Biomedical subjects

Paolo Viviani

Publications and source records attributed to Paolo Viviani.

7 recordsLinked to original sources

PubMed2005

Factors affecting the size of the detour effect in the kinaesthetic perception of Euclidean distance.

Three experiments investigated the mechanisms by which we estimate Euclidean distances on the basis of kinaesthetic cues. In all experiments, blindfolded participants followed straight and curvilinear paths with a stylus. Then, with a straight response movement, they estimated the distance between the end-points of the previously explored path. Experiment 1 was designed to validate the hypothesis-made on the basis of results from a previous study-that errors in the kinaesthetic estimations of distances (detour effect) originate from the difficulty to decompose the displacement vector into relevant and irrelevant components, which would become more severe at points of inflection. Using elliptic paths (no inflections), we demonstrated that errors are indeed reduced considerably. The role of the orientation of the work plane was investigated in Experiment 2 in which the same paths used in our previous study were oriented in the frontal rather than the horizontal plane. The results indicate that the detour effect is independent of the orientation. Moreover, despite the asymmetry that gravity introduces between upward and downward movements, errors in the two directions are almost identical. Experiment 3 addressed two issues. First, we demonstrated that introducing a delay between the exploration of the path and the response did not alter significantly the pattern of errors. By contrast, we demonstrated that errors are severely reduced when the number of paths to be explored is reduced by half. The results of the three experiments are discussed within the context of current theories of sensori-motor coding.

Adolescent↗

PubMed2005

Error parsing in visuomotor pointing reveals independent processing of amplitude and direction.

An experiment investigated systematic pointing errors in horizontal movements performed without visual feedback toward 48 targets placed symmetrically around two initial hand positions. Our main goal was to provide evidence in favor of the hypothesis that amplitude and direction of the movements are planned independently on the basis of the hand-target vector (vectorial parametric hypothesis, VP). The analysis was carried out mainly at the individual level. By screening a number of formal models of the potential error components, we found that only models compatible with the VP hypothesis provide an accurate description of the error pattern. A quantitative analysis showed that errors are explained mostly by a bias in the represented initial hand position (46% of the sum of squared errors) and a visuomotor gain bias (26%). Range effect (3%), directional biases (3%), and inertia-dependent amplitude modulations (1%) also provided significant contributions. The error pattern was incompatible with the view that movements are planned by specifying either a final posture or a final position. Instead, the results fully supported the view that, at least in the horizontal plane, amplitude, and direction of pointing movements are planned independently in a hand- or target-centered frame of reference.

Adult↗

PubMed2004

Planning short pointing sequences.

An experiment tested the hypothesis that fast, short sequences of movements are planned as a whole, before movement inception. The experimental task consisted of pointing to either one (one-step condition), or two (two-step condition) visual targets aligned along the mid-sagittal axis in a horizontal plane. There were nine possible arrangements of the targets resulting from all combinations of three distances (5, 10, 15 cm), and two trial orders (blocked or random). Performances were characterised by reaction time (RT), movement kinematics, and spatial accuracy. Compared with one-step trials, the first movements of two-step trials had longer RTs (length effect), particularly in random sessions, and when the sequences included short-distance targets. There were also differences in duration (one-target advantage), velocity profile and spatial accuracy that did not depend on the characteristics of the second movement. The results are inconsistent with the assumption that two-step sequences are planned as a whole. Instead, they are in keeping with the alternative hypothesis that part of the preparation of the second step takes place during the execution of the first step.

Adult↗

PubMed2004

Amplitude and direction errors in kinesthetic pointing.

