7/12/2006

Task-Switching: A Role for Inferior Parietal Cortex

Much recent research addresses "cognitive flexibility," which refers to the ability to flexibly switch between tasks and use mental resources appropriately. Task switching is a useful paradigm in which to measure flexibility, because people usually incur a "switch cost" - a slowing of reaction times - after switching tasks. However, as Badre and Wagner note in a recent issue of PNAS, the source of this switch cost remains controversial.

Some have conceptualized "switch cost" as arising from the need to inhibit the old task set. Others have suggested that it may arise from the need to more powerfully activate the new task set (in the absence of any directed inhibition against the old task set). Yet others believe it to be a mixture of the two. Badre and Wagner also mention what they term "reconfiguration theories" of switch cost, in which slowed reaction times are thought to arise from an intentional process of task set reconfiguration which is largely independent of target stimulus presentation.

As Badre and Wagner point out, all of these theories share the idea that a new task set must be activated. Citing fMRI evidence, the authors note that activity in ventrolateral prefrontal cortex (VLPFC) is frequently interpreted as serving to overcome proactive interference, in both semantic and episodic tasks. A network of regions appear active during the simplest task-switching contrast (between task-repeat and task-switch trials), including VLPFC as well as supplementary motor area (SMA) and inferior/superior parietal cortices. To Badre and Wagner, the fact that VLPFC is seen to be active in both reconfiguration as well as interference theories supports the idea that VLPFC is involved in overcoming interference.

Badre and Wagner thus sought to experimentally verify their hypothesis, by using measures of interference from a computational neural network model (named "CAM-TS") as quantitative predictions in an fMRI experiment. In the first experiment, subjects were given a cue as to what task they should perform on the next stimulus (either an odd/even judgment or a vowel/consonant judgment), and the stimulus was then presented (a digit/letter pair). On half of the trials, the judgment repeated from the previous trial, and on the other hald, the judgment switched. The authors also manipulated the delay between cue and stimulus (thus changing "preparedness") as well as the type of manual response (on some switch trials the correct response would require pressing the same button as the previous trial, despite the fact that the judgment required had changed).

As usual, task switching resulted in a switch cost, including an effect where switch cost was greatest on trials where the response had remained the same as the previous trial. In addition, shorter cue-stimulus intervals resulted in larger switch costs. These findings were then used to evaluate the neural network model mentioned earlier, such that the cue-stimulus interval was found to negatively correlate with switch cost only when the model included a "cognitive control" parameter that selectively up-regulated the gain on a "task" layer, which suggests that the idea of an intentional and effortful process of task set selection is a real part of task switching cognition. The network also successfully simulated the effect in which switch cost is greated in response-repeat trials, because the irrelevant task set information had always left it's strongest impression on the last response pathway that was used - resulting in additional proactive interference that must be overcome during a task-switch.

Using a comparison of the activity in competing units as a measure of conflict (on the output layer as a measure of "response conflict" and on the internal representations layer as a measure of "switch-related conflict"), the authors showed that switch-related conflict declined with increasing cue-stimulus intervals, whereas response conflict was seen to increase with increasing cue-stimulus intervals. This provided a prediction that was subsequently investigated in a replication of the first experiment.

In this second experiment, the only region to show negatively correlated activity with increasing CSI was mid-VLPFC, suggesting again that VLPFC activity indexes conceptual or switch-related conflict. In contrast, the only region to show a positive correlation with increasing CSI was inferior parietal cortex, suggesting that inferior parietal activity may index response related conflict.

This finding is consistent with other suggestions that inferior parietal activity may be more related to switching per se, or in other words may be specific to the remappings between stimulus and response. VLPFC, in contrast, is thought to have a more general role in active maintenance processes, which could be useful in overcoming high cognitive or conceptual conflict during a task-switch.

However, Badre and Wagner argue that task switching should be possible even with a damaged VLPFC, given that it is merely involved in overcoming cognitive conflict, and they cite relevant neuropsychological evidence that supports this claim. This assertion contrasts with many other models which tend to conceptualize task-switching as highly dependent on VLPFC maintenance to overcome the interference built up by experience with the preswitch task - particularly those which view age-related perseveration as resulting from a lack of prefrontal development. In contrast, Badre and Wagner might argue that age-related changes in task switching ability (i.e., remappings between stimulus and response) are more directly related to either inferior parietal development. In contrast, "learning rate" (i.e., plasticity) and prefrontal development are relevant only insofar as they counteracts the effects of interference from previous tasks.

Related Posts:
Task Switching in Prefrontal Cortex
The Rules in the Brain
Models of Active Maintenance as Oscillation
Selection Efficiency and Inhibition
An End to the Tyranny of Inhibition
Attention: The Selection Problem

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