Constanze Hesse

Memory mechanisms in grasping

The availability of visual information influences the execution of goal-directed movements. This is very prominent in memory conditions, where a delay is introduced between stimulus presentation and execution of the movement. The corresponding effects could be due to a decay of the visual information or to different processing mechanisms used for movements directed at visible (dorsal stream) and remembered (ventral stream) objects as proposed by the two visual systems hypothesis. In three experiments, the authors investigated grasping under full vision and three different delay conditions with increasing memory demands. Results indicate that the visuomotor information used for grasping decays rapidly. No evidence was found for qualitative changes in movement kinematics and the use of different representations for visually guided and memory guided movements. Findings rather suggest that delayed grasping is similar to grasping directed to larger objects under full vision. Therefore, the authors propose that grasping after a delay is guided by classic memory mechanisms and that this is reflected in an increasing maximum grip aperture in grasping.

The functional subdivision of the visual brain: Is there a real illusion effect on action? A multi-lab replication study

It has often been suggested that visual illusions affect perception but not actions such as grasping, as predicted by the two-visual-systems hypothesis of Milner and Goodale (1995, The Visual Brain in Action, Oxford University press). However, at least for the Ebbinghaus illusion, relevant studies seem to reveal a consistent illusion effect on grasping (Franz & Gegenfurtner, 2008. Grasping visual illusions: consistent data and no dissociation. Cognitive Neuropsychology). Two interpretations are possible: either grasping is not immune to illusions (arguing against dissociable processing mechanisms for vision-for-perception and vision-for-action), or some other factors modulate grasping in ways that mimic a vision-for perception effect in actions. It has been suggested that one such factor may be obstacle avoidance (Haffenden Schiff & Goodale, 2001. The dissociation between perception and action in the Ebbinghaus illusion: nonillusory effects of pictorial cues on grasp. Current Biology, 11, 177-181). In four different labs (total Nxa0=xa0144), we conducted an exact replication of previous studies suggesting obstacle avoidance mechanisms, implementing conditions that tested grasping as well as multiple perceptual tasks. This replication was supplemented by additional conditions to obtain more conclusive results. Our results confirm that grasping is affected by the Ebbinghaus illusion and demonstrate that this effect cannot be explained by obstacle avoidance.

Visuomotor performance based on peripheral vision is impaired in the visual form agnostic patient DF

The perception-action model states that visual information is processed in different cortical areas depending on the purpose for which the information is acquired. Specifically, it was suggested that the ventral stream mediates visual perception, whereas the dorsal stream primarily processes visual information for the guidance of actions (Goodale & Milner, 1992). Evidence for the model comes from patient studies showing that patients with ventral stream damage (visual form agnosia) and patients with dorsal stream damage (optic ataxia) show divergent performance in action and perception tasks. Whereas DF, a patient suffering from visual form agnosia, was found to perform well in visuomotor tasks despite her inability to use vision for perceptual tasks, patients with optic ataxia show usually the opposite pattern, i.e. good perception but impaired visuomotor control. The finding that both disorders seem to provoke a mirror-reversed pattern of spared and impaired visual functions, led to the belief that optic ataxia and visual form agnosia can be considered as complementary disorders. However, the visuomotor performance of patients with optic ataxia is typically only impaired when they are tested in visual periphery while being often preserved when tested in central vision. Here, we show that DFs visuomotor performance is also only preserved when the target is presented centrally. Her reaching and grasping movements to targets in peripheral vision are abnormal. Our findings indicate that DFs visuomotor performance is quite similar to the visuomotor performance of patients with optic ataxia which undermines previous suggestions that the two disorders form a double-dissociation.

Grasping remembered objects: Exponential decay of the visual memory

The accuracy with which goal-directed movements are executed depends substantially on the availability of accurate visuomotor information. When no visual information is available during movement execution, movement kinematics change and become more variable, indicating that the visual information about the movement environment is stored for a restricted period of time. However, little is known about the underlying decay characteristics. In this study we investigated how increasing memory demands change the kinematics of a grasping movement and whether these alterations reflect a continuous or an abrupt decay of the underlying visuomotor memory. Ten participants grasped differently sized objects under a full vision condition and four different delay conditions. Results show that the visuomotor information used for grasping decays rapidly after visual occlusion. The information decay over time became obvious in a decrease of movement accuracy and an increase in movement variability that were both well described by exponential decay models. Our findings suggest that visuomotor information is represented in some sort of short-term memory showing the same decay characteristics as observed in classical memory research.

Fixation locations when grasping partly occluded objects

When grasping an object, subjects tend to look at the contact positions of the digits (A. M. Brouwer, V. H. Franz, D. Kerzel, & K. R. Gegenfurtner, 2005; R. S. Johansson, G. Westling, A. Bäckström, & J. R. Flanagan, 2001). However, these contact positions are not always visible due to occlusion. Subjects might look at occluded parts to determine the location of the contact positions based on extrapolated information. On the other hand, subjects might avoid looking at occluded parts since no object information can be gathered there. To find out where subjects fixate when grasping occluded objects, we let them grasp flat shapes with the index finger and thumb at predefined contact positions. Either the contact position of the thumb or the finger or both was occluded. In a control condition, a part of the object that does not involve the contact positions was occluded. The results showed that subjects did look at occluded object parts, suggesting that they used extrapolated object information for grasping. Additionally, they preferred to look in the direction of the index finger. When the contact position of the index finger was occluded, this tendency was inhibited. Thus, an occluder does not prevent fixations on occluded object parts, but it does affect fixation locations especially in conditions where the preferred fixation location is occluded.

