Herbert Heuer

Intermanual interactions during programming of aimed movements: Converging evidence on common and specific parameters of control

SummaryReaction time for choice between left-hand and right-hand responses sometimes depends on whether the responses assigned to the two hands are the same or different, but sometimes it does not. It has been suggested that the result obtained indicates whether the variable on which the responses differ is determined by common or specific parameters of control. Common parameters apply to movements of both hands, but specific parameters can be set independently for left-hand and right-hand responses. The interpretation of choice RT in terms of intermanual interactions requires that the conclusions converge with conclusions that are based on studies of simultaneous movements, provided that interactions can indeed be attributed to common parameters. Results from the latter type of experiment suggest that duration of aimed movements is determined by common parameters, but amplitude is determined by specific parameters. In two experiments it is shown that the choice-task results are consistent with this conclusion: Choice RT increases when the choice is between aimed movements that differ in duration, but not if the choice is between movements of different amplitudes. Further, an assimilation of responses is found in the former case but not in the latter. These findings are taken as further support for the notion of intermanual interactions during motor programming.

Visual discrimination and response programming

SummaryChoice reaction time (RT) depends on the relationship between responses, and these dependencies are usually interpreted in terms of the “advance-specification assumption.” According to this assumption characteristics that are the same for the choice responses can be specified in advance of the response signal, thus allowing faster RTs. Since the advance-specification assumption is called into question by some of the available data, an alternative interpretation is suggested and formalized in terms of an accumulator model. According to the “programming-interactions assumption,” stimulus processing is tightly linked to response programming, so that both the possible responses are programmed as long as there is uncertainty with regard to stimulus identity. This gives rise to interactions between simultaneous processes of motor programming. Predictions of this assumption for the joint effects of signal similarity and the relationship between responses are tested and confirmed.

Choice Between Finger Movements of Different and Identical Form: The Effect of Relative Signal Frequency

SummaryBinary choice reaction time depends on the relationship between the alternative responses. In particular, it is longer when choice is between finger movements of either hand, which have different forms, than when choice is between movements of identical form. This effect can be interpreted in terms of conjunctive preparation. If everything common to both responses is prepared in advance of the response signal, the form of the movement can only be prepared if it is the same for both responses. An alternative interpretation is in terms of single preparation. Under this hypothesis either one or the other response is prepared completely, but in case the not prepared response is demanded by the signal the time needed to shift should be longer if the movements are of different form. Thus, an effect of the form-relation will be seen only in unprepared responses, while under the hypothesis of conjunctive preparation it should be present in prepared responses as well. To decide between both hypotheses relative signal frequency and thus the proportion of expected (and prepared) responses was varied in three experiments. It turned out that the effect of the form-relation is larger with low relative signal frequency than with high frequency. This indicates the existence of single preparation. However, single preparation appears to be incomplete. When the obtained data are extrapolated to a relative signal frequency of 1.0 an effect of the form-relation still remains.

On re-scaleability of force and time in aiming movements

SummaryEvidence for invariant relative timing is mostly based on comparisons between experimental conditions which differ in the average speed of the movements under study. The present experiment examines whether spontaneous variability of acceleration-time curves of aiming movements within a single experimental condition can also be described in terms of variable scaling parameters for both dimensions. This requires that the correlations between peak acceleration and peak deceleration, as well as those between time intervals defined by peak values and zero crossings, are high. In extensions of the elbow joint on a horizontal plane, the latter correlations turned out to be small or even negative. These results cannot be explained as being due to artifacts of several origins. To reconcile them with those based on comparisons between experimental conditions, a dual-level hypothesis of control in aiming movements is suggested. Only variability on one level can be described in terms of variable scaling parameters, and whether or not variability of acceleration-time curves can be described in this way depends on the relative contributions of both levels of control.

Apparent Motion in Depth Resulting from Changing Size and Changing Vergence

Two experiments are reported in which the effect of combining stimuli of changing size and changing vergence on the perception of motion in depth was examined. Changing size and changing vergence corresponded to in-phase or anti-phase sinusoidal motions of an outline circle, with different amplitudes. In-phase stimuli had approximately additive effects on the estimated peak-to-peak amplitude of apparent motion in depth. Anti-phase stimuli did not cancel each other; apparent motion was in-phase with one or the other stimulus. When apparent motion was in-phase with one stimulus, there was only a limited influence of the other stimulus. The results are discussed with regard to a model proposed by Regan and Beverley for the combination of changing size and changing disparity.

