Biomedical subjects

Dirk Kerzel

Publications and source records attributed to Dirk Kerzel.

At least 19 recordsLinked to original sources

PubMed2006

The spatio-temporal tuning of the mechanisms in the control of saccadic eye movements.

We compared the spatio-temporal tuning of perception to the mechanisms that drive saccadic eye movements. Detection thresholds were measured for Gabor-targets presented left or right of fixation (4 or 8deg eccentricity), at one of four spatial frequencies (1, 2, 4 or 8cpd) oscillating at one of three temporal frequencies (1, 8 or 16Hz). We then measured saccade latency to each target presented at various multiples of detection threshold. Consistent with previous research, latency decreased as a function of contrast. However, at equal detection performance, we found no systematic difference in saccadic latency and no difference in average oculometric performance (% correct saccade direction) across the different target spatio-temporal frequencies. Furthermore, position error remained fairly constant across all conditions. The results are consistent with the idea that the spatio-temporal signals used for perception are the same as those used by the mechanisms driving saccadic eye movements.

Adult↗

PubMed2006

Visually guided movements to color targets.

The pathways controlling motor behavior are believed to exhibit little selectivity for color, but there is growing evidence suggesting that color signals can be used to guide actions. We investigated this by having observers make a saccade or a rapid pointing movement to a small, peripherally flashed (100 ms) Gaussian target (SD=0.5 degrees) defined exclusively by luminance (maximum contrast) or color (from cardinal DKL red-green or blue-yellow axes, at maximum saturation). We found no difference in saccadic or pointing accuracy for luminance or color targets. The same was true using shutter goggles during pointing (to minimize the use of external cues), and when the luminance contrast of color targets was varied by up to +/-10%. In terms of response times, both eye and hand latencies increased with target eccentricity for R-G targets only, in a manner consistent with the sensitivity of this channel across eccentricity. We found little difference in response latencies between luminance and color targets once matched in terms of cone contrast. While RTs were longer when coupled with a goal directed pointing movement (versus a simple reaction without pointing), the difference was the same for color or luminance targets, suggesting that the spatial coding for the movements was also the same. In a final experiment we compared the accuracy of pointing to color-naming performance in a 4AFC procedure. The psychometric functions relating pointing accuracy (% correct quadrant) to color-naming (% correct color-name) were identical. Taken together, the results show that human observers can efficiently use pure chromatic signals to guide actions.

Analysis of Variance↗

PubMed2006

Estimating the quantitative relation between incongruent information and response time.

In Eriksen's flanker paradigm, participants' responses are slower and more error-prone when task-relevant and simultaneously available task-irrelevant cues are incongruent. The influence of task-irrelevant information decreases as its distance from the task-relevant information increases. Here, we manipulated the quantity of task-irrelevant information while keeping the distance constant. We asked whether when the impact on response selection processes was stronger the more incongruent information was available, or whether the impact on response selection depended only on its presence or absence. We conducted an experiment, in which subjects had to discriminate the direction of motion of a central point-light-walker that was flanked by two, four, or eight point-light-walkers at an equal distance from the center. The experiment showed that reaction times increased with the number of incongruent walkers. This effect was modulated by the total number of walkers, showing that the effect of incongruent information saturates when the display is cluttered.

Adult↗

PubMed2006

Distractor interference during smooth pursuit eye movements.

When 2 targets for pursuit eye movements move in different directions, the eye velocity follows the vector average (S. G. Lisberger & V. P. Ferrera, 1997). The present study investigates the mechanisms of target selection when observers are instructed to follow a predefined horizontal target and to ignore a moving distractor stimulus. Results show that at 140 ms after distractor onset, horizontal eye velocity is decreased by about 25%. Vertical eye velocity increases or decreases by 1 degrees /s in the direction opposite from the distractor. This deviation varies in size with distractor direction, velocity, and contrast. The effect was present during the initiation and steady-state tracking phase of pursuit but only when the observer had prior information about target motion. Neither vector averaging nor winner-take-all models could predict the response to a moving to-be-ignored distractor during steady-state tracking of a predefined target. The contributions of perceptual mislocalization and spatial attention to the vertical deviation in pursuit are discussed.

Adult↗

PubMed2006

Why eye movements and perceptual factors have to be controlled in studies on "representational momentum".

