Gregor Hardiess

Head and eye movements and the role of memory limitations in a visual search paradigm.

The image information guiding visual behavior is acquired and maintained in an interplay of gaze shifts and visual short-term memory (VSTM). If storage capacity of VSTM is exhausted, gaze shifts can be used to regain information not currently represented in memory. By varying the separation between relevant image regions, S. Inamdar and M. Pomplun (2003) demonstrated a trade-off between VSTM storage and gaze shifts, which were performed as pure eye movements, that is, without a head movement component. Here we extend this paradigm to larger gaze shifts involving both eye and head movements. We use a comparative visual search paradigm with two relevant image regions and region separation as independent variable. Image regions were defined by two cupboards displaying colored geometrical objects in roughly equal arrangements. Subjects were asked to find differences in the arrangement of the objects in the two cupboards. Cupboard separation was varied between 30 degrees and 120 degrees . Images were presented with two projectors on a 150 degrees x 70 degrees curved screen. Head and eye movements were simultaneously recorded with an ART head tracker and an ASL mobile eye tracker, respectively. In the large separation conditions, the number of gaze shifts between the two cupboards was reduced, while fixation duration increased. Furthermore, the head movement proportions negatively correlated with the number of gaze shifts and positively correlated with fixation duration. We conclude that the visual system uses increased VSTM involvement to avoid gaze movements and in particular movements of the head. Scan path analysis revealed two subject-specific strategies (encode left, compare right, and vice versa), which were consistently used in all separation conditions.

The pupillary light reflex pathway Cytoarchitectonic probabilistic maps in hemianopic patients

Objective: The anatomy of the human pupillary light reflex (PLR) pathway is a matter of debate. The aim of this study was twofold: namely, to investigate the association of a relative afferent pupillary defect (RAPD) in acquired suprageniculate lesions with the location and extent of the cerebral lesions. Further, we suggest a new strategy of lesion analysis by combining established techniques with the stereotaxic probabilistic cytoarchitectonic atlas developed by the Jülich group. Methods: Twenty-three patients with homonymous visual field defects participated in this study. The RAPD was quantified clinically by two independent examiners with graded neutral density filters (swinging flashlight test). Using MRI in each individual, cerebral regions commonly affected in patients with a RAPD but spared in patients without a RAPD were determined and subsequently assessed by using cytoarchitectonic probabilistic maps. Results: A RAPD was present in 10/23 patients. Comparison of patients showing a RAPD vs those not showing a RAPD revealed that a region including the course of the optic radiation at its early beginning in the temporal white matter is commonly associated with a RAPD. Conclusions: It was demonstrated that the pupillary light reflex (PLR) depends on the input of suprageniculate neurons, thus supporting the involvement of a cortical pathway also. The site of integration of cortical signals in relation to the PLR into the pupillomotor pathway may be located suprageniculately in the vicinity of the lateral geniculate nucleus. Moreover, the suggested combination of established lesion analysis techniques with the probabilistic cytoarchitectonic atlas turned out to be a very helpful amelioration of stroke data analyses.

Collision avoidance in persons with homonymous visual field defects under virtual reality conditions

The aim of the present study was to examine the effect of homonymous visual field defects (HVFDs) on collision avoidance of dynamic obstacles at an intersection under virtual reality (VR) conditions. Overall performance was quantitatively assessed as the number of collisions at a virtual intersection at two difficulty levels. HVFDs were assessed by binocular semi-automated kinetic perimetry within the 90° visual field, stimulus III4e and the area of sparing within the affected hemifield (A-SPAR in deg(2)) was calculated. The effect of A-SPAR, age, gender, side of brain lesion, time since brain lesion and presence of macular sparing on the number of collisions, as well as performance over time were investigated. Thirty patients (10 female, 20 male, age range: 19-71 years) with HVFDs due to unilateral vascular brain lesions and 30 group-age-matched subjects with normal visual fields were examined. The mean number of collisions was higher for patients and in the more difficult level they experienced more collisions with vehicles approaching from the blind side than the seeing side. Lower A-SPAR and increasing age were associated with decreasing performance. However, in agreement with previous studies, wide variability in performance among patients with identical visual field defects was observed and performance of some patients was similar to that of normal subjects. Both patients and healthy subjects displayed equal improvement of performance over time in the more difficult level. In conclusion, our results suggest that visual-field related parameters per se are inadequate in predicting successful collision avoidance. Individualized approaches which also consider compensatory strategies by means of eye and head movements should be introduced.

