location of phosphenes in the visual field. Previous experiments have shown that repetitive TMS reliably elicits brief flashes of white or coloured patches of light. Therefore, stimulation was performed with short trains of seven consecutive 15 Hz stimuli applied with an intensity of 1.3 times the motor threshold. Under such conditions, phosphenes occurred in 100% of subjects in Group 1, in 60% of Group 2 and in 20% of Group 3. Phosphene thresholds were normal, but the number of effective stimulation sites was significantly reduced in Groups 2 and 3. The results indicate that in blind subjects there is alteration in TMS-induced activation of the deafferented visual cortex or processes engaged in bringing the artificial cortex input to consciousness. The ability to elicit phosphenes is reduced in subjects with a high degree of visual deafferentation, especially in those without previous visual experience.

Adolescent↗