Biomedical subjects

Jens Schwarzbach

Publications and source records attributed to Jens Schwarzbach.

9 recordsLinked to original sources

PubMed2006

When sex meets syntactic gender on a neural basis during pronoun processing.

We used event-related functional magnetic resonance imaging (erfMRI) to investigate the neural basis of biological and syntactic gender integration during pronoun processing in German sentences about persons or things. German allows for separating both processes experimentally. Overall, syntactic processing activated areas adjacent to Broca's area (BA 44), whereas processing of the biological sex, in addition, involved the supramarginal gyrus (BA 39). A previously reported event-related potential study using identical material suggests that syntactic and semantic information is integrated 400-700 ms after target onset, visible in both cases as a P600 but with different effect sizes. The fMRI and ERP results illuminate that pronoun processing involves a highly dynamic spatiotemporal integration of syntactic and biological information depending on the type of the antecedent and whether or not a violation is involved. The results are discussed in the context of cognitive models of pronoun processing.

Adult↗

PubMed2005

Activation in visual cortex correlates with the awareness of stereoscopic depth.

Using event-related functional magnetic resonance imaging, we studied the activation correlating with the awareness of stereoscopic depth using a bistable slanted surface (slant rivalry). Bistability resulted from incongruence between two slant-defining cues: binocular disparity and monocular perspective. The stimulus was perceived as alternating between the perspective-dominated percept (monocular depth) and the disparity-dominated percept (stereopsis), while sensory input remained constant, enabling us to study changes in awareness of depth associated with either cue. Transient activation relating to perceptual alternations was found bilaterally in the caudal part of the intraparietal sulcus, in the right-hemispheric anterior intraparietal sulcus, within visual area V4d-topo, and inferior to area MT+. Transient activation correlating specifically with alternations toward the disparity-dominated percept was found in a number of visual areas, including dorsal visual areas V3A, V7, and V4d-topo and visual areas MT+ and lateral occipital complex. No activation was found for alternations toward the perspective-dominated percept. Our results show that of all visual areas responsive to disparity-defined depth, V4d-topo shows the most robust signal changes correlating with the instigation of stereoscopic depth awareness (stereopsis).

Awareness↗

PubMed2005

Receptive field size-dependent attention effects in simultaneously presented stimulus displays.

Neurophysiological studies in monkeys show that multiple stimuli presented within the receptive field of a neuron are not processed independently but rather act in a mutually suppressive way. Recently, such suppressive interactions have also been reported in human neuroimaging studies. This is seen as evidence that stimuli compete for neural representation. According to the 'biased competition' approach, attention can bias this competition in favor of the attended stimulus, relieving it of the suppressive influences of the distracters. In this paper, we report data that support these findings. Specifically, the effect of attention on stimuli with different spatial separations was investigated more thoroughly. The biased competition approach would predict that, for a given spatial separation and eccentricity, the difference between attended and unattended displays depends on the receptive field size of an area. In a blocked fMRI experiment, participants viewed four simultaneously presented, colorful pictures under different attention conditions (attended and unattended). Stimuli were separated either 2 degrees , 4 degrees or 7 degrees . In line with previous experiments, we found that the effect of attention correlated with the estimated receptive field size of an area. In areas V1, V2 and VP, where estimated receptive field sizes are small, no significant attention effects were found in any of the spatial separation conditions. In V4, there was a significant difference between attended and unattended conditions for the 2 degrees and 4 degrees spatial separations, but not for 7 degrees . Finally in area TEO, significant differences between attended and unattended conditions were observed for all spatial separations.

Adult↗

PubMed2004

Neural correlates of conscious perception in the attentional blink.

If attending to a target in a rapid stream of visual stimuli within the next 400 ms or so, a second target in the stream is frequently not detected by an observer. This so-called attentional blink can provide a comparison of neural signals elicited by identical stimuli that, in one condition, reach conscious awareness and, in the other, fail to be selected for awareness. In the present study, using event-related functional magnetic resonance imaging (fMRI), differences of neural activation were studied in an attentional blink experiment in order to identify brain regions putatively involved in controlling the access of information to consciousness. Subjects viewed a rapid stream of black letters in which the second target (T2) was either presented within or outside the attentional blink period, or not at all. We observed an increase in activation for detected as compared to missed T2 presented during the attentional blink in frontal and parietal cortices. In contrast, in occipitotemporal regions activation was increased for missed as compared to detected T2. Furthermore, in several frontal and parietal areas, missed targets were associated with increased activity if compared to the condition in which no second target was presented. Finally, a selective decrease in activation for detected T2 presented during the attentional blink was observed in areas associated with emotional and predominantly automatic processing. While activations in occipitotemporal regions might mainly reflect duration of attentive search, the frontoparietal areas seem to be involved in a highly distributed network controlling visual awareness.

