Stable visually guided reaching does not require an internal feedforward model to compensate for internal delay: Data and model.

Visually guided reaches are performed in ≈1s. Given unstable feedback control with neural transmission delay, stable visually guided reaching is assumed to require internal feedforward models that generate simulated feedback without delay that combines with actual feedback for stability. We investigated whether stable visually guided reaching requires internal models to handle such delay. Participants performed rapid targeted reaches in a virtual environment with different mappings between speeds of the hand and hand avatar. First, participants reached with visual guidance and constant mapping. Second, feedforward reaches were performed with constant mapping and hand avatar only visible at reach start and end. Reaches were accurate. Third, participants performed reaches with visual guidance and different mappings every trial. We expected performance as in the first condition. Finally, feedforward reaches with variable mapping yielded large errors showing visual guidance in the previous condition was successful despite an ineffective internal model. We simulated reaches using a proportional rate model with disparity Tau controlling the virtual Equilibrium Point in an Equilibrium Point (EP) model. The time dimensioned information and dynamic remained stable with delayed feedback. Finally, we fit movement times using the proportional rate EP model with 0msec, 50msec, and 100msec delay. With the fitted model parameters, we compared the model reach trajectories with the behavioral trajectories. Stable visually guided reaching did not require an internal feedforward model.

The effect of movement frequency on perceptual-motor learning of a novel bimanual coordination pattern.

The most widely known studies of rhythmic limb coordination showed that frequency strongly affects the stability of some coordinations (e.g. 180° relative phase) but not others (e.g. 0°). The coupling of such rhythmic limb movements was then shown to be perceptual. Frequency affected the stability of perceptual information. We now investigated whether frequency would impact the pickup of information for learning a novel bimanual coordination pattern (e.g. 90°) and the ability to sustain the coordination at various frequencies. Twenty participants were recruited and assessed on their performance of bimanual coordination at 0°, 180°, and 90° at five scanning frequencies before and after training at 90°, during which they were assigned to practice with either a high (2.5 Hz) or low (0.5 Hz) frequency until attaining proficiency. The results showed that learning was frequency specific. The best post-training performance occurred at the trained frequency. Although the coordination could be acquired through high frequency training, it was at the cost of a greater amount of training and most surprisingly, did not yield improved performance at lower frequencies that are normally thought to be easier. The findings suggest that movement frequency may determine whether visual or kinesthetic information is used for learning and control of bimanual coordination.

Investigation of optical texture properties as relative distance information for monocular guidance of reaching.

Reaches guided using monocular versus binocular vision have been found to be equally fast and accurate only when optical texture was available projected from a support surface across which the reach was performed. We now investigate what property of optical texture elements is used to perceive relative distance: image width, image height, or image shape. Participants performed reaches to match target distances. Targets appeared on a textured surface on the left and participants reached to place their hand at target distance along a surface on the right. A perturbation discriminated which texture property was being used. The righthand surface was higher than the lefthand one by either 2, 4 or 6 cm. Participants should overshoot if they matched texture image width at the target, undershoot if they matched image shape, and undershoot far distances and, depending on the overall eye height, overshoot near distances if they matched image height. In Experiment 1, participants reached by moving a joystick to control a hand avatar in a virtual environment display. Their eye height was 15 cm. For each texture property, distances were predicted from the viewing geometry. Results ruled out image width in favor of image height or shape. In Experiment 2, participants at a 50 cm eye height reached in an actual environment with the same manipulations. Results supported use of image shape (or foreshortening), consistent with findings of texture properties used in slant perception. We discuss implications for models of visually guided reaching.

Time for Space and the Stability of Prospective Control: Reaching-to-Grasp Gibson.

Gibson formulated an approach to goal-directed behavior using prospective information in the context of visually guided locomotion and manual behavior. The former was Gibson's paradigm case, but it is the rapidity of targeted reaching that has provided the special challenge for stable control. Recent treatments of visually guided reaching assume that internal forward models are required to generate stable behavior given delays caused by neural transmission times. Internal models are representations of the sort eschewed by Gibson in favor of prospective information. Reaching is usually described as guided using relative distances of hand and target, but prospective information is usually temporal rather than spatial. We describe proportional rate control models that incorporate time dimensioned prospective information and show they remain stable in the face of delays. The use of time-dimensioned prospective information removes the need for internal models for stable behavior despite neural transmission delays and allows Gibson's approach to prevail.

The role of intentionality in the performance of a learned 90° bimanual rhythmic coordination during frequency scaling: data and model.

Two rhythmic coordinations, 0° and 180° relative phase, can be performed stably at preferred frequency (~ 1 Hz) without training. Evidence indicates that both 0° and 180° coordination entail detection of the relative direction of movement. At higher frequencies, this yields instability of 180° and spontaneous transition to 0°. The ability to perform a 90° coordination can be acquired by learning to detect and use relative position as information. We now investigate the skilled performance of 90° bimanual coordination with frequency scaling and whether 90° coordination exhibits mode switching to 0° or 180° at higher frequencies. Unlike the switching from 180° to 0°, a transition from the learned 90° coordination to the intrinsic 0° or 180° modes would entail a change in information. This would seem to require intentional decisions during performance as would correcting performance that had strayed from 90°. Relatedly, correction would seem to be an intrinsic part of the performance of 90° during learning. We investigated whether it remains so. We tested bimanual coordination at 90° under both noninterference and correcting instructions. Under correcting instructions, bimanual 90° coordination remained stable at both low and high frequencies. Noninterference instructions yielded stable performance at lower frequencies and switching to 0° or 180° at higher frequencies. Thus, correction is optional and switching to the intrinsic modes occurred. We extended the Bingham (Ecol Psychol 16:45-53, 2004a; Advances in psychology, vol. 135, Time-to-contact, Elsevier Science Publishers, 2004b) model for 0° and 180° coordination to create a dynamical, perception-action account of learned 90° bimanual coordination, in which mode switching and correction were both initiated as the information required for performance of 90° fell below threshold. This means that intentional decisions about what coordination to perform and whether to correct occurred only before performance was begun, not during performance. The extended strictly dynamical model was successfully used to simulate performance of participants in the experiments.

Training 90° bimanual coordination at high frequency yields dependence on kinesthetic information and poor performance of dyadic unimanual coordination.

Two groups of participants were trained to be proficient at performing bimanual 90° coordination either at a high (2.5 Hz) or low (0.5 Hz) frequency with both kinesthetic and visual information available. At high frequency, participants trained for twice as long to achieve performance comparable to participants training at low frequency. Participants were then paired within (low-low or high-high) or between (low-high) frequency groups to perform a visually coupled dyadic unimanual 90° coordination task, during which they were free to settle at any jointly determined frequency to synchronize their rhythmic movements. The results showed that the coordination skill was frequency-specific. For dyads with one or both members who had learned the 90° bimanual coordination at low frequency, the performance settled at a low frequency (≈0.5 Hz) with more successfully synchronized trials. For dyads with both members who had learned the 90° bimanual coordination at high frequency, they struggled with the task and performed poorly. The dyadic coordination settled at a higher frequency (≈1.5 Hz) on average, but with twice the variability in settling frequency and significantly fewer synchronized trials. The difference between the dyadic coordination and bimanual tasks was that only visual information was available to couple the movements in the former while both kinesthetic and visual information were available in the latter. Therefore, the high frequency group must have relied on kinesthetic information to perform both coordination tasks while the low frequency group was well able to use visual information for both. In the mixed training pairs, the low frequency trained member of the pair was likely responsible for the better performance. These conclusions were consistent with results of previous studies.

Monocular guidance of reaches-to-grasp using visible support surface texture: data and model.

We investigated monocular information for the continuous online guidance of reaches-to-grasp and present a dynamical control model thereof. We defined an information variable using optical texture projected from a support surface (i.e. a table) over which the participants reached-to-grasp target objects sitting on the table surface at different distances. Using either binocular or monocular vision in the dark, participants rapidly reached-to-grasp a phosphorescent square target object with visibly phosphorescent thumb and index finger. Targets were one of three sizes. The target either sat flat on the support surface or was suspended a few centimeters above the surface at a slant. The later condition perturbed the visible relation of the target to the support surface. The support surface was either invisible in the dark or covered with a visible phosphorescent checkerboard texture. Reach-to-grasp trajectories were recorded and Maximum Grasp Apertures (MGA), Movement Times (MT), Time of MGA (TMGA), and Time of Peak Velocities (TPV) were analyzed. These measures were selected as most indicative of the participant's certainty about the relation of hand to target object during the reaches. The findings were that, in general, especially monocular reaches were less certain (slower, earlier TMGA and TPV) than binocular reaches except with the target flat on the visible support surface where performance with monocular and binocular vision was equivalent. The hypothesized information was the difference in image width of optical texture (equivalent to density of optical texture) at the hand versus the target. A control dynamic equation was formulated representing proportional rate control of the reaches-to-grasp (akin to the model using binocular disparity formulated by Anderson and Bingham (Exp Brain Res 205: 291-306, 2010). Simulations were performed and presented using this model. Simulated performance was compared to actual performance and found to replicate it. To our knowledge, this is the first study of monocular information used for continuous online guidance of reaches-to-grasp, complete with a control dynamic model.

Information for perceiving blurry events: Optic flow and color are additive.

Information used in visual event perception includes both static image structure projected from opaque object surfaces and dynamic optic flow generated by motion. Events presented in static blurry grayscale displays have been shown to be recognized only when and after presented with optic flow. In this study, we investigate the effects of optic flow and color on identifying blurry events by studying the identification accuracy and eye-movement patterns. Three types of color displays were tested: grayscale, original colors, or rearranged colors (where the RGB values of the original colors were adjusted). In each color condition, participants identified 12 blurry events in five experimental phases. In the first two phases, static blurry images were presented alone or sequentially with a motion mask between consecutive frames, and identification was poor. In Phase 3, where optic flow was added, identification was comparably good. In Phases 4 and 5, motion was removed, but identification remained good. Thus, optic flow improved event identification during and after its presentation. Color also improved performance, where participants were consistently better at identifying color displays than grayscale or rearranged color displays. Importantly, the effects of optic flow and color were additive. Finally, in both motion and postmotion phases, a significant portion of eye fixations fell in strong optic flow areas, suggesting that participants continued to look where flow was available even after it stopped. We infer that optic flow specified depth structure in the blurry image structure and yielded an improvement in identification from static blurry images.

Control of visually guided braking using constant-[Formula: see text] and proportional rate.

This study investigated the optical information and control strategies used in visually guided braking. In such tasks, drivers exhibit two different braking behaviors: impulsive braking and continuously regulated braking. We designed two experiments involving a simulated braking task to investigate these two behaviors. Participants viewed computer displays simulating an approach along a linear path over a textured ground surface toward a set of road signs. The task was to use a joystick as a brake to stop as close as possible to the road signs. Our results showed that participants relied on a weak constant-[Formula: see text] strategy (Bingham 1995) when regulating the brake impulsively. They used discrete [Formula: see text] values as critical values and they regulated the brake so as not to let [Formula: see text] fall below these values. Our results also showed that proportional rate control (Anderson and Bingham 2010, 2011) is used in continuously regulated braking. Participants initiated braking at a certain proportional rate value and controlled braking so as to maintain that value constant during the approach. Proportional rate control is robust because the value can fluctuate within a range to yield good performance. We argue that proportional rate control unifies the information-based approach and affordance-based approach to visually guided braking.

