Biomedical subjects

Frank Bremmer

Publications and source records attributed to Frank Bremmer.

14 recordsLinked to original sources

PubMed2006

Localization of visual targets during optokinetic eye movements.

We investigated localization of brief visual targets during reflexive eye movements (optokinetic nystagmus). Subjects mislocalized these targets in the direction of the slow eye movement. This error decreased shortly before a saccade and temporarily increased afterwards. The pattern of mislocalization differs markedly from mislocalization during voluntary eye movements in the presence of visual references, but (spatially) resembles mislocalization during voluntary eye movements in darkness. Because neither reflexive eye movements nor voluntary eye movements in darkness have explicit (visual) goals, these data support the view that visual goals support perceptual stability as an important link between pre- and post-saccadic scenes.

Fixation, Ocular↗

PubMed2005

Multisensory space representations in the macaque ventral intraparietal area.

Animals can use different sensory signals to localize objects in the environment. Depending on the situation, the brain either integrates information from multiple sensory sources or it chooses the modality conveying the most reliable information to direct behavior. This suggests that somehow, the brain has access to a modality-invariant representation of external space. Accordingly, neural structures encoding signals from more than one sensory modality are best suited for spatial information processing. In primates, the posterior parietal cortex (PPC) is a key structure for spatial representations. One substructure within human and macaque PPC is the ventral intraparietal area (VIP), known to represent visual, vestibular, and tactile signals. In the present study, we show for the first time that macaque area VIP neurons also respond to auditory stimulation. Interestingly, the strength of the responses to the acoustic stimuli greatly depended on the spatial location of the stimuli [i.e., most of the auditory responsive neurons had surprisingly small spatially restricted auditory receptive fields (RFs)]. Given this finding, we compared the auditory RF locations with the respective visual RF locations of individual area VIP neurons. In the vast majority of neurons, the auditory and visual RFs largely overlapped. Additionally, neurons with well aligned visual and auditory receptive fields tended to encode multisensory space in a common reference frame. This suggests that area VIP constitutes a part of a neuronal circuit involved in the computation of a modality-invariant representation of external space.

Acoustic Stimulation↗

PubMed2005

An fMRI study of optokinetic nystagmus and smooth-pursuit eye movements in humans.

Both optokinetic nystagmus (OKN) and smooth-pursuit eye movements (SPEM) are subclasses of so-called slow eye movements. However, optokinetic responses are reflexive whereas smooth pursuit requires the voluntary tracking of a moving target. We used functional magnetic resonance imaging (fMRI) to determine the neural basis of OKN and SPEM, and to uncover whether the two underlying neural systems overlap or are independent at the cortical level. The results showed a largely overlapping neural circuitry. A direct comparison between activity during the execution of OKN and SPEM yielded no oculomotor-related area exclusively dedicated to one or the other eye movement type. Furthermore, the performance of SPEM evoked a bilateral deactivation of the human equivalent of the parietoinsular vestibular cortex. This finding might indicate that the reciprocally inhibitory visual-vestibular interaction involves not only OKN but also SPEM, which are both linked with the encoding of object-motion and self-motion. Moreover, we could show differential activation patterns elicited by look-nystagmus and stare-nystagmus. Look-nystagmus is characterized by large amplitudes and low-frequency resetting eye movements rather resembling SPEM. Look-nystagmus evoked activity in cortical oculomotor centers. By contrast, stare-nystagmus is usually characterized as being more reflexive in nature and as showing smaller amplitudes and higher frequency resetting eye movements. Stare-nystagmus failed to elicit significant signal changes in the same regions as look-nystagmus/SPEM. Thus, less reflexive eye movements correlated with more pronounced signal intensity. Finally, on the basis of a general investigation of slow eye movements, we were interested in a cortical differentiation between subtypes of SPEM. We compared activity associated with predictable and unpredictable SPEM as indicated by appropriate visual cues. In general, predictable and unpredictable SPEM share the same neural network, yet information about the direction of an upcoming target movement reduced the cerebral activity level.

Adult↗

PubMed2005

Navigation in space--the role of the macaque ventral intraparietal area.

Goal-directed self-motion through space is anything but a trivial task. What we take for granted in everyday life requires the complex interplay of different sensory and motor systems. On the sensory side most importantly a target of interest has to be localized relative to one's own position in space. On the motor side the most critical step in neural processing is to define and perform a movement towards the target as well as the avoidance of obstacles. Furthermore, the multisensory (visual, tactile and auditory) motion signals as induced by one's own movement have to be identified and differentiated from the real motion of visual, tactile or auditory objects in the outside world. In a number of experimental studies performed in recent years we and others have functionally characterized a subregion within monkey posterior parietal cortex (PPC) that appears to be well suited to contribute to such multisensory encoding of spatial and motion information. In this review I will summarize the most important experimental findings on the functional properties of this very region in monkey PPC, i.e. the ventral intraparietal area.

Afferent Pathways↗

PubMed2005

What's next? Sequential movement encoding in primary motor cortex.

