The neuronal recycling hypothesis was proposed by Stanislas Dehaene in the field of cognitive neuroscience in an attempt to explain the underlying neural processes which allow humans to acquire recently invented cognitive capacities. This hypothesis was formulated in response to the 'reading paradox', which states that these cognitive processes are cultural inventions too modern to be the products of evolution. The paradox lies within the fact that cross-cultural evidence suggests specific brain areas are associated with these functions. The concept of neuronal recycling resolves this paradox by suggesting that novel functions actually utilize and 'recycle' existing brain circuitry. Once these cognitive functions find a cortical area devoted to a similar purpose, they can invade the existing circuit. Through plasticity (an ongoing characteristic of the brain's cortical structure to change and reorganize through learning), the cortex can adapt in order to accommodate for these novel functions.
History Up until recently, social scientists did not believe brain biology was relevant to their field, and thus never attempted to research the biological mechanisms of such cultural acquisitions as reading and arithmetic. Many early social scientists held tabula rasa (blank slate) views, which was the notion that individuals are born with no mental content, and that organization and function of the brain comes solely from life experiences. The standard social science model views the brain simply as a large domain-general structure, whose functions have evolved gradually through cultural input. Today, many scientists continue to view the brain as a black box, where only its inputs and outputs can be measured, but their internal mechanisms will never be known. One of the first discoveries relevant to this hypothesis came from a French neurologist, Joseph Jules Dejerine. He discovered that a stroke affecting a small area of the brain's left visual system left patients with selective impairments in reading. "Verbal blindness", a loss of the visual recognition of only letters and words, was the first diagnosis he made on a patient, and simultaneously the first conclusion regarding the cortical basis of reading in the brain. Dejerine's patient was still able to recognize numbers, which further implied the existence of separate areas of the brain being responsible for recognizing letters and words. Upon further study, the French neurologist found lesions affecting the posterior part of the left hemisphere, near the fusiform lobules in his patient. Currently, many patients have experienced these same symptoms of verbal blindness, but the term has been changed to pure alexia. It is now known to be the result of lesions to the occipitotemporal sulcus. A relevant theory to this hypothesis is the concept of exaptations from evolutionary theory, which states that several evolved characteristics were initially selected for other functions, but later adapted to their current role. In essence, evolutionary pressures acted on existing mechanisms to accommodate new functions which may be more culturally relevant.
Dehaene's neuronal recycling hypothesis Neuronal recycling is the idea that novel cultural cognitive processes invade cortical areas initially devoted to different, but similar functions. This cortical architecture presents biases prior to learning, but through neuronal recycling, novel functions may be acquired, so long as they find a suitable cortical area in the brain to accommodate it. This area is referred to a cognitive function's 'neuronal niche', which is analogous to biology's concept of an ecological niche. The novel cultural function must locate a cortical area whose prior function is similar and plastic enough to accommodate it. The concept of neuronal recycling is similar to exaptations in evolutionary theory, which states that several evolved functions are simply byproducts of an ancient biological mechanism. This process, however, is the reuse of biological mechanisms that occur as a result of brain plasticity, rather than evolutionary pressures on a population. Neuronal recycling produces changes in a matter of weeks to years which don't require a change in genome like evolutionary exaptations do. The neuronal recycling hypothesis relies on the following assumptions:
The organization of the human brain is subject to anatomical constraints from evolution and thus is not infinitely plastic. Neural maps are present in infancy which biases subsequent learning. Cultural tools like reading and writing are not present in the brain at birth, but rather must find a neuronal niche in the brain whose circuit is set up to perform a similar function and is sufficiently plastic to reorient itself enough to accommodate this novel use. The original organization of the cerebral cortex is never fully erased once these cultural tools invade the cortical areas. Instead, these initial neural constraints exert a powerful influence on what can be learned. Based on these assumptions, this hypothesis predicts the following:
Each cultural tool should be associated with specific cortical areas, consistent across individuals and cultures. Cultural variability regarding the acquired cognitive processes should be limited due to neural constraints. The speed and ease of cultural acquisitions should be predictable based on the amount and complexity of the recycling required.
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