Sensory Gating by Prefrontal Cortex

In a 2005 JOCN paper, Brad Postle argues that the dorsolateral prefrontal cortex is not responsible for storing information in short-term memory, but rather than it carries out control operations which include the "gating" or modulation of activity in more posterior regions. According to the argument in this paper, short-term storage of information is achieved by sustained firing in modality-specific posterior regions, and DLPFC's role is to make sure that distracting or irrelevant stimuli do not interfere with the maintenance of that information.

To support this argument, Postle puts 16 young adults in an fMRI scanner and presents them first with a target face, followed by a 7 second delay, followed by a probe face. On "memory present" trials, subjects had to identify whether the probe face was the same as the target face, disregarding any other faces that might have been presented during the delay period. For "memory absent" trials, subjects had to identify whether there were intervening distractor faces present during the delay period. The author estimated the hemodynamic response function for each participant, and identified two regions of interest on each subject's anatomical scans: the dorsolateral prefrontal cortex (DLPFC) and the inferior occipito-temporal cortex (IOTC).

The results indicate that DLPFC massively increases its activity during the delay period when a memory judgment is required, but interestingly, IOTC shows the opposite pattern, in that activity is actually slightly higher when a memory judgment is not required. Postle interprets this to show that DLPFC is suppressing or gating activity in posterior cortex to help it resist interference from distractors.

However, several caveats should be mentioned:

• For trials where memory judgments are not required, subjects were instead required to indicate whether distractor faces were present. For these trials, subjects may have actively recruited IOTC regions during the delay period, so as to increase their sensitivity to distractors; therefore, the difference between memory-absent and memory-present IOTC activity may reflect active recruitment rather than suppression of activity.
• Decreased activation in IOTC could also reflect "streamlined processing" of the maintained target face. In other words, DLPFC may be maintaining a "skeleton representation" of the target face during the delay, against which representations of the distractor faces would have to compete. In this way, DLPFC is not actively suppressing distracting information, but is doing so indirectly, by powerfully maintaining a specific representation with which distractors must compete.
• While DLPFC is clearly differentially engaged by memory absent and memory present trials, the case for differences in IOTC activity is not so clear. For example, DLPFC increased its activity by 139% while IOTC decreased its activity by only 7.5% on memory-present relative to memory-absent trials.
• Another alternative account is that increases in PFC activity for memory-present trials actually reflects dual tasking, in that subjects may be maintaining information about the target face and accidentally processing additional some aspects of the distractor faces. Although DLPFC activity was numerically stronger for distractor absent trials, this difference did not reach significance, suggesting the difference could be due to chance.
• This explanation is somewhat less parsimonious than the idea that DLPFC is storing the information, because it requires believing that IOTC is actually maintaining the target face information despite decreases in activity relative to trials where no memory is required, and then assuming that this decrease exists because distractor faces are suppressed. In contrast, one could explain the apparent decrease in IOTC activity as resulting from comparison to the task where the goal was to identify distractor faces.

Quibbles aside, this is a nice demonstration of how working memory might get done - through maintenance (or operations controlling that maintenance) as related to task demands. However, this study does not offer conclusive evidence about where information is actually stored, and how it is made robust to potential interference.