Discussion 11D: Ancient maps of Information

The first manifestation of this adaptive mammalian neural structure would be expressed as selective pressures migrate the previously segregated optokinetic and vergence imaging processes from the primordial optic tectum to this newly evolving cerebral cortex structure, synthesizing them into a unified vision system, in something of a neurally spectacular way: When the “pop-out” image of a moving object emerges in the optokinetic “circuitry” of the new cerebral vision system, the attention mechanism will, just as before, interrupt the cerebellum to command the eyes, head and neck to swing the optical apparatus toward the optokinetic image field, centering the object field in the middle of the binocular vergence band. Once centered, the optical apparatus will be commanded to adjust the binocular divergence of the eyes until the vergence vision system achieves a phase-coincidence “lock” on the image depth, at which point fine adjustments of the optical apparatus are made until there is a “trifecta” of phase-coincidences signaling that the vergence image field is sufficiently overlaid with, and is the one in the optokinetic imaging field, at which point conjugate eye movements can then commence, tracking the movement of the “object”.

This combined optokinetic/vergence imaging system has become so well-adapted that primate and human infants can make accurate visual pursuit movements as early as eighteen hours after birth.

Combining the optokinetic imaging circuitry with the vergence imaging circuitry into a unified vision assembly is not the only sensory modality that Nature would migrate to this developing cerebral cortex structure. (The bottom-up engineer should note that the vision systems in the primordial optic tectum would not be wholly abandoned, for, as cerebral cortex expands, the primordial optic tectum would become the “pre-vision” direction diverting mechanism for the attention process in a structure called the superior colliculus in extant mammals). The auditory and somatosensory modalities would also establish their domain maps in this cerebral cortex, via the thalamus, as these sensory modalities form the vertices of a neurological triangle. In the most evolved expression of cerebral cortex, the human brain, each hemisphere of the human brain reflects this triangular coalition of sense modalities, as the vision system in humans occupies the occipital vertex of the triangle, the auditory system occupies the temporal vertex, and the homuncular somatosensory modality occupies the parietal vertex.

Each one of the three sense modalities develop the various maps which express their specific domain semantics, or invariances, used to abstract perceptions within the organism. And in previous discussions, we have seen how these various domain semantics allowed the cerebellum to establish an egocentric systematic to translate those domain specific mappings into kinesthetic programs. But extracting information from an organisms’ environment is very different than just abstracting egocentric perceptions. The singular report of a sense invariant is not “information” in and of itself. It is merely sensation and stateless. It only becomes “information” when it is correlated with some other modal sensory report.

From the very beginning of primordial metazoans, all of the previous iterations of organic sensation could do no more than determine the “where” in instantaneous exteroception. But to extract true information from the environment, Nature will finally be confronted with developing structures to abstract neural patterns that signal the “what” in organic perception.

Which brings the conversation on organic sensation almost full-circle back to the ramified antennae of primordial metazoans, more than 500 million years ago. Back in discussion 9E, the dialog had only enough exposition space to barely introduce the development of the ubiquitous antennae of invertebrate species before moving on, but the dialog promised to return to their development in a later discussion. And for sure, the evolution of electro-chemical exteroception has continued since then, evolving into the non-dimensional senses of smell and taste in the mammals of our current discussion.

As principle components of the emotive complex, mammalian smell and taste have provided the organism with the only direct, exteroceptive sensations of quality in environmental signaling, which would certainly elevate their importance and might also promote their inclusion with the other three senses in the evolving cerebral cortex, creating a coalition of senses which might directly signal the “what” in organic perception.

But even though they are the most ancient of senses, there is an aspect to taste and smell that is fundamentally different than the visual, auditory and haptic senses, a difference which Nature has never learned to reconcile, which has kept the electro-chemical senses perpetually separate neuro-anatomically from the other three senses.

Where the exteroception in the three senses of vision, hearing and touch can express true metric dimensions for which Nature can fashion neural map structures, the electro-chemical exteroception of taste and smell is at best a nominal or ordinal measurement systematic, incapable of directly forming an integral scale or ordered sequence in measurement sampling.

For this reason, the contact senses of taste and smell will be forever segregated in the neural assemblies which have come to be characterized as the limbic structures in mammalian nervous systems.

Separate, but by no means less useful to the organism. Since the time of primordial metazoans, these two senses are the only exteroceptive means providing direct detection of the aversive/attractive quality to contact signaling. However, this diffraction of the immediate environment into dichotomous emotive characterizations does not provide the necessary signaling which can categorize the invariants being signaled by the other three senses of vision, hearing and touch.

As the neural “remodeling” continues in mammalian species, the exteroceptive senses of smell and taste will become the functional focus of this limbic system, as what the dialog has been characterizing as the “emotive complex” will evolve into several limbic structures, for which the principle structure is now called the amygdala.

And much like the metric senses, whose exteroceptive inputs to the thalamus were conducted by one-way afferents with no reciprocal projections back from the thalamus, the olfactory bulb in mammals is the only area that makes input to the amygdala and does not receive reciprocal projections back from the amygdala.

The amygdala in extant mammals will perform much the same functions as the emotive complex in primordial vertebrates, but as the sensory triad develops in early cerebral cortex, Nature would still need a method to integrate the metric maps of the cortex with the nominal or ordinal measurement signaling of aversion or attraction from the amygdala.

In extant mammals, there is a structure which is anatomically adjacent to the amygdala, and functionally part of the limbic system, but nestled closer to the cerebral cortex, which is called the hippocampus, and it would be the hippocampus that Nature would fashion to orchestrate the “texturing” of maps (created by the metric senses) with the emotive quality being signaled by the amygdala.