The neural correlates of impaired collision avoidance in hemianopic patients

Purpose:  The aim of this study was to assess the brain regions associated with impaired performance in a virtual, dynamic collision avoidance task, in a group of patients with homonymous visual field defects (HVFDs) because of unilateral vascular brain lesions.

View-based organization and interplay of spatial working and long-term memories.

Space perception provides egocentric, oriented views of the environment from which working and long-term memories are constructed. “Allocentric” (i.e. position-independent) long-term memories may be organized as graphs of recognized places or views but the interaction of such cognitive graphs with egocentric working memories is unclear. Here we present a simple coherent model of view-based working and long-term memories, together with supporting evidence from behavioral experiments. The model predicts that within a given place, memories for some views may be more salient than others, that imagery of a target square should depend on the location where the recall takes place, and that recall favors views of the target square that would be obtained when approaching it from the current recall location. In two separate experiments in an outdoor urban environment, pedestrians were approached at various interview locations and asked to draw sketch maps of one of two well-known squares. Orientations of the sketch map productions depended significantly on distance and direction of the interview location from the target square, i.e. different views were recalled at different locations. Further analysis showed that location-dependent recall is related to the respective approach direction when imagining a walk from the interview location to the target square. The results are consistent with a view-based model of spatial long-term and working memories and their interplay.

Gaze movements and spatial working memory in collision avoidance: a traffic intersection task

Street crossing under traffic is an everyday activity including collision detection as well as avoidance of objects in the path of motion. Such tasks demand extraction and representation of spatio-temporal information about relevant obstacles in an optimized format. Relevant task information is extracted visually by the use of gaze movements and represented in spatial working memory. In a virtual reality traffic intersection task, subjects are confronted with a two-lane intersection where cars are appearing with different frequencies, corresponding to high and low traffic densities. Under free observation and exploration of the scenery (using unrestricted eye and head movements) the overall task for the subjects was to predict the potential-of-collision (POC) of the cars or to adjust an adequate driving speed in order to cross the intersection without collision (i.e., to find the free space for crossing). In a series of experiments, gaze movement parameters, task performance, and the representation of car positions within working memory at distinct time points were assessed in normal subjects as well as in neurological patients suffering from homonymous hemianopia. In the following, we review the findings of these experiments together with other studies and provide a new perspective of the role of gaze behavior and spatial memory in collision detection and avoidance, focusing on the following questions: (1) which sensory variables can be identified supporting adequate collision detection? (2) How do gaze movements and working memory contribute to collision avoidance when multiple moving objects are present and (3) how do they correlate with task performance? (4) How do patients with homonymous visual field defects (HVFDs) use gaze movements and working memory to compensate for visual field loss? In conclusion, we extend the theory of collision detection and avoidance in the case of multiple moving objects and provide a new perspective on the combined operation of external (bottom-up) and internal (top-down) cues in a traffic intersection task.

