Biomedical subjects

Raymond H Cuijpers

Publications and source records attributed to Raymond H Cuijpers.

4 recordsLinked to original sources

PubMed2006

Grasping reveals visual misjudgements of shape.

There are many conditions in which the visually perceived shape of an object differs from its true shape. We here show that one can reveal such errors by studying grasping. Nine subjects were asked to grasp and lift elliptical cylinders that were placed vertically at eye height. We varied the cylinder's aspect ratios, orientations about the vertical axis and distances from the subject. We found that the subjects' grip orientations deviated systematically from the orientations that would give the mechanically optimal grip. That this is largely due to misjudging the cylinder's shape (rather than to selecting a comfortable posture) follows from the fact that the grip aperture was initially more strongly correlated with the maximal grip aperture (which is related to the expected contact positions) than with the final grip aperture (which is determined by the real contact positions). The correlation with the maximal grip aperture drops from 0.8 to 0.6 in the last 1% of the traversed distance (11% of movement time), showing that the grip aperture was anticipated incorrectly (it is automatically "corrected" at contact). The grip orientation was already strongly correlated with the grip orientation at the time of maximal grip aperture, half way through the movement (R > or = 0.7), showing that the suboptimal grip orientations were planned that way. We conclude that subjects plan their grasps using information that is based on the misperceived shape.

Form Perception↗

PubMed2006

Goals and means in action observation: a computational approach.

Many of our daily activities are supported by behavioural goals that guide the selection of actions, which allow us to reach these goals effectively. Goals are considered to be important for action observation since they allow the observer to copy the goal of the action without the need to use the exact same means. The importance of being able to use different action means becomes evident when the observer and observed actor have different bodies (robots and humans) or bodily measurements (parents and children), or when the environments of actor and observer differ substantially (when an obstacle is present or absent in either environment). A selective focus on the action goals instead of the action means furthermore circumvents the need to consider the vantage point of the actor, which is consistent with recent findings that people prefer to represent the actions of others from their own individual perspective. In this paper, we use a computational approach to investigate how knowledge about action goals and means are used in action observation. We hypothesise that in action observation human agents are primarily interested in identifying the goals of the observed actor's behaviour. Behavioural cues (e.g. the way an object is grasped) may help to disambiguate the goal of the actor (e.g. whether a cup is grasped for drinking or handing it over). Recent advances in cognitive neuroscience are cited in support of the model's architecture.

Attention↗

PubMed2004

On the relation between object shape and grasping kinematics.

Despite the many studies on the visual control of grasping, little is known about how and when small variations in shape affect grasping kinematics. In the present study we asked subjects to grasp elliptical cylinders that were placed 30 and 60 cm in front of them. The cylinders' aspect ratio was varied systematically between 0.4 and 1.6, and their orientation was varied in steps of 30 degrees. Subjects picked up all noncircular cylinders with a hand orientation that approximately coincided with one of the principal axes. The probability of selecting a given principal axis was the highest when its orientation was equal to the preferred orientation for picking up a circular cylinder at the same location. The maximum grip aperture was scaled to the length of the selected principal axis, but the maximum grip aperture was also larger when the length of the axis orthogonal to the grip axis was longer than that of the grip axis. The correlation between the grip aperture--or the hand orientation--at a given instant, and its final value, increased monotonically with the traversed distance. The final hand orientation could already be inferred from its value after 30% of the movement distance with a reliability that explains 50% of the variance. For the final grip aperture, this was only so after 80% of the movement distance. The results indicate that the perceived shape of the cylinder is used for selecting appropriate grasping locations before or early in the movement and that the grip aperture and orientation are gradually attuned to these locations during the movement.

Biomechanical Phenomena↗

PubMed2002

Illusions in action: consequences of inconsistent processing of spatial attributes.

Many authors have performed experiments in which subjects grasp objects in illusory surroundings. The vast majority of these studies report that illusions affect the maximum grip aperture less than they affect the perceived size. This observation has frequently been regarded as experimental evidence for separate visual systems for perception and action. In order to make this conclusion, one assumes that the grip aperture is based on a visual estimate of the object's size. We believe that it is not, and that this is why size illusions fail to influence grip aperture. Illusions generally do not affect all aspects of space perception in a consistent way, but mainly affect the perception of specific spatial attributes. This applies not only to object size, but also to other spatial attributes such as position, orientation, displacement, speed, and direction of motion. Whether an illusion influences the execution of a task will therefore depend on which spatial attributes are used rather than on whether the task is perceptual or motor. To evaluate whether illusions affect actions when they influence the relevant spatial attributes we review experimental results on various tasks with inconsistent spatial processing in mind. Doing so shows that many actions are susceptible to visual illusions. We argue that the frequently reported differential effect of illusions on perceptual judgements and goal-directed action is caused by failures to ensure that the same spatial attributes are used in the two tasks. Illusions only affect those aspects of a task that are based on the spatial attributes that are affected by the illusion.

Cues↗