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Isochronicity in neural networks Mental Synthesis theory: • the mechanism of imagining novel objects involves temporal SYNCHRONIZATION of independent neuronal ensembles. Synchronization no enhanced connections Asynchronously firing neuronal ensembles Perceived as two different objects Synchronously firing neuronal ensembles Perceived as one morphed image The lateral prefrontal cortex as a puppeteer PFC Puppeteer Puppets in the posterior cortex Synchronization Mental Synthesis • Remarkably, the volume of Brodmann area 10 (frontopolar cortex) is nearly 200% as large as expected for a primate brain of our size. • Neuronal density in human Brodmann area 10 was around half of that of other primates Brodmann area 10 in humans has a significantly greater portion of its volume dedicated to connections between neurons • Conclusion: Humans’ Brodmann area 10 is nearly 200% as large as expected for a primate brain of our size and that increase in size is due to greater number of connections and myelination 2. Mental Synthesis Hardware Puppeteer Puppets Synchronous connections • The Mental Synthesis theory predicts that to synchronize neuronal ensembles dispersed throughout the posterior cortex, the prefrontal cortex must rely on synchronous connections to those areas. • Without a mechanism that could equalize transit times, the signal from the prefrontal cortex would arrive to its targets in the posterior cortex at different times. • This synchronization mechanism poses a serious challenge that every human needs to solve during development: • These connections must be fine-tuned to become synchronous. • Only vertebrates have myelin. • In unmyelinated axons, the conduction velocity is proportional to the square root of the axon diameter. To have a conduction velocity that is 10 times faster, the diameter of an unmyelinated axon needs to be 100 times larger. • Accordingly, invertebrates often solve the problem of fast connection by using axons with large diameters. For example, squid giant axons which mediate its escape response have a diameter of up to 1mm, which is about 1000 times greater than the diameter of a typical vertebrate axon (normally only 1µm in diameter). • The stellate ganglion is connected to axons distributed throughout the mantle. The action potential reaches the muscles throughout the mantle simultaneously. The result is that the muscles contract synchronously, rapidly closing the mantle, and forcing water out of the mantle cavity. Water is expelled through the siphon, producing a jet action. • HOW can action potentials reach the muscles throughout the mantle simultaneously? • The longer the distance the thicker the axon. Thicker axons allow for faster conduction. In vertebrates conduction velocity is regulated by myelin • In the vertebrate brain, increasing the axonal diameter is an inferior option since it is packed with a large number of neurons myelination • Conduction velocity in myelinated fibers can be 100 times faster than conduction velocity in unmyelinated fibers • Nearly all neurons in the human brain are myelinated • The fastest axons have up to 160 layers of myelin • Myelin occupies nearly half of the human brain Fast conduction velocity Slow conduction velocity Greater differential myelination Slower conduction velocity Faster conduction velocity Slower conduction velocity Faster conduction velocity Faster conduction velocity Faster conduction velocity More myelin -> faster conduction velocity Less myelin -> slower conduction velocity • Hypothesis: myelination of the PFC connections is the primary factor producing uniform conduction time throughout the cortex and enabling precise (milliseconds) control of the posterior cortex by the PFC Hypothesis: myelination is the primary factor producing uniform conduction time EXPERIMENTAL EVIDENCE: 1. In the cat retina, axons from peripheral regions have a greater conduction velocity than axons from neurons at the center of the retina to assure simultaneous arrival of impulses in the brain (Stanford LR, 1987). 2. Experiments in rats show that myelination is the primary factor producing uniform conduction time (to within 1ms) in connections between the inferior olive nucleus in the medulla and the cerebellar cortex, despite wide variation in axon length (Sugihara I, 1993; Lang EJ, 2003 – JC2). 3. Isochronous activation of groups of cells distributed in distant cortical locations has been shown in the visual cortex (Gray et al., 1989) and even between the two hemispheres of the brain (Engel et al., 1991). inferior olive nucleus cerebellar cortex EXPERIMENTAL EVIDENCE (continued): 4. In the cerebral cortex, the layer V pyramidal neurons in the ventral temporal lobe innervate various subcortical regions. Chomiak and colleagues showed that isochronous action potential delivery to target regions located in the ipsilateral hemisphere is based on the differential conduction velocity in each fiber branch (Chomiak T, 2008). Chomiak’s observation on isochronicity is best explained by differential myelination (Kimura F, 2009 – JC1). 5. The amygdala connections to the perirhinal cortex play an important role in establishing fear memory. While the perirhinal cortex is an elongated structure, the small nucleus of the lateral amygdala is isochronically connected with a large portion of the perirhinal cortex (Pelletier JG, 2002). 