Something’s afoot when a massively parallel and distributed system shows a bottleneck in performance. We’ve known that numerous bottlenecks plague cognition since the 1940′s, but only with recent advances in neuroimaging have we been able to say whether these bottlenecks reflect the intrusion of executive operations (for managing goals and organizing cognitive processing) or a more passive “queueing” processes inherent to the selection of responses. Thanks to a number of very helpful (and interesting) reviews on a recent paper of mine, I’ve been pointed towards a fascinating study (by Jiang, Saxe and Kanwisher) suggesting that a queueing process might actually be to blame.
The particular “bottleneck” in cognitive performance investigated by Jiang et al is known as the “psychological refractory period,” which can be observed with a very simple manipulation: subjects must simply give separate responses to two different stimuli, presented in rapid succession (<500ms). Reaction times to the second stimulus are longer than when the stimuli are presented at a more leisurely pace (>500ms), as though the selection of responses is subject to a bottleneck in information processing. This bottleneck, or refractory period, persists even when the tasks require responses in different modalities (i.e., verbal and manual), suggesting that a relatively “central” source is to blame for the refractory period.
Jiang et al present two possible explanations for this effect: according to the passive queueing account, the second task “is held in a passive queue until the bottleneck is freed” from processing the first task. Alternatively, the active monitoring account holds that a number of executive operations must occur: the tasks must be ordered for processing, the first task must be checked for completion while the second task is “halted” (inhibited?), and the second task must be triggered when the first has passed through the bottleneck. Jiang et al assume that this active monitoring involves “significantly increased executive functions” and as such that they should be more invoked when the refractory period is observed.
Under the reasonable assumption that executive functioning of this kind would engage the prefrontal cortex, Jiang et al used neuroimaging during a refractory period paradigm. Their results show that the only part of the prefrontal cortex to show increased activity during short as opposed to long stimulus onset asynchrony was the right inferior frontal gyrus. From this, one might conclude that the right inferior frontal gyrus actually instantiates Jiang et al’s “active monitoring” process. Maybe.
Instead, Jiang et al hold the right inferior frontal gyrus to a higher standard: they predict that a region involved in active monitoring should show also greater BOLD response according to individual differences in the refractory period. Specifically, the region responsible for the refractory period should be greatest among those individuals showing the largest refractory period.
In a way, the prediction seems natural (particularly if you’re coming from an inhibitory perspective on this brain region). But what if the right inferior frontal gyrus actually reduces the psychological refractory period – say, if greater executive functioning actually helps you manage the “refractoriness”? After all, greater use of a putative executive function should probably improve task performance. Moreover, the refractoriness itself (i.e., the braking of motor output) might actually be the direct but involuntary result of a completely different, though connected region (say, the subthalamic nucleus), whereas the greater recruitment of “active monitoring” might help one detect the stimuli earlier, ultimately streamlining performance in the task. Of course, under this redefinition of monitoring, we’d expect the opposite effect: greater use of monitoring should be associated with a reduced bottleneck. Jiang et al observed this exact effect, but counter-intuitively interpreted it to mean that the activity in the right inferior frontal gyrus was not due to executive functions!
In a second experiment, they suggest that while the right inferior frontal gyrus does not implement an executive function, it may show greater activity due to effort. To manipulate effort, Jiang et al instructed subjects to employ a conservative strategy (i.e., “take your time”) in some trials and a “daring” strategy in others (i.e., “respond as quickly as possible!”).
A number of regions in the prefrontal cortex showed increased activation as a result of the effort manipulation (including the anterior cingulate, the pre-supplementary motor area, and bilateral middle frontal gyrus…) but the right inferior frontal gyrus was the only gray matter showing more activity both when the refractoriness was induced and when subjects were instructed to effortfully minimize it.
From this, Jiang et al conclude that “the effort to reduce the postponement is active” but suggest that the activity in the right inferior frontal gyrus is not related to executive functioning. I’d like to propose an alternative:
1) activity in the right inferior frontal gyrus is related to executive functions: it’s under conscious control, it’s engaged in a task-appropriate fashion, and this engagement predicts better task performance
2) activity in the right inferior frontal gyrus is related to an active monitoring process, and this is its executive function, but it’s of a different kind than considered by Jiang et al: right inferior frontal activity doesn’t reflect the monitoring of working memory (consistent with what Jiang et al seem to be arguing, and with previous work in primates (thanks, Reviewer #2 ;)), but does reflect active monitoring for the occurrence of stimuli in the environment. Detection of these stimuli provoke an involuntary “refractoriness” – perhaps by triggering activity in the subthalamic nucleus – but this itself is not the role of the right IFG (as reflected in the negative correlation between rIFG activity and the refractoriness observed here).
Of course, I don’t expect this account of right IFG function to be commonly accepted until there’s more neuroimaging data directly supporting it. Now, back to writing up that data of ours…