The role of neural plasticity in conscious perception
by Ger Post
For thousands of years, humans have tried to come to a definition of consciousness-in vain, say professor Victor Lamme and his colleagues. They want to explain conscious perception from an entirely different angle: from the cells in the brain. The neurobiological answer to the question of what exactly is consciousness usually comes down to the fact that consciousness somehow occurs through the interplay between different brain areas. In other words, consciousness depends on a complicated communication between areas that meld together information about various characteristics of an object into a whole: the scene you observe or the object you see. Thus far, research has focused primarily on finding brain areas that become more active when someone perceives something consciously, with the associative parts of the cortex as the usual suspects.
With the project, ‘The role of neural plasticity in conscious perception,' one of four projects awarded within the university research priority program Brain & Cognition, Lamme of the Faculty of Social and Behavioural Sciences (FMG) and his colleagues from the Faculties of Science (FNWI) and Humanities (FGW) want to approach consciousness from a different angle. According to the researchers, the key to consciousness lies in the way neurons change their mutual connections, so-called plasticity. ‘We want to prove that the way in which we consciously perceive visual pictures or sound depends mostly on how perception causes changes in the wiring of the brain,' the scientists write in their research proposal.
Using a series of experiments they want to test this hypothesis. First and foremost, the research focuses on the connections between neurons, following up on earlier research conducted by Lamme. The latter showed that a visual signal can be unconsciously processed within a hundred milliseconds. Someone can respond to this information, for example by catching a ball in a reflex, but it's not until after that-when higher and lower-level areas of the brain give each other feedback-that a conscious picture of catching the ball is formed. Now the expectation is that during this unconscious, rapid brain activity the wiring of the brain remains unaltered, while these neuronal connections do change during conscious feedback interactions.
A logical consequence of this expectation is that if these changes in neuronal connections (plasticity) can be blocked, this does not influence unconscious processes, but it does affect the necessary interactions for consciousness. Anouk van Loon will research this in humans. Cyriel Pennartz's group will look into this same question in rats.
Pennartz: ‘We will test this in rats by looking at whether their visual perception changes when we administer locally into the visual part of the cortex a pharmacological substance that blocks plasticity and a strong activation of neurons. This is how we'll study whether this so-called NMDA-receptor induces a so-called blind-sight effect. That is, whether rats still show an automatic behavioral response to a visual stimulus, but also-through their behavior-illustrate that they haven't seen this stimulus.'
If the researchers do find that blocking plasticity primarily influences conscious processes, this would yield, according to Pennartz, a radical new insight into how consciousness works and how it comes about. ‘One school of cognitive neuroscientists, for example, poses that consciousness comes about through the activation of networks in the higher associative regions of the cortex, which is also referred to as the workspace hypothesis. A confirmation of our hypothesis would shift the attention to the interaction between higher and lower visual areas in the cortex.'
A subsequent question the researchers want to answer: does learning require consciousness? Lamme and his colleagues think so and believe there is no such thing as unconscious learning. Aren't they neglecting piles of research that shows that people can be guided by unconscious stimuli-and actually learn things? No, say the researchers, because this unconscious guiding (priming) lasts only a short time and there is no solid evidence that such primes have any effect in the long term. Lamme: ‘When you carefully weed through the literature on unconscious learning, it turns out that all forms of "implicit learning" are in fact forms of learning without attention, but not without consciousness.'
In other words, the researchers see consciousness as an epiphenomenon of memory forming. This assumption appears to contradict findings from research involving Henry Molaison. This patient-known in the literature as H.M.-died a year and a half ago. His hippocampus, an area that is important for storing memories, had been removed. Because of this, Molaison couldn't remember new experiences, but he was in fact aware of what was happening around him. Pennartz: ‘Our proposal doesn't focus on the hippocampus. Our hypothesis that consciousness arises from synaptic plasticity applies to the neocortex. It revolves around the fact that, in conscious perception, feedback from higher areas of the cortex to the primary visual cortex goes hand in hand with the changing of connections. This could have very well been the case with H.M.'
Lamme adds: ‘It's about memory forming in the most general, biological sense. The hypothesis is: as soon as something changes in your brain, there's consciousness. In no way does this mean that you will explicitly remember this later on. Episodic memory is nothing but a very specific-and rather mysterious-form of memory.'
Whether learning necessarily goes along with consciousness-and not with attention-is the subject of a subproject by musicologist Henkjan Honing. Although all other subprojects focus on visual perception, Honing and his colleagues will focus on processing musical rhythms. In earlier experiments they found that a sense of rhythm appears to be ingrained in humans. Newborns, for example, already appeared to have a sense of rhythm.
‘In a series of listening experiments with non-musicians we also found that their rhythmic expectations were being processed even if they focused their attention on something else, for example when they were watching a video with subtitles,' Honing says. ‘In the planned experiments we will examine how open this rhythmic perception is to learning and to what extent attention or consciousness guides this.'
Lastly, research is planned into how visual perception is determined by similarities in the images we see daily. The idea is that our neural system has adapted to (hidden) statistical regularities in images of our environment, such as fields and forests. Lamme: ‘Even though all those images seem extremely different, they do display a hidden similarity in the way contrast is divided. We've discovered that our brain has adapted to this through learning, or perhaps even as part of evolution. So what has been ingrained by our experiences determines our conscious perception.'
People with different professions will participate in this research. In this way, they will test, for example, whether farmers learn outside scenes in a different way than office clerks who are inside all day. They'll also look to see whether people with indoor or outdoor jobs react differently to artificial images that have the same hidden contrast patterns as outside scenes. The expectation is that farmers are better at perceptual tasks in which they have to detect small hidden objects in a picture, for example, and show an elevated neural activity for artificial scenes with the same statistical regularities.
Arnold Smeulders from FNWI will join Sennay Ghebreab from FMG in conducting research into these "hidden" statistical regularities. Ghebreab explains: ‘If our expectations come true, this could mean that people can differentiate in a single glance an image of a beach from one of a street, because they react to the differences in global statistic regularities in these images. If we can prove that these statistical regularities provide for a conscious perception in our daily lives, this would mean that the current theories on attention and how we detect objects or navigate spatially must be reconsidered. Because those theories are all based on the assumption that consciousness arises from a bottom-up integration of parts of a picture-that an object is formed from various characteristics, and that these objects are subsequently being forged into scenes.‘
A neural definition of consciousness would provide a lot of answers to questions that are still open right now, according to Lamme. ‘You can then objectively determine whether there is consciousness in all those cases we've been wondering about for years. For example, in patients who are in a coma or a vegetative state. Wouldn't it be fantastic if you could determine whether someone who is in a vegetative state has conscious sensations of voices around him, or of the people who touch him? Until not so very long ago, a doctor would peer deep into the eyes of such a patient and would tell the family: "The lights are really off here."'
Now, brain scans are being done around the world, and patients who look unconscious from the outside are found to show enormous differences in brain activity, Lamme knows. ‘Some brains do react to voices; others don't. Some patients even react to directions such as "imagine walking through the room." You then see the same brain activity as in someone who is awake and is given the same direction. The key question related to those measurements is, of course: how do I know if brain activity signals consciousness or just unconscious processing? Such a neural definition would be the solution for that.'
Prof. dr. Victor Lamme (Psychology)
Prof. dr. Cyriel Pennartz (SILS)
Prof. dr. Anton Smeulders (ISLA)
Prof. dr. Henkjan Honing (ILLC)
The following institutes participate: