Vertebrate brain theory

7.3    The brightness analysis in the cerebellum

In addition to the Tectum, a second system also processed visual signals. This was the Cerebellum. And this also sent its output signals back to the tectum, but an efferent copy of the signals also reached the cortex. And as the range of signals expanded due to the creation of the red-green color channel, the cerebellum also received these new signals. While the CGL (probably) sent these color signals back to the tectum, so that a red-green difference image could also be created there for motor control, the output of the cerebellar color signal evaluation (via the thalamus) was fed to the visual cortex.

But the limbic system could also send the output from the temporal lobe to the primary visual cortex that resulted from processing visual signals. This will be described below.

In all these cases, a signal exchange would have to take place between the frontal cortex and the visual cortex. The signals would therefore have to be exchanged between the individual Lobi. The existence of innumerable intracortical connections between the individual lobi of the vertebrate brain has been known for a long time.

Like Brayn Kolb and Ian Q. Whishaw mention in their "Neuropsychology", experiments by Nakamura and his colleagues showed that visual blindness occurred when the anterior cortex, including the frontal lobe, was removed from the visual areas. However, individual recordings still showed the activity of the neurones in the visual region. Although the visual system was functioning, the animals were chronically blind. Therefore, the author also postulates the existence of visual signal processing in the frontal lobe and its input suppliers, especially the cerebellum.

We now want to analyse what happens with the light-dark signals that reach the nucleus olivaris from the tectum via the tractus tectoolivaris.

We know that there is a light-on signal and a dark-on signal for each pixel of each eye, both types of signal are inverse to each other. New information can only be obtained by combining inverse signal types. Thus, we hypothetically assume that in the nucleus olivaris the visual brightness signals are generally evaluated in pairs. One ipsilateral light-on signal may be evaluated with its corresponding ipsilateral dark-on signal. Both signals generally belong to the same pixel, but have inverse information contents. While the light-on signal has a higher firing rate with increasing brightness, the dark-on signal has the opposite tendency and fires more strongly with increasing darkness. This signal pair thus reaches the nucleus olivaris for each pixel.

Let's think of the visual part of the nucleus olivaris for brightness analysis as a grid of equidistant horizontal and vertical lines. In the grid points there are alternating neurons for the light-on signal and the dark-on signal. This was the initial state before the beginning of the signal divergence. Here, a failure of one or more neurons led to field of view failures from the respective side.

Again, we can assume that the principle of signal divergence was used to increase redundancy. Then the failure of individual neurons did not have such a serious effect. Therefore, additional projection neurons were formed between the paired neurons, which received their input from these two assigned input neurons. The initially square grid thus became rectangular. We assume an expansion in the horizontal direction. The squares became wide rectangles. Between the two input neurons of the signal pair, dozens or hundreds of projection neurons took over the signal transmission. This created a high degree of reliability. And as always, a distance-dependent attenuation occurred, which satisfied the cable equation for non-markless fibers. The nucleus olivaris was a non-markless nucleus, only its projection axons, the climbing fibers, had a myelin layer.

So there were several climbing fibres for this signal pair, which lay next to each other like a ribbon cable. And within this axon strand there was again a signal minimum. If the light-on signal of one pixel of one eye was as strong as the dark-on signal of the same pixel of the same eye, the excitation minimum was exactly in the middle between both input lines. If, on the other hand, the light-on signal fired more strongly, the excitation minimum shifted to the dark-on signal. A higher firing rate of the dark-on signal resulted in a minimum migration towards the light-on signal. The occurring excitation minimum thus encoded the fire rate ratio between the light-on signal and the dark-on signal of the same pixel. But this signal strength ratio means a brightness value. Thus, the nucleus olivaris was able to transmit the relative brightness value of the respective pixel of the common field of view to the spinocerebellum in a minimum coded way.

The signal inversion, which was the main task in the spinocerebellum, transformed this minimum coded brightness signal into a maximum coded brightness signal. In the subsequent thalamus, the receptive neighbor inhibition suppressed the weaker components, resulting in sparse coding. Therefore, the vertebrates could now not only recognize the colors white or black, but also the color gray in all its different brightness variants. Thus, information was wrested from the light that was of great advantage for the further survival of the vertebrates. Ideally, there was now a signal vector for each pixel of the field of view, which had a high rate of fire at one position, but a zero rate of fire at the other positions. And the maximum position encoded the brightness value of the pixel.

With the later development of colour receptors for the colour spectrum red-green and blue, an analogous ability came into play that led to colour vision. A simple colour vision had already been developed in primitive times with the help of the vertical divergence grids of the amygdala and its inversion systems.