Complex actions often can be decomposed into sequences of individual movements. Primate medial motor areas (SMA and pre-SMA) have been shown to be key players in the concert of such sequential actions. In this issue of Neuron, Lu and Ashe show for the first time that neurons in primate M1-the ultimate output stage of cortex-have anticipatory activity related to specific movement sequences. These findings challenge the traditional view of M1 as being a simple module for generating movements.

Animals↗

PubMed2004

The encoding of saccadic eye movements within human posterior parietal cortex.

Over the last few years, several functionally distinct subregions of the posterior parietal cortex (PPC) have been shown to subserve oculomotor control. Since these areas seem to overlap with regions whose activation is related to attention, we used functional magnetic resonance imaging to compare the cerebral activation pattern evoked by eye movements with different attentional loads, i.e., oscillatory saccades with different frequencies, as well as predictable, and unpredictable saccades. Our results show activation in largely overlapping networks with differing strength of activity and symmetry of involved areas. Predictable saccades having the shortest saccadic latency led to the most pronounced cerebral activity both in terms of cortical areas involved and signal intensity. Predictable and unpredictable saccades were dominated by activation within the right hemisphere, whereas oscillatory saccades showing the longest saccadic latency were dominated by activation within the left hemisphere. In all tasks, the centers of gravity of activation occurred within the posterior part of the intraparietal sulcus (IPS), while the predictable saccades additionally activated its anterior part. The enhanced activity during the execution of predictable saccades was probably related to top-down processing and/or the preparation of the upcoming eye movement. The hemispheric difference could arise from a predominant role of the right PPC for shifting spatial attention and the left PPC for shifting temporal attention. The differential encoding of saccadic eye movements within IPS indicates that the PPC splits up into different functional modules related to the particular demands of a saccade.

Adult↗

PubMed2003

Neural correlates of implied motion.

Current views of the visual system assume that the primate brain analyses form and motion along largely independent pathways; they provide no insight into why form is sometimes interpreted as motion. In a series of psychophysical and electrophysiological experiments in humans and macaques, here we show that some form information is processed in the prototypical motion areas of the superior temporal sulcus (STS). First, we show that STS cells respond to dynamic Glass patterns, which contain no coherent motion but suggest a path of motion. Second, we show that when motion signals conflict with form signals suggesting a different path of motion, both humans and monkeys perceive motion in a compromised direction. This compromise also has a correlate in the responses of STS cells, which alter their direction preferences in the presence of conflicting implied motion information. We conclude that cells in the prototypical motion areas in the dorsal visual cortex process form that implies motion. Estimating motion by combining motion cues with form cues may be a strategy to deal with the complexities of motion perception in our natural environment.

Animals↗

PubMed2003

Selectivity of macaque ventral intraparietal area (area VIP) for smooth pursuit eye movements.

In the posterior parietal cortex (PPC) of the macaque, spatial and motion signals arising from different sensory signals converge. One of the functional subregions within the PPC, the ventral intraparietal area (VIP), is thought to play an important role for the multisensory encoding of self- and object motion. In the present study we analysed the activity of area VIP neurons related to smooth pursuit eye movements (SPEMs). Fifty-three per cent of the neurons (123/234) were selective for the direction of the SPEMs. As evident from control experiments, activity observed during smooth eye movements was more closely related to extraretinal signals than visual parameters. In addition, we examined the sensitivity of area VIP neurons for the velocity of SPEMs. Seventy-four per cent of the pursuit-related neurons had a significant velocity tuning. There was a clear preference for high velocities. Eighty-six per cent of the neurons preferred the highest pursuit velocity (40 deg s-1) employed in our study. In everyday life, high pursuit velocities most frequently occur if the pursuit target is located in near-extrapersonal space, i.e. the action space of the head. Together with previous findings, the current results thus suggest that the information provided by VIP neurons may be used to encode motion in near-extrapersonal space and to guide and co-ordinate smooth eye and head movements within this very part of space.

Animals↗

PubMed2003

Seeing and acting at the same time: challenges for brain (and) research.

Traditionally, studies of the visual system of nonhuman primates have investigated neurons while the animal fixates a target in a static environment. Clearly, this is not what our everyday life is like; neither the environment nor we are stationary while we act in our world. On the contrary, we constantly move our eyes or limbs. Here we review a number of recent studies describing the brain in a more realistic mode of operation.

Animals↗

PubMed2003

Neural correlates of visual localization and perisaccadic mislocalization.

While reading this text, your eyes jump from word to word. Yet you are unaware of the motion this causes on your retina; the brain somehow compensates for these displacements and creates a stable percept of the world. This compensation is not perfect; perisaccadically, perceptual space is distorted. We show that this distortion can be traced to a representation of retinal position in the medial temporal and medial superior temporal areas. These cells accurately represent retinal position during fixation, but perisaccadically, the same cells distort the representation of space. The time course and magnitude of this distortion are similar to the mislocalization found psychophysically in humans. This challenges the assumption in many psychophysical studies that the perisaccadic retinal position signal is veridical.

Animals↗

PubMed2003

Discrimination of travel distances from 'situated' optic flow.

