The stopwatch in the brain

The brain is a highly complex information processing system. Whenever we see, hear or remember, information is transmitted from nerve cell to nerve cell in form of electrical signals – only in this way can we recognize images and understand language. But how is the information coded in the sequence of neuronal impulses? Is the crucial factor the number of impulses or is it rather their exact timing? Scientists around Clemens Boucsein, Bernstein Center for Computational Neuroscience and University of Freiburg, together with Martin Nawrot, Bernstein Center for Computational Neuroscience Berlin, have now addressed this central question of brain research in more detail. They showed: neuronal cells react with greater temporal precision than previously expected. Their work was published in the scientific journal "Frontiers in Neural Circuits".

The strength of the reaction of a downstream cell (black) to the activation of upstream cells in different regions of a tissue slice is shown color coded. Red: strong, blue: weak activation.
Bild: Clemens Boucsein.

All neuronal cells transmit information as a sequence of electrical impulses. But the way in which they code information in these signals and how this information is read out by downstream cells differs considerably from cell to cell. Some sensory cells and neurons that activate muscles use a so called "rate code": the more impulses they send per time interval, the brighter the light, louder the sound or stronger the evoked muscle contraction. Other cells, in contrast, use a "temporal code": it is not the number of impulses that matters, but rather their exact timing. It is crucial whether a cell sends an impulse a few milliseconds before or after another cell. Boucsein and his colleagues investigated which of the two strategies is used by cells in the cortex of the brain.

Every cell in the cortex receives many signals from upstream cells. If these cells would use a temporal code, they would also require the ability to respond to these upstream signals with high temporal precision. To test this, Boucsein and his colleagues used a new method developed in their laboratory. In a tissue slice, they measured the electrical activity of a cell, while at the same time activating its upstream cells in a precisely defined temporal sequence. To achieve this, they use a chemical component, which is released under the influence of light and which then stimulates the cells. With a laser and a mirror system, upstream cells are repeatedly switched on in the exact same temporal sequence. "We were surprised that the downstream cells react in such a reproducible and temporally precise manner to the sequence of upstream signals", says Boucsein. This is all but self-evident. Every signal of the upstream cell needs to be transported along far reaching cellular extensions, transferred to the downstream cell and  again transported along this cell’s  extensions to the cell body. All these processes could – in theory – lead to temporal imprecision. The fact that the cells nonetheless react so accurately shows that they are literally tailored for the use of a temporal code. If cells in the cortex would instead use a rate code, they would, according to these results, react in an unreliable manner.