Are the contents of conscious perception tied to individual brain cells? Can we use single neurons to determine someone’s subjective experience? A recent studies published in PNAS suggests otherwise: there are no specialized neurons that carry information about what enters awareness and what not. If one holds the picture of a butterfly in front of one eye and the image of an apple before the other, the brain gets into a deep dilemma. It must integrate the contradictory pictures to a meaningful whole. This situation inevitably leads to a fascinating phenomenon called “binocular rivalry“. During rivalry, perception alternates between the picture shown to one eye and that shown to the other one. In other words, the butterfly will be seen during some moments and the apple during others. It is as if the brain is unable to decide which eye to trust, and therefore constantly wavers back and forth between them.
stimulus-dependent activity patterns that are decisive for what reaches conscious awareness and what does notThere are neurons in the visual cortex, which become strongly active for certain pictures (such as that of a butterfly), but not for others. If one measures the activity of such a cell by means of tiny electrodes, it is often possible to determine which stimulus gets perceived at any moment during binocular rivalry. Interestingly, however, some cells do not seem to be affected by the spontaneous perceptual changes during binocular rivalry. Instead, they truthfully indicate that the stimulus that activates them is constantly present, regardless whether it is perceived or not. This observation raises the question of whether there are individual brain cells that differ in their characteristics from other cells so that their activity stands in direct connection with what we consciously perceive. To answer this question, scientists at the Max Planck Institute at Tuebingen, Germany repeated the experiment using another trick. After all, while one sees a certain image with one eye, one can replace the image in the other eye, without affecting awareness. The question was, would this manipulation nonetheless affect neuronal activity? To the surprise of the researchers, any exchange of the pictures had large effects on the activity of the recorded neurons. In fact, many cells lost their special characteristic and showed no more activity fluctuations during perceptual alternations. Other cells, however, which had been unaffected by perception before, began to indicate the perceptual alternation of the stimuli.
Whether an individual brain cell participates in conscious experiencing or not therefore seems to be flexibly determined. Based on these results, it is conceivable that it is not only fixed interconnections between neurons, but also stimulus-dependent activity patterns that are decisive for what reaches conscious awareness and what does not.
Alex Maier was a graduate student at the Max Planck Institute for Biological Cybernetics in Tübingen, Germany. He is now a researcher within the Unit on Cognitive Neurophysiology and Imaging at the National Institute of Mental Health in Bethesda, MD.
- Alexander Maier, Nikos K. Logothetis, and David A. Leopold Context-dependent perceptual modulation of single neurons in primate visual cortex, Proceedings of the National Academy of Sciences
ABSTRACT Some neurons in the visual cortex alter their spiking rate according to the perceptual interpretation of an observed stimulus, rather than its physical structure alone. Experiments in monkeys have suggested that, although the proportion of neurons showing this effect differs greatly between cortical areas, this proportion remains similar across different stimuli. These findings have raised the intriguing question of whether the same neurons always participate in the disambiguation of sensory patterns and whether such neurons might represent a special class of cortical cells that relay perceptual signals to higher cortical areas. Here we explore this question by measuring activity in the middle temporal cortex of monkeys and asking to what degree the percept-related responses of individual neurons depend upon the specific sensory input. In contrast to our expectations, we found that even small differences in the stimuli led to significant changes in the signaling of the perceptual state by single neurons. We conclude that nearly all feature-responsive neurons in this area, rather than a select subset, can contribute to the resolution of sensory conflict, and that the role of individual cells in signaling the perceptual outcome is tightly linked to the fine details of the stimuli involved.