A stratified process for the perception of objects: From optical transformations to 3D relief structure to 3D similarity structure to slant or aspect ratio.

Previously, we developed a stratified process for slant perception. First, optical transformations in structure-from-motion (SFM) and stereo were used to derive 3D relief structure (where depth scaling remains arbitrary). Second, with sufficient continuous perspective change (≥45°), a bootstrap process derived 3D similarity structure. Third, the perceived slant was derived. As predicted by theoretical work on SFM, small visual angle (<5°) viewing requires non-coplanar points. Slanted surfaces with small 3D cuboids or tetrahedrons yielded accurate judgment while planar surfaces did not. Normally, object perception entails non-coplanar points. Now, we apply the stratified process to object perception where, after deriving similarity structure, alternative metric properties of the object can be derived (e.g. slant of the top surface or width-to-depth aspect ratio). First, we tested slant judgments of the smooth planar tops of three different polyhedral objects. We tested rectangular, hexagonal, and asymmetric pentagonal surfaces, finding that symmetry was required to determine the direction of slant (AP&P, 2019, https://doi.org/10.3758/s13414-019-01859-5). Our current results replicated the previous findings. Second, we tested judgments of aspect ratios, finding accurate performance only for symmetric objects. Results from this study suggest that, first, trackable non-coplanar points can be attained in the form of 3D objects. Second, symmetry is necessary to constrain slant and aspect ratio perception. Finally, deriving 3D similarity structure precedes estimating object properties, such as slant or aspect ratio. Together, evidence presented here supports the stratified bootstrap process for 3D object perception. STATEMENT OF SIGNIFICANCE: Planning interactions with objects in the surrounding environment entails the perception of 3D shape and slant. Studying ways through which 3D metric shape and slant can be perceived accurately by moving observers not only sheds light on how the visual system works, but also provides understanding that can be applied to other fields, like machine vision or remote sensing. The current study is a logical extension of previous studies by the same authors and explores the roles of large continuous perspective changes, relief structure, and symmetry in a stratified process for object perception.

Bootstrapping a better slant: A stratified process for recovering 3D metric slant.

Lind et al. (Journal of Experimental Psychology: Human Perception and Performance, 40 (1), 83, 2014) proposed a bootstrap process that used right angles on 3D relief structure, viewed over sufficiently large continuous perspective change, to recover the scaling factor for metric shape. Wang, Lind, and Bingham (Journal of Experimental Psychology: Human Perception and Performance, 44(10), 1508-1522, 2018) replicated these results in the case of 3D slant perception. However, subsequent work by the same authors (Wang et al., 2019) suggested that the original solution could be ineffective for 3D slant and presented an alternative that used two equidistant points (a portion of the original right angle). We now describe a three-step stratified process to recover 3D slant using this new solution. Starting with 2D inputs, we (1) used an existing structure-from-motion (SFM) algorithm to derive the object's 3D relief structure and (2) applied the bootstrap process to it to recover the unknown scaling factor, which (3) was then used to produce a slant estimate. We presented simulations of results from four previous experiments (Wang et al., 2018, 2019) to compare model and human performance. We showed that the stratified process has great predictive power, reproducing a surprising number of phenomena found in human experiments. The modeling results also confirmed arguments made in Wang et al. (2019) that an axis of mirror symmetry in an object allows observers to use the recovered scaling factor to produce an accurate slant estimate. Thus, poor estimates in the context of a lack of symmetry do not mean that the scaling factor has not been recovered, but merely that the direction of slant was ambiguous.

Symmetry mediates the bootstrapping of 3-D relief slant to metric slant.

