Neurobiology For Dummies. Frank Amthor

in the fingertips.

       Increasing the number of processing stages in a hierarchy of “association” areas that are increasingly specific and powerful with respect to particular features. Examples are the fusiform face area that allows you to instantly identify thousands of faces that all have the same major features (eyes, nose, mouth) in the same relative positions.

The neocortex goes digital

      The expansion of the neocortex is reminiscent of the transition in the 1960s from analog to digital computers. When vacuum tubes and then transistors were made and handled individually, the most efficient control circuits were those in which a small number of devices modeled the control environment and generated a continuous control output from continuous inputs via the model.

      But when integrated digital circuits arose using thousands and millions of transistors, it became more efficient to represent the control environment on standard microprocessors using software. This provided the advantages of acuity (insensitivity to transistor parameter values) and adaptability (software can be changed and augmented easily). The commonality of the representation and transformation of information in the cortical minicolumn appear to be an essential basis of its success in taking over the brain, and in mammals, including humans, taking over the earth.

Perceiving the World, Thinking, Learning, and Remembering

      Different parts of the brain do different things. The front of the brain in the frontal lobe controls movement, the back and sides of the brain process sensory information. Specialized memory areas perform certain memory functions — the hippocampus and amygdala, for example. Chapters 10 through 14 deal with pathways and brain areas that process sensory information and memory, and produce motor output and thought.

      Looking at vision and audition

      The standard list of five major senses consists of vision, hearing, skin sensation, taste, and smell. The senses of limb position (called proprioception) and limb movement and acceleration (called kinesthesis) are built along similar lines and pathways with skin sensation.

Chapter 10 deals with vision and audition, starting with the peripheral receptors in the eye and ear, and marching up the projection pathways through the thalamus and onto primary, secondary, and higher-order cortical processing areas. In many mammals, particularly primates, visual processing dominates the brain — so much so that just less than 50 percent of the neurons in the brain have their activity modulated by visual input. In both vision and audition, high-order brain areas process complex inputs, such as face-specific neurons in inferotemporal cortex in vision, and language’specific areas in audition such as Wernicke’s area.

      Feeling, smelling, and tasting

      Skin sensation is composed of a group of different types of sensory capabilities controlled by different receptors in the skin and a few other places, such as the mouth and trachea. Mechanoreceptors detect shallow and deep pressure, applied constantly or intermittently. Cold and warm temperature receptors respond to skin temperatures below or above body temperature. Pain receptors respond to mechanical or chemical inputs likely to cause injury.

      The sense of smell is mediated by several thousand different receptor types in the olfactory organ in the roof of the nasal cavity. Evolutionary evidence suggests that some of the earliest neocortex in mammals may have been devoted to sorting out and identifying what produced the smells detected by the olfactory receptors.

      

A dog has about one billion olfactory receptors, a number comparable to the total number of neurons in its entire brain. I’m sure my dogs sniffing around the yard every morning know what was there last night, and probably when, and probably what each critter did, from the smells left over.

      The olfactory system is unique in being divided between a pathway that projects (although indirectly) through the thalamus, of which we are aware, and a pathway that is non-thalamic, which influences our behavior, but of which we are not directly aware.

      The sense of taste is mediated by salt, sweet, sour, and bitter receptors located mostly on the tongue. Some taste researchers also include the MSG taste, umami, as a fundamental taste.

      Learning and memory: Circuits and plasticity

      A behavioral hallmark of mammals is they can change their behavior through learning. High-level learning involves a neural representation of both an event and its context. Much of this representation occurs in the lateral prefrontal cortex as what is called working memory. This area of cortex has extensive connections with the hippocampus, where modifiable synapses containing NMDA receptors abound. Reciprocal connections between the hippocampus and the neocortical areas that originally represented that which is to be remembered instantiate the memory “trace” back in those areas of the neocortex.

The finding that the neocortex represents both original sensory input and its memory has profound implications for understanding what memory is. Many neuroscientists now believe that memory is intrinsically reconstructive — a hallucination, if you will. This is a very different metaphor from the token look up and address model taken from computer science. One important aspect of the reconstructive aspect of memory is that the act of reconstruction can distort the memory. Suggestions, guesses, and events after the memory can affect the reconstruction such that they become part of, and indistinguishable from, subsequent reconstructions.

      The neurobiology of memory depends both on modifiable synaptic weights, such as with NMDA receptors in hippocampus and cortex, and the creation of new neurons in memory areas such as the hippocampus. The discovery of the birth of neurons in the adult hippocampus overturns the old idea of zero neurogenesis in the adult brain. Some senile dementia and even depression appear to be associated with failure of this mechanism.

      The frontal lobes and executive brain

      The frontal lobes are responsible for planning and executing behavior. Generally speaking, the output of the frontal lobe is in its most posterior portion, the primary motor cortex. Neurons in primary motor cortex send their axons down the spinal cord (or out some cranial nerves) to drive motor neurons that cause muscles throughout the body to contract.

      Anterior to the primary motor cortex are the supplementary motor area and premotor cortex that organize the firing of groups of muscles. Anterior to those areas are the frontal eye fields and other areas called prefrontal cortex (even though they are in the frontal lobe) that are involved in more abstract aspects of planning.

      It is generally held that there is relative expansion of the frontal lobe compared to the rest of the brain in humans compared to other primates, and primates compared to other mammals. Some exceptional non-primate mammals such as the echidna have large frontal lobes, however. This has led to debate among neuroscientists about whether these frontal areas are really homologous across mammalian species. Whatever the result of that debate, we know that damage to prefrontal cortex in humans produces distinctive cognitive deficits such as impulsive behavior and profound changes in affect.

      Language, emotions, lateralization, and thought

      True grammatically ordered language distinguishes humans from all other species on earth. Recent evidence has suggested an important role for a gene called FOXP2 in generating language capability, although how this gene changes the brain to allow language isn’t clear.

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