Physiological Principles

Physiological Principles that distinguish the outer and inner worlds of our brain

The outer and inner sources of information can be distinguished not only on anatomical grounds, but on physiological grounds as well. The basic physiological division is embodied in the distinction that some neuroscientists draw between “channel” and “state” functions – the terms introduced by Mesulam  in 1998. His terminology is fairly idiosyncratic, but it denotes a relatively conservative concept, the physiological foundations of which are widely accepted. Other neuroscientists distinguish between the “contents” and “level” of consciousness – but these terms are less serviceable as they refer specifically to consciousness and thereby exclude the possibility of unconscious mental  processes. Mesulam's distinction between the channel and state functions of the brain is perhaps roughly equivalent to the distinction that psychoanalysis draw between mental representations (“traditional traces”) and mental energies (“quotas of affect”).

Brain functions (principally, forebrain functions) dependant on information derived from the external world are primarily channel-dependent functions. This means that the information processed by these systems comes in discrete bits and is communicated via distinct and specific pathways. Information transmitted from a particular source within a channel-dependent system is not widely distributed to the brain as a whole but, rather, is targeted with great accuracy to other discrete regions. For example, when information arrives at a particular location on the retinae (say 30 degrees below the horizontal and 20 degrees left of the vertical meridian), it projects to a highly specific area of primary visual cortex which represents that precise location on the retinae (and therefore in the external visual field). The coral aspects of this information then project to specific color areas, as do the motion aspects, and so on. In each case, a limited number of neurons directly “speak to” a limited number of other neurons some distance away, while the vast bulk of the brain is completely unaffected by the interaction. Thus Region A connects to Region B, which connects to Region C. Regions L, M, and N, which also connect with each other, are never involved in the interaction between Regions A, B, and C. Ring-fenced interaction of this kind occurs not only in the visual system, but in more or less all the externally directed functional systems of the brain.

This type of interaction between neurons involves three main neurotransmitters. The principal excitatory neurotransmitters are glutamate and aspartate. The principal inhibitory neurotransmitter is GABA (gamma-aminobutyric acid). These are the most common neurotransmitters in the brain, and they dominate the activity of all channel functions.

The internally directed brain structures, units for modulating cortical tone and arousal, operate in an entirely different way. Here, the means of communication is more gross and involves widespread and global effects that reflect changes in the state of the organism rather than in specific information-processing channels. The neurons of single brain-stem nuclei in the state-dependent systems project onto extremely large numbers of other neurons in the source nuclei.

The forebrain neurons thus affected are extremely widely distributed within the brain, so that a nucleus in the brainstem can influence neurons in all lobes of the forebrain simultaneously. In addition, forebrain neurons affected by one state-dependent nucleus can simultaneously be influenced by another one; in these systems there are no specific pathways (channels) but, rather, a number of overlapping “fields of influence”.  The specific serial linkages between regions in the channel systems are replaced by overlapping and interacting fields. Even more characteristic of the state-dependent systems is the fact that they are also open to influence by chemicals other than neurotransmitters, which link the brain directly with the visceral body.

Brain's distinctions is similar to the distinction that Freud drew between mental “quality” and mental “quantity”. Illustration by Elena.

The Brain and the Inner World, Introduction to Basic Concepts. Mark Solms, Oliver Turnbull.