In order to study memory of the final position of a smoothly moving target, Hubbard (e.g., Hubbard and Bharucha, 1988) presented smooth stimulus motion and used motor responses. In contrast, Freyd (e.g., Freyd and Finke, 1984) presented implied stimulus motion and used the method of constant stimuli. The same forward error was observed in both paradigms. However, the processes underlying the error may be very different. When smooth stimulus motion is followed by smooth pursuit eye movements, the forward error is associated with asynchronous processing of retinal and extraretinal information. In the absence of eye movements, no forward displacement is observed with smooth motion. In contrast, implied motion produces a forward error even without eye movements, suggesting that observers extrapolate the next target step when successive target presentations are far apart. Finally, motor responses produce errors that are not observed with perceptual judgments, indicating that the motor system may compensate for neuronal latencies.

Discrimination Learning↗

PubMed2005

Mislocalization of flashes during smooth pursuit hardly depends on the lighting conditions.

Targets that are briefly flashed during smooth pursuit eye movements are mislocalized in the direction of motion (forward shift) and away from the fovea (spatial expansion). Hansen [Hansen, R. M. (1979). Spatial localization during pursuit eye movements. Vision Research 19(11), 1213-1221] reported that these errors are not present for fast motor responses in the dark, whereas Rotman et al. [Rotman, G., Brenner, E., Smeets, J. B. (2004). Quickly tapping targets that are flashed during smooth pursuit reveals perceptual mislocalizations. Experimental Brain Research 156(4), 409-414] reported that they are present for fast motor responses in the light. To evaluate whether the lighting conditions are the critical factor, we asked observers to point to the positions of flashed objects during smooth pursuit either in the dark or with the room lights on. In a first experiment, the flash, which could appear at 1 of 15 different positions, was always shown when the eye had reached a certain spatial position. We found a forward bias and spatial expansion that were independent of the target and ambient luminance. In a second experiment, the flash was always shown at the same retinal position, but the spatial position of the eye at the moment of flash presentation was varied. In this case we found differences between the luminance conditions, in terms of how the errors depended on the velocity and position on the trajectory. We also found specific conditions in which people did not mislocalize the target in the direction of pursuit at all. These findings may account for the above-mentioned discrepancy. We conclude that although the lighting conditions do influence the localization errors under some circumstances, it is certainly not so that such errors are absent whenever the experiment is conducted in the dark.

Humans↗

PubMed2005

Effects of contrast on smooth pursuit eye movements.

It is well known that moving stimuli can appear to move more slowly when contrast is reduced (P. Thompson, 1982). Here we address the question whether changes in stimulus contrast also affect smooth pursuit eye movements. Subjects were asked to smoothly track a moving Gabor patch. Targets varied in velocity (1, 8, and 15 deg/s), spatial frequency (0.1, 1, 4, and 8 c/deg), and contrast, ranging from just below individual thresholds to maximum contrast. Results show that smooth pursuit eye velocity gain rose significantly with increasing contrast. Below a contrast level of two to three times threshold, pursuit gain, acceleration, latency, and positional accuracy were severely impaired. Therefore, the smooth pursuit motor response shows the same kind of slowing at low contrast that was demonstrated in previous studies on perception.

Contrast Sensitivity↗

PubMed2005

Effects of structured nontarget stimuli on saccadic latency.

It has been suggested that the remote distractor effect is the result of nontarget stimulation of a central region representing a collicular fixation zone near the time of target onset. The distributed network of the cells responsible for this effect is believed to extend over a large area, responding to distractors < or =10 deg in the periphery. Several studies also implicate the superior colliculus as the substrate behind an inhibited saccadic response arising from a display change. We investigated this further by using a patch of pink noise of various sizes as a nontarget stimulus. We show that the onset of a small patch (2.3 x 2.3 deg) of centrally displayed pink noise can produce a significant increase in saccadic latency to a simultaneously presented peripheral Gabor target. In contrast, a large patch (36 x 36 deg) of pink noise did not increase latency despite the fact that it also stimulated the region representing the fixation zone. Furthermore, only the large patch of noise facilitated latency when presented before target onset. We also examined the effect of patch sizes between these two extremes and found a steady decrease in latency as patch size increased. This confirms that nontarget stimulation of the region representing the fixation zone near the time of target onset is not in itself sufficient to produce the increase in latency typically found with remote distractors. The results are consistent with the idea that only a spatially confined object leads to a discharge of collicular fixation neurons.