Effective navigation requires knowledge of the direction of motion and of the distance traveled. Humans can use visual motion cues from optic flow to estimate direction of self-motion. Can they also estimate travel distance from visual motion?Optic flow is ambiguous with regard to travel distance. But when the depth structure of the environment is known or can be inferred, i.e., when the flow can be calibrated to the environmental situation, distance estimation may become possible. Previous work had shown that humans can discriminate and reproduce travel distances of two visually simulated self-motions under the assumption that the environmental situation and the depth structure of the scene is the same in both motions. Here we ask which visual cues are used for distance estimation when this assumption is fulfilled. Observers discriminated distances of visually simulated self-motions in four different environments with various depth cues. Discrimination was possible in all cases, even when motion parallax was the only depth cue available. In further experiments we ask whether distance estimation is based directly on image velocity or on an estimate of observer velocity derived from image velocity and the structure of the environment. By varying the simulated height above ground, the visibility range, or the simulated gaze angle we modify visual information about the structure of the environment and alter the image velocity distribution in the optic flow. Discrimination ability remained good. We conclude that the judgment of travel distance is based on an estimate of observer speed within the simulated environment.

Adult↗

PubMed2002

Visual-vestibular interactive responses in the macaque ventral intraparietal area (VIP).

Self-motion detection requires the interaction of a number of sensory systems for correct perceptual interpretation of a given movement and an eventual motor response. Parietal cortical areas are thought to play an important role in this function, and we have thus studied the encoding of multimodal signals and their spatiotemporal interactions in the ventral intraparietal area of macaque monkeys. Thereby, we have identified for the first time the presence of vestibular sensory input to this area and described its interaction with somatosensory and visual signals, via extracellular single-cell recordings in awake head-fixed animals. Visual responses were driven by large field stimuli that simulated either backward or forward self-motion (contraction or expansion stimuli, respectively), or movement in the frontoparallel plane (visual increments moving simultaneously in the same direction). While the dominant sensory modality in most neurons was visual, about one third of all recorded neurons responded to horizontal rotation. These vestibular responses were typically in phase with head velocity, but in some cases they could signal acceleration or even showed integration to position. The associated visual responses were always codirectional with the vestibular on-direction, i.e. noncomplementary. Somatosensory responses were in register with the visual preferred direction, either in the same or in the opposite direction, thus signalling translation or rotation in the horizontal plane. These results, taken together with data on responses to optic flow stimuli obtained in a parallel study, strongly suggest an involvement of area VIP in the analysis and the encoding of self-motion.

Action Potentials↗

PubMed2002

Heading encoding in the macaque ventral intraparietal area (VIP).

We recorded neuronal responses to optic flow stimuli in the ventral intraparietal area (VIP) of two awake macaque monkeys. According to previous studies on optic flow responses in monkey extrastriate cortex we used different stimulus classes: frontoparallel motion, radial stimuli (expansion and contraction) and rotational stimuli (clockwise and counter-clockwise). Seventy-five percent of the cells showed statistically significant responses to one or more of these optic flow stimuli. Shifting the location of the singularity of the optic flow stimuli within the visual field led to a response modulation in almost all cases. For the majority of neurons, this modulatory influence could be approximated in a statistically significant manner by a two-dimensional linear regression. Gradient directions, derived from the regression parameters and indicating the direction of the steepest increase in the responses, were uniformly distributed. At the population level, an unbiased average response for the stimuli with different focus locations was observed. By applying a population code, termed 'isofrequency encoding', we demonstrate the capability of the recorded neuronal ensemble to retrieve the focus location from its population discharge. Responses to expansion and contraction stimuli cannot be predicted based on quantitative data on a neuron's frontoparallel preferred stimulus direction and the location and size of its receptive field. These results, taken together with data on polymodal motion responses in this area, suggest an involvement of area VIP in the analysis and the encoding of heading.

Action Potentials↗

PubMed2002

Interaction of linear vestibular and visual stimulation in the macaque ventral intraparietal area (VIP).

Navigation in space requires the brain to combine information arising from different sensory modalities with the appropriate motor commands. Sensory information about self-motion in particular is provided by the visual and the vestibular system. The macaque ventral intraparietal area (VIP) has recently been shown to be involved in the processing of self-motion information provided by optical flow, to contain multimodal neurons and to receive input from areas involved in the analysis of vestibular information. By studying responses to linear vestibular, visual and bimodal stimulation we aimed at gaining more insight into the mechanisms involved in multimodal integration and self-motion processing. A large proportion of cells (77%) revealed a significant response to passive linear translation of the monkey. Of these cells, 59% encoded information about the direction of self-motion. The phase relationship between vestibular stimulation and neuronal responses covered a broad spectrum, demonstrating the complexity of the spatio-temporal pattern of vestibular information encoded by neurons in area VIP. For 53% of the direction-selective neurons the preferred directions for stimuli of both modalities were the same; they were opposite for the remaining 47% of the neurons. During bimodal stimulation the responses of neurons with opposite direction selectivity in the two modalities were determined either by the visual (53%) or the vestibular (47%) modality. These heterogeneous responses to unimodal and bimodal stimulation might be used to prevent misjudgements about self- and/or object-motion, which could be caused by relying on information of one sensory modality alone.

Animals↗