Discussion 11a: The distance between perception and cognition
There is an enormous volume of speculation regarding the environmental pressures that prompted the evolution of terrestrial tetrapods. Indeed, their origins were most likely the product of parallel evolutions, where tetrapods evolved numerous times, in different places.
This diverse phylogeny has unfortunately left an incomplete fossil record, and so the influences which produced the appearance of tetrapods are the subject of much debate, even the very transitional modes of breathing and eating are speculative.
Although these speculations are not the direct subject of this dialog, the Organon Sutra will join the debate with regards to the neurophysiology that evolved along with the varied anatomical adaptations that produced the superclass of four limbed vertebrates, which include the extant and extinct amphibians, reptiles, birds and mammals.
Tetrapods are now generally considered to have colonized land during the late Devonian and early Carboniferous periods, about 350 million years ago. There were a great many anatomical adaptations that provided for this terrestrial invasion, but this dialog will focus on two general forms of morphological evolution that drove the new neurophysiology of tetrapods.
Mobility on land requires a locomotive apparatus vastly different than the finned affair adapted for aquatic organisms. With streamlining no longer a form factor, weight bearing articulated limbs and the need for wide excursions of the head were the initial anatomical factors prompting the developments in early tetrapod neurophysiology, as it evolved from its lobe-finned, aquatic ancestors.
Even before taking their first step, land dwellers must be able to look down to see obstructions on the ground, after which they must also be able to raise their visual field of view in a constant search for predators, food, and attractive mates.
And each step in the four-limbed form of locomotion requires a motive neurophysiology significantly different than the aquatic ancestors of the tetrapods. And indeed, probably one of the greatest variations in forms for artificial agents will be in the expression of any mobility between individual agents.
There were certainly a great many other anatomical changes which contributed to tetrapod evolution, but the pana-view requirements of the head and the independently articulated limbs are the focal points of this discussion. With the incursion onto land, Nature now not only has an entirely new environment for which it must develop novel emotive sensory capabilities, but it must do this while evolving entirely unique methods of locomotion. The overall behavioral activity of tetrapodal neural arrays will have to change from aquatic life to life on the land.
Throughout the imagination scenario chronicling the evolution of the primordial metazoans, and through to the primordial pelagic vertebrates, overall behavioral activity has been portrayed as the result of phylogenetic programs built into what the dialog has labeled the “motoric complex”, most recently illustrated in Graphic 13. And certainly, with the requirement for an entirely novel form of locomotion, it will be this “motoric complex” that will first undergo the most fundamental change in tetrapodal neurophysiology.
Way back in discussion 9D, the dialog hinted that the motoric complex would differentiate into a structure which neuroscientists call the basal ganglia, and it is now time to examine that differentiation in a little more detail.
Throughout the dialog since discussion 9D, the whole play has been acted out with the basal ganglia backstage, supposedly developing “phylogenetic releasing mechanisms”, while the major scenes of the production have been played out onstage in a plot between the emotive complex and the developing visual apparatus. And while most of the evolutionary drama has unfolded onstage, the spotlight can now turn on the basal ganglia, because a terrestrial invasion will require a fundamental change in the locomotive machinery of land organisms, and the control of this new machinery will prompt a progression of our organisms’ neural array to that of a true nervous system.
The motoric complex in our primordial organisms has so far been involved with graded, feedback controlled movements that do not require a global synthesis of actions, operating essentially in a system where the serial order of events and actions are driven primarily by the immediate environmental conditions surrounding the organism, in what is characterized as a stimulus-bound behavioral system.
In extant mammals, the basal ganglia has an extensive feedback network with the Thalamus, and also receives a large input from the Amygdala and hippocampal formations of the limbic system (the primordial emotive complex), in addition to substantial projections from the cerebral motor cortex.
There are essentially three components to the input of the basal ganglia. The first group of afferents are comprised of projections from many regions of the cerebral cortex characterizing motoric intentions, and the second group arise from thalamic nuclei (as a projection of the perceptual diffraction which has been previously alluded to), and these two groups connect with specific inhibitive neural elements located in an area of the basal ganglia called the striatum. The combination of these two groups of inputs generate discrete patterns of activity within the striatum which can be considered to signal contingent actions, motoric actions which form intended movements of the organism, but which are not expressed until released by the third component of inputs.
Because of the inhibitory nature of the neurons mostly comprising the striatum, those discrete patterns of activity, the contingent actions, formed by the first two groups of inputs, cannot maintain sustained activations. Instead, the individual burst of activity resulting from the dual input will recurrently inhibit itself, in a self-dampening fashion, unless it is secondarily activated by the releasing input from the third component of the inputs.
