petach'te nme ohanaiim & TP athee es wi et he de delay of stimulation provides a means of measuring the speed of nerve con- duction. Helmhoitz used a_ reactiontime experiment to measure the speed of nerve transmission and found it to be between 50 and 60 meters per second. When localization is used to estimate the speed of conduction, the estimates are much greater. Von Békésy estimates the speed of transmission for vibrations on the skin to be 208 meters per second. He suggests that there are two speeds of neural conduction—a fast process for the inhibitory interactions that produce localization, and a slower process for the growth of a sensation. Whereas localization is determined within a few milliseconds after the onset of a stimulus, the time necessary for the growth of a sensa- tion may take from 20 milliseconds in hearing to more than 1000 milliseconds in taste, smell, and vibration. Von Békésy found that the speed of neural transmission is greatly affected by temperature and pain. He reports that an electric shock applied 10 seconds before an observation will cause a significant drop in the speed of nerve transmission. The lower speeds reported from animal studies may therefore be due to the effects of anesthesia, temperature, and pain which disturb the nervous system. Localization is a powerful method for. probing neural activity. Cyclic changesin localization when the tongue is stimulated indicate a periodicity in taste sensations. Localization phenome- na also divide the four basic taste effects that produce a sharp localization. Von Békésy’s contrast of the use of psychological and electrophysiological methodsto investigate semsory func- sistent with psychological experiments before electrophysiological measures of neural responses can be interpreted unambiguously. nesses of each approach. The limitations besetting each method make it important that sensory functions be studied by both electrophysiological Von Békésy discusses the difficulties of using psychological observations in the analysis of sensory functions. The foremost requirement is that methods of stimulation produce a well-defined and constant effect. The book amply attests to the difficulty of this requirement and to von Békésy’s skill in arranging experimental procedures that meet it. Von Békésy shows that psychology can be as precise in its meth- tions highlights the strengths and weak- and psychological experiments. Von Békésy suggests, for example, that localization of a stimulus is determined by the onset of nervefiring, the later firing serving to indicate the magnitude and quality of a sensation. The evidence here is psychological. A 2000-cycleper-second tone can be determined quite well from only two cycles of vibration; similarly, localization of a vibratory pattern on the skin of the strates the difficulty in wsing only electrophysiological methods to study sensory processes. Electrophysiological- on the particular sensation one is con- ly, there is at present no means for separating the initial burst from suc- ceeding spikes in neural transmission and for correlating initial bursts with the phenomenal property of localization and succeeding spikes with the properties of magnitude and quality. A central problem in the study of sensory processes is the relationship between neural responses and subjective attributes. Von Békésy’s discussion emphasizes that any hypothesis about the neurophysiological correlates of sen- bitter and sweet or sour and salt pro- ing divergence between electrophysio- two groups: and sweet-bitter. Simultaneous stimulation of the sides of the tongue with duces a single sensation in the middle of the tongue which can be moved from side to side by suitably timing the stimulations. A single sensation is not produced, however, for simultaneous stimulation with bitter and sour or salt and sweet. Thus there appears mentioned in the book, a most interest- logical and psychological studies con~ cerns the question of coding taste. Electrophysiological measures indicate that there are no receptors that are spe- cifically sensitive to taste qualities; rather, taste is determined by the pattern of neural activity. In contrast, von to be a closer relation between bitter and sweet and between salt and sour than between other pairs. An unusual Békésy’s psychological studies indicate that there are single receptors which are sensitive to specific tastes such as vibrators with frequencies of 20, 40, 80, 160, and 320 cycles per second sible that electrophysiological recordings from nerve fibers do not fully reflect the funneling action that occurs type of neural funneling is shown when are placed on the arm. Only the mid- die vibration of 80 cycles per second is felt. The presence of all the vibrators, however, increases the magnitude of the sensation through summation. This indicates that even a flat maxi- mum may set into action inhibitory 8 DECEMBER 1967 ever. The sensory effects produced by even a simple and precisely defined stimulus are often numerous, and the plete cycles are presented. This demon- sory attributes must remain tentative in the absence of corroborating psychological experiments. Though not into ods as electrophysiology. Precise experimental control is not sufficient, how- arm will occur when only two com- sour-salt sensations Experimental Methods salt, sour, bitter, and sweet. It is pos- at higher levels of the nervous system, or that the taste system in man differs from that of animals. Whatever the final resolution of this issue, it reveals clearly that electrophysiological recordings must be shown to be con- subject must be trained to report only cerned with and to inhibit other sensations. A careful analytic description of the possible percepts may even be necessary. Perceptual experiments for the purposes of sensory analysis must be guided by an introspective attitude that reduces cognitive and motivational factors to a minimum. Funneling and inhibition can also occur at a cortical level. A form of cortical inhibition is involved in the fact that we appear to observe discon- tinuously, taking in sensory information in temporal quanta. Through training one can also learn to inhibit stimulation. For example, a singer appar- ently can be trained not to hear the bone-conducted sounds coming from his throat and to hear his voice only through the air-conducted sounds. A complex case of funneling occurs in the projection of sensations outside the body. Von Békésy reports that when there is a time interval in stimulating the two knees with vibrators there is a jumping of sensation from one side to the other depending upon which knee is stimulated first. After several weeks of training, however, a subject can experience a continuous motion of the vibratory sensation from one knee to the other as a function of the time interval. Now when the knees are stim- ulated simultaneously, the vibrations are localized in the free space between them. Moreover, a displacement of the sensation in this free space will occur with suitable changes in the timing of stimulation. The projection of sensations into external space appears to be 1297