Neuro Lessons in Metaphysics

by Denis Larrivee

Given the current proliferation in neuro research byways and highways, from working out such esoteric parameters as metastability indices and frontoparietal activation, developing therapeutic strategies like myelin regrowth and cell cycle activators, and designing an assortment of implantable technologies, perceived prospects for neuro-rehabilitative outcome have entered a phase that, while not ebullient, are nonetheless shimmering with anticipation. By comparison, prospects for re-acquiring normal mobility in the 1970s and 1980s for patients with severe lumbar trauma, let alone cervical transection or higher cognitive impairments, were minimal for the former and virtually non-existent for the latter. Basic research initiatives undertaken then by private funding organizations like the Paralyzed Veterans of America, for example, focused on endogenous cellular mechanisms affecting nerve outgrowth and synaptic connections in spinal or peripheral nerves. Advances in basic research since, on most aspects of nervous system function, from single neuron to large scale multi network modules, have greatly improved this understanding. Significantly, this basic research understanding has greatly expanded the prospective range for neurotechnological intervention in the form of medical implant devices.

This latest is good news for patients, especially the swelling ranks of those past 60 years of age that World Health Organization demographics portend. Improvements in medical care have today significantly extended life expectancies, and are expected to increase population percentages of this age group to 20% globally by 2050 and 25% or more by 2030 in the most technically advanced nations. Dominating the health prognostications of this population sector, however, is the considerably increased risk of cognitive impairment; hence, also the increasing likelihood of long term, medical health care burden associated with it. WHO projections, for example, indicate a relatively uniform or slightly increasing percentage of those within this group suffering cerebral ischemia. Improved medical care, moreover, has substantially reduced the mortality to incidence ratio, making long term therapeutic assistance for substantially greater numbers of patients nearly inevitable.

Early approaches to neural implant devices appealed to the relatively simpler anatomical configuration and functioning of peripheral sensory and motor nerves, and the basic to and from communication between brain and body of the spinal cord. This anatomical simplicity limited the spectrum of technical obstacles that faced engineering researchers in this earlier developmental phase. The therapeutic objective of these implant devices was confined principally to the replacement of lost neural function by mimicking neural signals that would ordinarily be transmitted in the damaged neural cells to their respective target organs. Editors Jensen, Andersen, and Akay summarized this basic design strategy in their conference text titled “Replace, Repair, Restore, Relieve” taken from the II International Neuro-rehabilitative conference in 2014. That is, the design for device communication with nerves, termed interfacial design, was premised on an understanding of nerve function that was linearly related to nerve signals and was delivered by one way signal transmission. In the absence of signals, the presumption was that no meaningful activity was carried. In other words, the signals alone were the functionally significant elements.

Building on these earlier, successful designs, implant devices for the brain have gone on to adopt a physiological understanding of brain operation that, while acknowledging the brain's greater anatomical complexity, is, nonetheless, a direct extrapolation of that premised in the interfacial designs of peripheral nerve implants. Hall, Nazapour, and Jackson's 2014 paper, for example, is illustrative, where an external electrical signal called the local field potential, is exploited for the purpose of isolating nerve signals, known as spiking rates, from individual neurons. Neglected in the presuppositions underpinning these strategies, however, is a philosophy of nature with which they can be reconciled, one that is linked to the global role of the brain in guiding behavior. Humans, like all organisms, are autonomous, a capacity built into and mediated in large measure by the brain; that is, the brain has been evolutionarily crafted to enable the individual to be autonomous, and not vice versa. Due to these supra-physical constraints, a number of consequences ensue. For example, the brain assists in maintaining bodily homeostasis, and so is, necessarily, persistently active. Moreover, organisms are endowed with goal directedness, meaning that the whole organism, within the confines of its bodily perimeters, is subordinate to the brain's - and peripheral nervous system's - guidance. As a result of this crafting, there are significant constraints imposed on the way in which the brain can be organized that differentiate its organization and operation from a strictly feed forward, off on off working arrangement used in peripheral nerves for direct communication with a target organ.

Because the brain must remain continually active, for example, there is a constant stream of sensorial input as well as interregional communication that generates a prevalent, indeed persistent, circumstance of background noise. Transmission of signals through nerve networks, accordingly, always includes a mixture of signal and noise.

Accordingly, the brain's anatomy must be reconfigured to overcome this circumstance, one that is achieved through nerve feedback loops. In fact, nearly 95% of brain neurons possess such recurrent connections. Through this reconfiguration nerve signals are cycled through preferred, low resistance pathways that helps to create stable and reliable responses that can overcome the noisy background in which they are immersed. That is, a system of constraints is established that dynamically stabilizes the signal trajectory, known as an attractor motif. The use of this motif, rather than the spiking activity per se that is employed in peripheral nerves, becomes, instead, the relevant parameter for the execution of behavior, a circumstance that bears implication for implant design. This system of constraints, moreover, is likely to contain multiple elements, that in addition to spiking features and synaptic connections, may include influences of other cells like glial cells, physical characteristics, and the like. Significantly, glial cells in the brain are very large, elaborate compared to other locations in the body, and abundant relative to neuronal cells. Thus, numerous elements of the brain are designed to overcome this physically noisy state to enable the individual to act as an individual.

More telling still is the brain's ability to compile combinations of dynamical motifs to expand the range of behavioral activities that can be performed, from our highest cognitive abilities to our relatively simple motor movements. This combinatorial as well as hierarchical scaling of dynamical elements offers an explanation for another fundamental feature of living systems, that is, how living systems attain goals autonomously; that is, the overcoming of noise is not the only consideration that is evidenced by these broader realities of living, particularly human, organisms. Autonomy also implies that goal seeking must be integrated in light of the holistic nature of the organism. That is, the nervous system must be crafted to perform as an entity. To do otherwise, would eventually result in its complete disintegration, as demonstrated in physical, non-living systems. In fact, there is increasing evidence that the dynamics of the brain are so constructed to enable this unitary and integral character of the organisms to be prioritively exercised. The construction of stable elements like attractor motifs, notably, is akin to the construction of building blocks that can then be assembled to create global platforms that can elicit localized activity in the context of the whole or, alternatively, serve as the basis for personal identity. These universal characteristics of the individual are increasingly recognized by neuroscientists in the current proliferation of global models for such features as consciousness, sleep, personal identity, and control over motor behaviors.

What all of this seems to point to is that principled philosophical insight into physical nature is needed and cannot be only empirical or mechanistic, which is to say only partial or compositional, but rather needs to be synthetic and comprehensive. In a phrase, metaphysics is not just a bad penny that keeps showing its image indiscreetly, but a determinative feature of nature that anchors nature's appearance, bearing its own suite of repercussions when not considered. Neuro-engineers, take note!