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M von Grünau

Publications and source records attributed to M von Grünau.

At least 19 recordsLinked to original sources
PubMed2000

Stimulus segmentation in the visual search task.

Four experiments were conducted in order to study the segmentation process in a visual search task with relevant stimuli (target and distractors) randomly distributed among textural elements. The basic idea was that a parallel segmentation process of the relevant stimuli would contribute to the overall reaction time independently of the contribution of the number of relevant stimuli. In the first experiment, with relevant stimuli and textural elements that differed in the orientation of their component lines, texture presence interacted with number of relevant stimuli and with target presence. These results were not favorable to the parallel segmentation hypothesis. In the second and third experiments, in which the relevant and the textural stimuli differed in orientation and in the luminance contrast of their component lines, the results support a parallel segmentation process for the higher contrast conditions. In these experiments, the effect of texture presence was greater on target-absent than on target-present trials. Experiment 4 shows that the search can be restricted to the high-contrast relevant stimuli when the number of these stimuli is constant and the number of textural stimuli changes from trial to trial. The present results suggest that the relevant stimuli can be segmented in parallel and then submitted to a restricted analysis, even when they are scattered among textural stimuli.

Adolescent↗
PubMed1997

Illusory contour-motion arising from translating terminators.

Periodic grating patterns were created by phase shifting or eliminating vertical columns of a fine line carrier grating oriented 45 deg. Motion was created by translating the patterns parallel to the carrier grating. This veridical motion was seen when terminators (i) were created in low-frequency carriers; (ii) terminated short lines; and (iii) moved slowly. In the complementary conditions an illusory contour-motion was seen perpendicular to the orientation of the terminator-defined contours. A model involving a competition between second-layer filters (encoding the orientation and motions of the terminator defined contours) and double endstopped mechanisms (signalling the presence of terminators) was developed and found to be in quantitative agreement with these data. Experiments with plaids composed of two such patterns were generally consistent with the results of the one-dimensional cases. Coherent "subjective contour plaid" motion was almost always seen when the two subjective contours had the same orientation and were perfectly phase aligned.

Form Perception↗
PubMed1996

Measuring the attentional speed-up in the motion induction effect.

Motion induction is the illusory motion within an elongated stimulus, such as a bar or a line, when it is preceded by a priming stimulus next to one of its ends. Motion is away from this primer. The presentation of two priming spots at both ends of a stimulus bar results in motion away from both spots with a collision in the center of the bar. With a sufficiently long delay between the spots, motion will be seen only as away from the second spot. Similarly, in a bar with a luminance gradient an illusory motion is perceived as away from the high-luminance end, presumably due to the known dependence of neural processing speed on luminance. In the present study, these two illusory motions were made to oppose each other. The particular luminance gradient which would just cancel the motion induction effect when motion is seen optimally as away from the second spot (cancellation gradient) was determined, resulting again in a collision near the center of the bar. Furthermore, the luminance dependence of the reaction time to stimulus detection was measured in a separate experiment. Thus for each observer, the processing time difference associated with the cancellation gradient was established. This delta t then gives the amount of time by which processing is speeded up in motion induction due to the priming spot. In a simple model of motion processing it can also be identified as the built-in delay delta t of a typical Reichardt-type motion detector. With the present conditions, it varied between 14 and 19 msec for different observers for a bar length of 5.3 deg. In this way, we show not only that the priming effect in motion induction can be understood as a speed-up of neural processing, but also provide a way of measuring the times involved. In additional experiments, we examined the effect of bar length and luminance profile. These results allow us to estimate the gradients of the attentional fields.

Attention↗
PubMed1996

Two contributions of motion induction: a preattentive effect and facilitation due to attentional capture.

