- "Fingerprint" patterns of hemodynamic activity are readily observed with multi-voxel pattern analysis of functional MRI in most, if not all, areas of the human brain
- In animals, two-photon calcium imaging shows sparse brain activity patterns related to a broad range of sensory areas and body states: visual and auditory perception, awareness of spatial location, thirst, satiety, and in monkeys, visual working memory
- The breadth of the animal findings suggests sparse neural activity is a fundamental organizational principle that might manifest throughout the brain
- Research is needed to determine whether sparse coding contributes to the fingerprint patterns of activity evident in human fMRI studies and whether sparse activity in the human brain underlies perceptual, emotional, and cognitive functions
In the early 2000s, multi-voxel pattern analysis data showed "fingerprint" patterns of functional MRI (fMRI) activity in areas of the human brain that control vision. Numerous subsequent studies found patterns of hemodynamic activity across brain regions during various other stimuli and tasks: auditory perception, touch, visual and auditory working memory, semantic representations, emotional states, and decision-making. Thus, fMRI can help determine how information is represented in the human brain.
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In animals, single-neuron recordings have shown sparsely distributed brain activity—only a few neurons in a population strongly fire at a given time, possibly because of inhibitory mechanisms. Jyrki Ahveninen, PhD, an investigator in the Martinos Center for Biomedical Imaging at Massachusetts General Hospital, Martinos alumnus Iiro Jääskeläinen, now a professor at Aalto University, Finland, and colleagues recently reviewed this research for correspondences to what is known about fingerprint patterns.
In a commentary in NeuroImage, they discuss what human and animal findings imply for the quest by neuroscientists to discover whether a single, fundamental nervous system operational principle enables memory, focused attention, goal-directed behaviors, and the ability to experience emotions.
Sparse Brain Activity in Animal Studies
Sparsely distributed brain activity is a highly economical way of representing information in the brain. It presumably facilitates communication between different neural regions.
The recent introduction of two-photon calcium imaging (2PCI) allows simultaneous quantification of all neurons in large populations of thousands of neurons, a clear advantage over the traditional single-neuron recordings.
2PCI studies of animals have found that as few as 0.5% to 2.5% of all neurons in a population respond strongly during the presentation of a given visual image, even though most neurons in the populations studied respond to some of the images. Similar findings have been reported from studies of auditory perception, animals' awareness of their spatial location, thirst, satiety, and in monkeys, visual working memory.
The breadth of these findings suggests sparse coding is a fundamental organizational principle that might manifest throughout the brain.
Sparse Brain Activity and Fingerprint Patterns From fMRI
Calcium and hemodynamic signals are strongly correlated, and calcium influx gives rise to the hemodynamic responses measured by fMRI. Dr. Ahveninen's group believes the sparse neural activity depicted by 2PCI might contribute to the fingerprint patterns of activity evident in multi-voxel pattern analysis of fMRI.
Furthermore, the researchers propose that sparsely distributed representations in the human brain might influence each other, within and between brain regions, to enable perceptual, cognitive, and emotional functions.
The Path Forward
Compelling questions in this area of research are:
- Will advances in fMRI (7T spatial resolution and ultra-fast imaging) allow the detection of sparse brain activity in humans?
- Can fMRI reveal how fingerprint patterns support perception, emotions, and cognition?
Addressing these issues will require using 2PCI and fMRI in parallel in animal models and comparatively across humans and animal models.
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