Corrective processes in grasping after perturbations of object size.

Researchers proposed that humans may achieve grip adaptation to a new object size by reprogramming and substituting the initially planned motor program. The authors investigated corrective processes in grasping by using a size perturbation paradigm. In 3 experiments, they investigated how grip adjustments are influenced by different perturbation times (early or late), the visibility of the moving hand, and different perturbation sizes (small or large). Results indicated that individuals execute corrections faster after late perturbations. The availability of visual information about the hand had minimal effect on the corrections, suggesting that feedforward mechanisms are involved. Moreover, participants achieved adjustments mainly by smooth changes of the aperture over time, contradicting the researchers assumption that a new movement is programmed and superimposed.

Planning movements well in advance

It has been suggested that the metrics of grasping movements directed to visible objects are controlled in real time and are therefore unaffected by previous experience. We tested whether the properties of a visually presented distractor object influence the kinematics of a subsequent grasping movement performed under full vision. After viewing an elliptical distractor object in one of two different orientations participants grasped a target object, which was either the same object with the same orientation or a circular object without obvious orientation. When grasping the circular target, grip orientation was influenced by the orientation of the distractor. Moreover, as in classical visuomotor priming, grasping movements were initiated faster when distractor and target were identical. Results provide evidence that planning of visually guided grasping movements is influenced by prior perceptual experience, challenging the notion that metric aspects of grasping are controlled exclusively on the basis of real-time information.

Advance planning in sequential pick-and-place tasks.

It has been suggested that the kinematics of a reach-to-grasp movement, performed within an action sequence, vary depending on the action goal and the properties of subsequent movement segments (action context effect). The aim of this study was to investigate whether the action context also affects action sequences that consist of several grasping movements directed toward different target objects. Twenty participants were asked to perform a sequence in which they grasped a cylinder, placed it into a target area, and subsequently grasped and displaced a target bar of a certain orientation. We specifically tested whether the orientation of the target bar being grasped in the last movement segment influenced the grip orientation adapted to grasp and place the cylinder in the preceding segments. When all movement segments within the sequence were easy to perform, results indeed showed that grip orientation chosen in the early movement segments depended on the forthcoming motor demands, suggesting a holistic planning process. In contrast, high accuracy demands in specifying a movement segment reduced the ability of the motor system to plan and organize the movement sequence into larger chunks, thus causing a shift toward sequential performance. Additionally, making the placing task more difficult resulted in prolonged reaction times and increased the movement times of all other movement segments.

Contact points during multidigit grasping of geometric objects

We investigated the choice of contact points during multidigit grasping of different objects. In Experiment 1, cylinders were grasped and lifted. Participants were either instructed as to the number of fingers they should use, ranging from a two-finger grasp to a five-finger grasp, or could grasp with their preferred number of fingers. We found a strong relationship between the position of the fingertips on the object and the number of fingers used. In general, variability in the choice of contact points was low within- as well as between participants. The virtual finger, defined as the geometric mean position of fingers opposing the thumb, was in almost perfect opposition to the thumb, suggesting the formation of a functional unit using all contributing fingers in the grasp. In Experiment 2, four more complex shapes (rectangle, hexagon, pentagon, curved object) were grasped. Although we found some moderate between-participant variability in the choice of contact points, the within-participant variability was again remarkably low. In both experiments, participants showed a strong preference to use four or five fingers during grasping when left with free choice. Taken together, our findings suggest a preplanning of the grasping movement and that grasping results from a coordinated interplay between the fingers contributing to the grasp that cannot be understood as independent digit movements.

Biomechanical factors may explain why grasping violates Weber’s law

For grasping, Ganel, Chajut, and Algom (2008) demonstrated that the variability of the maximum grip aperture (MGA) does not increase with the size of the target object. This seems to violate Webers law, a fundamental law of psychophysics. They concluded that the visual representations guiding grasping are distinct from representations used for perceptual judgments. Webers law is however only relevant for one component of the measurable variability of MGA, namely the variability in the sensory system. We argue that when looking at the relationship between object size and grasping, the gain (often called slope) governing the relationship between target size and MGA can be used as an approximation to estimate the contribution of sensory noise to MGA variability. To test the idea that differences in gain modulate the relationship between target size and MGA variability, we examined grasping under a variety of conditions. We found that gain varied quite significantly across different tasks, but irrespective of gain Webers law could not be found in any of the grasping tasks. Instead we repeatedly found an inverse relationship between variability and object size, i.e. variability decreased for bigger objects. This trend may reflect the reduced biomechanical freedom found for movements at the end an effectors effective range of motion. MGA variability may thus be dominated by non-sensory factors and therefore may constitute a poor choice to estimate the variability of the visual signals used by the brain to guide our grasping actions.