Hockenbury Discovering Psychology 5th txtbk

68 CHAPTER 2 Neuroscience and BehaviorThe ForebrainKey Theme• The forebrain includes the cerebral cortex and the limbic system structures.Key Questions• What are the four lobes of the cerebral cortex and their functions?• What is the limbic system?• What functions are associated with the thalamus, hypothalamus,hippocampus, and amygdala?Figure 2.14 Evolution and the CerebralCortex The brains of these different animalspecies have many structures in common,including a cerebellum and cortex.However, the proportion devoted to thecortex is much higher in mammals than inspecies that evolved earlier, such as fishand amphibians. The relative size of thedifferent structures reflects their functionalimportance (Kaas & Collins, 2001).Situated above the midbrain is the largest region of the brain: the forebrain. In humans,the forebrain represents about 90 percent of the brain. In Figure 2.14, you can see howthe size of the forebrain has increased during evolution, although the general structure ofthe human brain is similar to that of other species (Clark & others, 2001). Many importantstructures are found in the forebrain region, but we’ll begin by describing the mostprominent—the cerebral cortex.CortexCerebellumCortexCerebellumCortexCerebellumCortexCerebellumFish Frog Bird HumanforebrainThe largest and most complex brain region,which contains centers for complex behaviorsand mental processes; also called thecerebrum.cerebral cortex(suh-REE-brull or SARE-uh-brull) The wrinkledouter portion of the forebrain, which containsthe most sophisticated brain centers.cerebral hemispheresThe nearly symmetrical left and right halvesof the cerebral cortex.corpus callosumA thick band of axons that connects thetwo cerebral hemispheres and acts as acommunication link between them.Corpus callosumFigure 2.15 The Cerebral Hemispheres and the CorpusCallosum The two hemispheres of the cerebral cortex canbe clearly seen in this side-to-side cross-sectional view jofthe brain. The main communications link connecting thetwo cerebral hemispheres is the corpus callosum, a thick,broad bundle of some 300 million myelinated neuron axons.The corpus callosum is often described as a C-shaped brainstructure, which is depicted more clearly in Figure 2.13.Cerebral hemispheresCerebral hemispheresThe Cerebral CortexThe outer portion of the forebrain, the cerebral cortex, is divided into twocerebral hemispheres. The word cortex means “bark,” and much like the barkof a tree, the cerebral cortex is the outer coveringof the forebrain. A thick bundle of axons,called the corpus callosum, connects thetwo cerebral hemispheres, as shown inFigure 2.15. The corpus callosum servesas the primary communication link betweenthe left and right cerebralhemispheres.The cerebral cortex is only about aquarter of an inch thick. It is mainly composedof glial cells and neuron cell bodiesand axons, giving it a grayish appearance—whichis why thecerebral cortex is sometimes describedas being composed of graymatter. Extending inward fromthe cerebral cortex are whitemyelinated axons that are sometimesreferred to as white matter.These myelinated axons connect thecerebral cortex to other brain regions.