Biomedical subjects

Frans A J Verstraten

Publications and source records attributed to Frans A J Verstraten.

At least 19 recordsLinked to original sources

PubMed2007

Disengaging attention sets the temporal limit of attentive tracking.

At first sight, recent studies investigating the temporal limits of attentive tracking show contradictory outcomes. Attentively tracking an object in an ambiguous apparent motion display can have an upper limit of around 0.4 revolutions per second (rps) [Horowitz, T. S., Holcombe, A. O., Wolfe, J. M., Arsenio, H. C., & DiMase, J. S. (2004). Attentional pursuit is faster than attentional saccade. Journal of Vision, 4, 585-603] or 1rps [Verstraten, F. A., Cavanagh, P., & Labianca, A. T. (2000). Limits of attentive tracking reveal temporal properties of attention. Vision Research, 40, 3651-3664.]. Here, we demonstrate that this difference depends on presentation conditions: an important determinant for the temporal limit of attentive tracking appears to be the duty cycle. Tracking performance at high(er) rates decreases to chance with increasing duty cycle, while at low rates duty cycle hardly has an effect on performance. Results are discussed in terms of (dis)engagement of attention.

Attention↗

PubMed2006

Time dilation in dynamic visual display.

How does the brain estimate time? This old question has led to many biological and psychological models of time perception (R. A. Block, 1989; P. Fraisse, 1963; J. Gibbon, 1977; D. L. I. Zakay, 1989). Because time cannot be directly measured at a given moment, it has been proposed that the brain estimates time based on the number of changes in an event (S. W. Brown, 1995; P. Fraisse, 1963; W. D. Poynter, 1989). Consistent with this idea, dynamic visual stimuli are known to lengthen perceived time (J. F. Brown, 1931; S. Goldstone & W. T. Lhamon, 1974; W. T. Lhamon & S. Goldstone, 1974, C. O. Z. Roelofs & W. P. C. Zeeman, 1951). However, the kind of information that constitutes the basis for time perception remains unresolved. Here, we show that the temporal frequency of a stimulus serves as the "clock" for perceived duration. Other aspects of changes, such as speed or coherence, were found to be inconsequential. Time dilation saturated at a temporal frequency of 4-8 Hz. These results suggest that the clock governing perceived time has its basis at early processing stages. The possible links between models of time perception and neurophysiological functions of early visual areas are discussed.

Acceleration↗

PubMed2006

The speed of visual attention: what time is it?

The time course of visual attention has been studied using a number of experimental designs. Here, we present a refined version of a technique first used by Wundt more than a century ago and demonstrate it as an effective method to measure the speed of visual attention. The method generates precise and robust data quickly and is flexible enough to be adapted into a variety of established paradigms. In the experiment, participants view an array of moving clocks and report the time on a target clock, which was indicated by a peripheral or central cue. We found latencies of around 140 ms when the target was cued peripherally and latencies of around 240 ms when the target was cued centrally. These values are in good agreement with previous literature and support the validity of the technique as a way to measure the speed of visual attention.

Attention↗

PubMed2006

The scope and limits of top-down attention in unconscious visual processing.

Attentional selection plays a critical role in conscious perception. When attention is diverted, even salient stimuli fail to reach visual awareness. Attention can be voluntarily directed to a spatial location or a visual feature for facilitating the processing of information relevant to current goals. In everyday situations, attention and awareness are tightly coupled. This has led some to suggest that attention and awareness might be based on a common neural foundation, whereas others argue that they are mediated by distinct mechanisms. A body of evidence shows that visual stimuli can be processed at multiple stages of the visual-processing streams without evoking visual awareness. To illuminate the relationship between visual attention and conscious perception, we investigated whether top-down attention can target and modulate the neural representations of unconsciously processed visual stimuli. Our experiments show that spatial attention can target only consciously perceived stimuli, whereas feature-based attention can modulate the processing of invisible stimuli. The attentional modulation of unconscious signals implies that attention and awareness can be dissociated, challenging a simplistic view of the boundary between conscious and unconscious visual processing.

Adaptation, Physiological↗

PubMed2006

Discrete color filling beyond luminance gaps along perceptual surfaces.

Perceived color at a point in space is not determined simply by the color directly stimulating the corresponding retinal position. Surface color is informed by flanking edge signals, which also serve to inhibit the intrusion of signals from neighboring surfaces. Spatially continuous local interactions among color and luminance signals have been implicated in a propagation process often referred to as filling-in. Here, we report a phenomenon of discrete color filling whereby color jumps over luminance gaps filling into disconnected regions of the stimulus. This color filling is found to be blocked at boundaries defined by texture. The color filling is also highly specific to the elements belonging to a common perceptual surface, even when multiple surfaces are transparently overlaid. Our results indicate that color filling can be governed by a host of visual cues outside the realm of first-order color and brightness, via their impact on perceptual surface segmentation and segregation.

