Biomedical subjects

Bart Krekelberg

Publications and source records attributed to Bart Krekelberg.

12 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↗

PubMed2006

Interactions between speed and contrast tuning in the middle temporal area: implications for the neural code for speed.

A car driving through the fog appears to move more slowly than one driving on a clear and sunny day. In the laboratory, this observation has been confirmed as a pronounced reduction of perceived speed caused by a reduction in contrast. We measured the influence of contrast on cells in the middle temporal area (MT) of the macaque, which has been hypothesized to underlie the perception of speed. The influence of contrast on the responsiveness and speed tuning of these cells was pervasive and highly regular. As expected, most cells responded less at low contrast. More importantly, the preferred speed of most cells shifted to lower speeds at lower contrasts. Moreover, approximately one-third of cells surprisingly responded more strongly to slow low-contrast stimuli than to slow high-contrast stimuli. Current models of speed perception suggest that each MT cell votes for its preferred speed, with a vote determined by its firing rate. We tested a number of these labeled-line models by entering the neural responses we recorded from MT and comparing the predictions of the models with the perceptual reports of human subjects and monkeys. Contrary to the perceptual reports, the labeled-line models predicted that perceived speed should increase when contrast is decreased. We therefore conclude that perceived speed is not based on a labeled-line interpretation of MT cells.

Animals↗

PubMed2006

Adaptation: from single cells to BOLD signals.

Functional magnetic resonance imaging adaptation (fMRIa) is an increasingly popular method that aims to provide insight into the functional properties of subpopulations of neurons within an imaging voxel. The technique relies on the assumption that neural adaptation reduces activity when two successive stimuli activate the same subpopulation but not when they stimulate different subpopulations. Here, we assess the validity of fMRIa by comparing single-cell recordings with functional imaging of orientation, motion and face processing. We find that fMRIa provides novel insight into neural representations in the human brain. However, network responses in general and adaptation in particular are more complex than is often assumed, and an unequivocal interpretation of fMRIa results can be achieved only with great care.

Adaptation, Physiological↗

PubMed2005

Adaptation in macaque MT reduces perceived speed and improves speed discrimination.

The visual system adapts to its environment. Some adaptive changes are detrimental-perception is no longer veridical. Others are beneficial-the ability to discriminate two stimuli improves. The latter may reflect the visual system's ability to zoom-in on the currently relevant properties of the environment. We studied the neural basis of adaptive changes in the middle temporal area (MT) of macaque monkey visual cortex. Our data show that brief adaptation to a moving stimulus reduces the magnitude of neural responses and reduces the width of speed tuning curves. Comparable with what has recently been reported in the direction domain, the response reduction was largest when the test speed was different from the adaptation speed. Using an ideal observer analysis, we show that these response changes in MT are consistent with a reduction in perceived speed as well as an improvement in speed discrimination. This supports the view that adaptive response changes in MT are not just a consequence of neural fatigue, but an active process that enhances the discrimination of speed.

Adaptation, Physiological↗

PubMed2005

Implied motion from form in the human visual cortex.

When cartoonists use speed lines--also called motion streaks--to suggest the speed of a stationary object, they use form to imply motion. The goal of this study was to investigate the mechanisms that mediate the percept of implied motion in the human visual cortex. In an adaptation functional imaging paradigm we presented Glass patterns that, just like speed lines, imply motion but do not on average contain coherent motion energy. We found selective adaptation to these patterns in the human motion complex, the lateral occipital complex (LOC), and earlier visual areas. Glass patterns contain both local orientation features and global structure. To disentangle these aspects we performed a control experiment using Glass patterns with minimal local orientation differences but large global structure differences. This experiment showed that selectivity for Glass patterns arises in part in areas beyond V1 and V2. Interestingly, the selective adaptation transferred from implied motion stimuli to similar real motion patterns in dorsal but not ventral areas. This suggests that the same subpopulations of cells in dorsal areas that are selective for implied motion are also selective for real motion. In other words, these cells are invariant with respect to the cue (implied or real) that generates the motion. We conclude that the human motion complex responds to Glass patterns as if they contain coherent motion. This, presumably, is the reason why these patterns appear to move coherently. The LOC, however, has different cells that respond to the structure of real motion patterns versus implied motion patterns. Such a differential response may allow ventral areas to further analyze the structure of global patterns.