6. Neurons in the thalamus send axons to a wide area of the somatosensory cortex through different trajectories of various travelling lengths. Nevertheless, action potentials in the thalamic neurons arrive almost simultaneously at each target cortical neuron. This isochronicity (to within 2ms) is achieved by changing the conduction velocity within the individual axons by differential myelination (Salami M, 2003 – JC1). Conclusion • • • David J. Linden (“The Accidental Mind): “The brain is not elegantly designed by any means: it is a cobbled-together mess, which, amazingly, and in spite of its shortcomings, manages to perform a number of very impressive functions. But while its overall function is impressive, its design is not.” Many structures are elongated and therefore located at various distances from the source of information. To vote on a particular issue, neurons have to synchronize. Synchronization is only possible, if the conduction time is uniform. • Fumitaka Kimura and Chiaki Itami: “myelination plays a major role in creating isochronicity. In other words, myelination is not merely insulation among neighboring cells, but is a method of regulating the timing of postsynaptic activations” (Kimura F, 2009). More myelin -> faster conduction velocity Less myelin -> slower conduction velocity • It is, in fact, possible that myelination is the primary factor producing uniform conduction time throughout the cortex Assuming myelination is producing uniform conduction time, How does the brain know how much myelin each fiber needs? • Genetic instructions alone are inadequate to specify the optimal conduction velocity in every fiber. • Therefore, synchronous connections are likely established in an experiencedependent manner. • Examples? • Nearly all neuronal connection are experience-dependent to some degree. • • • • Myelination is nearly completed by birth in most species in which the young are relatively mature and mobile from the moment of birth, such as wild mice and horses. In humans, myelination is delayed considerably. Few fibers are myelinated at birth and some brain regions continue myelination well into mid-life. The prefrontal cortex region is the last region to complete myelination. Myelination of fibers that connect the prefrontal cortex to other cortical areas does not reach full maturation until the third decade of life or later. Myelination in humans is delayed for years and therefore has significant potential to be experience-dependent! • In humans, myelination normally increases with age and with years of education Experimental evidence of the role of experience in myelination: 1. Rats: The number of myelin-forming cells in the visual cortex of rats increases by 30% when the rats are raised in an environment that is enriched by additional play objects and social interaction (Szeligo F, 1997). 2. Human infants: early experiences increase myelination in the frontal lobes in parallel with improved performance on cognitive tests (Als H, 2004). 3. On the contrary, children suffering from abuse and neglect show on average a 17% reduction in myelination of the corpus callosum, the structure comprised of connections between the neurons of the left and the right hemispheres of the brain (Teicher MH, 2004). 4. Extensive piano practicing in childhood is accompanied by increased myelination of axons involved in musical performance (Bengtsson SL, 2005): the white matter thickening increases proportionately to the number of hours each subject had practiced the instrument. Conclusions • Isochronicity in at least some neuronal networks seems to be achieved via differential myelination and myelination may be experience-dependent. • Considering the many variables affecting conduction delays in an adult brain, genetic instruction alone would seem inadequate to specify the optimal conduction velocity in every axon. • Neuronal ensembles encoding physical objects are located in the posterior sensory cortex, however the process of synchronization is executed by the prefrontal cortex . • The only way the prefrontal cortex could be capable of synchronizing ensembles of neurons distributed over the large area of the posterior sensory cortex is if the prefrontal cortex had isochronous connections to those cortical areas. • Therefore isochronicity is expected to develop in an experiencedependent manner via differential myelination during the limited period of prefrontal cortex plasticity What experience might be involved in the development of mental synthesis? • What system do we, humans, use to manipulate mental images stored in our memory into infinite number of novel mental pictures? • SYNTACTIC LANGUAGE (internal or external). • We converse with ourselves and each other and in the process create an infinite number of novel mental images. • Might the syntactic language be that very mechanism that provides the training necessary for the development of mental synthesis? • If the answer to this question is “yes,” then it might be expected that the lack of syntactic language during the sensitive period of prefrontal cortex plasticity would result in an underdevelopment of mental synthesis. Linguistically deprived children Feral children: 1. Genie, 1970 (Curtiss S, 1977) 2. Victor of Aveyron (1797) (Itard, 1962) Deaf linguistic isolates : 3. E.M. (Grimshaw GM, 1998) 4. Chelsea (Curtiss S, 1988) 5. Maria and Marcus (Morford J, 2003) 6. I.C. (Hyde DC, 2011) 7. Three adolescents (Ramírez NF, 2012) • Rescued after puberty. • Tested after many years of rehabilitation. • Can learn many new words. • Linguistic isolates do not understand complex syntax. • Linguistic isolates do not understand spatial prepositions such as in, on, under, over, beside, behind, in front. Normally children use syntactic language to fine-tune their connections Easily synchronize independent neuronal ensembles Linguistically deprived children end up with asynchronous connections Unable to synchronize neuronal ensembles • Use of infinite syntactic language with prepositions and verb tenses during critical period seems to provide vital training for normal brain development and mental synthesis acquisition. Syntactic language Asynchronous connections Synchronous connections • Genie (55 min): https://www.youtube.com/watch?v=hmdycJQ i4QA