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Biomedical subjects

Matthias Niemeier

Publications and source records attributed to Matthias Niemeier.

5 recordsLinked to original sources
PubMed2003

Stimulus-driven and voluntary saccades are coded in different coordinate systems.

We make fast, "saccadic" eye movements to view our surroundings, "voluntary" saccades when saccade targets are deliberately selected, and "stimulus-driven" saccades when a target suddenly appears. Saccades of patients with spatial neglect have been studied to identify the coordinate systems guiding such behavior. However, previous reports disagree on whether neglect involves an eye-centered deficit of (delayed and hypometric) saccades specifically when performed in the direction opposite the brain lesion or not. We show that this inconsistency is due to independent mechanisms underlying voluntary and stimulus-driven saccades. We used a new experimental procedure comparing identical saccades performed either during an exploratory search task or a stimulus-driven task, both of which required similar cognitive functions (Figure 1). Only the patients' stimulus-driven saccades showed the eye-centered deficit. The same saccades were intact when voluntarily performed. However, here the patients showed a head-centered deficit; their saccades ignored the left part of space. In none of our control subjects with or without brain lesions did the neglect patients' pattern of deficits occur. The results argue that the brain flexibly uses a system of distinct but interrelated neural circuits for visual orienting to optimally encode its sensorimotor functions in multiple behavioral situations.

Brain Injuries↗
PubMed2003

Optimal transsaccadic integration explains distorted spatial perception.

We scan our surroundings with quick eye movements called saccades, and from the resulting sequence of images we build a unified percept by a process known as transsaccadic integration. This integration is often said to be flawed, because around the time of saccades, our perception is distorted and we show saccadic suppression of displacement (SSD): we fail to notice if objects change location during the eye movement. Here we show that transsaccadic integration works by optimal inference. We simulated a visuomotor system with realistic saccades, retinal acuity, motion detectors and eye-position sense, and programmed it to make optimal use of these imperfect data when interpreting scenes. This optimized model showed human-like SSD and distortions of spatial perception. It made new predictions, including tight correlations between perception and motor action (for example, more SSD in people with less-precise eye control) and a graded contraction of perceived jumps; we verified these predictions experimentally. Our results suggest that the brain constructs its evolving picture of the world by optimally integrating each new piece of sensory or motor information.

Humans↗
PubMed2002

Simulating and testing visual exploration in spatial neglect based on a new model for cortical coordinate transformation.

Most studies of object and space perception have focused on neural representations of either object-centered or egocentric coordinate systems. But daily life requires interactions of both kinds of coordinates. We have recently proposed an 'integrated space-object (ISO-) map' combining both coordinate systems in one representation. Based on a lesioned version of this model, here we present results from visual search simulations demonstrating that the model accounts for contralesional neglect during space-centered and object-centered exploration tasks. Interestingly, the model simulations also predicted an amelioration of neglect symptoms during exploration with more ipsilesional object positions. By measuring the eye movements of neglect patients during different exploratory tasks, we confirmed all model predictions. These results corroborate the view that the brain might combine coordinates for object and space perception in an integrated coordinate system as suggested by the ISO-map model.

Aged↗
PubMed2002

Task-dependent differences in the exploratory behaviour of patients with spatial neglect.

The present study analysed task-dependent effects on the exploratory behaviour of neglect patients during their spontaneous search of the surroundings. We were asking whether different tasks would be associated with different structuring of the visual display and, therefore, would result in different forms of neglect in one and the same brain-damaged subjects. Neglect patients' eye and head movements were recorded when they searched for a target within a homogeneous stimulus array surrounding the subjects. Subsequently, they explored the same array which was now segmented into different areas. When the patients' attention was allocated to the whole surrounding space, all patients completely neglected the left hemispace and spontaneously attended to the right hemispace. No significant left-right asymmetry was detected in a selected segment located in the periphery of the attended, right hemispace. However, all patients completely ignored the left part of this segment when they had to concentrate visual search on this segment alone. The results suggest an important influence of task-dependent effects on the exploratory behaviour of neglect patients. They show that one and the same physical stimulus at one and the same location in a scene might be attended or, in another situation, neglected, just depending on the behavioural goal of the subject. The findings support the idea that the brain organises and reorganises continuously the representation of the same physical input according to the changing task requirements.

Aged↗