Color Perception↗

PubMed2006

Dynamics of visual recognition revealed by fMRI.

In our daily lives, recognizing a familiar object is an effortless and seemingly instantaneous process. Our knowledge of how the brain accomplished this formidable task, however, is quite limited. The present study takes a holistic approach to examining the neural processes that underlie recognition memory. A unique paradigm, in which visual information about the identity of a person or word is slowly titrated to human observers during a functional imaging session, is employed to uncover the dynamics of the visual recognition in the brain. The results of study reveal multiple unique stages in visual recognition that can be dissociated from one another based on temporal asynchronies and hemodynamic response characteristics.

Adult↗

PubMed2006

Attentional modulation of perceptual stabilization.

Perceptual priming is generally regarded as a passive and automatic process, as it is obtained even without awareness of the prime. Recent studies have introduced a more active form of perceptual priming in which priming for a subsequent ambiguous stimulus is triggered by the subjective percept, that is, interpretation of a previous ambiguous stimulus. This phenomenon known as stabilization does not require a conscious effort to actively maintain one perceptual interpretation. In this study, we show that distraction of attention, during and even after the prime presentation, interferes with the build-up of perceptual memory for stabilization. This implies that despite the apparent automaticity, stabilization involves an active attentional process for encoding and retention. The disruption during the encoding can be attributed to the reduction in sensory signals for the prime. However, the disruption during the retention suggests that the implicit memory trace of the prime necessitates the attentional resource to fully develop. The active nature of the build-up of perceptual memory for stabilization is consistent with the idea that perceptual memory increases its strength gradually over a few seconds. These findings suggest that seemingly automatic and effortless cognitive processes can compete with online perceptual processing for common attentional resources.

Attention↗

PubMed2006

Cerebral changes during performance of overlearned arbitrary visuomotor associations.

The posterior parietal cortex (PPC) is known to be involved in the control of automatic movements that are spatially guided, such as grasping an apple. We considered whether the PPC might also contribute to the performance of visuomotor associations in which stimuli and responses are linked arbitrarily, such as producing a certain sound for a typographical character when reading aloud or pressing pedals according to the color of a traffic light when driving a motor vehicle. The PPC does not appear to be necessary for learning new arbitrary visuomotor associations, but with extensive training, the PPC can encode nonspatial sensory features of task-relevant cues. Accordingly, we have tested whether the contributions of the PPC might become apparent once arbitrary sensorimotor mappings are overlearned. We have used functional magnetic resonance imaging to measure cerebral activity while subjects were learning novel arbitrary visuomotor associations, overlearning known mappings, or attempting to learn frequently changing novel mappings. To capture the dynamic features of cerebral activity related to the learning process, we have compared time-varying modulations of activity between conditions rather than average (steady-state) responses. Frontal, striatal, and intraparietal regions showed decreasing or stable activity when subjects learned or attempted to learn novel associations, respectively. Importantly, the same frontal, striatal, and intraparietal regions showed time-dependent increases in activity over time as the mappings become overlearned, i.e., despite time-invariant behavioral responses. The automaticity of these mappings predicted the degree of intraparietal changes, indicating that the contribution of the PPC might be related to a particular stage of the overlearning process. We suggest that, as the visuomotor mappings become robust to interference, the PPC may convey relevant sensory information toward the motor cortex. More generally, our findings illustrate how rich cerebral dynamics can underlie stable behavior.

Adult↗

PubMed2006

Visual transients reveal the veridical position of a moving object.

The position of a moving object is often mislocalised in the direction of movement. At the input stage of visual processing, the position of a moving object should still be represented veridically, whereas it should become closer to the mislocalised position at a later processing stage responsible for positional judgment. Here, we show that visual transients expose the veridical position of a moving object represented in early visual areas. For example, when a ring is flashed on a moving bar, the part of the bar within the ring is perceived at the veridical position, whereas the part outside the ring is perceived to be ahead of the ring as in the flash-lag effect. Our observations suggest that a filling-in process is triggered at the edges of the flash. This indicates that, in early cortical areas, moving objects are still represented at their veridical positions, and the perceived location is determined by the higher visual areas.

Humans↗

PubMed2006

Attention speeds binocular rivalry.

