by Robert DePaolo
This article discusses a pervasive brain function that has implications for language development, arousal modulation and autistic symptomatology. It revolves around the phenomenon known as the Mach band – which is a hypothetical description of an actual neural process known as lateral inhibition. It is this process that facilitates perceptual accuracy, prevents noise/uncertainty build up in the brain and prevents random uncontrollable excitation.
A Distinction Machine…
it is generally assumed that the evolution of a large brain was a watershed event in man’s ascendancy in nature. A large brain does provide obvious advantages. For example, vast neural connections allow for more associations, expansive memory and greater cognitive flexibility, all of which have enabled homo sapiens to adapt to more varied environments and circumstances than any other creature.
However there is an equally obvious disadvantage to having a large brain. It is a drawback that pertains to any highly complex, vast information system; a greater probability of noise interference. The more neurons and interconnections, the greater the potential for info-chaos and uncertainty. Perhaps that is why, despite our pan-adaptability humans seem to be the only animals with substantial degrees of mental illness and cognitive dysfunction . *
Fortunately, in most instances, there is a built-in mechanism that operates as a noise buster and enables humans to negotiate around a highly complex brain. It is called lateral or “surround” inhibition and it allows us to distinguish relevant inputs from irrelevant, peripheral inputs that would otherwise create interference patterns, prevent efficient response selection and accurate perception (Yantis, 2014). Without this mechanism a large brain would actually prove disadvantageous since the search for adaptive perceptions and behaviors would involve an excruciating needle in the haystack scenario.
Beyond preventing mass interference (noise) lateral inhibition provides the benefit of emotional equanimity, because it also modulates arousal levels. Just as an influx of visual and auditory stimuli are chaperoned by this mechanism for perceptual and behavioral clarity so too does it facilitate emotional control and specification. In both instances the impact of lateral inhibition on perception, behavior and emotional arousal levels is profound.
The Autistic Conundrum…
Questions have arisen as to why,some autistic individuals have exceptional (if somewhat narrow) skill levels in certain areas while others have more severe limitations. Lack of diagnostic accuracy might be one reason; bearing in mind that the autistic spectrum encompasses both those with fine motor deficits so profound as to render the use of eating utensils problematic and those whose fine motor and cognitive skills are intact enough to enable them to write lengthy auto-biographies.
However there are other possible explanations for what might be termed ‘autistic narrowness.’ It would appear both higher and lower functioning autistic individuals operate in an experiential tunnel and that deviation from this narrow vantage point comprises an aversive experience. This has relevance with regard to the question of why autistic individuals engage in self stimulatory behaviors, rely so heavily on routine and demonstrate task avoidance tendencies.
It is well established that one facet of autism is the “kindling” process – the tendency for neural activity to spike fairly chronically (Krista, Gilby et al 2013 ). One can assume the jolt resulting from spiking is aversive to the autistic individual, Since he or she is easily overwhelmed by spreading, unmodulated inputs and since all learning, particularly involving motor skills is accompanied by heightened arousal levels autistic learning deficits are not solely due to cognitive limitations but also in large part to arousal intolerance and task avoidance (Iverson 2015), (Liss, Saulnier et al 2006).
No one sits still with the impingement of aversive stimuli. Instead one responds adaptively to terminate or override the aversion. If true, it follows that self stimulatory behaviors (rocking, humming, finger play) are not awkward manifestations of neuro-pathology or lack of cognitive ability per se but arousal modulating, adaptive behaviors that conceivably substitute for defective lateral inhibition functions. In that context the narrow skill learning style of the higher functioning autistic individual could be considered a compromise learning style pitting personal tolerance vs. environmental vagaries.
