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.

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.

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.

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 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.

Breaking camouflage and detecting targets require optic flow and image structure information.

Use of motion to break camouflage extends back to the Cambrian [In the Blink of an Eye: How Vision Sparked the Big Bang of Evolution (New York Basic Books, 2003)]. We investigated the ability to break camouflage and continue to see camouflaged targets after motion stops. This is crucial for the survival of hunting predators. With camouflage, visual targets and distracters cannot be distinguished using only static image structure (i.e., appearance). Motion generates another source of optical information, optic flow, which breaks camouflage and specifies target locations. Optic flow calibrates image structure with respect to spatial relations among targets and distracters, and calibrated image structure makes previously camouflaged targets perceptible in a temporally stable fashion after motion stops. We investigated this proposal using laboratory experiments and compared how many camouflaged targets were identified either with optic flow information alone or with combined optic flow and image structure information. Our results show that the combination of motion-generated optic flow and target-projected image structure information yielded efficient and stable perception of camouflaged targets.

Motion-generated optical information allows event perception despite blurry vision in AMD and amblyopic patients.

Events consist of objects in motion. When objects move, their opaque surfaces reflect light and produce both static image structure and dynamic optic flow. The static and dynamic optical information co-specify events. Patients with age-related macular degeneration (AMD) and amblyopia cannot identify static objects because of weakened image structure. However, optic flow is detectable despite blurry vision because visual motion measurement uses low spatial frequencies. When motion ceases, image structure persists and might preserve properties specified by optic flow. We tested whether optic flow and image structure interact to allow event perception with poor static vision. AMD (Experiment 1), amblyopic (Experiments 2 and 3), and normally sighted observers identified common events from either blurry (Experiments 1 and 2) or clear images (Experiment 3), when either single image frames were presented, a sequence of frames was presented with motion masks, or a sequence of frames was presented with detectable motion. Results showed that with static images, but no motion, events were not perceived well by participants other than controls in Experiment 3. However, with detectable motion, events were perceived. Immediately following this and again after five days, participants were able to identify events from the original static images. So, when image structure information is weak, optic flow compensates for it and enables event perception. Furthermore, weakened static image structure information nevertheless preserves information that was once available in optic flow. The combination is powerful and allows events to be perceived accurately and stably despite blurry vision.

Transfer of learning between unimanual and bimanual rhythmic movement coordination: transfer is a function of the task dynamic.

Under certain conditions, learning can transfer from a trained task to an untrained version of that same task. However, it is as yet unclear what those certain conditions are or why learning transfers when it does. Coordinated rhythmic movement is a valuable model system for investigating transfer because we have a model of the underlying task dynamic that includes perceptual coupling between the limbs being coordinated. The model predicts that (1) coordinated rhythmic movements, both bimanual and unimanual, are organised with respect to relative motion information for relative phase in the coupling function, (2) unimanual is less stable than bimanual coordination because the coupling is unidirectional rather than bidirectional, and (3) learning a new coordination is primarily about learning to perceive and use the relevant information which, with equal perceptual improvement due to training, yields equal transfer of learning from bimanual to unimanual coordination and vice versa [but, given prediction (2), the resulting performance is also conditioned by the intrinsic stability of each task]. In the present study, two groups were trained to produce 90° either unimanually or bimanually, respectively, and tested in respect to learning (namely improved performance in the trained 90° coordination task and improved visual discrimination of 90°) and transfer of learning (to the other, untrained 90° coordination task). Both groups improved in the task condition in which they were trained and in their ability to visually discriminate 90°, and this learning transferred to the untrained condition. When scaled by the relative intrinsic stability of each task, transfer levels were found to be equal. The results are discussed in the context of the perception-action approach to learning and performance.

Perception of relative throw-ability.

Bingham et al. (J Exp Psychol Hum Percept Perform 15(3):507-528, 1989) showed that skilled throwers can perceive optimal objects for throwing to a maximum distance. Zhu and Bingham (J Exp Psychol Hum Percept Perform 34(4):929, 2008, 36(4):862-875, 2010) replicated this finding and then showed that felt heaviness is used to perceive this affordance (see also Zhu and Bingham in Evol Hum Behav 32(4):288-293, 2011; Zhu et al. in Exp Brain Res 224(2):221-231, 2013). Throwers pick the best weight for spherical projectiles in each graspable size. Bingham et al. (J Exp Psychol Hum Percept Perform 15(3):507-528, 1989) speculated that relative throw-ability might be perceptible. This would mean that the ordering of distances achieved by maximum effort throws of different objects could be judged. This affordance property is not the same as optimal throw-ability, because it requires all projectiles to be evaluated relative to one another with respect to ordinally scaled distances, not just a discrete optimum. We now used a magnitude estimation task to test this hypothesis, comparing the resultant ordering with that exhibited by distances of throws in previous studies. The findings show that participants were able to perform the perceptual task. However, discrimination among objects of different weight within a size was better than between sizes. The implications of these results for understanding of the information used to perform this task are discussed.

Information and control strategy to solve the degrees-of-freedom problem for nested locomotion-to-reach.

