5 : 3/ Electrical Potentials of the Brain 



95 



This error signal may be amplified and used to vary continuously the 

 rate of flow of oil to the furnace. In this case, the further from 68°F 

 the greater the change in heating rate. 



Instead of negative feedback, one may have positive feedback. Then 

 any small change will be added to and amplified by the controlling 

 element. Positive feedback is used in electronic oscillators. It is also 

 active at the membrane of nerve fibers. If their potential is slightly 

 decreased by external means, the membrane senses this and alters its 

 permeability to augment the decrease. Most feedback loops which 



Light Iris 



Retina 



J CMS. 



Analyzer 



C.N.S. 

 Comparator 



Tends to Close Iris 



Cranio-sacral 

 Division 



Tends to Open Iris 



^f 



Reference 



Hormones 



Emotions 



Thoraco - lumbar 

 Division 



Figure 4. Feedback control loop regulating iris opening. 



have been studied, and which involve more than one neuron, are of the 

 negative variety. 



For example, the iris diaphragm of the eye (see Chapter 2) is controlled 

 by nerve fibers of both the craniosacral and thoracolumbar divisions of 

 the autonomic nervous system. Its negative feedback loop is shown in 

 Figure 4. Light enters the eye through the iris diaphragm and is con- 

 verted to neural impulses in the retina. Some place in the retina or 

 brain both the total neural impulses and the maximum per area of the 

 retina are "computed." These are each "compared" to built-in values 

 and the nerve fibers to the iris are activated in such a fashion as to 

 decrease the differences ; that is, negative feedback occurs. The built-in 

 comparison value can be varied just as the thermostat setting. Both the 

 emotional state of the individual and the hormonal balance in the 

 blood alter the control values for the iris feedback loop. Exactly how 

 this process occurs within the brain is not fully understood, although it is 

 known that certain areas and fiber tracts are involved, and others are 

 not. 



A very impressive use of multiple negative feedback loops occurs in 



