Further evidence for, and some against, a planning–control dissociation

We summarize a number of recent results from our own experiments as well as those from other laboratories. Some of these results support Glover’s planning/control dissociation and some are at odds with it. We suggest that the model needs to be further refined and expanded. In his target article, Glover has provided a welcome alternative theory for the evidence related to the perception-action/dorsalventral visual stream controversy. Like many new theories, it leads to more questions than answers, but this is not necessarily a bad thing, if it motivates those of us doing sensorimotor research to delve more deeply into the issues surrounding this controversy. Indeed, because Glover’s planning–control dichotomy appears to map onto a circumscribed set of brain areas, it makes several predictions regarding activation patterns or the effects of lesions. For example, he suggests that transcranial magnetic stimulation (TMS) delivered to sensorimotor areas underlying planning or control should lead to corresponding deficits in these behaviors under appropriate conditions. A recent study from one of our labs has addressed this very question in the context of the perceptionaction debate (Lee & van Donkelaar 2002). Subjects were asked to point to the central circle in an Ebbinghaus display while TMS was delivered to either dorsal or ventral stream sites. Previously, we had shown that when the target circle appeared to be large, pointing movement times were shorter and velocities were greater than when the circle appeared to be small (van Donkelaar 1999). When TMS was delivered over either dorsal or ventral stream sites, this effect was significantly reduced. Interestingly, dorsal but not ventral stream stimulation also reduced the effects of target size even in a control condition without surrounding circles. We interpreted these findings, taken together, to suggest that the dorsal stream contribution to the effect was mainly related to the motor aspects of the task, a conclusion that does not appear to differ drastically from Glover’s point of view. A key issue with respect to his theory, however, is whether we were stimulating in the IPL or the SPL. According to the planning–control model, processing that occurs within the IPL is proposed to underlie the planning of the motor response and thus be susceptible to illusion effects. By contrast, SPL processing is thought to contribute to online control and to correct for any illusion effects induced during planning but to otherwise be immune to their influence. Therefore, according to the planning–control model, TMS over the IPL should significantly reduce the illusion effect because of a disruption of the planning process. By contrast, SPL stimulation should actually enhance the effect because of a disruption to the on-line control underlying the corrections in response to the illusion. Clearly, we observed the former result, which implies that we were in fact stimulating the IPL. Localizing TMS sites is not the most exact science, but various clues can be used to make good approximations. For example, the site of stimulation in our study was 7 cm posterior to the motor hot point. By comparison, in a study by Desmurget and colleagues (1999) examining the effects of TMS on on-line corrections within Commentary/Glover: Separate visual representations in the planning and control of action 52 BEHAVIORAL AND BRAIN SCIENCES (2004) 27:1 Figure 1 (Vaillancourt et al.). Visuomotor Process in IPL and SPL. The group functional map was obtained from a group Fisher test of the force with visual feedback minus rest t-map across the ten subjects. The group map was overlaid on a single subject’s Talairach transformed brain. The image is shown from a radiological perspective. the posterior parietal cortex (PPC), a site 4 cm posterior and 0.5 cm medial to the motor hot point was stimulated. This difference in the relative locations of the stimulation sites in the two studies and the typical underlying layout of the intraparietal region suggest that we were in fact affecting IPL processing. The results from our TMS study therefore appear to be consistent with Glover’s planning–control model. More generally, however, we feel that Glover’s theory does not place enough emphasis on the contributions to planning and control from areas such as the premotor cortex, supplementary motor area, and motor cortex. There is a tendency in the target article to focus on the functional distinctions between the SPL and IPL, with only a limited discussion of some of the areas to which these dorsal stream sites project. Indeed, perhaps a more appropriate approach is to think of planning/decision-making as a parietal-prefrontal process and control/execution as a motor area process with an evolution from planning to control as one moves from input to output areas. For example, one criterion for control/ execution should be the ability to elicit responses by direct stimulation. This definitely holds for motor areas such as the motor and premotor cortex, and (for eye movements) the frontal eye fields and superior colliculus. Moreover, there is clear evidence that control processes can occur within the motor cortex. In particular, Desmurget and colleagues (2001) have used brain imaging to demonstrate that the motor cortex (along with the cerebellum and PPC) is activated specifically during the on-line control of reaching movements. It would therefore seem that control can occur at even fairly low levels in the sensorimotor system. We think that it is vital to expand the model to include the contributions from these levels to the control process. Finally, although Glover explicitly states that the planning–control model is not meant to generalize to eye movements, we feel that the well-documented relationships between hand and eye movements can provide further insights with which to judge the model. For example, Glover uses the results of a study on the Roelofs illusion (Bridgeman et al. 1997) as evidence for his planning–control model of reaching – while the illusion does affect movements to remembered targets, movements with no delay are presumably corrected through on-line control so that the illusion’s effect on planning is eliminated. However, recent work here (Dassonville & Bala 2002) demonstrates that saccadic eye movements show the exact same pattern of accurate and inaccurate localizations for immediate and remembered saccades, respectively. Given the ballistic nature of saccades, though, on-line control cannot be used in an analogous way to explain the lack of illusory effects for immediate saccadic responses. It follows that either the saccadic system uses an altogether different mechanism to overcome the illusion than does the manual motor system, or that they both use a single mechanism that is not based on a planning–control distinction – to us, the latter possibility seems more parsimonious. Thus, a better understanding of the relations between eye and hand under illusory conditions (e.g., Binsted & Elliot 1999) will undoubtedly provide further insight into this issue. For now, though, it seems that the jury is still out.