Human brain theory

ISBN 978-3-00-068559-0

Monograph of Dr. rer. nat. Andreas Heinrich Malczan

4.6.  The sensory focus module 

Focal modules are used to control eye movements, control head rotation and coordinate trunk movements using visual signals, among other things.

We will demonstrate this using the example of a visual focus module for controlling eye movements. Focus modules serve the purpose of fixing a visual object with the eyes in such a way that it is positioned in the centre of the visual field. Movements of the object trigger eye movements so that the gaze follows the object. If eye movements alone are not sufficient, an additional head rotation or even a trunk movement takes place. This is controlled via centre of gravity modules.

Focus module - simplified representation

Figure 35: Focus module - simplified representation

 

In the above illustration, the magnocellular ganglion cells are symbolically represented in the retina. Each magnocellular ganglion cell is connected to a neuron in the focal module, whereby the arrangement of the ganglion cells is retinotopically transferred to the focal neurons. Only one of the many axons that transmit the signals from the retina to the focal module is drawn. This is for clarity.

Strictly speaking, there are two such layers in the retina, because there are the light-on ganglion cells and the dark-on ganglion cells. This has been omitted in the above illustration for simplicity.

Also omitted are the layers with the colour-sensitive receptors.

In this respect, a complete focal module in the visual area consists of the same layers as the visual thalamus. We will look at only one of the layers here as an example to explain the basic functioning of the visual focal module.

When a visual object excites one of these cells particularly strongly because it is perceived by its large receptive field, this excitation reaches the sensory centre of the third segment via optic nerve axons. There, a retinotopically ordered image of the retina exists. The class 4 input neuron, which receives the strong input from the retina, excites the interneurons in layer 4. The supplied excitation spreads over the area, with distance-dependent attenuation.

Lateral inhibition of retinal neurons from each other enhances the effect of this circuit.

We think of six output neurons in the form of a hexagon at the outer edge of the sensory nucleus, which forms a surface here. These six output neurons supply the six eye muscles of the eye on this side of the body with their signals. In principle, this focal circuit exists in the tectum opticum, called colliculi superiores in humans.

If the image of the visual object is in the centre of the retina, the ganglion cell there will project to the centre of the focal module. Then all six control neurons for the eye muscles are equally excited, because their distance to the centre is the same.

If the image of the object is off-centre, a retinal neuron located there is maximally excited and moves to the assigned location of the focal module. This location is now also off-centre at the same position as in the retina.

The control neuron that has the smallest distance to this centre-of-mass neuron receives the strongest excitation component, because the damping on the short distance is smaller. All the other 5 control neurons of the centre of gravity module receive a weakened excitation, which decreases with increasing distance to the centre of gravity neuron.

Therefore, the eye muscles receive different strength contraction signals. The eye muscle that contracts most strongly is the one that is the smallest distance from the focal neuron. This moves the eye so that the visual object moves towards the centre.

Here we should think of the muscles as being similar to small stepper motors. Each action potential leads to a small muscle shortening. Therefore, the eye muscle that receives the highest rate of fire will have the greatest shortening. The eye movement causes the image of the object to approach the centre of the retina.

Only when all eye muscles receive the same contraction signals, because the retinal image of the object is in the centre of the retina, i.e. centred, do all muscles receive equally strong contraction signals, so that no eye movement takes place, but only muscle tension is maintained.

In the above illustration, only one visual modality is shown. In reality, the different visual modalities are arranged in layers on top of each other, like in the visual thalamus. There one finds (in humans) the modalities magnocellular light-on and dark-on as well as the parvocellular modalities red-on/green-off, green-on/red-off as well as the modality blue-on/yellow-off.

If instead of the eye muscles those neck muscles are controlled that cause a head rotation, then the eye rotation can be supplemented by a head rotation. It is also possible to control trunk muscles that cause body rotation. For this purpose, the input neurons for these modalities form layers for the different muscle areas or the associated segments.

The sensory modules for eye rotation, head rotation and trunk rotation are simply arranged on top of each other, with the retinal signals then working their way down through these module layers, with the axons running vertically through the different layers. Here, the signals for controlling the eye muscles would be generated first, then - with a slight delay - the neck muscles and finally the trunk muscles. The whole thing would be a dynamic process because the first output signals already lead to eye movements, which in turn cause a repositioning of the visual object image.

If the retinal image also passes through the basal ganglia module, the eyes (involving neck and trunk muscles) can also follow moving objects.

Focal modules of this kind became possible because the originally quite large cavity of the neural tube in the head area, which was filled with ventricular fluid, was able to reduce its diameter, since the splitting of the neural tube at the level of the cerebellum meant that there was now also an outer ventricular space that could carry away the metabolic products of the nerve cells. Thus, the ventricular space above the thalamus almost completely disappeared, so that this type of focal module could develop. In simple vertebrates, the tectum opticum still forms a section of the hollow neural tube, the interior of which is formed by the ventricular space.

Monografie von Dr. rer. nat. Andreas Heinrich Malczan