Adult↗

PubMed2004

Control of object-based attention in human cortex.

Visual attention is a mechanism by which observers select relevant or important information from the current visual array. Previous investigations have focused primarily on the ability to select a region of space for further visual analysis. These studies have revealed a distributed frontoparietal circuit that is responsible for the control of spatial attention. However, vision must ultimately represent objects and in real scenes objects often overlap spatially; thus attention must be capable of selecting objects and their properties nonspatially. Little is known about the neural basis of object-based attentional control. In two experiments, human observers shifted attention between spatially superimposed faces and houses. Event-related functional magnetic resonance imaging (fMRI) revealed attentional modulation of activity in face- and house-selective cortical regions. Posterior parietal and frontal regions were transiently active when attention was shifted between spatially superimposed perceptual objects. The timecourse of activity provides insight into the functional role that these brain regions play in attentional control processes.

Acoustic Stimulation↗

PubMed2003

Different time courses for visual perception and action priming.

Visual stimuli may remain invisible but nevertheless produce strong and reliable effects on subsequent actions. How well features of a masked prime are perceived depends crucially on its physical parameters and those of the mask. We manipulated the visibility of masked stimuli and contrasted it with their influence on the speed of motor actions, comparing the temporal dynamics of visual awareness in metacontrast masking with that of action priming under the same conditions. We observed priming with identical time course for reportable and invisible prime stimuli, despite qualitative changes in the masking time course. Our findings indicate that experimental variations that modify the subjective visual experience of masked stimuli have no effect on motor effects of those stimuli in early processing. We propose a model that provides a quantitative account of priming effects on response speed and accuracy.

Adult↗

PubMed2003

Human cortical object recognition from a visual motion flowfield.

Moving dots can evoke a percept of the spatial structure of a three-dimensional object in the absence of other visual cues. This phenomenon, called structure from motion (SFM), suggests that the motion flowfield represented in the dorsal stream can form the basis of object recognition performed in the ventral stream. SFM processing is likely to contribute to object perception whenever there is relative motion between the observer and the object viewed. Here we investigate the motion flowfield component of object recognition with functional magnetic resonance imaging. Our SFM stimuli encoded face surfaces and random three-dimensional control shapes with matched curvature properties. We used two different types of an SFM stimulus with the dots either fixed to the surface of the object or moving on it. Despite the radically different encoding of surface structure in the two types of SFM, both elicited strong surface percepts and involved the same network of cortical regions. From early visual areas, this network extends dorsally into the human motion complex and parietal regions and ventrally into object-related cortex. The SFM stimuli elicited a face-selective response in the fusiform face area. The human motion complex appears to have a central role in SFM object recognition, not merely representing the motion flowfield but also the surface structure of the motion-defined object. The motion complex and a region in the intraparietal sulcus reflected the motion state of the SFM-implicit object, responding more strongly when the implicit object was in motion than when it was stationary.

Adult↗

PubMed2003

Attentional inhibition of visual processing in human striate and extrastriate cortex.

Allocating attention to a spatial location in the visual field is associated with an increase in the cortical response evoked by a stimulus at that location, compared to when the same stimulus is unattended. We used event-related functional magnetic resonance imaging to investigate attentional modulation of the cortical response to a stimulus probe at an attended location and to multiple probes at unattended locations. A localizer task and retinotopic mapping were used to precisely identify the cortical representations of each probe within striate (V1) and extrastriate cortex (V2, VP, V3, V4v, and V3A). The magnitude and polarity of attentional modulation were assessed through analysis of event-related activity time-locked to shifts in spatial attention. Attentional facilitation at the attended location was observed in striate and extrastriate cortex, corroborating earlier findings. Attentional inhibition of visual stimuli near the attended location was observed in striate cortex, and attentional inhibition of more distant stimuli occurred in both striate and extrastriate cortex. These findings indicate that visual attention operates both through facilitation of visual processing at the attended location and through inhibition of unattended stimulus representations in striate and extrastriate cortex.

Adult↗

PubMed2002

Transient neural activity in human parietal cortex during spatial attention shifts.

Observers viewing a complex visual scene selectively attend to relevant locations or objects and ignore irrelevant ones. Selective attention to an object enhances its neural representation in extrastriate cortex, compared with those of unattended objects, via top-down attentional control signals. The posterior parietal cortex is centrally involved in this control of spatial attention. We examined brain activity during attention shifts using rapid, event-related fMRI of human observers as they covertly shifted attention between two peripheral spatial locations. Activation in extrastriate cortex increased after a shift of attention to the contralateral visual field and remained high during sustained contralateral attention. The time course of activity was substantially different in posterior parietal cortex, where transient increases in activation accompanied shifts of attention in either direction. This result suggests that activation of the parietal cortex is associated with a discrete signal to shift spatial attention, and is not the source of a signal to continuously maintain the current attentive state.

Adult↗