By combining the paradigms of motion induction (presentation of an inducing stimulus, followed after a short delay by the presentation of an elongated bar next to it) and visual search (many-item displays with or without a pop-out target), it was possible to demonstrate the existence of two separate contributions to the motion induction effect. Illusory motion in the test bar could be produced either preattentively or by facilitation due to attentional capture. The former effect is fast, independent of the delay between the inducers and the test bar and operating simultaneously at all locations across the visual display, the latter is slower (full strength in 200-300 msec) and confined to the vicinity of the pop-out inducer. The two possibly also differ in their spatial extent, the attentional capture effect extending over a larger area around the inducer. We conclude that the motion induction effect can be used to show the existence of several effects due to the sudden presentation of a visual stimulus.

Attention↗
PubMed1995

Processing speed in the motion-induction effect.

The motion-induction effect, where an illusory motion is perceived within a bar when it is shown next to a spot presented slightly earlier, was studied with respect to the idea that it is based on differential processing speeds between the two ends of the bar. First, by using just a bar with a luminance gradient, the existence of a motion illusion (gradient motion) within such a bar was demonstrated, presumably due to the different processing speeds of differential luminances. When such a bar was used in the motion-induction effect, it was shown to modulate, for short delays, the strength of the effect up or down, according to the direction of the gradient with respect to the position of the spot. When the same bar was used in the double-motion-induction effect (split priming), in which motion is usually away from the later spot, it totally determined the perceived direction of illusory motion, independently of gradient direction with respect to the later spot or the time between the two spots. These results demonstrate, on the one hand, that differential local processing speed is a likely mechanism to underlie the motion-induction effect. On the other hand, they also suggest the involvement of other more global (and perhaps top-down) processes.

Analysis of Variance↗
PubMed1995

The detection of gaze direction: a stare-in-the-crowd effect.

A visual-search paradigm was used to explore the relative ease with which the direction of gaze can be detected. Straight-gaze stimuli were presented as targets within a variable number of distractors with left-averted or right-averted gaze. Reaction time in this case was compared with that when either the left-averted or right-averted gaze stimuli were the targets among distractors of the two remaining gaze directions. The data were examined for the existence of a search asymmetry favoring the straight-gaze targets. Such an asymmetry was found with stimuli that were realistically drawn renditions of pairs of human eyes, as well as with similar schematic stimuli representing pairs of human eyes. The asymmetry, however, was not found with geometric control stimuli, which also presented the critical feature in the central, the left-lateral, or the right-lateral position within the stimulus, but were not eyelike. It was also not found for schematic stimuli consisting of only one eye. It was concluded that the straight gaze direction is a special stimulus with eyelike stimuli, which the visual system is set up to process faster and with fewer errors than averted gaze directions. The results are discussed in terms of the evolutionary significance of the straight gaze direction.

Adult↗
PubMed1994

Intraattribute and interattribute motion induction.

The phenomenon of motion induction occurs, for example, when a bar that is presented next to a spot, which itself was presented slightly earlier, is not correctly perceived to appear everywhere simultaneously, but seems to grow out of the spot. The spot is said to prime one end of the bar. Experiments have been designed to throw more light on the local and global aspects of this phenomenon, in particular to establish whether this illusory motion percept can be observed when the spot and the bar stimuli are defined with respect to the background by one of a variety of attributes, such as luminance, color, stereodepth (crossed and uncrossed), texture, and motion (start and stop). It was found that all attribute combinations supported motion induction readily, but that the strength of the perceived motion (as measured by magnitude estimation) varied and depended more on the attribute defining the bar than on the attribute of the spot. Luminance and color gave the most vivid effects, whereas motion and depth showed the least vivid effects. The influence of the amount of luminance and color contrast on the strength of the effect was also determined and it was found that these variables affected motion induction most at very low contrast levels close to detection threshold. It is concluded that the illusory motion in this effect depends only slightly on the particular visual attribute channel that carries the stimulus information. This is consistent with the contention that it is a high-level, attention-related effect, phenomenologically similar to polarized gamma movement.

Attention↗
PubMed1994

The interaction of depth parameters in motion integration with polar plaids.

