Regular articleWhen the same response has different meanings:: recoding the response meaning in the lateral prefrontal cortex
Introduction
One of the most fascinating properties of intelligent behavior is the human ability to apply a restricted behavioral repertoire to an infinite number of different task situations. Pressing a light switch, for example, can have different outcomes (switch the light on or off) depending on the context. To be able to use the same physical response (pressing the light switch) to achieve different goals (switching the light on or off) one has to recode the response meaning when switching from one task situation to the next Meiran, 2000, Schuch and Koch, 2003. We will refer to this process as “response recoding.” Recently it was found that neurons in the prefrontal cortex of monkeys represent the response meaning depending on a given task context (Asaad et al., 2000). In this study prefrontal cortex neurons were found to be activated in preparation of a specific response but only when the response occurred in a given task context. These results suggest that the prefrontal cortex might be involved in processing task-context specific response information (the response meaning).
In cognitive psychology the influence of the task context on task performance is usually investigated in so-called task-switching paradigms. A number of neuroimaging studies have used this paradigm to investigate task-related control processes Brass and von Cramon, 2002, Dove, 2000, Dove et al., 2000, Kimberg et al., 2000, Omori, 1999, Pollmann et al., 2000, Sohn et al., 2000. However, to our knowledge recoding the response meaning was never directly investigated with functional MRI. Some previous studies have manipulated the stimulus-response mapping (S-R mapping) between two tasks, which probably involves response recoding Dove et al., 2000, Nagahama, 2001, Pollmann et al., 2000. Reversing the S-R mapping yielded activation in the prefrontal cortex. However, because these studies did not manipulate response recoding independently from task switching, it is not clear whether the prefrontal activation was due to general task-set reconfiguration or to response recoding.
The aim of the present study was to directly investigate the cortical basis of our ability to use the same physical response to achieve different goals. To do so, we compared a situation in which subjects were required to recode the meaning of their responses while switching from one task to the next with a situation in which they were not required to recode the response meaning.
Section snippets
Experimental design
In the study we used a paradigm in which subjects had to switch between two simple spatial tasks (Meiran, 1996). In one task (task A) they were required to decide whether a square, which was presented in a two-by-two grid, was in the upper or lower half of the grid, and in the other task (task B) subjects had to decide whether the square was on the left or on the right side of the grid (Fig 1). The task executed in a given trial was indicated by two arrows presented either above or below the
Behavioral results
The behavioral data replicate previous studies; they show that subjects were significantly slower in switch trials (transition from task A to B or task B to A) than in repetition trials (repetition of task A or task B), F(1,17) = 82.3, P < 0.001. This switch effect (635 vs. 594 ms) reflects the costs of switching from one task to the next. This effect was larger in the bivalent (51 ms) than in the univalent condition (31 ms), F(1,17) = 12.2, P < 0.01, indicating, that the switch costs were
Discussion
By manipulating the overlap of response sets in the univalent and bivalent condition, we demonstrate that the LPFC mediates the change of response meaning when the person switched from one task to the next. An alternative explanation of this interpretation might be that participants, instead of recoding the response in switch trials, represent the response in a multidimensional response space. However, in incongruent trials this is not a reasonable strategy, since the same stimulus requires two
Acknowledgements
This work was supported by the German-Israeli Foundation for Scientific Research and Development (Grant Number: G-635-88.4/1999).
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