Perceptual attraction in tool use: evidence for a reliability-based weighting mechanism

Humans are well able to operate tools whereby their hand movement is linked, via a kinematic transformation, to a spatially distant object moving in a separate plane of motion. An everyday example is controlling a cursor on a computer monitor. Despite these separate reference frames, the perceived positions of the hand and the object were found to be biased toward each other. We propose that this perceptual attraction is based on the principles by which the brain integrates redundant sensory information of single objects or events, known as optimal multisensory integration. That is, 1) sensory information about the hand and the tool are weighted according to their relative reliability (i.e., inverse variances), and 2) the unisensory reliabilities sum up in the integrated estimate. We assessed whether perceptual attraction is consistent with optimal multisensory integration model predictions. We used a cursor-control tool-use task in which we manipulated the relative reliability of the unisensory hand and cursor position estimates. The perceptual biases shifted according to these relative reliabilities, with an additional bias due to contextual factors that were present in experiment 1 but not in experiment 2 The biased position judgments variances were, however, systematically larger than the predicted optimal variances. Our findings suggest that the perceptual attraction in tool use results from a reliability-based weighting mechanism similar to optimal multisensory integration, but that certain boundary conditions for optimality might not be satisfied.NEW & NOTEWORTHY Kinematic tool use is associated with a perceptual attraction between the spatially separated hand and the effective part of the tool. We provide a formal account for this phenomenon, thereby showing that the process behind it is similar to optimal integration of sensory information relating to single objects.

Selective fatigue in the human motor system

SummaryIf the form of simple movements as well as the muscles involved are coded by place, it should be possible selectively to fatigue form-specific and muscle-specific neurons, whereby muscle-specific units are supposed to control homologous muscles on both sides of the body. In two experiments both kinds of selective fatigue effects were observed by the use of movements of different forms and with different fingers of both hands. These effects showed up in the variability of movement times only, but not in their mean.

Advance specification and programming interactions: A reply to Rosenbaum, Barnes, and Slotta (1988)*

SummaryChoice reaction time often depends on the relation between the responses from which subjects have to choose. Rosenbaum, Barnes, and Slotta (1988) criticized the interpretation of a particular set of such effects which can be observed when subjects have to choose between left-hand and right-hand responses that have same or different spatio-temporal patterns. This set of effects had been interpreted in terms of a programming interactions (PI) hypothesis rather than in terms of the more popular advance specification (AS) hypothesis which is the foundation of a major portion of motor-programming research. It is made clear that the controversy is not about whether the one or the other hypothesis is correct in general, but about whether there are exceptions which cannot be interpreted in terms of the AS hypothesis. Further it is argued that the particular set of effects represents such an exception. It is shown that Rosenbaum et al. claim that the data are consistent with the AS hypothesis and inconsistent with the PI hypothesis is untenable.

Aftereffects of Sustained Convergence: Some Implications of the Eye Muscle Potentiation Hypothesis

Aftereffects of sustained convergence are generally explained as the result of eye muscle potentiation (EMP). Three implications of this hypothesis for aftereffects in distance perception were tested: (i) According to the EMP hypothesis the aftereffects are based on oculomotor distance cues only. The addition of secondary cues during the test period should therefore reduce or eliminate the aftereffects. This expectation was confirmed. (ii) According to the EMP hypothesis the critical variable determining the aftereffect should be the distance of the inspection stimulus from the observer but not from the test stimulus. In confirmation of the expectation the aftereffects of certain inspection stimuli were of the same direction for test stimuli which bounded the range of inspection distances on both sides. (iii) When the aftereffects are based on oculomotor distance cues only, the effect of secondary cues during the test period should remain unchanged when they are present during the inspection period as well. Contrary to this expectation the aftereffects did not depend on the cues available during the test. This suggests that they are based on secondary cues, too, provided they are present during the inspection period.

Kinematic cross-correlation induces sensory integration across separate objects

In a basic cursor‐control task, the perceived positions of the hand and the cursor are biased towards each other. We recently found that this phenomenon conforms to the reliability‐based weighting mechanism of optimal multisensory integration. This indicates that optimal integration is not restricted to sensory signals originating from a single source, as is the prevailing view, but that it also applies to separate objects that are connected by a kinematic relation (i.e. hand and cursor). In the current study, we examined which aspects of the kinematic relation are crucial for eliciting the sensory integration: (i) the cross‐correlation between kinematic variables of the hand and cursor trajectories, and/or (ii) an internal model of the hand‐cursor kinematic transformation. Participants made out‐and‐back movements from the centre of a semicircular workspace to its boundary, after which they judged the position where either their hand or the cursor hit the boundary. We analysed the position biases and found that the integration was strong in a condition with high kinematic correlations (a straight hand trajectory was mapped to a straight cursor trajectory), that it was significantly reduced for reduced kinematic correlations (a straight hand trajectory was transformed into a curved cursor trajectory) and that it was not affected by the inability to acquire an internal model of the kinematic transformation (i.e. by the trial‐to‐trial variability of the cursor curvature). These findings support the idea that correlations play a crucial role in multisensory integration irrespective of the number of sensory sources involved.