Locomoting-to-reach to a target is a common visuomotor approach behavior that consists of two nested component actions: locomotion and reaching. The information and control strategies that guide locomotion and reaching in isolation are well studied, but their interaction during locomoting-to-reach behavior has received little attention. We investigated the role of proportional rate control in unifying these components into one action. Individuals use this control strategy with hand-centric disparity-based τ information to guide seated reaching (Anderson and Bingham in Exp Brain Res 205:291-306. doi: 10.1007/s00221-010-2361-9 , 2010) and use it with sequential information to perform targeted locomotion to bring an outstretched arm and hand to a target; first with eye-centric τ information and then hand-centric τ information near the target (Anderson and Bingham in Exp Brain Res 214:631-644. doi: 10.1007/s00221-011-2865-y , 2011). In the current study, participants performed two tasks: locomoting to bring a rigidly outstretched arm and hand to a target (handout), and locomoting to initiate and guide a reach to a target (locomoting-to-reach). Movement trajectories were analyzed. Results show that participants used proportional rate control throughout both tasks, in the sequential manner that was found by Anderson and Bingham (Exp Brain Res 214:631-644. doi: 10.1007/s00221-011-2865-y , 2011). Individual differences were found in the moment at which this information switch occurred in the locomoting-to-reach task. Some participants appeared to switch to proportional rate control with hand-τ once the hand came into view and others switched once the reaching component was complete and the arm was fully outstretched. In the locomoting-to-reach task, participants consistently initiated reaches when eye-τ specified a time-to-contact of 1.0 s. Proportional rate control provides a solution to the degrees-of-freedom problem in the classic manner, by making multiple things one.

The dynamics of sensorimotor calibration in reaching-to-grasp movements.

Reach-to-grasp movements require information about the distance and size of target objects. Calibration of this information could be achieved via feedback information (visual and/or haptic) regarding terminal accuracy when target objects are grasped. A number of reports suggest that the nervous system alters reach-to-grasp behavior following either a visual or haptic error signal indicating inaccurate reaching. Nevertheless, the reported modification is generally partial (reaching is changed less than predicted by the feedback error), a finding that has been ascribed to slow adaptation rates. It is possible, however, that the modified reaching reflects the system's weighting of the visual and haptic information in the presence of noise rather than calibration per se. We modeled the dynamics of calibration and showed that the discrepancy between reaching behavior and the feedback error results from an incomplete calibration process. Our results provide evidence for calibration being an intrinsic feature of reach-to-grasp behavior.

Perceived 3D metric (or Euclidean) shape is merely ambiguous, not systematically distorted.

Many studies have reported that perceived shape is systematically distorted, but Lind et al. (Inf Vis 2:51-57, 2003) and Todd and Norman (Percept Psychophys 65:31-47, 2003) both found that distortions varied with tasks and observers. We now investigated the hypothesis that perception of 3D metric (or Euclidean) shape is ambiguous rather than systematically distorted by testing whether variations in context would systematically alter apparent distortions. The task was to adjust the aspect ratio of an ellipse on a computer screen to match the cross-section of a target elliptical cylinder object viewed in either frontoparallel elliptical cross-section (2D) or elliptical cross-section in depth (3D). Three different groups were tested using two tasks and two different ranges of aspect ratio: Group 1) 2D(Small) → 3D(Large), Group 2) 2D(Large) → 3D(Small), Group 3a) 2D(Small) → 3D(Small), and Group 3b) 2D(Large) → 3D(Large). Observers performed the 2D task accurately. This provided the context. The results showed the expected order of slopes when judged aspect ratios were regressed on actual aspect ratios: Group 1 (SL) < Group 3 (SS and LL) < Group 2 (LS). The ambiguity of perceived 3D aspect ratios allowed the range of aspect ratios experienced in the 2D task to affect the 3D judgments systematically. Nevertheless, when the 2D and 3D ranges of aspect ratios were the same (LL and SS) and the 2D were judged accurately, this did not yield accurate 3D judgments. The results supported the hypothesis that perceived 3D metric shape is merely ambiguous rather than systematically distorted.

Perceptuo-motor learning rate declines by half from 20s to 70/80s.

This study examined perception-action learning in younger adults in their 20s compared to older adults in their 70s and 80s. The goal was to provide, for the first time, quantitative estimates of perceptuo-motor learning rates for each age group and to reveal how these learning rates change between these age groups. We used a visual coordination task in which participants are asked to learn to produce a novel-coordinated rhythmic movement. The task has been studied extensively in young adults, and the characteristics of the task are well understood. All groups showed improvement, although learning rates for those in their 70s and 80s were half the rate for those in their 20s. We consider the potential causes of these differences in learning rates by examining performance across the different coordination patterns examined as well as recent results that reveal age-related deficits in motion perception.

Felt heaviness is used to perceive the affordance for throwing but rotational inertia does not affect either.