During binocular rivalry, incompatible images presented dichoptically compete for perceptual dominance. It has long been debated whether binocular rivalry can be controlled by attention. Most studies have shown that voluntary control over binocular rivalry is limited. We sought to remove attention from binocular rivalry by presenting a concurrent task. Diverting attention slowed the rivalry alternation rate, and did so in proportion to the difficulty of the concurrent task. Even a very demanding distractor task, however, did not arrest rivalry alternations completely. Given that diverting attention was equivalent to lowering the contrast of the rival stimuli, the ability of attention to speed binocular rivalry is most likely due to an increase in the effective contrast of the stimuli through boosting the gain of the cortical response. This increase in effective contrast will ultimately lead to a perceptual switch, thereby limiting voluntary control. Thus, attention speeds rivalry alternations, but has no inherent control over the rivalry process.

Attention↗

PubMed2005

Perceptual manifestations of fast neural plasticity: motion priming, rapid motion aftereffect and perceptual sensitization.

Visual neurons show fast adaptive behavior in response to brief visual input. However, the perceptual consequences of this rapid neural adaptation are less known. Here, we show that brief exposure to a moving adaptation stimulus-ranging from tens to hundreds of milliseconds-influences the perception of a subsequently presented ambiguous motion test stimulus. Whether the ambiguous motion is perceived to move in the same direction (priming), or in the opposite direction (rapid motion aftereffect) varies systematically with the duration of the adaptation stimulus and the adaptation-test blank interval. These biases appear and decay rapidly. Moreover, when the adapting stimulus is itself ambiguous, these effects are not produced. Instead, the percept for the subsequent test stimulus is biased to the perceived direction of the adaptation stimulus. This effect (perceptual sensitization) builds gradually over the time between the adaptation and test stimuli. Our results indicate that rapid adaptation plays a role mainly within early motion processing, whereas a slow potentiation controls the sensitivity at a later stage.

Adaptation, Physiological↗

PubMed2005

Adaptive center-surround interactions in human vision revealed during binocular rivalry.

We used binocular rivalry as a psychophysical probe to explore center-surround interactions in orientation, motion and color processing. Addition of the surround matching one of the rival targets dramatically altered rivalry dynamics. For all visual sub-modalities tested, predominance of the high-contrast rival target matched to the surround was greatly reduced-a result that disappeared at low contrast. At low contrast, addition of the surround boosted dominance of orientation and motion targets matched to the surround. This contrast-dependent modulation of center-surround interactions seems to be a general property of the visual system and may reflect an adaptive balance between surround suppression and spatial summation.

Color Perception↗

PubMed2005

Center-surround inhibition and facilitation as a function of size and contrast at multiple levels of visual motion processing.

Visual context often plays a crucial role in visual processing. In the domain of visual motion processing, the response to a stimulus presented to a neuron's classical receptive field can be modulated by presenting stimuli to its surround. The nature of these center-surround interactions is often inhibitory; the neural response decreases when the same direction of motion is presented to center and surround. Here we use binocular rivalry as a tool to study center-surround interactions. We show that magnitude of surround suppression varies as a function of luminance contrast and surround width. Increasing the size of surround motion increased surround suppression at high contrast. Furthermore, large, high-contrast surrounds facilitated opposite-direction motion in the center. For stimuli presented at low contrast, surround suppression peaked at a smaller surround width. In addition, we provide evidence that surround inhibition occurs at multiple levels of visual processing: Surround inhibition in motion processing is likely to originate from both monocular and binocular processing stages.

Contrast Sensitivity↗

PubMed2005

The motion aftereffect of transparent motion: two temporal channels account for perceived direction.

Adaptation to orthogonal transparent patterns drifting at the same speed produces a unidirectional motion aftereffect (MAE) whose direction is opposite the average adaptation direction. If the patterns move at different speeds, MAE direction can be predicted by an inverse vector average, using the observer's motion sensitivity to each individual pattern as vector magnitudes. These weights are well approximated by the duration of each pattern's MAE, as measured with static test patterns. However, previous efforts to use the inverse-vector-average rule with dynamic test patterns have failed. Generally, these studies have used spatially and temporally broadband test stimuli. Here, in order to gain insight into the possible contribution of temporal channels, we filtered our test pattern in the temporal domain to produce five ideal, octave-width pass-bands. MAE durations were measured for single-component stimuli drifting at various adaptation speeds and tested at a range of temporal frequencies. Then, two components with orthogonal directions and different speeds were combined and the direction of the resulting MAE was measured. The key findings are that: (i) for a given adaptation speed, the duration of a single component's MAE is dependent on test temporal frequency; (ii) the direction of MAEs produced by transparent motion (i.e., bivectorial adaptation) also varies strongly as a function test temporal frequency (by up to 90 degrees for some speed pairings); and (iii) the inverse-vector-average rule predicts the direction of the transparent MAE provided the MAE durations used to weight the vector combination were obtained from stimuli matched in adaptation speed and test temporal frequency. These results are discussed in terms of the number and shape of temporal channels in our visual system.

