ISBN
978-3-00-064888-5
Monograph of Dr. rer. nat. Andreas Heinrich Malczan
The time-sensitive difference mapping in the nucleus ruber for motion analysis should not remain the only one. The nucleus ruber projected descending to the motor neurons. Thus, this difference mapping for motion detection in the primordial brain, i.e. above the nucleus ruber, was unfortunately not directly available.
Only the frontal cortex received the differential image from the nucleus ruber via the detour of the spinocerebellum, whose input was provided by the nucleus ruber via the nucleus olivaris. It could recognize movements. The other cortical lobi were not involved in this signaling pathway.
This was disadvantageous because there were also signals that only reached the primordial brain above the nucleus ruber, such as visual signals.
The ability to detect the movement of visually perceptible objects (motion vision) could give the creature a tremendous advantage.
Nature proved to be able to bring about this advantage. It developed a new subsystem, expanding and further developing existing subsystems.
To understand this, we need to remember the achievement that produced the cerebellum: the ability to induce signal inversion.
With the sense of sight a peculiarity developed. There, signal inversion took place directly in the visual receptors - from a certain evolutionary stage onwards. Receptor signals could be inverted directly on site at so-called band synapses.
At first there was only the light-dark vision. But this already provided two types of output signals: on-signals and off-signals.
While with on-signals the fire rate increases with increasing signal strength (strictly monotonically increasing function), it decreases with off-signals (strictly monotonically decreasing function).
An on signal can be converted into an off signal by signal inversion. The first thing to do is to switch over to an inhibiting transmitter. Secondly, an exciting average signal is required. Thirdly, an inversion neuron is needed, which is excited by the mean signal while it is relatively inhibited by the inhibiting signal. This reverses the monotony process. The weaker the input signal becomes, the stronger the residual signal is, because the signal average is now less inhibited.
In the visual system, signal inversion is already realized at the receptor level by so-called band synapses. The signal is then present as a signal pair, one on-variant and one off-variant.
We want to show how the primal brain of the earliest vertebrates was already able to detect movements in both the on-signaling and off-signaling pathways. This means, for example, that it was able to detect the movement of light objects, but also the movement of dark objects. And both simultaneously and independently of each other.
If you take the red/green+ signal as the on-signal and the opposite red/green+ signal as the off-signal, a living being could now perceive both the movement of red objects and the movement of green objects. Both channels were separate, completely independent of each other.
How could such a movement analysis be realized in the primordial brain?
In a very abstract way we choose two signal channels that belong together in pairs because they consist of an on-signal and an off-signal. Both types of signal leave the retina and move - in a well-ordered way, pixel by pixel - to the input segment of the visual level of the rope ladder system. This corresponds to the visual thalamus. This sensory center now projects on the one hand to the motor center via the horizontal commissure neurons of class 3. The visual signals thus reach the motor center. They are also transmitted to class 4 connective neurons on the uppermost level - i.e. the cortical level. But there is a third goal.
Let us remember: All signals are transmitted to the different averaging systems. Including the dopaminergic substantia nigra pars compacta. This applies both to the visual on-signals of our example and to the off-signals. Class 5 neurons are used for the transmission, which also project downwards to the mean nuclei.
However, the substantia nigra pars compacta of the early primordial brain projected excitingly back to the striosomes. Pixel by pixel, one layer of GABAergic striosome neurons was contacted and excited for both the on-signals and the off-signals.
Thus, the striosomes contained, among other things, two new images of the retina: one neuron layer represented the on-signals, the other the off-signals.
In later evolutionary times there was such an on-layer for each color channel and an off-layer in the striatum, as well as for the brightness channel.
The GABAergic striosomal neurons projected in descending order towards the substantia nigra pars compacta. However, they were unable to contact the neurons from which the signal was coming. In that case, the signal transmission would have broken down. But the substantia nigra pars compacta contained inhibitory interneurons There, these were originally used for lateral inhibition, i.e. to increase the contrast between the signals. For this purpose, they were excited by the dopaminergic neurons, since this nucleus was also a mean nucleus.
These signals returning from the striosomes were inhibitory. They recruited the inhibitory interneurons in the mean nucleus of the SNpc. In the course of a prolonged evolutionary process, these interneurons separated from the substantia nigra pars compacta and formed their own nucleus, the substantia nigra pars reticularis. This consisted of GABAergic, i.e. inhibitory projection neurons and projected inhibitively into the thalamus. The reason for this was the relationship of the signals, the primal signals came from there. However, these inhibitory neurons still received the mean excitation of the substantia nigra pars compacta. They were tonically excited. Therefore a signal inversion took place in them. The on-signals thus became off-signals, and the previous off-signals were transformed into on-signals.