Adult↗

PubMed2005

Visual short-term memory during smooth pursuit eye movements.

Visual short-term memory (VSTM) was probed while observers performed smooth pursuit eye movements. Smooth pursuit keeps a moving object stabilized in the fovea. VSTM capacity for position was reduced during smooth pursuit compared with a condition with eye fixation. There was no difference between a condition in which the items were approximately stabilized on the retina because they moved with the pursuit target and a condition in which the items moved across the retina because they were stationary in space. The reduction of capacity for position was eliminated when miniature items were presented on the pursuit target. Similarly, VSTM capacity for color did not differ between smooth pursuit and fixation. The results suggest that visuospatial attention is tied to the target during smooth pursuit, which impairs VSTM for the position of peripheral objects. Sensory memory during smooth pursuit was only slightly impaired.

Adult↗

PubMed2005

Is direction position? Position- and direction-based correspondence effects in tasks with moving stimuli.

Five experiments were carried out to test whether (task-irrelevant) motion information provided by a stimulus changing its position over time would affect manual left-right responses. So far, some studies reported direction-based Simon effects whereas others did not. In Experiment 1a, a reliable direction-based effect occurred, which was not modulated by the response mode--that is, by whether participants responded by pressing one of two keys or more dynamically by moving a stylus in a certain direction. Experiments 1a, 1b, and 2 lend support to the idea that observers use the starting position of target motion as a reference for spatial coding. That is, observers might process object motion as a shift of position relative to the starting position and not as directional information. The dominance of relative position coding could also be shown in Experiment 3, in which relative position was pitted against motion direction by presenting a static and dynamic stimulus at the same time. Additionally, we explored the role of eye movements in stimulus-response compatibility and showed in Experiments 1b and 3a that the execution or preparation of saccadic eye movements--as proposed by an attention-shifting account--is not necessary for a Simon effect to occur.

Adolescent↗

PubMed2004

Motion-induced illusory displacement reexamined: differences between perception and action?

The position of a drifting sine-wave grating enveloped by a stationary Gaussian is misperceived in the direction of motion. Previous research indicated that the illusion was larger when observers pointed to the center of the stimulus than when they indicated the stimulus position on a ruler. This conclusion was reexamined. Observers pointed to the position of a small Gabor patch on the screen or compared its position to moving patches, stationary lines, or flashed lines. With moving patches, the illusion was larger with probe than with motor judgments; with stationary lines, the illusion was about the same size; and with flashed lines, the illusion was smaller with probe than with motor judgments. Thus, the comparison between perceptual and motor measures depended strongly on the methods used. Further, the target was mislocalized toward the fovea with motor judgments, whereas the target was displaced away from the fovea relative to line probes.

Humans↗

PubMed2004

Spatial distortions and processing latencies in the onset repulsion and Fröhlich effects.

In the Fröhlich illusion, the first position of a moving target is mis-localized in the direction of motion. In the onset repulsion effect, the opposite error occurs. To reconcile these conflicting error patterns, we improved previous methods by using natural pointing movements and a large range of target velocities. Displacement was found to increase in the direction of motion, but the linear function relating velocity and displacement was shifted opposite to the direction of target motion. The results suggest that onset localization may be determined by two independent factors: first, an (attentional) delay that accounts for the increase of displacement in the direction of motion with increasing velocity. This delay is visible in motor and probe judgments and explains the Fröhlich illusion. Second, motor judgments are offset opposite to the direction of target motion. This bias is unique to motor judgments (pointing) and may be partially explained by attentional repulsion.

Humans↗

PubMed2004

The trial context determines adjusted localization of stimuli: reconciling the Fröhlich and onset repulsion effects.

It is known that observers make localization errors in the direction of motion when asked to localize the perceived onset position of a moving target (Fröhlich effect). However, recent studies also revealed the contrary: In the onset repulsion effect, the error is opposite to the direction of motion. In four experiments we demonstrate that the conflict between these findings is resolved by considering the trial context: when the stimuli appeared at predictable positions to the left or right of fixation, pointing responses to the perceived onset position were displaced in movement direction. In contrast, when the stimuli appeared at unpredictable positions in the visual field, pointing errors were displaced opposite to motion or at least drastically reduced. Thus, localization of the perceived onset position varies with the trial context.

