By 1950, the results of attempts to relate brain processes to mental experience appeared rather discouraging. Such variations in size, shape, chemistry, conduction speed, excitation threshold, and the like as had been demonstrated in nerve cells remained negligible in significance for any possible correlation with the manifold dimensions of mental experience.

Near the turn of the century, it had been suggested by Hering that different modes of sensation, such as pain, taste, and color, might be correlated with the discharge of specific kinds of nervous energy. However, subsequently developed methods of recording and analyzing nerve potentials failed to reveal any such qualitative diversity. It was possible to demonstrate by other methods refined structural differences among neuron types; however, proof was lacking that the quality of the impulse or its conduction was influenced by these differences, which seemed instead to influence the developmental patterning of the neural circuits. Although qualitative variance among nerve energies was never rigidly disproved, the doctrine was generally abandoned in favor of the opposing view, namely, that nerve impulses are essentially homogeneous in quality and are transmitted as "common currency" throughout the nervous system. According to this theory, it is not the quality of the sensory nerve impulses that determines the diverse conscious sensations they produce, but rather the different areas of the brain into which they discharge, and there is some evidence for this view. In one experiment, when an electric stimulus was applied to a given sensory field of the cerebral cortex of a conscious human subject, it produced a sensation of the appropriate modality for that particular locus, that is, a visual sensation from the visual cortex, an auditory sensation from the auditory cortex, and so on. Other experiments revealed slight variations in the size, number, arrangement, and interconnection of the nerve cells, but as far as psychoneural correlations were concerned, the obvious similarities of these sensory fields to each other seemed much more remarkable than any of the minute differences.

However, cortical locus, in itself, turned out to have little explanatory value. Studies showed that sensations as diverse as those of red, black, green, and white, or touch, cold, warmth, movement, pain, posture, and pressure apparently may arise through activation of the same cortical areas. What seemed to remain was some kind of differential patterning effects in the brain excitation: it is the difference in the central distribution of impulses that counts. In short, brain theory suggested a correlation between mental experience and the activity of relatively homogeneous nerve-cell units conducting essentially homogeneous impUlses, through homogeneous cerebral tissue. To match the multiple dimensions of mental experience psychologists could only point to a limitless variation in the spatiotemporal patterning of nerve impulses.

21. The author suggests that, by 1950, attempts to correlate mental experience with brain processes would probably have been viewed with

(A) indignation
(B) impatience
(C) pessimism
(D) indifference
(E) defiance




22. The author mentions "common currency" primarily in order to emphasize the

(A) lack of differentiation among nerve impulses in human beings
(B) similarity of the sensations that all human beings experience
(C) similarities in the views of scientists who have studied the human nervous system
(D) continuous passage of nerve impulses through the nervous system
(E) recurrent questioning by scientists of an accepted explanation about the nervous system




23. The description in lines 32-38 of an experiment in which electric stimuli were applied to different sensory fields of the cerebral cortex tends to support the theory that

(A) the simple presence of different cortical areas cannot account for the diversity of mental experience
(B) variation in spatiotemporal patterning of nerve impulses correlates with variation in subjective experience
(C) nerve impulses are essentially homogeneous and are relatively unaffected as they travel through the nervous system
(D) the mental experiences produced by sensory nerve impulses are determined by the cortical area activated
(E) variation in neuron types affects the quality of nerve impulses




24. According to the passage, some evidence exists that the area of the cortex activated by a sensory stimulus determines which of the following?

I. The nature of the nerve impulse
II. The modality of the sensory experience
III. Qualitative differences within a modality

(A) II only
(B) III only
(C) I and II only
(D) II and III only
(E) I, II, and III




25. The passage can most accurately be described as a discussion concerning historical views of the

(A) anatomy of the brain
(B) manner in which nerve impulses are conducted
(C) significance of different cortical areas in mental experience
(D) mechanics of sense perception
(E) physiological correlates of mental experience




26. Which of the following best summarizes the author's opinion of the suggestion that different areas of the brain determine perceptions produced by sensory nerve impulses?

(A) It is a plausible explanation, but it has not been completely proved.
(B) It is the best explanation of brain processes currently available.
(C) It is disproved by the fact that the various areas of the brain are physiologically very similar.
(D) There is some evidence to support it, but it fails to explain the diversity of mental experience.
(E) There is experimental evidence that confirms its correctness.




27. It can be inferred from the passage that which of the following exhibits the LEAST qualitative variation?

(A) Nerve cells
(B) Nerve impulses
(C) Cortical areas
(D) Spatial patterns of nerve impulses
(E) Temporal patterns of nerve impulses