From Perception To Action: The Role of the P3b in Binding

Last week I discussed how central dopamine levels appear to correlate with how strongly actions are bound to particular visual features. I presented this as part of "the binding problem," but in fact the topic runs must deeper: cognitive neuroscience has yet to reveal the mechanisms by which the motor system "links up" with the perceptual system. This is the topic covered by Verleger et al's 2005 J. Psychophysiology article, which points not to dopamine, nor to multiplexed neural synchrony, but rather to the "P3b" component of scalp electrical activity as the "glue" that binds together action and perception.

Verleger et al. begin their article with a review of the P3b, a component of scalp electrical activity that is typically seen in response to task-relevant but relatively infrequent or contextually novel events. The classic example of the P3 response comes from the oddball paradigm, where subjects must ignore a series of frequent and infrequent stimuli to detect a relatively infrequent target. Increased positivity in fronto-parietal electrical potentials are found anywhere between 300 and 600 msec after the onset of the rare stimulus, whether that stimulus is novel (e.g., "beep beep beep rulp"), contextually infrequent (e.g., "go go go stop"), or is grammatically incorrect (e.g., "the horse is run.") In general, this positivity is higher in amplitude if the subject is actively attending for rare stimuli.

Based on this wide variety of "eliciting conditions," some have proposed that the P3 component reflects the updating of context representations in working memory - as if to say "hey, there's something unusual and possibly important going on here." Verleger et al. suggest this theory falls short of explaining the available evidence, partly because it is not well-specified and partly because a wide variety of brain regions (including those that are not often thought to be involved in working memory, such as the temporo-parietal junction) also show P3-like electrical potentials, as recorded from electrodes implanted in epileptics.

Also unclear is the relationship of P3 to motor responses. Some have showed that delays in responding are accompanied by delays in P3 potentials only when features of a stimulus change, and yet others have shown nearly the opposite. One possible reason for these contradictory findings is that the P3 (and even the P3b subcomponent) response may actually consist of two smaller components, one of which has been interpreted to reflect "stimulus updating" and the other "response updating".

In contrast, Verleger et al. suggest that the P3 response may reflect a decision process at the intersection of stimulus processing and response preparation. In other words, the P3 itself may reflect the binding of perception and action. The authors made three prediction to test this hypothesis:

1) The P3 component should have equally large amplitudes regardless of whether the electrical activity is aligned to the onset of stimuli, and then summed, as when aligned to the execution of a response and then summed.

2) The first prediction should be equally true of any set of response times, including those that are particularly fast and those that are particularly slow.

3) The peak latency of P3 responses should vary with response times, even when an individual's P3 activity is analyzed on their fastest and slowest trials independently.

Methodological details follow in italics:

To test these predictions, Verleger et al. presented 12 subjects with a visual and an auditory Simon task. In the visual task, subjects had to press a left key when they saw an "A" and a right key if they saw a "B," but these letters could appear either on the right or the left side of the screen. In the auditory task, subjects had to press a left key if they heard a high tone, and a right key if they heard a low tone, but these tones could originate from either the left or the right. Thus, in both tasks, subjects could encounter a compatiable stimulus-response mapping (e.g., "B" on the right or a low tone from the right) or an incompatiable stimulus-response mapping (e.g., "B" on the left or a low tone on the left). While the subjects completed the task, Verleger et al. recorded both scalp electrical activity as well as "response force" on each of the keys (which increases steadily as subjects prepare to give a response, and thus gives a good index of response readiness).

All of the authors' predictions were confirmed, consistent with a role for the P3 component to mediate the relationship of stimulus processing with response preparation.

Interestingly, the same was not true of an early P3 component, known as the P3a, which is thought to originate from ventrolateral prefrontal cortex and to be purely stimulus-related. P3a's were larger when stimulus-aligned than when response-aligned, and the latency of P3a's varied when response-aligned but did not vary when stimulus-aligned. All of these findings put it in direct contrast with the more prominent P3b component that Verleger et al. argue is involved in connecting action to perception.

Conversely, the "lateralized readiness potential" (LRP) recorded from premotor cortex is thought to be involved in purely response-related processes, and sure enough, it showed the opposite trends as the P3a discussed above. Instead, the LRP was larger when response-aligned than when stimulus-aligned, and the peak latency remained constant when response-aligned but not when stimulus-aligned.

The authors conclude that the P3 component may reflect "some monitoring process" which operates when stimuli are nonstandard, "infrequent, spread out in time, or relevant." At the end of their article, Verleger et al. note that this interpretation of P3 is not far removed from the "context updating" hypothesis, in the sense that "monitoring" may be a good precursor to any updating of working memory.