Analysis of Variance↗

PubMed2004

Motion mechanisms in macaque MT.

The macaque middle temporal area (MT) is exquisitely sensitive to visual motion and there is a large amount of evidence that neural activity in MT is tightly correlated with the perception of motion. The mechanisms by which MT neurons achieve their directional selectivity, however, have received considerably less attention. We investigated the motion-energy model as a description of motion mechanisms in macaque MT. We first confirmed one of the predictions of the motion-energy model; macaques-just like humans-perceive a reversed direction of motion when a stimulus reverses contrast with every displacement (reverse-phi). This reversal of perceived direction had a clear correlate in the neural responses of MT cells, which were predictive of the monkey's behavioral decisions. Second, we investigated how multiple motion-energy components are combined. Psychophysical data have been used to argue that motion-energy components representing opposite directions are subtracted from each other. Our data show, however, that the interactions among motion-energy components are more complex. In particular, we found that the influence of a given component on the response to a stimulus consisting of multiple components depends on factors other than the response to that component alone. This suggests that there are subthreshold nonlinear interactions among multiple motion-energy components; these could take place within MT or in earlier stages of the motion network such as V1. We propose a model that captures the complexity of these component interactions by means of a competitive interaction among the components. This provides a better description of the MT responses than the subtractive motion opponency envisaged in the motion-energy model, even when the latter is combined with a gain-control mechanism. The competitive interaction extends the dynamic range of the cells and allows them to provide information on more subtle changes in motion patterns, including changes that are not purely directional.

Animals↗

PubMed2004

Neural correlates of saccadic suppression in humans.

When you look into a mirror and move your eyes left to right, you will see that you cannot observe your own eye movements. This demonstrates the phenomenon of saccadic suppression: during saccadic eye movements, visual sensitivity is much reduced. Given that humans make more than 100,000 eye movements each day, it is clear why suppression is needed: without it, the motion on the retina would prevent us from seeing anything at all. Psychophysical data show that suppression is stimulus selective: it is strongest for the kind of stimuli that preferentially activate magnocellular thalamic neurons. This has led to the hypothesis that saccadic suppression selectively targets the magnocellular stream. We used fMRI to find brain areas with a stimulus-selective suppression of the BOLD signal that matches the psychophysical data. We found such a neural correlate of saccadic suppression in the dorsal stream (hMT+, V7) and in ventral area V4. These areas receive magnocellular input; hence our findings are consistent with the magnocellular hypothesis. The range of effects in our data and in single cell data, however, argues against a single thalamic mechanism that suppresses all cortical input. Instead, we speculate that saccadic suppression relies on multiple mechanisms operating in different cortical areas.

Adult↗

PubMed2004

Attention enhances adaptability: evidence from motion adaptation experiments.

Several previous psychophysical and neurophysiological studies have investigated the separate effects of attention and adaptation on visual processing. Here, we investigated the combined effects of attention and adaptation on motion processing by measuring the effects of spatial attention on the duration of the motion after-effect (MAE) over a wide range of stimulus contrasts. MAE duration was compared between two conditions: full-attention, subjects were required to pay attention to the adapting motion stimulus, and poor-attention, subjects performed a difficult vowel detection task at the center of gaze and ignored the adapting motion stimulus. Attention was found to increase the MAE duration by a factor of 1.4, which was approximately constant over a wide range of stimulus contrasts (3.22-80.6%). Notably, this included contrasts for which the MAE duration had reached its asymptotic value. We show that a quantitative model based on known properties of directionally selective MT neurons can explain these results by assuming that attention enhances the effects of adaptation, a phenomenon we refer to as "adaptation gain". Specifically, attending to an adapting motion stimulus shifts the semi-saturation point (C50) of the underlying contrast response function (CRF) of motion detectors roughly 1.4-fold more to the right than does ignoring that same stimulus. By enhancing the effects of adaptation in this fashion, attention is predicted to enhance the adaptability of the visual motion system.

Adaptation, Physiological↗

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

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

Sound and vision.

When a brief flash appears at the same position as a moving object, the flash is perceived to lag behind. This so-called flash-lag effect tells us something about the perception of space and time: where is the moving object when the flash appears? A recent paper by Alais and Burr on auditory and crossmodal flash-lag effects indicates that our (often implicit) models of the perception of space and time might be flawed.

Journal Article↗