Human brain theory

4.4.  The disparity module 

All visual divergence modules require input from mostly four retinal ganglion cells, whether magnocellular or parvocellular.

When the fields of vision began to overlap in early prehistoric times, because the eyes were no longer arranged totally sideways but their position shifted forward, the brain was able to compare the signals of the left and right fields of vision where there was a common field of vision.

Initially, the ganglion cells of the common visual field acquired statistical signal relatedness because they received the same signals more frequently (albeit randomly). Signal-related neurons converge, so the neurons of the common visual field began to overlap in the visual thalamus. They simply lay on top of each other there, so that a certain pixel of the common visual field from the left eye lay exactly under the assigned pixel of the right eye.

Since in the course of evolution a splitting of the visual receptors occurred, among other things through gene duplication, the jointly overlapping input layer also split into sub-layers, each of which was again adjacent in a retinotopically ordered manner. This also affected the on-off splitting, as well as the splitting into parvocellular and magnocellular layers.

In humans - and perhaps other mammals - the superimposed layers to the same image point separated again when reaching the cortex. However, something peculiar happened.

In the primary visual cortex, double rows of neurons formed that received their input from the left retina. Underneath one such double row was another double row of neurons receiving input from the right retina.

These double rows were important. The orientation columns always needed input from a retinal square. This consisted of four neurons. Therefore, double rows of retinal cells always projected into the cortex and maintained this arrangement there. This was noticed when the ganglion cells of an eye were provided with a retrograde marker substance. This diffused via the axons to the cortex. There, it became visible that one eye had a strip-shaped projection area. The projection area of the other eye lay between these stripes.

With this, however, there were also squares in the primary visual cortex - seen from above - in which the upper two corners received input from the left, but the lower ones from the right. Here it was possible to compare the signals from both sides of the body. We call the corresponding cortex cubes disparity modules.

If both eyes focused on a certain point in space, the left and right eyes delivered completely identical brightness and colour signals to this image point in the cube corners of the corresponding disparity module, so that excitation maxima could only form there at the points x = 0 and y = 0, the z-coordinates of the maximally excited neurons encoded the concrete colour and brightness.

If the signals of the left and right eye did not match, the excitation maximum was asymmetrically shifted, x = 0 and y = 0 then no longer applied.

Thus, all neurons of the disparity module - a new submodality - fired particularly strongly on the coordinates x = 0 and y = 0, if the same information arrived in the respective pixel of the eye. The greater the deviation from these target coordinates, the greater the disparity.

In all divergence modules of the cortex we observe - this has already been proven for visual modalities and is transferred by me to all modalities - a special phenomenon, which here is given the name return phenomenon. It only becomes understandable when one has understood the function of the basal ganglia, which, however, will be explained later.

Return phenomenon

The return phenomenon occurs when a time-delayed return signal from the basal ganglia not only triggers a new secondary modality field, which adds the property "moved" to the previous modality (brightness, colour, angle), but also the area of origin of its origin and generates its own return neuron there.

Incoming axons should (in the cortical maturation phase) lead to the formation of new neurons in the cortex. The return neuron generated by a return signal in the primary cortex areas takes over from basal ganglia signal signal compatibility with the original signal, so that it also tries to acquire its input.

Thus, a new differential neuron is created in the original module, which receives the excitatory cortex signal and the inhibitory and time-delayed cortex signal at the same time. Therefore, it is movement-selective.

Therefore, there are also neurons in the primary visual cortex that react to line elements, but also to movements of line elements, as well as end-inhibited visual neurons. These are found by default in the respective higher-level module. The fact that such neurons also exist in the primary cortex areas is due to nature's urge to build up reserves. If the higher-level module fails, these replacement neurons can step in. This explains the phenomenon that primary neurons react to stimuli that are only detected in secondary areas.