Many parameters have been investigated as to their effect on the way in which the visual system is able to integrate different motion directions at the same visual location. Of special interest have been parameters that determine the depth relationship between surfaces, such as disparity, relative contrast, and occlusion versus transparency. The preferred stimulus for this research has been the 'plaid', usually constructed from two linear gratings. The present study concentrated not on these Cartesian plaids, but on polar plaids, made from a combination of concentric circles and radial gratings. These kinds of plaids also have a special theoretical significance: within the Lie Transformation Group approach to visual pattern processing, Cartesian and polar stimuli represent different invariances in the visual world. This study compared Cartesian, polar and hybrid plaids as to their propensity to be perceived as coherently moving stimuli. Cartesian and polar plaids were similar in terms of the effects of intersection luminance and relative contrast on coherence, polar plaids being consistently less coherent. Hybrid plaids did not usually cohere at all. Adaptation to an unambiguously coherent plaid decreased perceived coherence when tested with a bistable plaid from the same, and not from the other Lie group, i.e. there was within-group adaptation but no between-group adaptation. Polar plaids also offer the possibility of studying the influence of another depth parameter on motion integration: expansion or contraction of circular gratings, which represent motion-in-depth toward or away from the observer. This motion-in-depth was tested for interaction with disparity or relative contrast in the determination of motion integration. The results were negative under the present conditions. Thus not all depth parameters contribute equally to the determination of the stimulus depth relations affecting the motion integration process.

Depth Perception↗
PubMed1994

Local and global factors of similarity in visual search.

Effects of the similarity between target and distractors in a visual search task were investigated in several experiments. Both familiar (numerals and letters) and unfamiliar (connected figures in a 5 x 5 matrix) stimuli were used. The observer had to report on the presence or absence of a target among a variable number of homogeneous distractors as fast and as accurately as possible. It was found that physical difference had the same clear effect on processing time for familiar and for unfamiliar stimuli: processing time decreased monotonically with increasing physical difference. Distractors unrelated to the target and those related to the target by a simple transformation (180 degrees rotation, horizontal or vertical reflection) were also compared, while the physical difference was kept constant. For familiar stimuli, transformational relatedness increased processing time in comparison with that for unrelated stimulus pairs. It was further shown in a scaling experiment that this effect could be accounted for by the amount of perceived similarity of the target-distractor pairs. For unfamiliar stimuli, transformational relatedness did have a smaller and less pronounced effect. Various comparable unrelated distractors resulted in a full range of processing times. Results from a similarity scaling experiment correlated well with the outcome of the experiments with unfamiliar stimuli. These results are interpreted in terms of an underlying continuum of perceived similarity as the basis of the speed of visual search, rather than a dichotomy of parallel versus serial processing.

Adult↗
PubMed1994

Visual search asymmetry for viewing direction.

In visual search experiments, we examined the existence of a search asymmetry for the direction with which three-dimensional objects are viewed. It was found that an upward-tilted target object among downward-tilted distracting objects was detected faster than when the orientation of target and distractors was reversed. This indicates that the early visual process regards objects tilted downward with respect to the observer as the situation that is more likely to be encountered. That is, the system is set up to expect to see the tops of these objects. We also found a visual field anisotropy, in that the asymmetry was more pronounced in the lower visual field. These findings are consistent with the idea that the tops of objects are usually situated in the lower visual field and less often in the upper field. Examination of the conditions under which the asymmetry and the anisotropy occur demonstrated the importance of the three-dimensional nature of the stimulus objects. Early visual processing thus makes use of heuristics that take into account specific relationships between the relative locations in space of the observer and 3-D objects.

Attention↗
PubMed1993

Ambiguous plaids: switching between coherence and transparency.

A plaid pattern consisting of two differently oriented moving gratings can be seen as two alternative percepts: transparency, in which the two gratings are seen to slide over each other in their respective directions, or coherence, in which one integrated pattern (the plaid) is seen to move in a new direction. With prolonged inspection, an observer switches between these two alternatives. It was found here that adaptation to unambiguous coherence reduces the time that coherence is seen with an ambiguous test stimulus. Similarly, adaptation to transparency reduces the time transparency is seen. Analysis of the duration of consecutive episodes revealed that the underlying processes are adapted independently. Control experiments confirmed that adaptation occurred to the coherent plaid and not to the intersections, and that this adaptation was not simply a directional motion aftereffect. It is concluded that switching occurs between motion processes at different cortical levels that can be adapted independently.