These signals had received a time delay due to the long transit time on the longer route and were therefore the inhibiting, time-delayed component of the original signals, in addition, a type mix-up occurred: On became Off, Off became On.
The output of the substantia nigra pars reticularis reaches the thalamus in an ascending manner and docks there again in a precisely fitting manner. The on-signals of the SNpr dock in the thalamus to neurons that also received the visual on-signal from the retina. Since they arrived from the SNpr on GABAergic axons, they have an inhibitory effect. In addition, they show a time delay. Therefore, a time-sensitive difference image was created for the on-signal type. Point by point, pixel by pixel, a signal remained in the difference map only where the past signal from the SNpr differed from the present signal. All moving objects of type On become visible. In the case of Hell-On signals, this means the bright objects that have moved in the meantime.
However, if the On signal was the Red+/Green color signal, all red objects that have moved remain visible.
Similarly, there is a thalamus layer of differential neurons that received the off original signal from the retina as the excitatory variant and additionally the inhibitory and time-delayed off signal from the substantia nigra pars reticularis. This differential mapping contains all objects of this signal type that have moved in the meantime. For example, the dark objects or the green ones.
We assume that thalamic difference neurons formed their own populations, arranged in their own layers and nuclei. Thus, the previous projections to the cortex were preserved, which had existed since primeval times. In addition, there were now these differential neurons, which were used for motion analysis. And we assume that the projection locations were also different. The previous projection neurons moved into a separate cortex area, which served the object analysis (What is that?).
The projection axons of the differential neurons moved to another, independent cortex area, which served for the analysis of location and movement. Both types of projection and cortex areas are also separated in modern vertebrates.
In humans, visual shape and color perception is transmitted via the ventral path leading to the inferior temporal cortex. The signals generated for spatial localization and motion analysis reach the posterior parietal cortex via the dorsal path.
The substantia nigra pars reticularis is a descendant of the inhibitory interneurons of the substantia nigra pars compacta. It is used for the inversion of those cortical signals which are present in their on and off variants, it is therefore an inversion nucleus. The signal type is exchanged by the signal inversion. An obstructive rear projection to the thalamic origin of the signals and a type-correct superposition with the original signals generates an (additional) time-sensitive difference image for motion analysis.
The time delay occurs on the detour of the signals from the thalamus to the substantia nigra parc compacta, on to the striosomes of the striatum, from there to the substantia nigra pars compacta with signal inversion and back to the thalamic point of origin.
Since not only visual signals are present in an on and an off variant, but other receptor systems also formed such a splitting into on and off signals, the substantia nigra pars reticularis is also reached by these signals. The terminal regions of their output are then of course not located in the visual thalamus, but often also in the ventral thalamus. However, the signal processing principle is the same. First, a time delay in the dopaminergic subsystem of the SNpc, back projection into the striosomes of the striatum, switching to GABA, signal inversion in the SNpr and inhibitory back projection to the thalamus, where the difference mapping in the on and off variant takes place.
If a signal type is present in the on-off variant, the formation of a time-sensitive differential image for motion detection for this modality is achieved by the involvement of the substantia nigra pars compacta, the striosomes of the striatum, the substantia nigra pars reticularis and the thalamus.
First, the signal is projected from the thalamus via the cortex into the substantia nigra pars compacta. This acts as a delay and switching core. After switching to dopamine, the excitatory projection into the striosomes of the striatum and switching to GABA takes place. This inhibitory signal is inverted in the substantia nigra pars reticularis by inhibitory inversion neurons, which receive their tonic excitation from the substantia nigra pars compacta as well as from the nucleus subthalamicus. The relative inhibition of this excitation corresponds to signal inversion. Thus the on signal becomes an off signal and the off signal becomes an on signal. Both have a time delay due to the long transmission path and are therefore past signals.
They reach the thalamus in an ascending and inhibiting manner and are superimposed on the output signals there, which represent the excitatory present signals, according to type. As a result, a time-sensitive difference mapping is created for each signal category, both for the on and off variant. This enables the movement detection of objects of this modality in both signal categories.
In later evolutionary time, the substantia nigra pars reticularis should realize this function also for other types of signals, which in this monograph are called extreme value coded signals. This coincides with the formation of the matrix in the striatum and is described from chapter 6 on.
Monograph of Dr. rer. nat. Andreas Heinrich Malczan