92 Electrical Potentials of the Brain /5 : 3 



called the cerebral hemispheres near the olfactory area. In mammals, and 

 to the greatest extent in man, these cerebral hemispheres are a major 

 part of the brain. The cerebral cortex which covers the hemispheres is 

 so folded around and over the brain stem in mammals that the eeg 

 potentials on the skull are related to the cerebral cortex only, and 

 probably only to the outermost layers of the cerebral cortex. (By 

 placing electrodes within the brain, eeg potentials can be measured as a 

 function of the part of the brain nearest the electrodes, rather than of the 

 outer layers of the cortex.) 



The portion of the brain stem connected directly to the cerebral 

 cortex is called the thalamus. The sensory pathways all have synapses 

 in the thalamus. Certain thalamic regions are believed associated with 

 emotional responses. Thus, if an electrode is placed in the appropriate 

 spot in a rat's thalamus, it will pull a lever to shock itself in preference 

 to eating food. Other areas in the thalamus produce just the opposite 

 effect when stimulated. It appears proper to consider all mammals, and 

 possibly all vertebrates, as having emotions homologous to ours and 

 represented by thalamic centers. 



Thought, memory, conscious sensations, and conscious motor activity 

 are all associated with the cerebral cortex. The cerebrum is attached to 

 the thalamus. In the relative size and complexity of his cerebral cortex, 

 man is unique among the animals. As illustrated in Figure 2, certain 

 areas can be associated with specific functions. However, the role of 

 many areas of the cerebral cortex is not known, nor is it known how man 

 analyzes, or thinks, or remembers. Because it reflects, in some sense, the 

 activity of this part of the brain, the eeg has attracted the interest of 

 many investigators. 



However, if the cerebral cortex is removed, similar eeg patterns 

 remain. Even fishes, whose cerebral cortices are negligible, possess 

 typical eeg patterns similar to man's. The eeg is a vertebrate pheno- 

 menon; insect ganglia do not exhibit comparable potentials. The eeg 

 must, in some way, be related to the structure and function of the verte- 

 brate central nervous system. 



3. Feedback Loops and the Nervous System 



It is possible that the eeg potentials reflect, in some manner, feedback 

 loops within the central nervous system. Whether or not this is the case 

 is a moot point, but there is no doubt that feedback loops are important 

 in all coordinated animals and, in particular, in the over-all action of the 

 nervous system. The basic elements of a feedback loop are shown in 

 Figure 3. They consist of (a) a quantity being controlled, such as the 