Empirical studies have always shown 3-D slant and shape perception to be inaccurate as a result of relief scaling (an unknown scaling along the depth direction). Wang, Lind, and Bingham (Journal of Experimental Psychology: Human Perception and Performance, 44(10), 1508-1522, 2018) discovered that sufficient relative motion between the observer and 3-D objects in the form of continuous perspective change (≥45°) could enable accurate 3-D slant perception. They attributed this to a bootstrap process (Lind, Lee, Mazanowski, Kountouriotis, & Bingham in Journal of Experimental Psychology: Human Perception and Performance, 40(1), 83, 2014) where the perceiver identifies right angles formed by texture elements and tracks them in the 3-D relief structure through rotation to extrapolate the unknown scaling factor, then used to convert 3-D relief structure to 3-D Euclidean structure. This study examined the nature of the bootstrap process in slant perception. In a series of four experiments, we demonstrated that (1) features of 3-D relief structure, instead of 2-D texture elements, were tracked (Experiment 1); (2) identifying right angles was not necessary, and a different implementation of the bootstrap process is more suitable for 3-D slant perception (Experiment 2); and (3) mirror symmetry is necessary to produce accurate slant estimation using the bootstrapped scaling factor (Experiments 3 and 4). Together, the results support the hypothesis that a symmetry axis is used to determine the direction of slant and that 3-D relief structure is tracked over sufficiently large perspective change to produce metric depth. Altogether, the results supported the bootstrap process.

Change in effectivity yields recalibration of affordance geometry to preserve functional dynamics.

Mon-Williams and Bingham (Exp Brain Res 211(1):145-160, 2011) developed a geometrical affordance model for reaches-to-grasp, and identified a constant scaling relationship, P, between safety margins (SM) and available apertures (SM) that are determined by the sizes of the objects and the individual hands. Bingham et al. (J Exp Psychol Hum Percept Perform 40(4):1542-1550, 2014) extended the model by introducing a dynamical component that scales the geometrical relationship to the stability of the reaching-to-grasp. The goal of the current study was to explore whether and how quickly change in the relevant effectivity (functionally determined hand size = maximum grip) would affect the geometrical and dynamical scaling relationships. The maximum grip of large-handed males was progressively restricted. Participants responded to this restriction by using progressively smaller safety margins, but progressively larger P (= SM/AA) values that preserved an invariant dynamical scaling relationship. The recalibration was relatively fast, occurring over five trials or less, presumably a number required to detect the variability or stability of performance. The results supported the affordance model for reaches-to-grasp in which the invariance is determined by the dynamical component, because it serves the goal of not colliding with the object before successful grasping can be achieved. The findings were also consistent with those of Snapp-Childs and Bingham (Exp Brain Res 198(4):527-533, 2009) who found changes in age-specific geometric scaling for stepping affordances as a function of changes in effectivities over the life span where those changes preserved a dynamic scaling constant similar to that in the current study.

Training children aged 5-10 years in manual compliance control to improve drawing and handwriting.

A large proportion of school-aged children exhibit poor drawing and handwriting. This prevalence limits the availability of therapy. We developed an automated method for training improved manual compliance control and relatedly, prospective control of a stylus. The approach included a difficult training task, while providing parametrically modifiable support that enables the children to perform successfully while developing good compliance control. The task was to use a stylus to push a bead along a 3D wire path. Support was provided by making the wire magnetically attractive to the stylus. Support was progressively reduced as 3D tracing performance improved. We report studies that (1) compared performance of Typically Developing (TD) children and children with Developmental Coordination Disorder (DCD), (2) tested training with active versus passive movement, (3) tested progressively reduced versus constant or no support during training, (4) tested children of different ages, (5) tested the transfer of training to a drawing task, (6) tested the specificity of training in respect to the size, shape and dimensionality of figures, and (7) investigated the relevance of the training task to the Beery VMI, an inventory used to diagnose DCD. The findings were as follows. (1) Pre-training performance of TD and DCD children was the same and good with high support but distinct and poor with low support. Support yielded good self-efficacy that motivated training. Post training performance with no support was improved and the same for TD and DCD children. (2) Actively controlled movements were required for improved performance. (3) Progressively reduced support was required for good performance during and after training. (4) Age differences in performance during pre-training were eliminated post-training. (5) Improvements transferred to drawing. (6) There was no evidence of specificity of training in transfer. (7) Disparate Beery scores were reflected in pre-training but not post-training performance. We conclude that the method improves manual compliance control, and more generally, prospective control of movements used in drawing performance.

