S.R. Caljouw

The impact of task constraints on the planning and control of interceptive hitting movements

On the basis of research in self-paced aiming movements, Glover [S. Glover, Separate visual representations in the planning and control of action, Behav. Brain Sci., 27 (2004) 3-24] proposed a dichotomy between visual systems that accommodate planning and on-line control of action. Specifically, the planning-versus-control model posits that the on-line control system solely adjusts the spatial parameters of a movement. We examined whether this proposition is also adequate for interceptive hitting movements that require being at a specific location at the right time. Participants had to hit an approaching ball by first moving the bat away from the interception point (preparatory phase) and subsequently make the hitting movement (strike phase). The ball had to be projected to a landing location that could be near or far. To examine the ability of actors to use online visual information to adapt to unexpected changes in impact requirements, we perturbed the distance of the landing location from near to far during the unfolding of the movement. No adjustments were observed when the perturbation occurred at the onset of the strike phase. When the perturbation occurred at the onset of the preparatory phase the impact velocity increased and, therefore, the ball traveled a larger distance. This was realized by decreasing the duration of the strike phase and increasing the movement amplitude. So, both spatial and temporal characteristics of the hitting movement were adjusted to comply with an online perturbation of target distance. This observation is incongruent with Glovers recent proposition that the online visual system only accommodates spatial parameters.

A dynamical neural network for hitting an approaching object

Abstract.Besides making contact with an approaching ball at the proper place and time, hitting requires control of the effector velocity at contact. A dynamical neural network for the planning of hitting movements was derived in order to account for both these requirements. The model in question implements continuous required velocity control by extending the Vector Integration To Endpoint model while providing explicit control of effector velocity at interception. It was shown that the planned movement trajectories generated by the model agreed qualitatively with the kinematics of hitting movements as observed in two recent experiments. Outstanding features of this comparison concerned the timing and amplitude of the empirical backswing movements, which were largely consistent with the predictions from the model. Several theoretical implications as well as the informational basis and possible neural underpinnings of the model were discussed.

Chapter 19 The fallacious assumption of time-to-contact perception in the regulation of catching and hitting

Abstract Interceptive actions, such as catching and hitting, were thought to be timed on the basis of a critical value of perceived time-to-contact (TTC), provided by the optical variable τ(). This optical variable is a monocular invariant that specifies, given a constant approach velocity, the time-to-contact of the ball with the observers eye. The present Chapter shows that the original formulations of this TTC model were fatally flawed in many ways and that they are subjected to revision. The second part of this Chapter presents alternative models for the information-based regulation of interceptive actions. We no longer present the perception of TTC as an intermediate phase in interceptive timing. Instead, we propose that information directly regulates action. Furthermore, we suggest that not a sole variable, but multiple variables might constrain natural interceptive actions. The question what information contributes to interceptive timing cannot be posed in isolation from the question how information is used to regulate timing. The original TTC model assumed a critical timing strategy, but there is also online regulation. The final section of the present chapter discusses different online control models and their informational inputs.

Visual constraints in the development of action

The chapters aim is to understand the role of visual information in the control of avoidance and interception behaviors in infancy from the ecological psychology approach to perception and action. We show that during infancy developmental change in action is associated with the use of different information sources and that this process of attunement promotes the perceived action possibilities (affordances). In the final section, we position these findings within Milner and Goodales two-visual system model, which holds that perception and action are mediated by two functionally and neuron-anatomically separate visual (sub-)systems.