The view that the various kinds of sensory experience, such as touch, taste, and sight, are dependent on the specific nature of the nerve fibers involved.This doctrine was proposed in 1826 by Johannes Muller, (1801-1858) often called “the father of experimental physiology.” It had actually been suggested 15 years earlier by Charles Bell (1774- 1842), but Muller gained acceptance for the concept by including it in his Handbook of Physiology (1838), the first important compendium ever published in this field. Some of its aspects had also been foreshadowed by Aristotle, Descartes and Locke in their efforts to explain the relation between the mind and the external world.The specific energy theory is an attempt to give a scientific answer to some basic epistemological questions: How do we know the environment? What actually happens in the process of perceiving? The commonsense answer is that minute replicas of objects are directly presented to the brain. A somewhat more sophisticated explanation is the Greek view that the messages which reach the brain are not identical to the objects but are “simulative representations” of them. In contrast to these ideas, Muller and others developed the view that we are never directly aware of objects, but only of the nerve excitation stimulated by them. Nerves, then, are intermediate between the mind and perceived objects, and impose limitations of their own. As an example, we respond only to a restricted range of wavelengths in sight and hearing, due to the limitations of our sense organs, and are blind to infrared waves and deaf to frequencies which a dog or a porpoise can hear. What we term reality is therefore merely that portion of energy to which our nerves happen to be sensitive.In elaborating this doctrine, Muller held that each sensory nerve has its own specific quality, and this quality determines the characteristic sensations we experience when it is aroused. This means that receptors will always react in their own specific way to any effective stimulus—for example, the optic nerve responds to light and also to pressure (as it does when we “see stars” after being hit over the head). In other words, the different sense qualities we perceive depend on the nature of the receptor rather than merely on the source of stimulation.Even if we grant that we are only aware of the activity of our nerves and that different receptor cells yield different sensory qualities, we still have to explain how these qualities are elaborated into our perceptual world. At this point there is a divergence in theory. Muller held that the nerves deliver qualitatively different types of impulses to the brain and that this is sufficient to explain the different kinds of perception and the different properties of objects in the external world. His contemporary, Bell, however, held that the specific nature of sensory experience is not the product of the impulse by itself, but depends on the particular part of the brain to which it is delivered. Subsequent research has proved that Bell was right on this point. Studies have also shown that all nerve impulses are similar, since they are all electrochemical waves moving along the fibers—in fact, if we were able to cross-connect the optical and auditory nerves, we would probably be able to hear light and see sounds. The quality of the sensations we experience is therefore not determined by the impulses but by the specific area of the cortex which they stimulate. In short, we see because our optic nerve carries patterns of excitation to the occipital cortex, where they are decoded and elaborated into visual experience in some as yet unknown way. Likewise, we hear because our auditory nerve carries impulses to the temporal lobe.Both interpretations had a profound effect on physiological psychology. The idea that different receptors respond to different forms and ranges of energy led to the discovery of specific nerve endings for taste, touch, warmth, cold and pressure, as well as to the identification of structures in the eye and ear that respond to waves of light or air. And the view that the specificity of response can be explained in terms of the brain rather than the nerves spurred research into the localization of sensory functions in the cortex.Recent research, however, suggests that both the stimulation of the nerve fibers and the processes that take place in the brain are far more complex than these early investigators anticipated. We have learned, for example, that the activities of the reticular formation enable us to receive stimuli that are important in ongoing behavior and to shunt aside inputs from less important sources. In other words, much can happen to a nerve impulse on its way to a brain center. Moreover, it is now possible to record the activity of a single nerve fiber, and such recordings have shown that a single taste fiber will fire in response to all types of substances, sweet, salt, bitter and acid. It has therefore been suggested that the combined activity of many cells sets up a complex code that is different for each taste we experience (Pfaff- man, 1959). Similarly, there is evidence that the same group of neurons connected to the retina may signal different events through differences in the frequency of impulses they deliver to the brain.These and other findings have rendered the doctrine of specific fiber types and specific energies obsolete. Nevertheless, these ideas served the important function of focusing attention on physiological investigation as contrasted with philosophic speculation.