SENSORY DISCRIMINATION 



1449 



supported them by evidence based upon careful 

 observations. 



The main points of Miiller's doctrine were the 

 following. 



a) Sensation is an awareness of the states of the 

 sensory neural pathways and not of the environment 

 directly. Miiller gave strong emphasis to this part 

 of his theory, stating it in slightly different forms in 

 several of his 10 laws. He undoubtedly felt that this 

 emphasis was necessary in order to refute the theory 

 which had been accepted with little change by most 

 scholars and scientists from the time of Empedocles, 

 namely that an image of the stimulus entered the 

 sense organ and in some manner was transmitted 

 to the 'sensorium' or sensing center in the brain. 



b) When a given sensory nerve is excited, the 

 same kind of sensory experience results no matter 

 what the nature of the stimulus. For example, when 

 the optic nerve is excited by light waxes striking 

 the retinal receptors, by mechanical pressure (ex- 

 ternally or internally produced) directly affecting 

 the optic nerve or by electric shock applied to the 

 nerve, the result is a visual sensation. For the audi- 

 tory nerve, the result is always a sensation of sound, 

 and so on for the other sensory 1 1 h k [ . 1 1 i t ic> 



c) The same physical stimulus applied to dif- 

 ferent sense organs or sensory nerves gives rise to 

 different sensations, in each ease to the sensation 

 appropriate to the organ or nerve in question. 

 For example, electric shock applied to the optic 

 nerve produces a visual sensation; to a tactual nerve, 

 the sensation of touch. 



Miiller accepted the concepts of 'adequate stim- 

 ulus' and 'specific irritability' of sense organs which 

 had been expressed by other biologists before him. 

 He gave a straightforward answer to die very im- 

 portant question: where does the 'specific energy' 

 reside, is it a characteristic of the peripheral nerve 

 or of centers in the central nervous svsiem.' His 

 answer was that he did not know. He explicitly 

 stated: "The peculiar mode of reaction of each 

 sense, after the excitement of its nerve, may be due 

 to either of two conditions. Either the nerves them- 

 selves may communicate impressions different in 

 quality to the sensorium, which in every instance 

 remains the same; or the vibrations of the nervous 

 principle may in every nerve be the same and vet 

 give rise to the perception of different sensations in 

 the sensorium owing to the parts of the latter with 

 which the nerves are connected having different 

 properties" (50, p. 166). 



The doctrine that activity in separate peripheral 



nerves or in their central connections forms the 

 basis for different sensations was soon extended 

 not only to account for vision, hearing, touch, smell 

 and taste but for the sense qualities within each of 

 these modalities. 4 As we have noted above, Thomas 

 Young is often credited with having suggested a 

 specific nerve energy theory a number of years 

 before Miiller's first publications on the subject. 

 Young's suggestion was not made in an attempt to 

 account for differences among the major senses but 

 to account for the different colors perceived visually. 

 He proposed the notion that there might be three 

 kinds of receptors and connecting nerve fibers sub- 

 serving t he three primary colors. 



Helmholtz made use of the principle of specific 

 nerve energies to explain different qualities of sensa- 

 tion in both vision and hearing. He elaborated upon 

 Young's earlier hypothesis and, giving due credit to 

 Young, postulated specific nerve energies for the 

 primary colors. The theory which he put forward is 

 still referred to as the Young-Hclmholtz theory of 

 color vision. 



In hearing, Helmholtz sought to explain pitch 

 perception in terms of the specific nerve energv 

 principle. The Helmholtz resonance theory of hear- 

 ing stated that resonators in the cochlea analyze 

 sounds so thai sensory receptors and their connecting 

 nerve libers are selectively stimulated, and that each 

 discriminablc pitch is represented I >v a specific 

 receptor or receptors and corresponding neural 

 connections. 



In the search for specific receptors for individual 

 sense qualities, Blix in ifWj and Goldscheider in 



1884 explored the surface of the skin for spots ha\ ing 

 specific sensitivit) to the qualities pressure, warmth, 

 cold and pain. They, and later von Ftev , were able 

 to identifv spots for pressure, warmth and cold, with 

 the question of spots for pain being a matter of some 

 controversy. Von Frey also attempted to identifv 

 particular receptor endings in the skin for each of tin- 

 separate qualities. 



For the other senses, a direct attempt to identify 

 separate receptors for different sensory qualities was 

 less easy. That the sensory receptors for taste were 

 associated with the papillae on the tongue had been 



4 Here and in the discussion which follows, modality is used 

 in referring to one of the principal senses, namely to vision, 

 hearing, somesthetic sense, kinesthetic sense, vestibular sense, 

 taste or smell. The term 'attribute' will be used to refer to any 

 discriminable dimension within a given sense, e.g. differences 

 in intensity, in space, in duration or in quality. For a definition 

 of 'sense quality,' see p. 1457. 



