Matthias Niemeier

Changes of Cerebrovascular CO2 Reactivity During Normal Aging

BACKGROUND AND PURPOSE During the past decade, transcranial Doppler sonography has widely been used to assess blood flow velocities in the basal intracranial arteries and cerebrovascular reactivity (CR) to various stimuli. Although numerous studies have shown a decline of cerebral blood flow velocity with age, the age dependency of CR, including cerebrovascular CO2 reactivity, however, is controversial. Recently, we have reported a significant sex-related difference in CR, stressing the need to study the relation between normal aging and CR in both sexes separately. METHODS By means of transcranial Doppler sonography, CR was determined in 100 healthy, nonsmoking volunteers (age 20 to 70 years, 10 men and 10 women per decade). RESULTS In men, no change of CR with increasing age could be observed (P=0.98). In contrast, CR in women declined significantly, with a step decrease from the 4th to the 5th decades (F=4.413; P<0.01) and was significantly higher in the 3rd and 4th compared with the 5th, 6th, and 7th decades (P<0.05). Information on hormone replacement therapy (HRT) in women of the 6th and 7th decades was obtained retrospectively. HRT was associated with enhanced CR (HRT, n = 7 versus non-HRT, n = 13; P<0.001), with values similar to those found in premenopausal women. CONCLUSIONS There are no changes of CR during normal aging in men, whereas CR declines significantly from the 4th to the 5th decades in women. HRT in postmenopausal women appears to enhance CR.

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.

Transsaccadic integration of visual features in a line intersection task

Transsaccadic integration (TSI) refers to the perceptual integration of visual information collected across separate gaze fixations. Current theories of TSI disagree on whether it relies solely on visual algorithms or also uses extra-retinal signals. We designed a task in which subjects had to rely on internal oculomotor signals to synthesize remembered stimulus features presented within separate fixations. Using a mouse-controlled pointer, subjects estimated the intersection point of two successively presented bars, in the dark, under two conditions: Saccade task (bars viewed in separate fixations) and Fixation task (bars viewed in one fixation). Small, but systematic biases were observed in both intersection tasks, including position-dependent vertical undershoots and order-dependent horizontal biases. However, the magnitude of these errors was statistically indistinguishable in the Saccade and Fixation tasks. Moreover, part of the errors in the Saccade task were dependent on saccade metrics, showing that egocentric oculomotor signals were used to fuse remembered location and orientation features across saccades. We hypothesize that these extra-retinal signals are normally used to reduce the computational load of calculating visual correspondence between fixations. We further hypothesize that TSI may be implemented within dynamically updated recurrent feedback loops that interconnect a common eye-centered map in occipital cortex with both the “dorsal” and “ventral” streams of visual analysis.

Exploratory saccades show no direction-specific deficit in neglect

Article abstract In patients with spatial neglect, contralesional reflexive saccades toward suddenly appearing targets show direction-specific deficits. We examined whether these deficits also occur during free exploration of space. Neglect patients’ voluntary eye movements showed reduced amplitudes for saccades in all directions but no direction-specific deficit. The results argue against an interpretation of spatial neglect as a general deficit to disengage attention or to program saccades in contralesional direction.

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.

Spatial frequency-specific effects on the attentional bias: Evidence for two attentional systems

Using a gratingscales task as a sensitive measure of the attentional bias, we have recently observed a new form of frequency-specific cross-over; people showed left-biased preferences when comparing the high spatial frequency (HiSF) components of the task and rightward biases when comparing low spatial frequencies (LoSFs). Here we investigated which mechanisms underlie the cross-over. (1) We found that leftward and rightward biases were positively correlated, suggesting that the same set of mechanisms are involved in both versions of the task. (2) When we cued attention to the left or right side we found transient effects on gratingscales biases that were symmetrical for the LoSF condition but asymmetrical for the HiSF condition. This indicates that the HiSF condition itself biased stimulus-driven attention more to the left side than the LoSF condition. (3) When we lowered the contrast of the HiSF or the LoSF stimulus components, specifically the latter case made HiSF and LoSF conditions more different. This suggests that HiSF and LoSF conditions differ because HiSF components are more salient and more likely stir stimulus-driven attention. Our data are consistent with the idea that the attentional bias results from right-dominant control mechanisms of stimulus-driven attention potentially interacting with voluntary control mechanisms.

