the gateway to the neocortex, since all senses — except for some of olfaction (the sense of smell) — relay through it. But the neocortex projects back extensively to the thalamus. These back projections come from “higher” cortical areas, as well as from the primary areas that receive inputs from that area of the thalamus.
The neocortex
The neocortex is one of the most important “inventions” of mammals. It dominates the mammalian brain in volume, particularly in primates. One of the most remarkable properties of the neocortex is that it has the same six-layered structure virtually everywhere, with the same cell types in what appears to be the same general minicolumn circuit. This is in stark contrast to the rest of the brain, where each area tends to have its own distinct set of cell types and neural circuits.
Neocortical processing power is primarily a function of area. Increased area in neocortex has two main uses:
Increasing “acuity,” whereby, for example, a larger area can support a higher density of peripheral receptors, such as retinal ganglion cells in the fovea or mechanoreceptors 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 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.
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