In order to see how Nature accomplished this orchestration, it is important for the bottom-up engineer to understand the fundamental essence in the disparity between the metric senses of vision, hearing and touch and the nominal senses of smell and taste. The cerebellum can perform phase space transformations with the metric vision, auditory and haptic senses by exploiting the invariances between the maps which these senses develop, but the nominal or ordinal sampling senses of smell and taste cannot resolve invariances within the total “set” of their sampling.

Nature developed the hippocampus to synthetically develop a unique form of invariance in the nominal sampling of smell and taste episodes. In the three metric senses of vision, hearing and touch, invariance was expressed by transformational semantics of the topologic maps for each modality, so, to create invariance for the nominal electro-chemical modalities, Nature would have to create virtual, non-dimensional “maps” and their semantics for these two senses, which would transcend the lack of an integral scale in their unordered sampling regime.

These topologically unique maps, collectively referred to as association maps, forming in the neural barycenter of the cerebral triangle of metric senses, would provide the evolving mammalian central nervous system with an ability to abstract topologic frames of reference, which are not egocentric to the organism, but allocentric in their transformational semantics, and their abstraction would be accomplished with the mechanisms of metabotropic state synthesis, expressed in Natures’ specialization of the pyramidal cell, developing in the cerebral cortex, hippocampus and amygdala of our primordial mammalian species.

This latest metabotropic state synthesis, neurophysiologically identical to the metabotropic processes of previous discussions, but now modulated by the primordial mammals’ evolving attention mechanism, would establish the neurologic substrate for cognitive abstraction in the evolution of central nervous systems.

These association maps are established within the barycentric association area of the cerebral cortex metric sensory triangle by forming metabotropic persistences between temporally coincident episodes of an invariant signaled by one of the metric senses (or, more typically, the phasic association of more than one metric sense) with a specific incident of smell or taste. For this, the hippocampus serves as the neural assembly which biases the metabotropic synapses in the cortical association area, much like (what is now the locus coeruleus) biases many of the metabotropic synapses in the primary sensory areas themselves.

Now, these episodic associations are just metabotropic persistences, and although they are spatial abstractions, to have survival value, they must be “re-associated” with a future encounter of either the metric invariant(s) in the association, or the specific smell or taste which it was associated with. And this temporal reinforcement is a learning process, for which we have seen, is a specialized function of the cerebellum.

The bottom-up engineer will recall the recent discussion on how the cerebellum developed to perform the learning of novel motor programs through the constructive repetition of previously learned sub-programs. And surely Nature can utilize this temporal sequencing talent to constructively reinforce association spaces, or the learning of temporally separate sequences having repetitive associations of the same metric and limbic invariants. Where the cerebellum was previously employed to develop “muscle memory”, Nature can surely recruit it to create “episodic memory” from these association maps.

As the bottom-up engineer should recognize that memory is the ultimate expression of temporal invariance, and because this “episodic memory” was formed from an association having a non-metric, nominal or ordinal component, the abstraction comprises an allocentric frame of reference to those invariants, providing the primordial mammal with much of the neural machinery to perform non-phylogenic adaptive assimilation of environmental conditions.

But an organism that assimilates every episodic memory of its environment will soon find itself on the wrong side of the entropic equation. This capability to assimilate episodic memories cannot just run willy-nilly. As much as this capability provides the organism with a survival advantage, it must be marshaled before the variety in the environment overwhelms it. And as successful as the hippocampus was at creating these association maps in the cerebral cortex triangle of sensory integration, which provided our primordial mammal with the “film” to assimilate selective “snapshots” of its environment, Nature would soon find the need for a structure to control the shutter in this neurological camera.

Neuroanatomists have identified an area in mammalian central nervous systems which they have labeled the retrosplenial cortex. Its location adjacent to the hippocampus and close to the barycentric associative area of the metric sensory triangle suggests that it has a role in mediating associative memory functions. Specifically, neurophysiological studies of the retrosplenial cortex show evidence that it participates in the translation between egocentric sensory maps and allocentric association maps, providing the mammalian species with a mechanism to normalize the allocentric invariants across episodic associations. These allocentric invariants would become the neurological equivalents of sensory “landmarks” in an organisms’ environment having reinforced emotive salience, in addition to providing a gating mechanism in the hippocampus for the selective association of “novel” salient perceptions.

This neural arrangement will provide the primordial mammalian species with the necessary mechanisms to assimilate those sensory perceptions in their immediate environment having survival advantage, and thus alleviating the need for genetic adaptations, but it seems that Nature is always having to play catch-up with the infinite variety that the environment presents to her creations.

As this survival advantage allows many mammalian species to grow in both body size and neural complexity, the geographical area in which these growing species inhabit will also expand. And a growing area of habitation will result in a geometric increase in the salient invariants, or “landmarks”, that the central nervous system in our primordial mammal will have to deal with, an increase that will eventually tax the neurologic capability of the retrosplenial cortex. Indeed, in extant mammals, the relative size of the retrosplenial cortex compared to the overall cortical surface varies significantly in an inverse manner as the body size varies among species. In rats, it is one of the largest cortical regions of that species overall cortex, reducing in comparative size as species become larger, where in humans, it comprises only 0.3% of the cortical surface. As functional as the retrosplenial cortex was for smaller mammals, as their habitat expanded, Nature would need an expanded solution to the metering of hippocampal associations and the creation of episodic memory.



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