Prefrontal cortex involved in conscious vision
Use of ambiguous visual stimuli to disambiguate the neural correlates of consciousness
An international group of researchers demonstrate that prefrontal cortex is critically involved in the emergence of conscious visual perception. Their discoveries constitute a major conceptual advance in consciousness research, providing important new data for the fervent debate about the neural basis of consciousness. The results have implications for our understanding of what consciousness is and how it arises.
Why and how can conscious perception emerge from the brain – from a mere collection of interconnected neurons? This question fascinates philosophers and neuroscientists. Finding out what happens at the neural level, i.e. how many and which structures are involved in the process of self-organization of brainwide neural networks, when we consciously experience the world around us is vital for understanding the nature of consciousness.
“The prefrontal cortex is at the heart of the debate on consciousness”, states Vishal Kapoor, previously a researcher at the Department for Physiology of Cognitive Processes of the Max Planck Institute for Biological Cybernetics in Tübingen and now a principal investigator at the International Center for Primate Brain Research in Shanghai. Kapoor is – together with his colleague Abhilash Dwarakanath, who is now a researcher at NeuroSpin, Paris – the lead author of a study which demonstrates that conscious visual perception is correlated to neural activity in the prefrontal cortex. The prefrontal cortex, a brain area in the forehead, is in charge of many high-level brain functions; for example, it is responsible for decision-making and is connected with the motor cortex.
Previous studies had pointed towards a role for prefrontal activity in conscious vision, but the evidence those studies afforded remained disputed: one major difficulty is that when test subjects report their perception, the act of reporting triggers activity in the prefrontal cortex, since decision-making and motor control are involved. It is thus impossible to distinguish if conscious vision or reporting about it causes the prefrontal activity. To add to this challenge, when visual stimuli change, there will also be activity related to transmission of the visual data, not necessarily its conscious perception.
Changing impressions of ambiguous visual stimuli
To rule out measuring prefrontal activity unrelated to conscious perception, the researchers had to devise an experiment that required neither perceptual reports, nor changes in the visual stimuli, but that would nevertheless produce measurable changes in conscious perception.This is why the authors came up with the idea of presenting monkeys with a visual stimulus that could be interpreted in two different ways. “Think, for example, of the well-known picture that allows you to see either a vase or instead the contours of two human profiles facing each other,” explains Theofanis Panagiotaropoulos, previously a project leader at the Max Planck Institute for Biological Cybernetics, where he supervised the study, and now a principal investigator for Inserm and the Human Brain Project. “This ambiguous stimulus will prompt your conscious perception to shift automatically from one interpretation to the other, sometimes perceiving the faces, at other times the vase. When combined with simultaneous recordings of neuronal populations, this type of ambiguous images is an excellent tool to dissociate conscious from non-conscious seeing and provide important new data that can inform theoretical models of consciousness.”
The researchers presented moving horizontal grids whose motion could be interpreted as upwards or downwards, with the monkey’s perception being judged from their eye movement pattern, thus eliminating the need for voluntary reports. They combined this with recording the activity of prefrontal neurons. This approach utilizing ambiguous stimuli has been fundamental in the search for the single neuron correlates of conscious perception and pioneered by Nikos K. Logothetis, senior author of the study, in whose facilities in Tübingen the data were collected, before he moved his lab to Shanghai.
Shaping the idea of what it means to be a conscious being
The results of this carefully thought out experiment were impressive: the researchers were able to observe from large populations of neurons and with hitherto unprecedented precision that the prefrontal cortex represented what was consciously perceived, and changed its activity pattern when conscious perception switched. The precision of their measurements even allowed them to decode the ongoing contents of consciousness from neural activity without a report.
“Our results do not imply that the prefrontal cortex is the seat of consciousness,” Kapoor cautiously interprets their results. “Together with previous work, these findings suggest a distributed representation of what is consciously perceived; we cannot point to one region as the only key player. Knowing that the prefrontal cortex is undoubtedly involved in conscious perception, however, has the potential to shape our idea of how we consciously experience this world, maybe what consciousness is, and perhaps one day we can understand what it means to be a conscious being.”