Vertebrate brain theory

5.10    The body side comparison in difference illustrations

Many of the neural circuits presented so far use the principle of signal transformation. This consists of neurons that are linked together.

Signal-related neurons (usually) have the same targets, the target is passed on in the signal chains.

If the basal ganglia or amygdala nuclei generate a time-delayed and inhibitory signal from an excitatory primary signal, this signal will target the same targets as the primary signal because of the signal relationship. One explanation would be the transfer of marker control of the axons from the predecessor neuron to the successor neuron. Another would be the existence of electrical synapses that connect the neurons in signal chains.

If now the primary signals have several targets, the derived secondary signals will (mostly) drive to the same targets.

From these assumptions it can be deduced that there must be bilateral differential mapping, i.e. mapping in which the signals of one half of the body are superimposed on those of the other half.

Why is that? Because each type of signal reaches the opposite side via sideload cores.

It was shown that the inhibitory and time-delayed signal, which is generated in the amygdala system and the basal ganglia system, is superimposed on the excitatory primary signal in a differential nucleus for motion analysis.

This secondary signal is signal related to the primary signal because it was derived from it. Therefore it will drive the same signal targets.

We know that the primary signal is also made available to the opposite side via the side change core. It is therefore not surprising that the secondary signal also drives the side change cores.

Therefore the paging cores also receive the inhibited and time-delayed output used for motion detection. Here, however, the excitation component consists of the signals from the opposite side. In this way, inhibitory past signals of one side of the body are compared with the excitatory present signals of the other side of the body.

For example, it can be determined whether the left or right olfactory signals are stronger. From this, a correction signal can be derived which curves the body of a fish in such a way that its approach to the prey is ensured.

It should be remembered here that all side comparison kernels exist bilaterally.

The same signal comparison between olfactory signals of the present and the (immediate) past also takes place on the other half of the body. Depending on whether the olfactory signal is stronger on the left or right side, a residual signal remains in the left or right midline thalamus, which is now supplied to the trunk muscles of the same side of the body and results in a curvature of the trunk in the direction of the prey. This is how predatory fish find their prey via the scent. If, on the other hand, the residual signal was used to trigger the trunk muscles of the opposite side, the trunk bent in the opposite direction, i.e. away from the scent object, i.e. the predator. Friedfish and predatory fish therefore use the residual signal from the signal comparison between left and right olfactory strength in the opposite direction. This was definitely an evolutionary advance. Later, with the development of the cerebellum, a learning ability was added that could also change such reactions.

In the course of evolution, separate comparison nuclei were formed, which worked bilaterally and compared the signals of the left and right side of the body. They were also mostly bilateral, although in the course of evolution there was a splitting according to modalities.

The comparison of the signals of the left and right half of the body in comparison cores is an important algorithm. It can still be observed today, both in the motor and visual sense. The principle is always the same. An ipsilateral and excitatory image of the world by means of a modality is merged with a contralateral, inhibitory and time-delayed image of the world in a topologically well-sorted manner and, through additive superposition, produces a differential image to this modality. The time delay causes the dopaminergic system. This differential image is the zero signal in every point of the image if there were no changes during the time of the signal delay by the dopaminergic center. However, if a signal change occurred during this (rather short time span), only this change remained in the difference image. Moving objects could thus be filtered out.

The most elementary generation of such a differential mapping in grey primeval times was made possible by the formation of an efference copy that was switched to dopamine and returned directly to inhibit the signals of the output neurons via excitatory D2 receptors after switching to GABA. In this way, temporal changes on the same side of the body could be determined. If, on the other hand, the topologically assigned neuron of the opposite side was inhibited, an intensity comparison between the two halves of the body was possible, which also allowed the direction of change to be detected.

A basal ganglion system consisting of striatum, nucleus accumbens or substantia nigra pars reticularis for switching to the inhibitory transmitter GABA developed only later in the course of evolution. Even today, such D2-receptors are still present in the thalamus, the amygdala and the hippocampus, for example, and bear witness to this first early form of generating a differential image for the detection of changes and movement.

The olfactory system, which was extended as described above, was thus able to detect odour mixtures, perform a side-by-side comparison of left and right odour signals and rotate the signals in the limbic loop as a hippocampal theta. The latter served olfactory memory and enabled prey capture or escape even minutes or hours after the olfactory signal had already ebbed.

This principle of comparing an excitatory present signal with a dopaminergic time-delayed and inhibitory signal of the immediate past by additive superposition became a basic principle in the course of evolution. In the resulting differential image, only the signal changes remained. Thus, movements could be detected from visual signals, resulting in two visual signalling pathways in the brain areas. One signaling pathway was used to recognize visual objects, thus answering the question of "what". The other signaling pathway was used to recognize movements and changes, thus answering the question of "how" and "where". This also applied to other modalities, such as hearing, but also smelling. The basic principle of inhibiting the signals of the present by signals of the past for the purpose of recognizing movement was already postulated in 1961 by Werner Reichhardt from Tübingen, who successfully researched the functioning of insect brains. The circuit he proposed became known as the Reichhardt detector. We will refer to the circuit in the vertebrate brain presented here as bilateral difference formation with time delay. We will encounter it several times in the brain.

Theorem of time-sensitive difference mapping

Time-sensitive differential mapping is used for motion detection of objects that are perceived with the help of receptors of different modalities.

In the limbic system, the dopaminergic VTA took over the function of a delay nucleus, while in the rest of the brain system the dopaminergic substantia nigra pars compacta became the delay nucleus.

Analog signals were time-delayed in the dopaminergic system and excited GABAergic neurons, which formed the inhibitory component that was superimposed on the output signals. In the striatum the inhibitory neurons formed the matrix with the D2-receptors, in the nucleus accumbens the nucleus and in the amygdala the central nucleus.

ON-Off signals were time-delayed and inverted once, in order to then be superimposed on the excitation output signals in the correct type. In the striatum the matrix neurons with the D1-receptor took over the signal inversion, in the amygdala analog neurons in the central nucleus and in the nucleus accumbens the neurons of the shell.

Maximum coded signals required a double signal inversion.

The second inversion nucleus was the medial nucleus of the amygdala in the limbic system and the globus pallidus in the nonlimbic system. In the limbic system, the tonic excitation input originated from the magnocellular basal nucleus of the amygdala, in the nonlimbic system from the nucleus subthalamicus. The latter received the mean value signals from the fifth layer of the cortex and generated the mean value signal required for inversion.

The output of the second inversion kernel was the inhibiting signal component for the difference mapping for extreme value coded signals. If it was compared with the ipsilateral excitation signals, change and motion detection was monolateral. If, on the other hand, the contralateral excitation signals were inhibited by the inhibitory ipsilateral excitation signals, the difference mapping produced was bilateral and no longer limited to one half of the body.

The monolateral comparison nucleus for analog signals was the nucleus ruber, for the other signals, however, a thalamus nucleus. For bilateral comparison, separate nuclei were formed (e.g. the nucleus isthmi for visual difference imaging, in mammals the nucleus parabigeminalis).

The reference nuclei delivered excitatory signals during movements or signal changes, which reached the motor neurons as additional output, thus generating motor response reactions to movements and changes that could be detected by various modalities.