"Simplicity is the ultimate sophistication." - Leonardo Da Vinci
"The aim of science is to seek the simplest explanation of complex facts. We are apt to fall into the error of thinking that the facts are simple because simplicity is the goal of our quest. The guiding motto in the life of every natural philosopher should be ``Seek simplicity and distrust it.'' - Alfred North Whitehead
No one ever said the brain was going to be simple - and this is precisely why there is a need to use simple tasks in cognitive neuroscience; even the "simplest" tasks may be subserved by bewildering complex neural processing. Accordingly, those who research "executive function" - the processes required for the coordination of behavior and planning to achieve a high-level goal - are often focused on alarmingly simple tasks. A salient example is the stop signal task, in which subjects must merely make a simple choice between two possibilities, unless they are told to stop, in which case they should refrain from choosing. This task is thought to be a relatively pure measure of inhibition.
Of course, it's possible to simplify our tasks to the point where they are no longer useful, but this does not seem true of stop signal. In fact, stop signal can predict performance on a wide variety of more complex tasks, including abstract reasoning tasks often used to estimate IQ. To the contrary, stop signal may still be too complicated for an accurate understanding of the neural processes that determine performance. A 2004 paper by Burle et al in Brain and Cognition focuses on the simple act of choosing between two stimuli, and shows that inhibition seems to be involved even in that process.
To test their claim, Burle et al. required subjects to flex their left or right thumb in response to a variety of visual stimuli. In the first experiment, a visual stimulus apppeared on either the right or left side of the screen; subjects should respond with the right thumb for stimuli on the right, and with the left thumb for those on the left. In this task, Burle et al stimulated the median nerve (at the wrist) of each subject some amount of time after the onset of the visual stimulus, but before they could give a response, while recording the skin electrical potentials on each thumb. The results showed that stimulation of the nerve closer to the time of response was associated with a larger reflex in the "correct side" thumb and a smaller reflex in the "incorrect side" thumb. Burle et al. interpret this to reflect increasing disinhibition of the correct thumb, and increasing inhibition of the incorrect thumb, as subjects approach the point where they will ultimately give a response.
In a second experiment, the authors asked subjects to give a right thumb response to stimuli of one color, and a left thumb response to stimuli of another color. Simultaneously they stimulated primary motor cortex with a magnetic coil (known as transcranial magnetic stimulation, or TMS). TMS of areas in motor cortex thought to be involved in triggering hand movements was associated with a "silent period" of electrical activity at the skin. This silent period was of increasingly shorter duration on the side of the scalp opposite the correct answer on that trial, and of increasingly greater duration on the scalp opposite to the incorrect answer for that trial, depending on how late after the presentation of the visual stimulus TMS was applied.
The authors also reviewed results from a third experiment in which subjects performed a simple Stroop task. The task was to respond with the left thumb to any word written in one ink color, with the other thumb to any word written in another ink color, and to give no response to any word written in third ink color. Critically, the words themselves were color words (of the form RED. The results showed that negative electrical activity grows over the motor cortex opposite the correct response for a given trial, and symmetrical positive electrical activity grows over the motor cortex opposite the incorrect response for that trial. Critically, a positive wave also grows over both motor cortices on those trials where subjects are not supposed to respond at all.
The authors note in conclusion that these results do not dissociate between a process of active suppression of inappropriate responses and a process whereby activation is "stolen" from the uninvolved cortex and implemented instead on the side responsible for giving a response. In fact, they emphasize the importance of cross-callosal connections for such tasks. On the other hand, they did observe some apparent correlates of inhibition without concomitant excitation (such as the growing positivity on trials in the third experiment where no response was required), but this could simply reflect activity in another region "stealing" activation from motor cortex.
Although Burle et al. only demonstrate inhibition in motor tasks, they suggest that similar mechanisms may hold in more complex executive function tasks. It's not clear whether they believe this inhibition to be solely related to responses, or whether abstract concepts might be actively inhibited as well. While inhibition is clearly an important part of neural processing, Burle et al. do not present strong evidence that directed inhibition occurs independent of concomitant activation.
Neural network models suggest that inhibition is important because it prevents run-away patterns of neural activity. Without the "automatic braking" function of lateral inhibition, the brain would simply excite itself into oblivion, through unrestrained positive feedback in reciprocally connected neural circuits. In contrast, the metabolic and architectural requirements for implementing directed inhibition are overshadowed by the fact that this can be accomplished more elegantly and efficiently through lateral inhibition.
Simplicity is important, both in experimental design (as demonstrated here by Burle et al) but also in terms of understanding energetic evolved systems like the brain. It seems unlikely that directed inhibition exists if cortical anatomy, artificial neural network models, and metabolic efficiency all seem to argue against it. Nevertheless it is important to question these conclusions, as Burle et al have done, because there is no guarantee that nature operates according to simple principles of parsimony.
"So, if I've accomplished nothing else in this talk, I hope at least that the next time you're tempted to consider parsimony as a desirable aspect of whatever you are doing, you'll give some thought to whether you really want to advocate a simplistic and nonexistent parsimony, rather than an appropriately complicated and meaningful psychology."
William Battig, as quoted in McDaniel & Einstein 2007
A Benefit of Ignorance: Inhibition of Return and the Stroop Effect
Backward Inhibition: Evidence and Possible Mechanisms
Inhibition from Excitation: Reconciling Directed Inhibition with Cortical Structure