Bingham et al. discovered a perceptible affordance property, composed of a relation between object weight and size, used to select optimal objects for long-distance throwing. Subsequent research confirmed this finding, but disconfirmed a hypothesis formulated by Bingham et al. about the information used to perceive the affordance. Following this, Zhu and Bingham investigated the possibility that optimal objects for throwing are selected as having a particular felt heaviness. The results supported this hypothesis. Perceived heaviness exhibits the size-weight illusion: to be perceived as equally heavy, larger objects must weigh more than smaller ones. Amazeen and Turvey showed that heaviness perception is determined by rotational inertia. We investigated whether rotational inertia would determine both perceived heaviness and throw-ability when spherical objects were held in the hand and wielded about the wrist. We found again that a particular judged heaviness corresponded to judged throw-ability. However, rotational inertia was found to have no effect on either judgment, suggesting that rotational inertia does not determine perceived heaviness of spherical objects held in the hand, as it did for the weighted-rod-type objects used by Amazeen and Turvey.

With an eye to low vision: optic flow enables perception despite image blur.

PURPOSE: From static blurry images, it is difficult to perceive objects because high spatial frequency details are filtered out. However, in the context of events (defined as objects in motion), motion generates optic flow, which provides a depth map of 3D layout and allows good event perception. Visual motion measurement uses low spatial frequencies that remain available in blurry images, making events perceivable. Optic flow and image structure are intrinsically related in vision because optic flow takes one image to the next. Optic flow is powerful in specifying depth structures and it calibrates the degraded image structure; image structure is persistent and it preserves events perceived with ongoing motion, after it stops. Might optic flow and image structures interact and allow events to be perceived despite poor quality images? The answer to this question has implications for event perception with low vision. METHODS: Twenty blurry images depicting each of eight daily events were used as stimuli. Ten normally sighted participants perceived the stimuli and described the events in five ordered conditions: (1) when single frames were presented, (2) when all frames were presented with motion masks, (3) when all frames were presented without motion masks, (4) when single frames were presented, and (5) when single frames were presented 5 days later. RESULTS: With blurry static images alone, participants were unable to identify events. Events were perceptible when the blurred images were played in sequence, making motion-generated information available. Subsequently, when given the original blurry static images again, post-motion performance was vastly superior to the pre-motion performance. Furthermore, the high rate of recognition persisted after 5 days. CONCLUSIONS: Optic flow calibrates low-quality image structure to allow accurate event perception during and after motion. This implies that low-vision observers should perform much better than allowed by weakened image structure information alone.

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.

Locomoting-to-reach: information variables and control strategies for nested actions.

Locomoting-to-reach is a basic perception/action behavior that requires visual information for the control of both locomotion and reaching components. We investigated the visual information and the control strategies used to guide both the head and the hand on approach to a target in a locomotion-to-reach task. In this study, participants were required to locomote in the dark to a lit target in three different conditions: monocular vision/target with image size, binocular vision/target with image size, and binocular vision/point-light target (without image size). In task one, participants brought their eyes to the target. In task two, participants brought their outstretched hand to the target. Movement trajectories for both tasks were analyzed. Results show that participants were significantly more accurate when binocular information was present. In both tasks, participants were found to use a proportional rate control strategy rather than a constant τ strategy. In the walk-to-reach task, they used monocular and/or binocular τ information to guide the head and then switched to using relative disparity τ to guide the hand to final target acquisition, switching when the hand centric τ became less than the head centric τ. Dynamical models of the information and control strategies were used to perform simulations that were found to fit the data well. The conclusion is that proportional rate control is used sequentially with head centric, then hand-centric τ-based information, using at each moment the τ with the smallest value.

Discovering affordances that determine the spatial structure of reach-to-grasp movements.

Extensive research has identified the affordances used to guide actions, as originally conceived by Gibson (Perceiving, acting, and knowing: towards an ecological psychology. Erlbaum, Hillsdale, 1977; The ecological approach to visual perception. Erlbaum, Hillsdale, 1979/1986). We sought to discover the object affordance properties that determine the spatial structure of reach-to-grasp movements--movements that entail both collision avoidance and targeting. First, we constructed objects that presented a significant collision hazard and varied properties relevant to targeting, namely, object width and size of contact surface. Participants reached-to-grasp objects at three speeds (slow, normal, and fast). In Experiment 1, we explored a "stop" task where participants grasped the objects without moving them. In Experiment 2, we studied "fly-through" movements where the objects were lifted. We discovered the object affordance properties that produced covariance in the spatial structure of reaches-to-grasp. Maximum grasp aperture (MGA) reflected affordances determined by collision avoidance. Terminal grasp aperture (TGA)--when the hand stops moving but prior to finger contact--reflected affordances relevant to targeting accuracy. A model with a single free parameter predicted the prehensile spatial structure and provided a functional affordance-based account of that structure. In Experiment 3, we investigated a "slam" task where participants reached-to-grasp flat rectangular objects on a tabletop. The affordance structure of this task was found to eliminate the collision risk and thus reduced safety margins in MGA and TGA to zero for larger objects. The results emphasize the role of affordances in determining the structure and scaling of reach-to-grasp actions. Finally, we report evidence supporting the opposition vector as an appropriate unit of analysis in the study of grasping and a unit of action that maps directly to affordance properties.