In mammals, the third component is comprised of metabotropic afferents primarily trafficking in the neurotransmitter dopamine, and these non-specific and spatially diffuse dopaminergic inputs serve to maximize or minimize sequential behaviors based on the immediate and prior (prior because remember, metabotropic neurons express temporal properties) environmental situation. Say, for example, the emotive complex declares the presence of a predator and the cortex orders an immediate evasive motor sequence. The resultant contingent activation in the striatum would self-quench unless the dopaminergic inputs signal (or affirm) that the motor sequence currently ordered would indeed successfully lower the threat, in which case the contingent activation would be released to actuate muscles, as the dopamine signaling would maximize and further extend the motor sequence.
In an opposite scenario, where the unsuppressed inputs from the metabolic complex signal that, say, the smell of food was detected, further initiating a contingent activation to close in on the source, the dopaminergic inputs would act to maximize or minimize that motor sequence accordingly, behaving in a simple feedback guidance mechanism.
The two primary input groups would represent movement control computations of direction and force in the first, and representations of motivation and salience in the second order, which for our primordial organisms are activations which have been phylogenetically developed.
The learning capabilities of the primordial and extant basal ganglia is limited to linearly separable sets of inputs, as in a simple neural network, but with the feedback loops to the emotive complex in primordial species, or with feedback loops to the cortex in extant species, the neural structure as a whole allows the organism to perform exploratory behaviors of its environment, based on metabotropic predictions of future success, and to execute survival enhancing evasive behaviors in an aversive environment.
And for a hundred million years, this motoric complex provided a simple interface with the phylogenetically adoptive sensory and state complexes of the neural array. However, with the massive adaptations required for land locomotion, this simple interface will get much more complicated.
The primordial motoric complex has been characterized as a stimulus-bound behavior system, in which the response-related activity patterns are defined entirely within the “motor space” of the organism. Although beginning with an external stimulus, once perceived, the sensory apparatus triggers the motor sequence, and the sequence progression and any feedback employed is neurologically defined within the phasic envelopes of the organisms’ musculature apparatus.
But now, as things get more complicated with terrestrial locomotion, the neural control of motor activity must adopt a different response-related activity pattern, one in which the preparation and organization of movements are referenced to a much more inclusive space, enveloping not just the organisms motor space, but simultaneously, its visual space as well.
These new selective pressures will serve to evolve two entirely different neural assemblies than the neural mechanisms expressing the primordial basal ganglia, assemblies which will come to dominate organic neurophysiology, and which will promote our primordial neural array to the league of nervous systems. (However, in all of this neurophysiological upheaval, the basal ganglia will not be simply abandoned. For one thing, Nature is never so wasteful, and is always re-using especially successful designs. But more importantly, Nature cannot afford to abandon the phylogenetically evolved aversive and attractive response programs developed for the emotive complex, programs developed over millions of years and shaped by untold organic experiments).
The first new neural assembly needed for terrestrial organisims, which is called the cerebellum, has actually been present in a primitive structure developed to coordinate the growing number of voluntary muscle groups evolving around the locomotive tail fin in the primordial vertebrate, such as pectoral fins and ever complicated muscles controlling the jaw and tongue. But the cerebellum of the tetrapods will require capabilities beyond mere coordination of disparate muscle groups, and it will need to operate in a cooperative manner with the remainder of the evolving nervous system in a “super-system” of interlocking neural assemblies comprising itself, the currently evolved neural array, and the next new neural assembly which the unique requirements of tetrapodal survival will adopt. That next neural assembly is what has been collectively termed as “cerebral cortex”.
The selective pressures which will evolve these new assemblies come about by a systemic divergence from the dimensionally shallow motor space of the basal ganglia and its rigid interface to the emotive complex, a divergence demanded as tetrapodal locomotion splits motoric control into three major, interdependent control systems.
Although labeled in various ways, these three motor control systems can be categorized as:
> The medial motor system
> The lateral motor system
> The visuo-tactic motor system
The medial motor system refers to the neuro-muscular controls which regulate the weight-bearing force distribution required in the maintenance of overall equilibrium and posture, providing balance and stance during non-locomotive voluntary processes such as sitting and standing, in addition to providing the necessary components of tension and force to maintain overall equilibrium during locomotive and independent limb movements.
The lateral motor system provides the highly stereotyped pattern of limb movement during locomotive processes such as walking and running, in addition to the organization of multi-joint movements acting in concert during independent limb traversals.