We investigated the accuracy with which, in the absence of vision, one can reach again a 2D target location that had been previously identified by a guided movement. A robotic arm guided the participant's hand to a target (locating motion) and away from it (homing motion). Then, the participant pointed freely toward the remembered target position. Two experiments manipulated separately the kinematics of the locating and homing motions. Some robot motions followed a straight path with the bell-shaped velocity profile that is typical of natural movements. Other motions followed curved paths, or had strong acceleration and deceleration peaks. Current motor theories of perception suggest that pointing should be more accurate when the homing and locating motion mimics natural movements. This expectation was not borne out by the results, because amplitude and direction errors were almost independent of the kinematics of the locating and homing phases. In both experiments, participants tended to overshoot the target positions along the lateral directions. In addition, pointing movements towards oblique targets were attracted by the closest diagonal (oblique effect). This error pattern was robust not only with respect to the manner in which participants located the target position (perceptual equivalence), but also with respect to the manner in which they executed the pointing movements (motor equivalence). Because of the similarity of the results with those of previous studies on visual pointing, it is argued that the observed error pattern is basically determined by the idiosyncratic properties of the mechanisms whereby space is represented internally.

Adult↗

PubMed2004

Perceptual asynchronies for biological and non-biological visual events.

Four experiments investigated the hypothesis that different attributes of a visual scene are processed by independent channels working asynchronously. Experiment 1 considered the attributes of colour, form, and movement of simple geometrical configurations. In each of three conditions, two of these attributes switched simultaneously between two fixed values (Green/Red, Circle/Square, Fixed/Moving). Participants indicated which of the two attributes changes was perceptually closer in time to a sound signal. Response probabilities varied as a function of the time of occurrence of the sound, showing that the processing of the movement channel is delayed with respect to the other two. A smaller but significant difference was also detected between the processing times for colour and form. Comparing Experiments 1 and 2 showed that movement velocity does not affect the delay with which movement onset is perceived with respect to colour. Experiment 3 contrasted colour and movement in the perception of a biological movement. The stimuli were video clips of a coloured ball being lifted by a hand. The colour of the ball changed a variable amount of time before or after the ball started moving. Participants indicated which of the two changes had occurred first. We found that, unlike in Experiments 1 and 2, movement perception no longer lagged colour perception. Experiment 4 tested the hypothesis that the disappearance of the asynchrony is due to perceptual anticipation. We discuss the implications of the results vis-à-vis current theories on perceptual binding and on the coding of dynamic events.

Adolescent↗

PubMed2003

The kinaesthetic perception of Euclidean distance: a study of the detour effect.

An experiment investigated the mechanisms by which humans estimate Euclidean distances on the basis of kinaesthetic cues. Blindfolded participants followed straight and curvilinear paths with a hand-held stylus (encoding phase). Then, with a straight movement, they estimated the Euclidean distance between the start- and end-points of the path (response phase). The experiment contrasted an On-axis condition, in which encoding and response movements were spatially aligned, and an Off-axis condition, in which they were displaced laterally. Performances were slightly more accurate in the On-axis condition than in the Off-axis condition. In both conditions, however, errors were consistently smaller when the path covered a larger surface. The results showed that small paths yielded an overestimation of the Euclidean distance, the relative errors increasing with the length of curvilinear paths. The findings are compared with results of other studies in which distances were estimated on the basis of haptic cues.

Arm↗

PubMed2002

Altering the visuomotor gain. Evidence that motor plans deal with vector quantities.

Two experiments investigated the effects of providing nonveridical knowledge of the results (KR) in a visuomanual task in which participants pointed to briefly (200 ms) presented targets without seeing their hand. By showing after each trial the movement endpoint displaced radially with respect to its true position, we were able to alter progressively the gain of the visuomanual loop. In experiment 1, the KR was provided only for transversal movements and for one target distance, but the effect generalized to all directions and all distances. Moreover, it also generalized to the other hand that had never been biased. In experiment 2, nonveridical KR was supplied for movements along the two major diagonals which require sharply different muscle synergies. The transfer to other directions and to the other hand was equally substantial. It is argued that the results support the vector coding hypothesis, which holds that the input to the motor execution stage is supplied by specifying independently the amplitude and the direction of the vector from the initial to the final position in an extrinsic frame of reference. We also discuss the possible brain structures involved in the biasing action of the KR.

Adaptation, Psychological↗