Nuts and Bolts…
Lateral inhibition entails the capacity of excitatory nerve cells to suppress surrounding nerve cells. One of its purposes is to create contrast in facilitating sensory perception so that edges in the visual domain and details in other sensory areas can be accurately detected. Most of this process originates in the higher center of the brain – the cerebral cortex (Coppola, Parisi et al (2013) but extends to a mid brain structure called the thalamus (Lavallee, Deschenes 2014). It is an encoding mechanism that, as mentioned above prevents noise from overwhelming the brain and has parallels to the “slow potential wave micro-structure” regulatory, memory-encoding process referenced by Karl Pribram (1991).
Lateral inhibition is usually associated with sensory systems such as touch, vision and hearing but since this mechanism is permeates the cortex and thalamus (the latter of which acts as a relay station transmitting neural signals to a variety of brain sites) it can be presumed to be widespread around the brain. As discussed above, since it dampens surrounding interference patterns, making sure that relevant inputs are highlighted, it must operate reciprocally with memory. That is because deciding on relevant vs. irrelevant inputs involves learning in the first place. While some sensory phenomena are built in to the central nervous system – for example perception of angles, edges and perhaps the human face,(Cowen, Chun et al. 2014 ) there can be said to be a reciprocal interaction between memory and the “noise busting” aspect of lateral inhibition. That has implications for autism.
For many autistic individuals anxiety is a central problem; not just because of specific fears but, also because they have difficulty consolidating memory and anchoring experiences to the point of comfortable familiarity, either for lack of adequate lateral inhibition, or because they do not have the perceptual and/or emotional clarity to readily establish memories in the first place. While they can certainly memorize information. memory entails two processes; consolidation and retrieval. Problematic lateral inhibition could create noise within neural interactions and prevent memory from functioning smoothly – thus the need for repetition and routine as adaptive/compensatory measures.
Expression, Comprehension and Cadence…
Deficient lateral inhibition could also have implications for language development. Language is a vast encoding process that might well be dependent on lateral inhibition. It is not just a social-communicative skill. It also allows us to separate one experience, event or object from another. The minute we label a color “red” it automatically creates a distinction between “red” and “blue.” In that context superior human social perception might ultimately derive from what Spence called discrimination learning (1936). As an aside, this implies a connection between language and the hyper-social nature of our species. For example, to be as social as we are requires a greater capacity to distinguish one person or group from another categorically.
While lateral inhibition is a natural byproduct of neural development it can only function effectively by being able to sift through neural interference patterns at variable rates in the course of child development. In child development vertically arranged pathways are followed by horizontally arranged pathways resulting in a cross grid morass of vast interconnections. As inter-connectivity expands noise levels increase in the child’s brain with age (which might explain why, prior to decline around the age of two some autistic children appear normal). As a result, in the course of child development, lateral inhibition would have to keep pace. Just how this occurs is unknown but one key might lie in the regulatory functions of a site often referred to as the brain’s central computer.
The cerebellum is an interesting brain structure located above the hind brain with a unique cellular topography. It has extensive connections to various other parts of the brain, which speaks to its functional influence. it is also the most uniform of the major brain sites, with only two types of cells – Pyramidal and Purkinje cells – which has led some to liken it to a computer (Ito 1979). That comparison is based on the fact that its smooth, uniform structure has a programmed, regulatory topography apparently designed to anchor other parts of the brain – particularly with regard to motor and cognitive memories. It appears to be a low noise structure, providing the luxury of automatic responding. In other words, once stored in the cerebellum, a behavior or cognitive memory no longer requires sifting and retrieval – an enormously helpful function, not only because it enhances stability but because by ameliorating the search for memories it conserves energy in the brain.
There seems to be evidence of dysfunctional development in the pathways of the cerebellum of autistic individuals, specifically fairly consistent signs of cerebellar hypoplasia (lack of cellular maturation) (Hampsom,Blatt 2015).
Developmental deficiency in the cerebellum would mitigate against automaticity. which coincides with the autistic child’s tendency to behave as though each experience is new and in many instances threatening. In not being able to store experiences in automatic memory the autistic individual would appear to be subject to an excruciating, chronic level of hyper-vigilance and anxiety.
