Addressing the question of visual attention, a Franco-American team led by CNRS researcher Rufin VanRullen, from CERCO,1 developed a method to measure attention strategy. Their study2 has found that in fact attention acts like a stroboscope, a beam that flashes from one target to another at a rate of about seven times per second.
The process of visual attention is a complex one. For the past 30 years, the general consensus was that it acted like a searchlight–a single, scanning beam that lit up one part of the field of vision after another. The theory was that each “target” that was lit up by the searchlight was analyzed in succession by the brain.
But increasing evidence now suggests that attention can in fact simultaneously select multiple targets. What is uncertain, however, is whether this is performed by continuously allocating attention to different targets (a process known as a “parallel strategy”), or whether attention switches rapidly between the targets (known as a “sampling strategy”).
VanRullen and his colleagues studied the reactions of eight people, five males and three females, between the ages of 25 and 32. The subjects performed two versions of a task, in separate sessions, which involved them detecting a decrement in contrast, using disk stimuli on a grey screen in a dimly-lit room. The team used a simple experimental method to distinguish which strategy–parallel or sampling–was used. The performance of an observer can be linked to the duration of a stimulus, here a single visual target, and extrapolated to the observer’s performance when subsequently presented with several targets. The predicted performances differ according to whether the observer uses one or several attention “beams.” By comparing the predictions with the actual human performance, it was possible to determine what strategy was used.
Not only did the study find that attention to different targets was employed using a stroboscopic, sampling process–much like a super-rapid slide show–but also that this process applied when attention was focused on just one target.
The discovery may now help to understand the neuronal mechanisms involved in attention, in particular the link between attention and the known oscillations of electric cortical activity.
1. Centre de recherche cerveau et cognition (CNRS / Université Paul Sabatier),
2. R. VanRullen et al. “The Blinking Spotlight of Attention” PNAS, 2007. 104: 19204-9.