Adolescent↗

PubMed2004

A Simon effect with stationary moving stimuli.

To clarify whether motion information per se has a separable influence on action control, the authors investigated whether irrelevant direction of motion of stimuli whose overall position was constant over time would affect manual left-right responses (i.e., reveal a motion-based Simon effect). In Experiments 1 and 2, significant Simon effects were obtained for sine-wave gratings moving in a stationary Gaussian window. In Experiment 3, a direction-based Simon effect with random-dot patterns was replicated, except that the perceived direction of motion was based on the displacement of single elements. Experiments 4 and 5 studied motion-based Simon effects to point-light figures that walked in place--displays requiring high-level analysis of global shape and local motion. Motion-based Simon effects occurred when the displays could be interpreted as an upright human walker, showing that a high-level representation of motion direction mediated the effects. Thus, the present study establishes links between high-level motion perception and action.

Color Perception↗

PubMed2004

Attentional load modulates mislocalization of moving stimuli, but does not eliminate the error.

Localization of the onset and offset of a moving target is subject to a number of errors that have to be attributed to events following or preceding the target event. Apparently, observers are unable to ignore the spatiotemporal context surrounding the target event. In two experiments, observers' attention was directed toward a single position along a trajectory, two positions along a single trajectory, or two positions along two different trajectories. In the latter condition, attention to details of a single trajectory was reduced. At the same time, motion type was manipulated by varying the temporal interval between successive target presentations. The localization error was not affected by attentional load; however, effects of motion type were eliminated when two trajectories had to be attended to. It may be sufficient to notice that the target has moved for localization errors to occur, while specifics of the trajectory are ignored.

Attention↗

PubMed2003

Neuronal processing delays are compensated in the sensorimotor branch of the visual system.

Moving objects change their position until signals from the photoreceptors arrive in the visual cortex. Nonetheless, motor responses to moving objects are accurate and do not lag behind the real-world position. The questions are how and where neural delays are compensated for. It was suggested that compensation is achieved within the visual system by extrapolating the position of moving objects. A visual illusion supports this idea: when a briefly flashed object is presented in the same position as a moving object, it appears to lag behind. However, moving objects do not appear ahead of their final or reversal points. We investigated a situation where participants localized the final position of a moving stimulus. Visual perception and short-term memory of the final target position were accurate, but reaching movements were directed toward future positions of the target beyond the vanishing point. Our results show that neuronal latencies are not compensated for at early stages of visual processing, but at a late stage when retinotopic information is transformed into egocentric space used for motor responses. The sensorimotor system extrapolates the position of moving targets to allow for precise localization of moving targets despite neuronal latencies.

Humans↗

PubMed2003

Asynchronous perception of motion and luminance change.

Observers were asked to indicate when a target moving on a circular trajectory changed its luminance. The judged position of the luminance change was displaced from the true position in the direction of motion, indicating differences between the times-to-consciousness of motion and luminance change. Motion was processed faster than luminance change. The latency difference was more pronounced for a small (116-134 ms) than for a large luminance decrement (37 ms). The results show that first-order motion is perceived before an accurate representation of luminance is available. These findings are consistent with current accounts of the flash-lag effect. Two control experiments ruled out that the results were due to a general forward tendency. Localization of the target when an auditory signal was presented did not produce forward displacement, and the judged onset of motion was not shifted in the direction of motion.

Acoustic Stimulation↗

PubMed2003

Attention maintains mental extrapolation of target position: irrelevant distractors eliminate forward displacement after implied motion.

Observers' judgments of the final position of a moving target are typically shifted in the direction of implied motion ("representational momentum"). The role of attention is unclear: visual attention may be necessary to maintain or halt target displacement. When attention was captured by irrelevant distractors presented during the retention interval, forward displacement after implied target motion disappeared, suggesting that attention may be necessary to maintain mental extrapolation of target motion. In a further corroborative experiment, the deployment of attention was measured after a sequence of implied motion, and faster responses were observed to stimuli appearing in the direction of motion. Thus, attention may guide the mental extrapolation of target motion. Additionally, eye movements were measured during stimulus presentation and retention interval. The results showed that forward displacement with implied motion does not depend on eye movements. Differences between implied and smooth motion are discussed with respect to recent neurophysiological findings.

Adolescent↗