And the visuo-tactic motor system supervises the head, neck and eye movements which are required from a new need to share the visual system with its previous emotive sensory duties. The visual system must now be multi-tasked to additionally provide immediate environmental envelopes for lateral motor intentions, which require feed-forward control in their visually guided trajectories. In the previous discussions, the dialog demonstrated that once the optokinetic mechanisms of the visual system was functionally capable of detecting predators, genetic processes would have the luxury of phylogenetically developing all other response-related behaviors in the neural array of the aquatic vertebrates, organisms occupying one of the least changing habitats on Earth. But in a land world offering a physical terrain of infinite variety, the terrestrial genome would no longer have that luxury.
Once again, the selective pressures of an infinitely variable environment will promote adaptations to the primordial tetrapod vision apparatus just as all of the other anatomical developments are evolving.
These adaptations to the tetrapod vision system take up where we left off at the end of the previous discussion. Recall that the optokinetic process in pelagic vertebrates had developed the ability to resolve the visual field of those things that moved relative to their background, a capability that automatically “deleted” the background from the visual stream of contrast boundaries (those “pop-out” images) being impressed onto the neural maps of the organisms’ optic tectum.
This optokinetic response would surely be just as serviceable in the continual vigilance that the new land dwellers would need to demonstrate, because Nature would certainly evolve predatory tetrapods, and terrestrial organisms would renew the never-ending dance between predators and prey that has been playing out in the world’s oceans for hundreds of millions of years.
However, the optokinetic process of aquatic vertebrates would not provide the primordial tetrapods with imagery that could neurologically discriminate the infinitely varying terrain which their newly land-mobile locomotion apparatus must navigate. In that terrain, there would be no moving imagery relative to the background, and the optokinetic process would just present a meaningless jumble of ever-changing contrast boundaries to the optic tectum when the tetrapod’s head lowers to examine the solid earth of its new surroundings.
The previous discussion ended with a brief characterization of the visual process that pelagic vertebrates were to adapt in the detection of obstacles to their locomotion, and in that characterization, the neural circuitry responding to the narrow band in front of the fish, where the visual fields of each eye overlap, developed a sensitivity to anything resolving in that band, which would subsequently trigger an avoidance reaction on the part of the organism.
This sensitivity by itself would not provide the primordial tetrapods with the discrimination needed to resolve individual obstacles in the visual field, but as this sensitivity would have increased survival value, the neuroanatomy of the visual apparatus would be subject to some subtle adaptations.
Recall again that the optic fibers from each vertebrate eye cross over to serve the contralateral hemisphere of the optic tectum. However, because of the selective pressures brought about from this sensitivity in the overlapping visual fields, a small number of ganglionic axons from each eye that fall within this overlapping field would not cross over, but will terminate in the optic tectum on the same side.
Within this small population of neurons in each hemisphere of the optic tectum receiving stimulation from both eyes, there would be local assemblies that are stimulated from simultaneous, and identical contrast signaling coming from separate eyes, and it would not take Nature long to develop intermediary neural aggregates, which do not respond to the contrast signaling of the retinal ganglionic cells directly, but which respond to the phase coincidence of those neurons which are being stimulated with identical contrast signaling themselves. The response of these intermediary aggregates are not signaling derivatives of the image so much as they are signaling a temporal state of phase coincidence in the overlapping neural assemblies. Most neurologists are familiar with ocular dominance columns in the primary visual areas of mammalian cortex. Although the functional architecture of these neural assemblies serve multiple purposes, as is typical with many of Natures’ designs, their neurophysiological structure directly supports the neural circuits developing from this new form of visual imaging.
And since they are signaling a temporal state, the signaling is an abstraction, as the dialog stated in Discussion 10C, although in this situation, the state was produced mechanically instead of metabotropically. But abstraction it is in any case, and Nature will be quick to phylogenetically corral this abstraction as another form of invariance in the many processes of visual imaging, an additional tool that will lead her designs to cognitive abstraction, and ultimately, to gestalt abstraction.
And if the bottom-up engineer is quick to realize that, because of its temporal nature, this expression of invariance will form the very basis for cognitive abstraction itself, then that realization is very astute indeed.
However, in order to get there, Nature would have to exploit this new invariance. It will not be so straight-forward to combine the metabotropically derived spatial invariance of the optokinetic process with the mechanically derived temporal invariance of this new “vergence process”. And as a matter of evolutionary history, the synthesis of these two imaging processes will not actually occur for hundreds of millions of years, when an extraterrestrial visit to the habitat of our terrestrial tetrapods will literally upset their whole world.
Until that time, as these two vision processes within the optic tectum remain independent vision systems, the neurologic separation between the optokinetic imaging processes and the “vergence imaging” processes will represent the conceptual distance between perception and cognition. And not coincidentally, the separation between the lateral eyes of vertebrates will present a similar phylogenetic distance.
GO TO NEXT DISCUSSION
GO TO TOP OF DISCUSSION
Copyright © 2019 All rights reserved