Neurons are constantly performing complex calculations to process sensory information and infer the state of the environment. For example, to locate a sound or to recognize the direction of visual movement, individual neurons are thought to multiply two signals. However, how such a calculation is made has been a mystery for decades. Researchers from the Max Planck Institute for Biological Intelligence have now discovered the biophysical basis in fruit flies that allows a specific type of neuron to multiply two incoming signals. This provides fundamental information about the algebra of neurons – the calculations that can underlie countless processes in the brain.
We easily recognize objects and the direction in which they are moving. The brain calculates this information based on local changes in light intensity detected by our retina. Computations occur at the level of individual neurons. But what does it mean when neurons calculate? In a network of communicating nerve cells, each cell must calculate its outgoing signal based on a multitude of incoming signals. Some types of signals will increase and others will reduce the outgoing signal, a process neuroscientists call ‘excitation’ and ‘inhibition’.
Theoretical models assume that seeing motion requires the multiplication of two signals, but how these arithmetic operations are performed at the level of single neurons was previously unknown. Researchers from Alexander Borst’s department at the Max Planck Institute for Biological Intelligence have now solved this puzzle in a specific type of neuron.
Recording from T4 cells
The scientists focused on the so-called T4 cells of the fruit fly’s visual system. These neurons only respond to visual movement in a specific direction. Lead authors Jonatan Malis and Lukas Groschner succeeded in measuring both incoming and outgoing signals from T4 cells for the first time. To do this, the neurobiologists placed the animal in a miniature cinema and used tiny electrodes to record the electrical activities of the neurons. Since T4 cells are among the smallest of all neurons, the successful measurements were an important methodological step.
With computer simulations, the data revealed that the activity of a T4 cell is constantly inhibited. However, if a visual stimulus moves in a certain direction, the inhibition is briefly lifted. In this short window of time, an incoming excitatory signal is amplified: Mathematically, a constant inhibition is equivalent to a division; removal of inhibition results in multiplication. “We have discovered a simple basis for complex computation in a single neuron”, explains Lukas Groschner. “The inverse operation of a division is a multiplication. Neurons seem to be able to exploit this relationship”, adds Jonatan Malis.
The ability of T4 cells to multiply is linked to a certain receptor molecule on its surface. “Animals lacking this receptor misperceive visual movement and fail to maintain a steady course in behavioral experiments,” says co-author Birte Zuidinga, who analyzed the walking trajectories of fruit flies in a walking pattern. virtual reality. This illustrates the importance of this type of calculation for animal behavior.
“Until now, our understanding of the basic algebra of neurons has been rather incomplete,” explains Alexander Borst. “However, the fruit fly’s relatively simple brain has allowed us to better understand this seemingly unsolvable puzzle.” The researchers speculate that similar neural calculations underlie, for example, our abilities to localize sounds, focus our attention or orient ourselves in space.
Dendrites can help neurons perform complicated calculations
Lukas N. Groschner et al, A biophysical account of multiplication by a single neuron, Nature (2022). DOI: 10.1038/s41586-022-04428-3
Provided by the Max Planck Society
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