Optimal inference explains dimension-specific contractions of spatial perception

It is known that people misperceive scenes they see during rapid eye movements called saccades. It has been suggested that some of these misperceptions could be an artifact of neurophysiological processes related to the internal remapping of spatial coordinates during saccades. Alternatively, we have recently suggested, based on a computational model, that transsaccadic misperceptions result from optimal inference. As one of the properties of the model, sudden object displacements that occur in sync with a saccade should be perceived as contracted in a non-linear fashion. To explore this model property, here we use computer simulations and psychophysical methods first to test how robust the model is to close-to-optimal approximations and second to test two model predictions: (a) contracted transsaccadic perception should be dimension-specific with more contraction for jumps parallel to the saccade than orthogonal to it, and (b) contraction should rise as a function of visuomotor noise. Our results are consistent with these predictions. They support the idea that human transsaccadic integration is governed by close-to-optimal inference.

A right hemisphere dominance for bimanual grasps

To find points on the surface of an object that ensure a stable grasp, it would be most effective to employ one area in one cortical hemisphere. But grasping the object with both hands requires control through both hemispheres. To better understand the control mechanisms underlying this “bimanual grasping”, here we examined how the two hemispheres coordinate their control processes for bimanual grasping depending on visual field. We asked if bimanual grasping involves both visual fields equally or one more than the other. To test this, participants fixated either to the left or right of an object and then grasped or pushed it off a pedestal. We found that when participants grasped the object in the right visual field, maximum grip aperture (MGA) was larger and more variable, and participants were slower to react and to show MGA compared to when they grasped the object in the left visual field. In contrast, when participants pushed the object we observed no comparable visual field effects. These results suggest that grasping with both hands, specifically the computation of grasp points on the object, predominantly involves the right hemisphere. Our study provides new insights into the interactions of the two hemispheres for grasping.

Short- and long-term plasticity of eye position information: examining perceptual, attentional, and motor influences on perisaccadic perception.

Spatial vision requires information about eye position to account for eye movements. But integrating eye position information and information about objects in the world is imperfect and can lead to transient misperceptions around the time of saccadic eye movements most likely because the signals are prone to temporal errors making it difficult to tell when the retinas move relative to when retinal images move. To clarify where this uncertainty comes from, in four experiments we examined influences of eye posture, attentional cueing, and trial history on perisaccadic misperceptions. We found evidence for one longer-term modulation of perisaccadic shift that evolved over the time of the test session due to biased eye posture. Another, short-term influence on perisaccadic shift was related to eye posture during preceding trials or the direction of the preceding saccade. Both perceptual effects could not be explained with visual delays, influences of attention or changes in saccade metrics. Our data are consistent with the idea that perisaccadic shift is caused by neural representations of eye position or space that are plastic and that arise from non-motor, extraretinal mechanisms. This suggests a perceptual system that continuously calibrates itself in response to changes in oculomotor and muscle systems to reconstruct a stable percept of the world.

The perceptual consequences of the attentional bias: evidence for distractor removal

A fundamental question of attentional research concerns the perceptual consequences of attention. Spatial attention can enhance stimuli within the focus of attention relative to stimuli outside; or attention can remove the influence of distracting stimuli and other forms of external noise inside the focus of attention. It is known that both strategies apply depending on how attention is cued to a location in space. Here we asked which strategy applies in an uncued situation in which people show a spontaneous bias of attention to the left side. To measure bias, we used a gratingscales task with stimuli corrupted by pixel noise. If biased attention resulted in biased stimulus enhancement its effect should be largest when there is little noise or few distractors within the attended region, and bias should decline with increasing noise. If, however, bias caused distractors to be removed asymmetrically, larger bias should show up with noisy stimuli. We found that bias rose exponentially as noise increased, in agreement with the external noise removal model, and we found evidence that noise modified interhemispheric competition between attentional systems. Our data offer new insights into the neural mechanisms of the right-hemisphere dominance in spatial and attentional tasks.