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.

The distinctionbetween the premovement planning of an action and its on-line control has a long history (e.g., Jeannerod, 1988;Keele & Posner, 1968;Woodworth, 1899). Here, we demonstrate that the earlier portions of a grasping movement are more affected by the Ebbinghaus illusion than are the latter portions. These results provide further support for a planning/control model (Glover, 2001;Glover & Dixon, 2001a, 2001b, 2001d in which planning is more susceptible to illusions than control. The results do not support a perception/action model (e.g., Aglioti, DeSouza, & Goodale, 1995;Bridgeman, 1999;Bridgeman, Peery, & Anand, 1997;Goodale & Milner, 1992;Milner & Goodale, 1995), in which both planning and control are thought to be less susceptible to illusions than are perceptions.Woodworth (1899) was the first to demonstrate the distinctionbetween the premovement planning and the on-line control of action. Woodworth found that when participants were asked to draw lines of particular lengths, they required sufficient time to correct the actions on line. For example, accuracy was worse at a drawing rate of 400 msec per line than at slower rates. Furthermore, if the linedrawing task was done without vision, participants' performance at all speeds was just as poor as when the task was done quickly with vision. Woodworth reasoned from this that actions were composed of two stages: an "initial impulse" stage that reflected the premovement planning of the action and a subsequent "current control" stage that reflected the on-line correction of an action via feedback mechanisms.Since Woodworth's (1899) seminal study, much research has gone into characterizingthese two stages of action (e.g., Abrams & Pratt, 1993;Elliot, Binsted, & Heath, 1999;Flash & Henis, 1991;Keele & Posner, 1968;Khan, Franks, & Goodman, 1998;Meyer, Abrams, Kornblum, Wright, & Smith, 1988;Pratt & Abrams, 1996), and some distinctions between the two stages have been elucidated. For example, planning appears to be a relatively slow and deliberate process. The minimum time required to initiatea movement has been found to be around 250 msec (Stark, 1968). Conversely, on-line control appears to operate relatively quickly. In contrast to the 400-msec lag between planning and control stages hypothesized by Woodworth on the basis of his original study, the benefits of visual and proprioceptive feedback during on-line control have more recently been found to occur in as little as 70-150 msec (e.g., Evarts & Vaughn, 1978;Lee & Tatton, 1975;Smeets, Erkelens, & van der Gon, 1990;Zelaznik, Hawkins, & Kisselburgh, 1983).We have hypothesized that another difference between planning and control may be how each is affected by context-induced visual illusions. In the planning/control model (Glover, 2001;Glover & Dixon, 2001a, 2001b, 2001d, context-induced illusions affect the planning process, but not the on-line control process. We use the term context-inducedto refer to distortions that arise owing to the visual context surrounding the target, as opposed to those that re...

Action affordances can be activated by non-target objects in the visual field as well as by word labels attached to target objects. These activations have been manifested in interference effects of distractors and words on actions. We examined whether affordances could be activated implicitly by words representing graspable objects that were either large (e.g., APPLE) or small (e.g., GRAPE) relative to the target. Subjects first read a word and then grasped a wooden block. Interference effects of the words arose in the early portions of the grasping movements. Specifically, early in the movement, reading a word representing a large object led to a larger grip aperture than reading a word representing a small object. This difference diminished as the hand approached the target, suggesting on-line correction of the semantic effect. The semantic effect and its on-line correction are discussed in the context of ecological theories of visual perception, the distinction between movement planning and control, and the proximity of language and motor planning systems in the human brain.