Biomedical subjects

Jens-Max Hopf

Publications and source records attributed to Jens-Max Hopf.

8 recordsLinked to original sources

PubMed2006

The neural site of attention matches the spatial scale of perception.

What is the neural locus of visual attention? Here we show that the locus is not fixed but instead changes rapidly to match the spatial scale of task-relevant information in the current scene. To accomplish this, we obtained electrical, magnetic, and hemodynamic measures of attention from human subjects while they detected large-scale or small-scale targets within multiscale stimulus patterns. Subjects did not know the scale of the target before stimulus onset, and yet the neural locus of attention-related activity between 250 and 300 ms varied according to the scale of the target. Specifically, maximal attention-related activity spread from a high-level, relatively anterior visual area (the lateral occipital complex) for large-scale targets to include a lower-level, more posterior area (visual area V4) for small-scale targets. This rapid change indicates that the neural locus of attention in visual cortex is not static but is instead determined rapidly and dynamically by means of an interaction between top-down task information and local information about the current visual input.

Adult↗

PubMed2005

Selectivity for speed gradients in human area MT/V5.

Cortical area MT/V5 in the human occipito-temporal cortex is activated by visual motion. In this study, we use functional imaging to demonstrate that a subregion of MT/V5 is more strongly activated by unidirectional motion with speed gradients than by other motion patterns. Our results suggest that like the monkey homolog middle temporal area (MT), human MT/V5 contains neurons selective for the processing of speed gradients. Such neurons may constitute an intermediate stage of processing between neurons selective for the average speed of unidirectional motion and neurons selective for different combinations of speed gradient and different motion directions such as expanding optical flow patterns.

Adult↗

PubMed2005

The temporal flexibility of attentional selection in the visual cortex.

Visual attention operates by biasing competitive interactions between neural representations, favoring relevant over irrelevant visual inputs. Attention can enhance the processing of relevant information using location-based, feature-based or object-based selection mechanisms. Studies using event-related potential and event-related magnetic field recordings, together with functional magnetic resonance imaging, show that the temporal sequencing of these different selection mechanisms is flexible. Depending on the specific processing demands of the experimental task, location-based, feature-based or object-based selection might be given temporal priority on a time scale of tens of milliseconds.

Animals↗

PubMed2004

Attention to features precedes attention to locations in visual search: evidence from electromagnetic brain responses in humans.

Single-unit recordings in macaque extrastriate cortex have shown that attentional selection of nonspatial features can operate in a location-independent manner. Here, we investigated analogous neural correlates at the neural population level in human observers by using simultaneous event-related potential (ERP) and event-related magnetic field (ERMF) recordings. The goals were to determine (1) whether task-relevant features are selected before attention is allocated to the location of the target, and (2) whether this selection reflects the locations of the relevant features. A visual search task was used in which the spatial distribution of nontarget items with attended feature values was varied independently of the location of the target. The presence of task-relevant features in a given location led to a change in ERP/ERMF activity beginning approximately 140 msec after stimulus onset, with a neural origin in the ventral occipito-temporal cortex. This effect was independent of the location of the actual target. This effect was followed by lateralized activity reflecting the allocation of attention to the location of the target (the well known N2pc component), which began at approximately 170 msec poststimulus. Current source localization indicated that the allocation of attention to the location of the target originated in more anterior regions of occipito-temporal cortex anterior than the feature-related effects. These findings suggest that target detection in visual search begins with the detection of task-relevant features, which then allows spatial attention to be allocated to the location of a likely target, which in turn allows the target to be positively identified.

Adult↗

PubMed2004

Popout modulates focal attention in the primary visual cortex.

