10/25/2006

Inhibition From Excitation: Reconciling Directed Inhibition with Cortical Structure

A frequent interpretation of Stroop tasks is that successful performance (i.e., ink color naming) requires inhibition of the prepotent response (i.e., word reading). However, this interpretation is frequently criticized given the relative lack of long-range inhibitory projections in the neocortex. Instead, inhibition seems to be mostly local, occuring within cortical microcircuits. How then can we reconcile the intuitively appealing idea that Stroop requires inhibition with known features of cortical structure? This is the question answered in a recent JoCN paper by Herd, Banich, and O'Reilly.

One theoretical perspective of cognitive control (such as involved in Stroop) suggests that top-down exictatory signals can suffice for the override of prepotent responses - thus the bias to read the word in a Stroop task is thus overcome by a strong signal which favors "color perception" and "color naming" processes. These then inhibit competing representations through local lateral inhibition.

However, this proposal requires that "color processes" directly compete with word processing - although these are thought to have distinct neural substrates. fMRI data has shown that incongruent Stroop trials (e.g., red) show more activity in regions involving the to-be-ignored stimulus feature than in neutral trials, which might also be interpreted as supporting "directed inhibition." This is a straightforward but problematic interpretation, given that relatively few long-range inhibitory connections exist in cortex.

The computational model presented by Herd et al. can explain these results without invoking the concept of directed inhibition. In short, it is able to accomplish this because a "color" task set is constantly active during both color naming and word reading trial types. This causes activity throughout the network to be increased in incongruent trials relative to neutral trials as a result of increased competition. In this case, increased activity reflects increased competition - not "directed inhibition" as might be presumed. This is reflected both in the network's slower cycle settling times in incongruent relative to neutral trials (which is analogous to slower reaction time in humans).

The model architecture consists of 2 input layers (colors and words respectively, with 3 units each), which send activation to color and word hidden layers, respectively (each with 3 units). These are bidirectionally connected with each other (excitatory only), as well as to the output layer (consisting of 3 units). Finally, a PFC layer (3 units, representing the "ink color naming" "word reading" and "color" task sets) sends top-down exictatory biasing signals to both hidden layers. Each hidden layer, as well as the output, implements local inhibition to create competition for representation. The network was randomly initialized and then trained with Hebbian learning on word-reading and color-naming tasks, with 1.67 times more word-reading than color-naming trials (to roughly mirror human experience with these tasks and instill a prepotent bias for word-reading). The model was then run on the Stroop task, with both neutral and incongruent trials presented as input, and activity in the prefrontal layer clamped to represent the current task (1.0 if color-naming on incongruent trials, .85 if color-naming on congruent trials or if word-reading, and the general "color" task set unit was set to .5 throughout).

The network demonstrated a nice fit to human behavioral data, although very few parameters were modified from their standard values. Several further explorations showed that removing the bidirectional exictatory projections between the color and word processing hidden layers resulted in 0% performance on incongruent trials - showing that, in effect, these excitatory connections were paradoxically helping the network to inhibit the prepotent response.

This result has profound implications for the way cognitive control is discussed in the broader sense. As the authors note, it is potentially confusing to refer to "inhibition" as a construct if it is actually accomplished through exictation that is itself supported by task-relevant representations in PFC. This collapses the distinction between supposed "inhibitory" processes and those of active maintenance, and brings "directed inhibition" accounts into harmony with known characteristics of inhibitory interneurons in cortex.

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