The Pleasure of Specificity…
With regard to questions on language development, and in a larger context, the evolutionary origins of human language no clear answers have been provided. Numerous theories abound but the closest thing to an empirical sense of human language development has typically been derived from infant/toddler observations. One typical observation noted by parents and linguists is that children tend to develop simultaneously the skills of pointing and speaking. Some have surmised that this indicates a primarily social aspect to language development; more specifically a ‘theory of mind’ capability that allows toddlers to approximate what others might be thinking. In other words, the child purportedly seeks to know if: “What I see is what you see” as a sort of interpersonal confirmation ( Korkmaz 2011).
Certainly that is an important aspect of language development but perhaps not the core factor. For example one could just as readily assume the child points and speaks not because he is wondering what another might be seeing or thinking but because he has so many neurons in his brain (most of which are not quite interconnected – an infant’s brain has as many neurons as an adults but early on the wiring is not meshed) that the act of identifying an object is a noise busting, information attaining experience that gives him great joy- just as closure and resolution might please an adult at the climax of a who-done-it movie. In this scenario the child is not asking for confirmation of his observations but instead communicating his joy over neuro-experiential resolution (closure) as a kind of aha experience.
If “closure theory” has any merit than it would tie in with the importance of lateral inhibition in the developing brain and In part resolve the conflict between the nature and nurture theories of language acquisition – particularly regarding the question of why children can learn language so quickly (a phenomenon that tends to favor nature theorists).
If, built into the brain is a noise busting mechanism, reinforced by a persistent closure/pleasure experience (internal feedback) then both the nature and nurture theories of language acquisition could be accommodated. In that sense instinct and learning might be seen as two sides of the same coin because for lateral inhibition to operate effectively, the process must rely on learned memories in determining what are relevant vs. peripheral pathways – lateral inhibition being bio-natural while specific words, dialects etc. are learned.
On the other hand if noise busting was not fluid due to defective lateral inhibition then instead of being pleasurable, language reception and expression would be aversive. That would mean that part of the reason autistic individuals have trouble speaking is because they find both listening and speaking too overwhelming, the search for phonics, words and phrases too difficult. That would mean language and learning deficits are caused in part by an emotional (noise) avoidance reaction.
Observations of autistic individuals would seem to indicate that the auditory channel in particular is effected by this deficit (Leekam, Nieto et al 2007 ). Not being able to effectively parse (laterally inhibit) various sounds would make learning of language and other skills difficult. By the same token the auditory stimulus of music – which is synthesized by melody and cadence (both can to an extent compensate for deficient lateral inhibition via rhythmic encoding) might be more tolerable, even enjoyable.
It all suggests autism is largely a stimulus avoidance disorder, requiring chronic, adaptive (albeit limiting) noise remedial behaviors, including extreme adherence to routine, over reliance on specific learning styles, use of stimulus control behaviors to override chronic CNS exhaustion (because of the endless sifting and sensory contrast demands placed on the brain) a need for brain/body rest due to that exhaustion and opportunities to control their sensorium by being able to terminate tasks prior at the point of intolerable arousal (analogous to what Goldstein called the “catastrophic reaction”(2012 ). It is a life style characterized by a ‘less is more’ paradigm that necessarily sacrifices integrative/conceptual volume learning for neuro-functional equanimity.
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Goldstein catastrophic reaction reference… (2012) Medical Eponyms – retrieved from Medical Dictionary – The Free Dictionary
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* Mental illness – animal reference…Article in Leviathan Process references observations by various field researchers and zoologists that while animal psychopathology does occur in captivity it has not been observed in the wild. and only because the animals are not able to exert natural behaviors- rather than having psychic conflicts.
Pribram, K. 1991) Brain and perception: holonomy and structure in figure processing, Hillsdale N.J. Laurence Erlbaum Associates
Spence,K.W. (1936) The nature of discrimination learning in animals Psychology Review 43 (5) 427-449
Yantis, S. (2014) Sensaton and Perception, New York.NY Worth Publishing p. 77