Searching for invariance: Geographical and optical slant.

When we move through rigid environments, surface orientations of static objects do not appear to change. Most studies have investigated the perception of optical slant which is dependent on the perspective of the observer. We investigated the perception of geographical slant, which is invariant across different viewing perspectives, and compared it to optical slant. In Experiment 1, participants viewed a 3D triangular target surface with triangular phosphorescent texture elements presented at eye level at one of 5 slants from 0° to 90°, at 0° or 40° tilt. Participants turned around to adjust a 2D line or a 3D surface to match the slant of the target surface. In Experiment 2, the difference between optical and geographical slant was increased by changing the height of the surface to be judged. In Experiment 3, target surfaces were rotated by 50° (±25°) and viewed in both a dark and lighted room. In Experiment 1, the overall pattern of judgments exhibited only slight differences between response measures. In Experiment 2, slant judgments were slightly overestimated when the surface was at a low height and at 0° tilt. We compared optical slants of the surfaces to geographical slants. While sometimes inaccurate, participants' slant judgments remained invariant across changes in viewing perspective. In Experiment 3, judgments were the same in the dark and lighted conditions. There was no effect of target motion on judgments, although variability decreased. We conclude that participants' judgments were predicted by geographical slant, not optical slant.

Information about relative phase in bimanual coordination is modality specific (not amodal), but kinesthesis and vision can teach one another.

How is information from different sensory modalities coordinated when learning an action? We tested two hypotheses. The first is that the information is amodal. The second is that the information is modality specific and one modality is used in first learning the action and then is used to teach the other modality. We investigated these hypotheses using a rhythmic coordination task. One group of participants learned to perform bimanual coordination at a relative phase of 90° using kinesthesis. A second group used vision to learn unimanual 90° coordination. After training, performance using the alternate modality was tested in each case. Snapp-Childs, Wilson, and Bingham (2015) had found transfer of 50% of learned performance of 90° coordination between the unimanual and bimanual tasks when each had included use of vision. Now, we found essentially no transfer (≈5%) indicating that the information was modality specific. Next, post-training trials performed using the untrained modality were alternated with trials in which kinesthesis and vision were used. The result was that performance using the untrained modality progressively improved. We concluded that trained modality was used to teach the untrained modality and that this likely represents the way information from different sensory modalities is coordinated in performance of actions.

Large continuous perspective change with noncoplanar points enables accurate slant perception.

Perceived slant has often been characterized as a component of 3D shape perception for polyhedral objects. Like 3D shape, slant is often perceived inaccurately. Lind, Lee, Mazanowski, Kountouriotis, and Bingham (2014) found that 3D shape was perceived accurately with perspective changes ≥ 45°. We now similarly tested perception of 3D slant. To account for their results, Lind et al. (2014) developed a bootstrap model based on the assumption that optical information yields perception of 3D relief structure then used with large perspective changes to bootstrap to perception of 3D Euclidean structure. However, slant perception usually entails planar surfaces and structure-from-motion fails in the absence of noncoplanar points. Nevertheless, the displays in Lind et al. (2014) included stereomotion in addition to monocular optical flow. Because stereomotion is higher order, the bootstrap model might apply in the case of strictly planar surfaces. We investigated whether stereomotion, monocular structure-from-motion (SFM), or the combination of the two would yield accurate 3D slant perception with large continuous perspective change. In Experiment 1, we found that judgments of slant were inaccurate in all information conditions. In Experiment 2, we added noncoplanar structure to the surfaces. We found that judgments in the monocular SFM and combined conditions now became correct once perspective changes were ≥ 45°, replicating the results of Lind et al. (2014) and supporting the bootstrap model. In short, we found that noncoplanar structure was required to enable accurate perception of 3D slant with sufficiently large perspective changes. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Training children aged 5-10 years in compliance control: tracing smaller figures yields better learning not specific to the scale of drawn figures.

