At the edge of awareness

From a purely intuitive point of view, it is easy to believe that our ability to actively pay attention to a target is inextricably connected with our capacity to consciously perceive it. However, this proposition remains the subject of extensive debate in the research community, and surprising new findings from a team of scientists in Japan and Europe promise to fuel the debate.

Resolving how these aspects of perception are managed requires a detailed understanding of how the visual centers in our brain process information. A region known as V1 has been investigated as it represents the first portion of the visual cortex to receive and process signals transmitted from the retina.

Many researchers favor a model in which functions pertaining consciousness are widely spread among the whole visual system, including V1. The classical model, which assumes that the neural mechanism of consciousness is integrated into a narrow subset of brain structures, referred to as a homunculus, or ‘little human’, is almost defunct. However, a modern version of this model is under debate. It proposes that the neural mechanism of consciousness is a privileged set of cortical areas, a subpopulation of neurons, or certain neural dynamics (e.g. oscillations); while there are also visual systems that have nothing to do with conscious vision, explains Masataka Watanabe a researcher investigating brain function at the University of Tokyo, Japan.

Watanabe cites studies proposing that visual attention as processed within V1 may be only minimally impacted by conscious perception; but, the experimental data have been contradictory. For example, studies using a technique called functional magnetic resonance imaging (fMRI) to map brain activity have indicated that V1 contributes to both attention and awareness in humans. However, invasive electrophysiological studies in non-human primates yielded different results. “You would find only 10 to 15% of neurons in V1 that are barely modulated by awareness, and 85% or so that are not modulated at all,” says Watanabe. To resolve this ambiguity, he, Kang Cheng from the RIKEN Brain Science Institute, Wako, and their colleagues designed an experiment that examined both processes independently. Surprisingly, their results may lend support the modern homunculus model¹.

The difficulty in separately testing awareness and attention in an experimental setting is a likely cause for these contradictory results. “I believe previous studies were most likely ‘contaminated by attention’,” says Watanabe. 

Watanabe, Cheng and their colleagues used a technique known as continuous flash suppression (CFS) to address the issue of awareness. The concept is based on ‘binocular rivalry’: when each eye is presented with a distinct image, perception rapidly shifts between the two images rather than melding them together. However, by making one of these stimuli extremely ‘busy’ and dynamic, it becomes possible to entirely suppress conscious perception of the other, thereby selectively eliminating awareness.

In their study, the researchers used a patterned ring as the visual target. They displayed this to subjects along with an equally sized ring consisting of multiple patches of dynamically shifting patterns. When both target and ring were projected in an alternating pattern to the same eye, the subjects could consciously perceive both the ring and the target at its center. When the two were transmitted to different eyes, however, the dynamic ring suppressed perception of the target (Fig. 1).

For both visible and invisible target conditions, the subjects participated in two kinds of trials. In one, they were asked to pay attention to and report the visibility of the target. In the other, they were asked to shift attention away from the target region so they could follow a rapidly changing series of letters appearing within the surrounding ring pattern. Using fMRI, the researchers could specifically track changes in neuronal activity in each subject’s V1 that were induced by different combinations of attention and visual awareness under all four conditions.