Many strands are woven into the ideas and work of Jeffrey Gray. From a background of classical languages and a spell in military intelligence spent honing skills in languages and typing, he took two BA degrees (in modern languages and psychology) at Oxford University. He then trained as a clinical psychologist at the Institute of Psychiatry (IOP), London, capping this with a PhD on the sources of emotional behaviour.

Evidence for a dichotomy between the planning of an action and its on-line control in humans is reviewed. This evidence suggests that planning and control each serve a specialized purpose utilizing distinct visual representations. Evidence from behavioral studies suggests that planning is influenced by a large array of visual and cognitive information, whereas control is influenced solely by the spatial characteristics of the target, including such things as its size, shape, orientation, and so forth. Evidence from brain imaging and neuropsychology suggests that planning and control are subserved by separate visual centers in the posterior parietal lobes, each constituting part of a larger network for planning and control. Planning appears to rely on phylogenetically newer regions in the inferior parietal lobe, along with the frontal lobes and basal ganglia, whereas control appears to rely on older regions in the superior parietal lobe, along with the cerebellum.

Consistent with the planning–control model, recent fMRI data reveal that the inferior parietal lobe, the frontal lobes, and the basal ganglia are involved in motor planning. Inconsistent with the planning–control model, however, recent behavioral data reveal a spatial repulsion effect, indicating that the visual context surrounding the target can sometimes influence the on-line control of goal-directed action.

The planning/control distinction is an important tool in the study of sensorimotor transformations. However, published data from our laboratories suggest that, contrary to what is predicted by the proposed model, (1) structures in the superior parietal lobe of both monkeys and humans can be involved in movement planning; and (2) fast pointing actions can be immune to visual illusions even if they are performed without visual feedback. The planning–control model as proposed by Glover is almost certainly too schematic.

This commentary focuses on issues related to Glover's suppositions regarding the information available to the on-line control system and the behavioral consequences of (visual) information disruption. According to the author, a “highly accurate,” yet temporally unstable, visual representation of peripersonal space is available for real-time trajectory corrections. However, no direct evidence is currently available to support the position.

The perception/planning–control conception has a direct predecessor in a cognitive/sensorimotor scheme, where the cognitive branch includes Glover's perception and planning functions. The sensorimotor branch corresponds to Glover's control function. The cognitive/sensorimotor scheme, like the perception/planning–control scheme, differentiates between motor planning and direct motor control, which is inaccessible to awareness or to long-term memory.

We argue that one can explain why the influence of illusions decreases during a movement without assuming that different visual representations are used for planning and control. The basis for this is that movements are guided by a combination of correctly perceived information about certain attributes (such as a target's position) and illusory information about other attributes (such as the direction of motion). We explain how this can automatically lead to a decreasing effect of illusions when hitting discs that move in an illusory direction, and when grasping objects of which the apparent size or orientation has been changed by an illusion.

Some data concerning visual illusions are hardly compatible with the perception–action model, assuming that only the perception system is influenced by visual context. The planning–control dichotomy offers an alternative that better accounts for some controversy in experimental data. We tested the two models by submitting the patient I. G. to the induced Roelofs effect. The similitude of the results of I. G. and control subjects favoured Glover's model, which, however, presents a paradox that needs to be clarified.

Glover's planning–control model accommodates a substantial number of findings from subjects who have motor deficits as a consequence of brain lesions. A number of consistently observed and robust findings are not, however, explained by Glover's theory; additionally, the claim that the IPL supports planning whereas the SPL supports control is not consistently supported in the literature.

Very young children occasionally commit scale errors, which involve a dramatic dissociation between planning and control: A child's visual representation of the size of a miniature object is not used in planning an action on it, but is used in the control of the action. Glover's planning–control model offers a very useful framework for analyzing this newly documented phenomenon.

Several lines of evidence indicate that the on-line control of rapid target-aiming movements can be influenced by the visual context in which the movements are performed. Although this may result in movement error when an illusory context is introduced, there are many situations in which the control system must know about context in order to get the limb to the target rapidly and safely.

Glover's planning–control model is based on his finding that visual illusions exert a larger effect in early phases than in late phases of a movement. But evidence for this dynamic illusion effect is weak, because: (a) it appears difficult to replicate; (b) Glover overestimates the accuracy of his results; and (c) he seems to underestimate the illusion effect at late phases.

We address three issues that might be important in evaluating the validity of the planning–control model: (1) It could be artificial to distinguish between control and planning when control involves the re-planning of a new corrective submovement that overlaps with the initial response; (2) experiments involving illusions are not totally compelling; (3) selectively implicating the superior parietal lobe in movement control and the basal ganglia in movement planning, appears questionable.

Our commentary focuses, first, on Glover's proposal that only motor planning is sensitive to cognitive aspects of the target object, whereas the on-line control is completely immune to them. We present behavioural data showing that movement phases traditionally (and by Glover) thought to be under on-line control, are also modulated by object cognitive aspects. Next, we present data showing that some aspects of cognition can be coded by means of movement planning. We propose a reformulation of Glover's theory to include both an influence of cognition on on-line movement control, and a mutual influence between motor planning and some aspects of cognition.

It is our contention that the concept of planning in Glover's model is too broadly defined, encompassing both action/goal selection and the programming of the constituent movements required to acquire the goal. We argue that this monolithic view of planning is untenable on neuropsychological, neurophysiological, and behavioural grounds. The evidence demands instead that a distinction be made between action planning and the specification of the initial kinematic parameters, with the former depending on processing in the ventral stream and the latter on processing in the dorsal stream.

Glover suggests that representational systems for planning versus control are mapped exclusively to the inferior (IPL) versus superior (SPL) parietal lobules respectively. Yet, there is ample evidence that the IPL and SPL both contribute to action planning and control. Alternatively, I distinguish between the parietal-frontal systems involved in the representation of acquired manual skills versus nonskilled actions.

Glover argues that separate representations underlie the planning and the control phase of actions, and he contrasts his model with Goodale and Milner's perception/action model. Is this representation indeed an independent representation within a more general action system, or is it an epiphenomenon of the interaction between the perception/action systems of the Goodale–Milner model?

Studies with primates in sequence production tasks reveal that chimpanzees make action plans before initiating responses and making on-line adjustments to spatially exchanged stimuli, whereas such planning isn't evident in monkeys. Although planning may rely on phylogenetically newer regions in the inferior parietal lobe – along with the frontal lobes and basal ganglia – it dates back to as far as five million years ago.

Understanding neural anatomy and physiology depends on first understanding the behaviour being mediated. Glover, in his review of earlier work suggesting various dichotomies in visual processing, shows how there is a tendency to oversimplification if this approach is ignored. His own new proposals demonstrate the advantages of allowing function to drive anatomical analysis. Nevertheless, the new planning–control dichotomy he proposes, though a valuable advance, is itself an oversimplification of what must be a multi-channel system.

We question the generalizability of Glover's model because it fails to distinguish between different forms of planning. The highly controlled experimental situations on which this model is based, do not reflect some important factors that contribute to planning. We discuss several classes of action that seem to imply distinct planning mechanisms, questioning Glover's postulation of a single “planning system.”