Adaptation, Ocular↗
PubMed1993

The effect of disparity on motion coherence.

Many moving plaid stimuli are ambiguous, and perception switches between a coherent plaid pattern and two transparent gratings. Here, experiments are reported that examined the effect of stereodepth between the two gratings of the moving plaid stimulus on the perception of coherence or motion transparency. Increasing disparity increased the percentage of time that two independently drifting transparent gratings were perceived. This was studied for plaids with various levels of intersection luminance. Using intersection luminances beyond conditions of physical transparency increased the percentage of time that one coherent plaid was seen. These two opposing influences could be pitted against each other to achieve constant levels of coherence. An adaptation paradigm was also used in which observers adapted to a stationary stimulus with either zero, crossed or uncrossed disparity between the gratings, and then indicated the occurrence of coherence and motion transparency in test stimuli of drifting plaids with zero, crossed or uncrossed disparity. Adaptation to crossed and uncrossed stereo-depth increased relative perceived coherence equally, especially for zero test disparity. An analysis of the length of the episodes of coherence and motion transparency indicated that the effect of adaptation was to decrease the length of motion transparency episodes, while the length of coherence episodes did not change. It is concluded that mechanisms involved in the processing of stereo-depth must have an input to the integration stage of the motion channel and that pattern and component motion mechanisms can operate quite independently.

Adaptation, Ocular↗
PubMed1992

Comparing local and remote motion aftereffects.

A new method, using phase-reversing sinusoidal gratings to cancel perceived motion, was developed to measure the motion aftereffect (MAE). This technique was used to show the existence of a remote MAE, i.e. an MAE in areas that were not directly stimulated during adaptation. In several experiments, this remote MAE was compared to the local MAE. The remote effect was generally weaker and of shorter duration. It showed no directional tuning within the investigated range, as compared to a tuning of +/- 60 deg of the local MAE. There was no adaptation effect to the component gratings of a plaid, indicating that the plaid was treated as a coherent pattern. The local MAE showed clear spatial frequency tuning, whereas the remote MAE varied little with spatial frequency difference, although there was a tendency towards frequencies lower than the adaptation frequency. The possibility is considered that both local and remote MAEs are generated in extrastriate areas.

Adaptation, Ocular↗
PubMed1989

Interattribute apparent motion.

Apparent motion can be seen between two alternating stimuli even if they are defined with respect to their background by attributes other than luminance (such as color, or texture). We measured motion strength as the maximum separation between two alternating stimuli which produced an impression of motion, for conditions in which the two stimuli were defined by the same attribute (intra-attribute) as well as conditions in which they were defined by different attributes (interattribute). The attributes used to define the stimuli were luminance, color, texture, relative motion, or stereopsis. The results indicate that motion was seen for all the intra-attribute conditions about equally well. The results also show that interattribute motion could be seen for all combinations studied. The motion strength in these cases was about 80% of that for the intra-attribute conditions. The process responsible for this motion perception must therefore be able to combine information from different attributes.

Adult↗
PubMed1986

Differences of visual field representation in the medial and lateral banks of the suprasylvian cortex (PMLS/PLLS) of the cat.