Previously we developed a method that supports active movement generation to allow practice with improvement of good compliance control in tracing and drawing. We showed that the method allowed children with motor impairments to improve at a 3D tracing task to become as proficient as typically developing children and that the training improved 2D figure copying. In this study, we expanded the training protocol to include a wider variety of ages (5-10-year-olds) and we made the figures traced in training the same as in figure copying, but varied the scale of training and copying figures to assess the generality of learning. Forty-eight children were assigned to groups trained using large or small figures. All were tested before training with a tracing task and a copying task. Then, the children trained over five sessions in the tracing task with either small or large figures. Finally, the tracing and copying tasks were tested again following training. A mean speed measure was used to control for path length variations in the timed task. Performance on both tasks at both baseline and posttest varied as a function of the size of the figure and age. In addition, tracing performance also varied with the level of support. In particular, speeds were higher with more support, larger figures and older children. After training, performance improved. Speeds increased. In tracing, performance improved more for large figures traced by children who trained on large figures. In copying, however, performance only improved significantly for children who had trained on small figures and it improved equally for large and small figures. In conclusion, training by tracing smaller figures yielded better learning that was not, however, specific to the scale of drawn figures. Small figures exhibit greater mean curvature. We infer that it yielded better general improvement.

Perception of time to contact of slow- and fast-moving objects using monocular and binocular motion information.

The role of the monocular-flow-based optical variable τ in the perception of the time to contact of approaching objects has been well-studied. There are additional contributions from binocular sources of information, such as changes in disparity over time (CDOT), but these are less understood. We conducted an experiment to determine whether an object's velocity affects which source is most effective for perceiving time to contact. We presented participants with stimuli that simulated two approaching squares. During approach the squares disappeared, and participants indicated which square would have contacted them first. Approach was specified by (a) only disparity-based information, (b) only monocular flow, or (c) all sources of information in normal viewing conditions. As expected, participants were more accurate at judging fast objects when only monocular flow was available than when only CDOT was. In contrast, participants were more accurate judging slow objects with only CDOT than with only monocular flow. For both ranges of velocity, the condition with both information sources yielded performance equivalent to the better of the single-source conditions. These results show that different sources of motion information are used to perceive time to contact and play different roles in allowing for stable perception across a variety of conditions.

Predicting the duration of reach-to-grasp movements to objects with asymmetric contact surfaces.

The duration of reach-to-grasp movements is influenced by the size of the contact surfaces, such that grasping objects with smaller contact surface areas takes longer. But what is the influence of asymmetric contact surfaces? In Experiment 1a, participants reached-to-lift wooden blocks off a table top, with the contact locations for the thumb and index finger varying in surface size. The time taken to lift the block was driven primarily by the thumb contact surface, which showed a larger effect size for the dependent variable of movement duration than the index finger's contact surface. In Experiment 1b participants reached-to-grasp (but not lift) the blocks. The same effect was found with duration being largely driven by contact surface size for the thumb. Experiment 2 tested whether this finding generalised to movements towards conical frusta grasped in a different plane mounted off the table top. Experiment 2 showed that movement duration again was dictated primarily by the size of the thumb's contact surface. The thumb contact surface was the visible surface in experiments 1 and 2 so Experiment 3 explored grasping when the index finger's contact surface was visible (participants grasped the frusta with the index finger at the top). An interaction between thumb and finger surface size was now found to determine movement duration. These findings provide the first empirical report of the impact of asymmetric contact surfaces on prehension, and may have implications for scientists who wish to model reach-to-grasp behaviours.