Biomedical subjects

M A Schoenfeld

Publications and source records attributed to M A Schoenfeld.

9 recordsLinked to original sources

PubMed2007

Spatio-temporal analysis of feature-based attention.

The cortical mechanisms of feature-selective attention to color and motion cues were studied in humans using combined electrophysiological, magnetoencephalographic, and hemodynamic (functional magnetic resonance imaging) measures of brain activity. Subjects viewed a display of random dots that periodically either changed color or moved coherently. When attention was directed to the color change it elicited enhanced neural activity in visual area V4v, previously shown to be specialized for processing color information. In contrast, when dot movement was attended it produced enhanced activity in the motion-specialized area human MT. Parallel recordings of event-related electrophysiological and magnetoencephalographic responses indicated that the attention-related facilitation of neural activity in these specialized cortical areas occurred rapidly, beginning as early as 90-120 ms after stimulus onset. We conclude that selection of an entire feature dimension (motion or color) boosts neural activity in its specialized cortical module much more rapidly than does selection of one feature value from another (e.g., one color from another), as reported in previous electrophysiological studies. By combining methods with high spatial and temporal resolution it is possible to analyze the precise time course of feature-selective processing in specialized cortical areas.

Adult↗

PubMed2006

Causal visual interactions as revealed by an information theoretic measure and fMRI.

In the present study, we evaluated the direction of the effective connectivity between fMRI activations in neural structures mediating preserved visual function in a patient with homonymous hemianopsia due to a posterior cerebral artery stroke. Although the lesion affected the primary visual cortex, the visual abilities of this patient included above-chance verbal reports of movement and color change as well as the discrimination of movement direction in his hemianopic field. These abilities were coupled with awareness (Riddoch syndrome). The strength and the direction of the interactions between visual regions were assessed by applying directed transinformation (T), a nonparametric information theoretic causal measure sensitive to linear as well as to nonlinear interactions. In the healthy hemisphere, T identified a strong flow of information from visual area V1 to V5 during stimulation by visual movement and from V1 to V4/V8 during stimulation by color change. In addition, during color change stimulation, a bi-directional flow was observed between V4/V8 and V5, suggesting crosstalk between these regions. In the lesioned hemisphere, the color change stimulation evoked a stronger flow from V5 to V4/V8 and a flow from V4/V8 to V2. These observations provide support for the hypothesis that visual information is mediated via subcortical pathways that bypass V1 and project first to higher-tier visual areas V5 and V4/V8 then subsequently to lower-tier area V2.

Adult↗

PubMed2006

Direct neurophysiological evidence for spatial suppression surrounding the focus of attention in vision.

The spatial focus of attention has traditionally been envisioned as a simple spatial gradient of enhanced activity that falls off monotonically with increasing distance. Here, we show with high-density magnetoencephalographic recordings in human observers that the focus of attention is not a simple monotonic gradient but instead contains an excitatory peak surrounded by a narrow inhibitory region. To demonstrate this center-surround profile, we asked subjects to focus attention onto a color pop-out target and then presented probe stimuli at various distances from the target. We observed that the electromagnetic response to the probe was enhanced when the probe was presented at the location of the target, but the probe response was suppressed in a narrow zone surrounding the target and then recovered at more distant locations. Withdrawing attention from the pop-out target by engaging observers in a demanding foveal task eliminated this pattern, confirming a truly attention-driven effect. These results indicate that neural enhancement and suppression coexist in a spatially structured manner that is optimal to attenuate the most deleterious noise during visual object identification.

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PubMed2004

Functional magnetic resonance tomography correlates of taste perception in the human primary taste cortex.

The present study investigated the functional magnetic resonance tomography correlates of taste perception in the human primary taste cortex. There is conflicting evidence in the literature about chemotopical organization in this brain region. The topography of hemodynamic activity elicited by five taste stimuli (sweet, sour, salty, bitter and umami) was analyzed on the flattened cortical surfaces of six single subjects. A high inter-individual topographical variability had to be noted. The results showed different patterns of hemodynamic activity for the investigated tastes with some considerable overlap. However, the taste specific patterns were stable over time in each subject. Such an individual taste specific pattern was also found for the umami taste within the primary taste cortex of each subject. These results suggest that input from glutamate receptors on the tongue might be processed in an exclusive way in the primary taste cortex rather than as a combination of inputs from the classical taste receptors.

Adult↗

PubMed2003

Dynamics of feature binding during object-selective attention.

Objects in the environment may be attended selectively and perceived as unified ensembles of their constituent features. To investigate the timing and cortical localization of feature-integration mechanisms in object-based attention, recordings of event-related potentials and magnetic fields were combined with functional MRI while subjects attended to one of two superimposed transparent surfaces formed by arrays of dots moving in opposite directions. A spatiotemporal analysis revealed evidence for a rapid increase in neural activity localized to a color-selective region of the fusiform gyrus when the surface moving in the attended direction displayed an irrelevant color feature. These data provide support for the "integrated-competition" model of object-selective attention and point to a dynamic neural substrate for the rapid binding process that links relevant and irrelevant features to form a unified perceptual object.

Adult↗

PubMed2003

Form-from-motion: MEG evidence for time course and processing sequence.

