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The nerve endings in your fingertips can perform complex neural computations that were thought to be carried out by the brain, according to new research published in the journal Nature Neuroscience.
The processing of both touch and visual information involves computations that extract the geometrical features of objects we touch and see, such as the edge orientation. Most of this processing takes place in the brain, which contains cells that are sensitive to the orientation of edges on the things we touch and see, and which pass this information onto cells in neighbouring regions, that encode other features.
The brain has outsourced some aspects of visual processing, such as motion detection, to the retina, and the new research shows that something similar happens in the touch processing pathway. Delegating basic functions to the sense organs in this way could be an evolutionary mechanism that enables the brain to perform other, more sophisticated information processing tasks more efficiently.
Your fingertips are among the most sensitive parts of your body. They are densely packed with thousands of nerve endings, which produce complex patterns of nervous impulses that convey information about the size, shape and texture of objects, and your ability to identify objects by touch and manipulate them depends upon the continuous influx of this information.
The nerve endings in the fingertips contain two different types of touch receptor organs - Meissner corpuscles and Merkel discs, which are sensitive to fast and slow deformations across the skin, respectively. Each nerve ending branches just beneath the skin surface to form an elliptical receptive field with an area of about 5mm2, each with up to 8 highly sensitive zones distributed unevenly within it.
Andrew Pruszynski and Roland Johansson of Umeå University in Sweden predicted that nerve endings in the fingertips can encode the orientation of edges on touched objects in the intensity and timing of the electrical impulses they produce.
To test this, they recorded impulses from nerve fibres in the arms of 44 volunteers in response to various kinds of touch sensations applied to their fingertips. The participants sat with their arms firmly attached to a table, with their palms facing upwards, while Pruszynski and Johansson applied touch sensations using a rotating drum with raised edges, and recorded the responses of 47 individual neurons that extend into the fingertips with a tungsten electrode inserted into the median nerve.
The researchers found that all the cells responded to most of the different touch sensations, but that each one was most responsive to sensations moving in a specific direction. Each neuron produced high frequency “trains” of impulses when the pattern of skin deformation coincided precisely with the spatial arrangement of the sensitive zones in the cell’s receptive field, but fired less intensely in response to touch sensations in other directions.
Thus, the timing of nervous impulses produced by nerve endings in the fingertips signal the orientation of edges to cells in the spinal cord, which then relay these signals up to the brain. Until now, this function was believed to be performed by orientation-selective neurons in the somatosensory cortex, one of several brain regions that process touch information.
Earlier work has shown that people are better at perceiving touch patterns than would be predicted by the size of the receptive fields in the fingertips and the spacing between them. These results provide an explanation for why this might be: While a neuron with multiple sensitive zones is only capable of signalling touch stimuli within its own receptive field, neighbouring cells with overlapping fields would provide touch sensations with a higher resolution.
Pruszynski and Johansson suspect that the same nerve endings might perform other complex computations that are currently attributed to the brain, such as signalling information about object curvature and motion direction, and are now carrying out further experiments to see if this is the case. “Another line of research is more theoretical,” says Pruszynski. “We’d like to get a better handle on how feature extraction [in the fingertips] helps the brain do its job.”
Pruszynski adds that the new findings may also have implications for rehabilitation. “If you are working towards regrowing peripheral nerves in people with nerve injury, it is important to consider not just how many neurons grow back, but also how they grow back, as this may be critical to the type of information they send the brain.”
Reference:
Pruszynski, J. A. & Johansson, R. S. (2014). Edge-orientation processing in first-order tactile neurons. Nature Neurosci. doi: 10.1038/nn.3804
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