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PubMed2001

The Duncker illusion and eye-hand coordination.

A moving background alters the perceived direction of target motion (the Duncker illusion). To test whether this illusion also affects pointing movements to remembered/extrapolated target locations, we constructed a display in which a target moved in a straight line and disappeared behind a band of moving random dots. Subjects were required to touch the spot where the target would emerge from the occlusion. The four directions of random-dot motion induced pointing errors that were predictable from the Duncker illusion. Because it has been previously established that saccadic direction is influenced by this illusion, gaze was subsequently recorded in a second series of experiments while subjects performed the pointing task and a similar task with eye-tracking only. In the pointing task, subjects typically saccaded to the lower border of the occlusion zone as soon as the target disappeared and then tried to maintain fixation at that spot. However, it was particularly obvious in the eye-tracking-only condition that horizontally moving random dots generally evoked an appreciable ocular following response, altering the gaze direction. Hand-pointing errors were related to the saccadic gaze error but were more highly correlated with final gaze errors (resulting from the initial saccade and the subsequent ocular following response). The results suggest a model of limb control in which gaze position can provide the target signal for limb movement.

Fixation, Ocular↗
PubMed2001

Throwing accuracy in the vertical direction during prism adaptation: not simply timing of ball release.

In a previous study, others have hypothesized that the variance in vertical errors that occurs while throwing at visual targets is caused by changes in any of three throw parameters: hand location in space, hand translational velocity, and hand orientation. From an analysis of skilled throwers, those authors concluded that vertical error is best correlated with variance in hand orientation, which in turn is related to the timing of ball release. We used a vertical prism adaptation paradigm to investigate which of these throwing parameters subjects use when adapting to external perturbation. Our subjects showed no correlation between hand position or hand translational velocity and ball impact height in normal, over-practiced throwing. However, video-based motion analysis showed that modifications both of position and speed of the hand play an important role when subjects are forced to compensate for a vertically shifting prism perturbation during a dart-like throw (these factors contribute approximately 30% of the adaptation). We concluded that, during adaptation, more degrees of freedom and more sources of potential error are modified to achieve the gaze-throw recalibration required to hit the target than are employed in this type of throw during normal conditions.

Adaptation, Physiological↗
PubMed2002

Dynamic coordination of body parts during prism adaptation.

We studied coordination across body parts in throwing during adaptation to prisms. Human subjects threw balls at a target before, during, and after wearing laterally shifting prism eyeglasses. Positions of head, shoulders, arm, and ball were video-recorded continuously. We computed body angles of eyes-in-head, head-on-trunk, trunk-on-arm, and arm-on-ball. In each subject, the gaze-throw adjustment during adaptation was distributed across all sets of coupled body parts. The distribution of coupling changed unpredictably from throw to throw within a single session. The angular variation among coupled body parts was typically significantly larger than angular variation of on-target hits. Thus coupled body parts changed interdependently to account for the high accuracy of ball-on-target. Principal components and Monte Carlo analyses showed variability in body angles across throws with a wide range of variability/stereotypy across subjects. The data support a model of a dynamic and generalized solution as evidenced by the distribution of the gaze-throw adjustment across body parts.

Adaptation, Physiological↗
PubMed2006

Intracochlear pressure and organ of corti impedance from a linear active three-dimensional model.

Intracochlear pressure and basilar membrane (BM) velocity are calculated from a physiologically based chinchilla cochlea model . The model includes three-dimensional viscous fluid and the pectinate zone of the elastic BM with dimensional and material property variation along its length. The passive response shows excellent agreement with measurement at high sound pressure levels. The active process is represented by adding the motility of the outer hair cells (OHCs) to the passive model with the feed-forward approximation of the organ of Corti (OC), as was done previously. The current model explains recent observations including: (1) agreement with characteristic frequency (CF)-to-place map, (2) CF shift in the active model, (3) BM displacement gain from OHC motility, (4) lower intracochlear pressure gain than BM displacement gain, and (5) OC impedance (Z(OC)).

Animals↗
PubMed1991

Lateralized anomalous perceptual experiences in schizophrenia.

Four cases of schizophrenia are described, in which subjects reported anomalous perceptual experiences confined to one visual field, in all cases the left visual field. The pattern, content and laterality of such anomalous perceptions point to the presence of right hemisphere dysfunction in schizophrenia. Further evidence to this effect comes from examining the type of anomalous perceptual experiences reported by subjects with focal lesions in either hemisphere. Those with right hemisphere damage but not those with left hemisphere damage report almost identical experiences to those seen in the 4 schizophrenics described here.

Adult↗
PubMed1992

The cocaine experience: refuting the concept of a model psychosis?

Eighteen patients admitted to a drug dependency unit for opiate detoxification and who had recently used cocaine were asked to describe the experience based on a questionnaire. A comparison was made with 18 schizophrenic patients who completed the same questionnaire. Results showed a qualitative difference in the subjective phenomena described by the two groups. There appeared to be certain phenomena present in a cocaine induced state which appear to be specific to this state. Amongst these features are: increased intensity of colours, prominence of certain colours in illusions, i.e. red and green, and changes in light intensity. Objects appear to be more vivid and stand out, macropsia and micropsia also seem to occur in this group and paranoia if present is of a transient nature. These particular symptoms do not seem to be characteristic of a schizophrenic illness. This may therefore contradict the idea of a drug induced psychosis (in this case cocaine) as being indistinguishable from schizophrenia (the model psychosis theory).

Adolescent↗
PubMed2005

Ineffectiveness of prism adaptation on spatial neglect signs.

BACKGROUND AND PURPOSE: To evaluate the effect of prism adaptation (PA) on spatial neglect signs. METHODS: Ten patients (hemianopia in 6) and 8 control subjects were included. Tasks were reading single words, nonwords, and a text, bell cancellation, line bisection, and scene drawing, before and after adaptation, with either deviating or neutral prisms. RESULTS: Errors were more frequent in patients but without any specific effect of PA. We found partial improvement at the late sessions, independent of prisms. CONCLUSIONS: We did not confirm the efficacy of PA on spatial neglect.

Adaptation, Physiological↗
PubMed2002

Curvature in hand movements as a result of visual misjudgements of direction.

The path that our hand takes when moving from one position to another is often slightly curved. Part of this curvature is caused by perceptual errors. We examine here whether this is so for the influence that a surface's orientation has on the approaching hand's path. When moving our hand towards a point on a surface we tend to follow a path that makes the final approach more orthogonal to the surface at that point. Doing so makes us less sensitive to imperfections in controlling our movements. Here we show that this tendency is also present when moving towards a point along an edge of a drawing of an oriented bar. The influence of the bar's orientation is no smaller when people are explicitly asked to move as straight as possible, than when they are instructed to move as fast as possible. The bar's orientation also influences perceptual judgements of a straight path, but this influence is only as large as it is on the curvature of the hand's path for judgements of the direction from the hand's initial position to the target. We conclude that the influence of the bar's orientation on the curvature of the hand's path is caused by a misperception of the initial direction in which the hand has to move to reach the target.

Hand↗
PubMed1997

Display fidelity: link between psychophysics and contrast discrimination models.

In medical images, the data often represent information with a quantitative meaning, such as the radiation absorption by tissues. Therefore, it is crucial that this quantitative information is accurately transferred into brightness impressions from luminance patterns on the display. Perceptual linearization was proposed by Pizer (1981) as a way to guarantee the display fidelity in as much as equal steps in the grey value evoke equal steps in brightness sensation. In this study, the fidelity of achromatic displays prior to and after perceptual linearization was investigated. First, magnitude estimation of brightness differences between grey square patches embedded in a uniform background was used to make equal-interval brightness series. Then, the brightness contrast discrimination models of Whittle (1986, 1992), and Kingdom and Moulden (1991) for grey patches in a uniform background were extended to the suprathreshold data. A good fit was found. Finally, the look-up tables that provide perceptual linearization for the grey patches were applied to complex images. Brightness matching with a scale of reference grey patches was used to estimate grey levels at specified image locations. The experimental results indicate that the accuracy of this task is not necessarily affected by perceptual linearization.

Adult↗
PubMed2001

Compensation of corneal aberrations by the internal optics in the human eye.

The objective was to study the relative contribution of the optical aberrations of the cornea and the internal ocular optics (with the crystalline lens as the main component) to overall aberrations in the human eye. Three sets of wave-front aberration data were independently measured in the eyes of young subjects: for the anterior surface of the cornea, the complete eye, and internal ocular optics. The amount of aberration of both the cornea and internal optics was found to be larger than for the complete eye, indicating that the first surface of the cornea and internal optics partially compensate for each other's aberrations and produce an improved retinal image. This result has a number of practical implications. For example, it shows the limitation of corneal topography as a guide for new refractive procedures and provides a strong endorsement of the value of ocular wave-front sensing for those applications.

Accommodation, Ocular↗
PubMed2004

Mislocalization of targets flashed during smooth pursuit depends on the change in gaze direction after the flash.

Subjects mislocalize the position of a target that is flashed while they are making pursuit eye movements. This mislocalization is in the direction of pursuit. However, it is not clear whether it is the movement of the eyes or the movement of the pursuit target that matters. Neither is it clear whether it is the movement after the flash or the movement before the flash that matters. To resolve these issues, we asked subjects to pursue a disk that regularly changed its movement direction. Each change was followed by a change in the direction of gaze movement. Subjects were asked to tap targets that were flashed close to the moment at which the pursuit disk changed direction. We measured the movements of the eyes, head, and index finger. Subjects did not make saccades to the position they tapped but kept pursuing the disk. We compared the direction of the mislocalization with the changes in gaze and in target position during different intervals relative to the flash. We found that the mislocalization is related to the change in gaze after the flash.

Cues↗