The neural mechanisms and role of attention in the processing of visual form defined by luminance or motion cues were studied using magnetoencephalography. Subjects viewed bilateral stimuli composed of moving random dots and were instructed to covertly attend to either left or right hemifield stimuli in order to detect designated target stimuli that required a response. To generate form-from-motion (FFMo) stimuli, a subset of the dots could begin to move coherently to create the appearance of a simple form (e.g., square). In other blocks, to generate form-from-luminance (FFLu) stimuli that served as a control, a gray stimulus was presented superimposed on the randomly moving dots. Neuromagnetic responses were observed to both the FFLu and FFMo stimuli and localized to multiple visual cortical stages of analysis. Early activity in low-level visual cortical areas (striate/early extrastriate) did not differ for FFLu versus FFMo stimuli, nor as a function of spatial attention. Longer latency responses elicited by the FFLu stimuli were localized to the ventral-lateral occipital cortex (LO) and the inferior temporal cortex (IT). The FFMo stimuli also generated activity in the LO and IT, but only after first eliciting activity in the lateral occipital cortical region corresponding to MT/V5, resulting in a 50-60 msec delay in activity. All of these late responses (MT/V5, LO, and IT) were significantly modulated by spatial attention, being greatly attenuated for ignored FFLu and FFMo stimuli. These findings argue that processing of form in IT that is defined by motion requires a serial processing of information, first in the motion analysis pathway from V1 to MT/V5 and thereafter via the form analysis stream in the ventral visual pathway to IT.

Attention↗

PubMed2003

Clinical criteria for the switch of treatment strategies in Parkinson's disease.

Along the years the treatment of Parkinson's disease with L-dopa has revealed unfavorable effects in general after 5-10 years. This has led to the present criteria for treatment of de novo patients that mainly relay on the age, the general strategy being to delay the use of L-dopa as long as possible. However, this practical approach lacks a scientific basis. In a retrospective study data of 155 patients with Parkinson's disease were analyzed with the goal of finding a clinical marker for the critical time point when L-dopa needs to be administrated. The clinical stage of the patients was assessed using the Hoehn and Yahr (H&Y) scale and the severity of the symptoms was measured using the UPDRS score. The results show that there was no relationship between the age of the patients and the therapy (L-dopa vs. no L-dopa) with regard to the clinical outcome. A significant interaction was found however, between the clinical stage (H&Y) and the therapy. Further analysis of this interaction showed that in the H&Y Stages 1-2.5 the UPDRS scores were lower in the patient groups treated without L-dopa. Remarkably, in the H&Y stages 3 and higher the UPDRS scores were lower in the patient groups treated with L-dopa. These results suggest that the clinical stage of the disease (H&Y) might be a better criterion than the age for the time point when L-dopa needs to be administered in de novo patients.

Adult↗

PubMed2002

Unmasking motion-processing activity in human brain area V5/MT+ mediated by pathways that bypass primary visual cortex.

Most models of the human visual system argue that higher-order motion-processing cortical regions receive their inputs only via the primary visual cortex (striate cortex), rather than also via direct projections from the thalamus that bypass primary visual cortex. However, recent evidence in non-human primates, along with some evidence in humans with damaged primary visual cortex (e.g., "blindsight" for motion in the blind visual hemifield), have argued for the existence of a direct thalamic-to-extrastriate projection for motion processing. This evidence remains controversial. Here we tested the idea that direct thalamic input to extrastriate motion processing areas exists in humans but might be masked in scalp recordings by activity from early visual areas. To do this, we employed stimuli that induced strong refractory effects in primary visual cortex--thereby creating a brief "reversable lesion" in primary visual cortex--immediately before the presentation of a motion stimulus. Under these conditions, we then assessed whether motion areas of cortex were still able to process the motion stimuli by recording event-related potentials (ERPs) and event-related magnetic fields (ERFs/MEG). We found robust motion-related activity in extrastriate motion processing areas in the ERP and MEG signals even when primary visual cortex was heavily suppressed by our manipulation. This finding provides evidence for a direct thalamic functional pathway to extrastriate visual cortical motion processing areas in the human that bypasses primary visual cortex.

Adult↗

PubMed2000

Amantadine influences cognitive processing in patients with multiple sclerosis.

We investigated the effect of amantadine on cognitive processing in patients with multiple sclerosis (MS) and fatigue with objective electrophysiological measures. Behavioral methods (Reaction Time, RT) and two different Event Related Potential (ERP) components measuring i) stimulus selection (Selection Negativity, SN) and ii) response selection (Lateralized Readiness Potential, LRP) were employed. Twenty-four patients with clinical definite MS (10 relapsing remitting and 14 secondary progressive) and confirmed fatigue in the past three months (Fatigue Severity Scale (FSS) > 4) were included. Patients were randomized in a double-blind, placebo-controlled cross-over design. We found a difference between the two treatments for ERP measures to stimuli with relevant colour starting at about 200 ms. This negativity had a higher amplitude during amantadine treatment regardless of treatment order. The RT did not differ significantly between the treated and untreated groups. Additional analysis indicated that patients with a disease duration of less than 7 years had a significant test position (practice effect), but no treatment effect, while patients with a longer MS duration showed no practice effect, but rather an improved reaction speed and increased ERP amplitude effects when treated with amantadine. The present findings suggest that amantadine exerts beneficial effects on early cognitive processes in patients with MS, but appears to be limited to subjects with a longer duration of the disease.

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