The influence of context-dependent interactions on attention-related neural activity was studied in the human primary visual cortex (V1) with event-related fMRI. Retinotopic field-sign mapping was used to determine the localization of V1 with respect to adjacent retinotopic areas. Observers reported the orientation of a Gabor patch at pre-cued extrafoveal locations when it was salient among distractor Gabors and when it was not. Saliency was caused by local orientation contrast between Gabors-a mechanism that is thought to arise from context-dependent interactions in the V1 proper. A comparison of the attention-related BOLD response for salient and non-salient stimuli in V1 revealed that salient Gabors caused a significantly smaller BOLD response than non-salient Gabors. This differential effect was not observed in higher-order visual areas (V3/V3A, MT+/LO, IPS). When attention was not focused onto the target, the size of the BOLD response was generally reduced in all visual areas, and no difference was seen in V1 for salient and non-salient Gabors. These findings suggest that contextual interactions underlying saliency influence attentional modulations in V1 and support the view that perceptual and attentional mechanisms share neural circuits at this early stage of visual processing.

Adult↗

PubMed2003

Is human sentence parsing serial or parallel? Evidence from event-related brain potentials.

In this ERP study we investigate the processes that occur in syntactically ambiguous German sentences at the point of disambiguation. Whereas most psycholinguistic theories agree on the view that processing difficulties arise when parsing preferences are disconfirmed (so-called garden-path effects), important differences exist with respect to theoretical assumptions about the parser's recovery from a misparse. A key distinction can be made between parsers that compute all alternative syntactic structures in parallel (parallel parsers) and parsers that compute only a single preferred analysis (serial parsers). To distinguish empirically between parallel and serial parsing models, we compare ERP responses to garden-path sentences with ERP responses to truly ungrammatical sentences. Garden-path sentences contain a temporary and ultimately curable ungrammaticality, whereas truly ungrammatical sentences remain so permanently--a difference which gives rise to different predictions in the two classes of parsing architectures. At the disambiguating word, ERPs in both sentence types show negative shifts of similar onset latency, amplitude, and scalp distribution in an initial time window between 300 and 500 ms. In a following time window (500-700 ms), the negative shift to garden-path sentences disappears at right central parietal sites, while it continues in permanently ungrammatical sentences. These data are taken as evidence for a strictly serial parser. The absence of a difference in the early time window indicates that temporary and permanent ungrammaticalities trigger the same kind of parsing responses. Later differences can be related to successful reanalysis in garden-path but not in ungrammatical sentences.

Adult↗

PubMed2002

Analysis of pathways mediating preserved vision after striate cortex lesions.

This study investigated the neural substrates of preserved visual functioning in a patient with homonymous hemianopsia and Riddoch syndrome after a posterior cerebral artery stroke affecting the primary visual cortex (area V1). The limited visual abilities of this patient included above-chance verbal reports of movement and color change as well as discrimination of movement direction in the hemianopic field. Functional magnetic resonance imaging showed that motion and color-change stimuli presented to the hemianopic field produced activation in several extrastriate areas of the lesioned hemisphere that were defined using retinotopic mapping. Magnetoencephalographic recordings indicated that evoked activity occurred earlier in the higher-tier visual areas V4/V8 and V5 than in the lower-tier areas V2/V3 adjacent to the lesion. In addition, the functional magnetic resonance imaging analysis showed an increased functional connectivity between areas V4/V8 and V5 of the lesioned hemisphere in comparison with the same areas in the intact hemisphere during the presentation of color changes. These results suggest that visual perception after the V1 lesion in Riddoch syndrome is mediated by subcortical pathways that bypass V1 and project first to higher-tier visual areas V5 and V4/V8 and subsequently to lower-tier areas V2/V3.

Adult↗

PubMed2002

Localizing visual discrimination processes in time and space.

Previous studies of visual processing in humans using event-related potentials (ERPs) have demonstrated that task-related modulations of an early component called the "N1" wave (140-200 ms) reflect the operation of a voluntary discrimination process. Specifically, this component is larger in tasks requiring target discrimination than in tasks requiring simple detection. The present study was designed to localize this discriminative process in both time and space by means of combined magnetoencephalographic (MEG) and ERP recordings. Discriminative processing led to differential ERP and MEG activity beginning within 150 ms of stimulus onset. Source localization of the combined ERP/MEG data was performed using anatomical constraints from structural magnetic resonance images. These analyses revealed highly reliable and focused activity in regions of inferior occipital-temporal cortex. These findings indicate that the earliest measurable correlates of discriminative operations in the visual system appear as neural activity in circumscribed regions of the ventral processing stream.

Adult↗