Adaptation, Physiological↗

PubMed2005

Center-surround inhibition deepens binocular rivalry suppression.

When dissimilar stimuli are presented to each eye, perception alternates between both images--a phenomenon known as binocular rivalry. It has been shown that stimuli presented in proximity of rival targets modulate the time each target is perceptually dominant. For example, presenting motion to the region surrounding the rival targets decreases the predominance of the same-direction target. Here, using a stationary concentric grating rivaling with a drifting grating, we show that a drifting surround grating also increases the depth of binocular rivalry suppression, as measured by sensitivity to a speed discrimination probe on the rival grating. This was especially so when the surround moved in the same direction as the grating, and was slightly weaker for opposed directions. Suppression in both cases was deeper than a no-surround control condition. We hypothesize that surround suppression often observed in area MT (V5)-a visual area implicated in visual motion perception-is responsible for this increase in suppression. In support of this hypothesis, monocular and binocular surrounds were both effective in increasing suppression depth, as were surrounds contralateral to the probed eye. Static and orthogonal motion surrounds failed to add to the depth of rivalry suppression. These results implicate a higher-level, fully binocular area whose surround inhibition provides an additional source of suppression which sums with rivalry suppression to effectively deepen suppression of an unseen rival target.

Humans↗

PubMed2005

Perceptual alternation induced by visual transients.

When our visual system is confronted with ambiguous stimuli, the perceptual interpretation spontaneously alternates between the competing incompatible interpretations. The timing of such perceptual alternations is highly stochastic and the underlying neural mechanisms are poorly understood. We show that perceptual alternations can be triggered by a transient stimulus presented nearby. The induction was tested for four types of bistable stimuli: structure-from-motion, binocular rivalry, Necker cube, and ambiguous apparent motion. While underlying mechanisms may vary among them, a transient flash induced time-locked perceptual alternations in all cases. The effect showed a dependence on the adaptation to the dominant percept prior to the presentation of a flash. These perceptual alternations show many similarities to perceptual disappearances induced by transient stimuli (Kanai and Kamitani, 2003 Journal of Cognitive Neuroscience 15 664-672; Moradi and Shimojo, 2004 Vision Research 44 449-460). Mechanisms linking these two transient-induced phenomena are discussed.

Adaptation, Psychological↗

PubMed2004

Attention-based motion perception and motion adaptation: what does attention contribute?

Attention-based motion perception refers to the phenomenon that a stimulus with ambiguous motion energy can be seen to move in a direction that is under attentive control of the observer. The role of attention is obvious when the stimulus is ambiguous: it makes the stimulus move in one direction. The goal of the current experiment is to investigate what the contribution of attention under attentive tracking conditions actually is, especially while viewing-time progresses. We had our observers look at a circular array of four evenly spaced discs whose motion direction was biased in the clockwise direction. Observers either viewed the stimulus moving around a circular path passively or actively. In the latter case they attentively tracked one of the discs. The observer's task was to indicate the perceived direction of motion. As time progresses, this kind of stimulus will undergo spontaneous motion direction reversals. We analyzed the time course of the reversals and show that actively attentive tracking the stimulus massively delays the reversal time. These results suggest that attention can temporarily overrule lower level adaptation.

Adaptation, Physiological↗

PubMed2004

Center-surround interactions in visual motion processing during binocular rivalry.

When each eye is confronted with a dissimilar stimulus, the percept will generally alternate between the two. This phenomenon is known as binocular rivalry. Although binocular rivalry occurs at locations where targets overlap spatially, the area surrounding rivalrous targets can modulate their dominance. Here we show that during binocular rivalry of oppositely moving gratings, a surrounding grating moving in the same direction as one of the two leads to increased dominance of the opposite direction of motion in the center. This increased dominance of the opposite direction in the center was observed irrespective of the eye to which the surround was presented. Inspection of the results for different conditions reveals that the preference for the opposite direction of motion cannot be explained by a single mechanism operating beyond binocular fusion. We therefore suggest that this phenomenon is the outcome of center-surround interactions at multiple levels along the pathway of visual motion processing.

Contrast Sensitivity↗