Patients suffering from “hemineglect” ignore things presented to their left side. However, sometimes these ignored stimuli may be processed without awareness.

In a paper published in Cortex Issue 6, Sackur and colleagues reported that unconscious processing in hemineglect is not limited to low level features of the stimuli. They showed that the brain may extract the meaning of symbols that the patient has not consciously perceived. Thus, digits or number words presented on the left side were not detected by hemineglect patients, but still their numerical value influenced the way these patients performed on a numerical task presented shortly thereafter.

Until now, few studies have directly addressed the issue of the unconscious access to semantics. Sackur and colleagues engaged four patients with unilateral left spatial neglect in a number comparison task. Each target number was preceded by a lateralized number prime, either in the intact or neglected hemifield (HF).

Both group analyses and the intensive study of a single patient show that left (neglected) as well as right (consciously perceived) number primes affect performance: primes representing quantities that fall on the same side of the reference as the target lead to faster categorization. This congruency effect is highly suggestive of numerical semantic processing of neglected stimuli. Absence of conscious perception of neglected primes was evaluated using a combination of subjective and objective measures of performance in forced-choice tasks.

This study demonstrates that in hemineglect the left part of the world is not a 'blind' region: in a way, patients read unconsciously what is there. However, the patients cannot make conscious use of this information.

Article: Sackur J, Naccache L, Pradat-Diehl P, Azouvi P, Mazevet D, Katz R, Cohen L, and Dehaene S: Semantic processing of neglected numbers. Cortex 2008, 44, 673-682.