We have studied the orderliness of representation of visual space in the medial and lateral banks of the middle suprasylvian sulcus. Penetrations were made either parallel to the sulcus, in one bank or the other, or vertical, thus crossing the sulcus between the postero-medial (PMLS) and posterolateral (PLLS) divisions of this area. In some cases we found clear evidence for topographical order in the representation of the visual field with a tendency (greater in PMLS than in PLLS) for the receptive fields of cells recorded deeper in the walls of the sulcus to lie closer to the area centralis, but along many penetrations the receptive fields were so large and so scattered that no retinotopic arrangement could be discerned. In PMLS the receptive fields of the majority of units we studied were centered below and close to the horizontal meridian, whereas in PLLS they were distributed over both the upper and lower visual fields with an over-representation of the upper field. Receptive fields were significantly larger in PLLS (mean field area = 442.2 deg2) than in PMLS (mean area = 154.4 deg2); there was also less clear correlation between receptive field size and eccentricity in PLLS (correlation coefficient = +0.25) than in PMLS (corr. coeff. = +0.72). Analysis of the distance between the receptive field centres of consecutively recorded units demonstrated that the mean scatter in both PMLS and PLLS amounts to about half the average receptive field diameter. In summary the topographical representation of visual space is less orderly in PLLS, and may involve a wider area of the visual field. These findings may relate to the segregated visual cortical and extrageniculate thalamic connections that the medial and lateral banks of the LS receive.

Animals↗
PubMed1983

Double-opponent-process mechanism underlying RF-structure of directionally specific cells of cat lateral suprasylvian visual area.

For the experiments reported in this study, recordings were obtained from 246 single units in the middle lateral suprasylvian visual area (LS) of 13 cats. 49 of these cells were subjected to detailed quantitative analysis. The receptive field (RF) organization was examined for directionally specific cells by presenting moving single spots on large moving random dot backgrounds. A cell's response to an optimal spot (in terms of size, direction, velocity) moving on a stationary background inside the excitatory RF (ERF) was compared to in-phase (same direction, same velocity) and anti-phase (opposite direction, same velocity) movement of spot and background. In-phase movement resulted in inhibition of the cell's response (3-100%) in 94% of the cells, while anti-phase movement led to reduced inhibition in 52% of the cells or to facilitation (0.5-327%) in 39% of the cells. By changing the direction of background motion with respect to that of the spot, the directional tuning of the in-phase inhibition and anti-phase facilitation effects was determined. We were able to manipulate the size of the background effects by masking out the background for various proportions of the ERF, and maximizing them by restricting background stimulation to the large inhibitory RF (IRF) surrounding the ERF. These results could be best accounted for by a double-opponent-process mechanism with both RF center and RF surround being directionally selective, but with opposite polarity. It is suggested that this type of mechanism could be involved in the processing of object motion.

Animals↗
PubMed1980

Functional amblyopia in kittens with unilateral exotropia. I. Electrophysiological assessment.

In two cats in which surgically induced, unilateral divergent strabismus had led to behaviourally determined amblyopia, a variety of electrophysiological parameters were determined in search of neuronal correlates of squint amblyopia. Tests that assess global neuronal excitability along the pathways from the two eyes to the visual cortex (areas 17 and 18) failed to reflect the functional inferiority of the squinting eye: retinographic responses and cortical evoked potentials elicited by Ganzfeld-stimulation and by stimulation of the optic nerves were identical for the two eyes. The ocular dominance distribution of neurons in area 17 showed the expected disruption of binocularity but failed to provide clear evidence for a functional inferiority of the squinting eye. At other levels of analysis, however, a clear difference between the two eyes was apparent: 1. Responses to optimally aligned light stimuli tended to be more sluggish and the under-representation of neurons with vertically oriented receptive fields was more pronounced in neurons driven from the deviated eye than in cells dominated by the normal one. 2. Interocular inhibition as assessed from electrically evoked potentials was found to be asymmetric; responses evoked from the amblyopic eye were suppressed more readily and over longer periods by conditioning shocks applied to the normal nerve than vice versa. 3. Numerous abnormalities reflecting the functional inferiority of the squinting eye became apparent in cortical potentials evoked by phase reversal of gratings of variable spatial frequency and contrast. A laminar analysis of these field potentials suggests impaired transmission along the intracortical pathways which relay activity to supragranular layers as a major cause for abnormal responses from the squinting eye. It is concluded that squint amblyopia is associated with a variety of neuronal changes at various levels of the visual levels of the visual system, the present data providing evidence for alterations at the cortical level.

Amblyopia↗