When TMS Helps: Does Parietal Cortex Cause Binding Errors?

Synaesthesia involves the inappropriate binding of one perception to another - for example, color-grapheme synaesthetes might perceive the letter "h" to be noticeably red, and are actually slower to identify the letter "h" when it is green than when it is red or gray. This "inappropriate binding" of color to other percepts can be disrupted in synaesthetes through transcranial magnetic stimulation of the parietal lobe.

While interesting, this work does not say much about how binding is accomplished in normal subjects, where various colors and various shapes need to be regularly and temporarily integrated with one another to allow for our coherent perceptual impression of the world. A new article in Neuroimage by Estermann, Vertsynen and Robertson addresses this shortcoming by applying the same technique to normal adults in a task that requires exactly this form of visual binding.

Estermann et al had 24 subjects complete a simple task where they had to identify the color of either a "7" or an "L" (only one of those would appear in each trial) which was adjacent to a distractor "O." Subjects often make errors in this task where they incorrectly report the color of the "7" or the "L" as the color of the distractor "O" item, but correctly indicate whether a "7" or "L" was present. This is known as an illusory conjunction error, since subjects correctly identified the target but mistakenly recombined its color with the color of another object - in other words, this represents a failure of binding processes. During a practice session, Estermann et al. optimized the display time and eccentricity of these displays to maximize each subject's probability of committing these errors.

During a subsequent testing session, the subjects underwent anatomical MRI to identify the exact locations of their transverse-occipital and intraparietal sulci. The subjects then completed the task again while having one of these regions targeted with transcranial magnetic stimulation (TMS) - a giant iron-cored magnet which seems to "scramble" or otherwise disrupt activity in a certain region of the brain. The strength of the TMS was calibrated to 115% of the power necessary to cause a subject's fingers to twitch when the motor cortex is stimulated, a standard threshold for TMS calibration.

The results showed that the likelihood of illusory conjunction errors decreased when TMS was targeted towards the right intraparietal sulcus. However, neither TMS of the left intraparietal sulcus nor of the right transverse occipital sulcus caused a similar decrease in illusory conjunction errors.

Who would have thought that better performance could occur as a result of "scrambling" activity in a certain part of the brain?

The authors suggest that their counterintuitive result makes more sense if one considers that right parietal cortex may play a role in "spreading or distributing attention over a wide area of the visual field" and further suggest that "inhibiting this function may create a bias to focus attention more locally, which would in turn decrease illusory conjunction errors."