Acquisition vs. Memorization Trade-Offs Are Modulated by Walking Distance and Pattern Complexity in a Large-Scale Copying Paradigm

In a “block-copying paradigm”, subjects were required to copy a configuration of colored blocks from a model area to a distant work area, using additional blocks provided at an equally distant resource area. Experimental conditions varied between the inter-area separation (walking distance) and the complexity of the block patterns to be copied. Two major behavioral strategies were identified: in the memory-intensive strategy, subjects memorize large parts of the pattern and rebuild them without intermediate visits at the model area. In the acquisition-intensive strategy, subjects memorize one block at a time and return to the model after having placed this block. Results show that the frequency of the memory-intensive strategy is increased for larger inter-area separations (larger walking distances) and for simpler block patterns. This strategy-shift can be interpreted as the result of an optimization process or trade-off, minimizing combined, condition-dependent costs of the two strategies. Combined costs correlate with overall response time. We present evidence that for the memory-intensive strategy, costs correlate with model visit duration, while for the acquisition-intensive strategy, costs correlate with inter-area transition (i.e., walking) times.

Task-dependent representation of moving objects within working memory in obstacle avoidance.

Purpose: The purpose of the study was the quantitative analysis of the working memory representation of dynamic objects related to gaze movement behavior. Methods: Eighteen subjects participated in a virtual street-crossing paradigm. The primary task was collisions avoidance. To investigate the representation format, during a sub-task subjects were asked to reconstruct the traffic scene from memory. Results: The distribution of cars positioned during the sub-task reveals a task-dependent (i.e., collision-relevant) representation of about four cars. In contrast, analysis of gaze behavior did not show a preference for collision-prone cars. Conclusion: Subjects avoided collisions efficiently by applying a gaze strategy adequate to create a representation that fulfills the demands of the task. Collision-prone cars are more likely to be represented in memory, but not more likely to be fixated.

The perception of ego-motion change in environments with varying depth: Interaction of stereo and optic flow.

When estimating ego-motion in environments (e.g., tunnels, streets) with varying depth, human subjects confuse ego-acceleration with environment narrowing and ego-deceleration with environment widening. Festl, Recktenwald, Yuan, and Mallot (2012) demonstrated that in nonstereoscopic viewing conditions, this happens despite the fact that retinal measurements of acceleration rate-a variable related to tau-dot-should allow veridical perception. Here we address the question of whether additional depth cues (specifically binocular stereo, object occlusion, or constant average object size) help break the confusion between narrowing and acceleration. Using a forced-choice paradigm, the confusion is shown to persist even if unambiguous stereo information is provided. The confusion can also be demonstrated in an adjustment task in which subjects were asked to keep a constant speed in a tunnel with varying diameter: Subjects increased speed in widening sections and decreased speed in narrowing sections even though stereoscopic depth information was provided. If object-based depth information (stereo, occlusion, constant average object size) is added, the confusion between narrowing and acceleration still remains but may be slightly reduced. All experiments are consistent with a simple matched filter algorithm for ego-motion detection, neglecting both parallactic and stereoscopic depth information, but leave open the possibility of cue combination at a later stage.

Modality dependence and intermodal transfer in the Corsi Spatial Sequence Task: Screen vs. Floor.

Abstract Four versions of the Corsi Spatial Sequence Task (CSST) were tested in a complete within-subject design, investigating whether participants’ performance depends on the modality of task presentation and reproduction that put different demands on spatial processing. Presentation of the sequence (encoding phase) and the reproduction (recall phase) were each carried out either on a computer screen or on the floor of a room, involving actual walking in the recall phase. Combinations of the two different encoding and recall procedures result in the modality conditions Screen–Screen, Screen–Floor, Floor–Screen, and Floor–Floor. Results show the expected decrease in performance with increasing sequence length, which is likely due to processing limitations of working memory. We also found differences in performance between the modality conditions indicating different involvements of spatial working memory processes. Participants performed best in the Screen–Screen modality condition. Floor–Screen and Floor–Floor modality conditions require additional working memory resources for reference frame transformation and spatial updating, respectively; the resulting impairment of the performance was about the same in these two conditions. Finally, the Screen–Floor modality condition requires both types of additional spatial demands and led to the poorest performance. Therefore, we suggest that besides the well-known spatial requirements of CSST, additional working memory resources are demanded in walking CSST supporting processes such as spatial updating, mental rotation, reference frame transformation, and the control of walking itself.