the phenomena of two-point stimulation and Mach bands, The properties of a geometric model of Mach bands on the skin and in the eye are examined in some detail. Von Békésy shows that if Mach bandsare to occurthe inhibitory area has to be larger than the sensory area, and that an adequate description Book Reviews of observed Mach bands is obtained Man as Sensor Sensory Inhibition. GEORG von BEKEsy. Princeton University Press, Princeton, N.J., 1967. 277 pp., ulus. $8.50. This book is based upon the Herbert S. Langfeld Lectures delivered by von Békésy at Princeton University in the fall of 1965. Von Békésy’s report of his own extensive experiments in the processes of sensory inhibition represents an important substantive and methodological contribution. On the substantive side, the formulation of the idea of “funneling” as a general prin- ciple underlying the inhibitory interactions that filter information from sensory receptors clarifies the relationships between phenomenaof perceptual resolution, sensitivity, and integration. On the methodological side, it is appropriate that, in a period increasingly dominated by electrophysiological approaches to sensory research, the ad- vantages and unique achievements of psychological experiments be explicitly Stated. Von Békésy’s concern with inhibition goes back to 1928 and his study of the mechanical properties of the basilar membrane. Depending upon the frequency of vibration, different sections of the basilar membrane vibrate at maximum amplitude, but the maximum is relatively flat. In order to explain the precision of pitch discrimination, von Békésy suggested that the lateral fibers of the basilar membrane sharpen resolution by suppressing the response of all the nerves but those stimulated near the maxima. In 1930, he proposed that inhibition is also a factor in the perception of the direction of a sound. The inhibition that occurs in the basilar membrane and in directional hearing, however, differs in nature from the inhibition of motor responses, in that it is accompanied also by summation. Though a large binaural time difference localizes a sound completely in one ear, the loudness that is perceived is greater when both ears are stimulated than when only one ear is stimulated. Von Békésy conceives of this simultan1296 eous action of inhibition and summation in sensory processes as a funneling of laterally spreading stimulation into a localized neural pathway. Funneling by filtering and amplifying the neural response serves the important function of increasing the signal-to-noise ratio. Von Békésy’s view of funneling ts far-ranging and varied. He shows it to be a general characteristic of the nervous system that may occur at many different levels. Funneling is a function of the spatial and temporal distribution of stimuli, the magnitude, frequency, and abruptness of a stimulus, and the density of neural interconnections. The experiments he re- ports show that it affects sensitivity, resolution, and the integration of spatial and temporal patterns of stimulation. A large amount of funneling produces a low absolute threshold, a farge difference threshold, a lower rate of increase in sensation magnitude with increase of stimulus intensity, and precise apparent localization. Pursued in depth, the processes of funneling raise many fascinating and fundamental questions. A basic type of funneling action is that responsible for Mach bands. In Mach bands, a graded distribution of light is transformed into a much sharper distribution of sensation. Von Békésy demonstrates that Mach bands are not restricted to the eye but occur on the skin for both direct pressure and vibration. The basic effects of excitation and inhibition that produce Mach bands are derived from observations on the sensory impressions produced by two points of stimulation on the skin as their distance is increased. The summation of adjacent stimuli and the inhibition of more distant stimuli lead von Békésy to propose that every stimulus produces an area of sensation surrounded by an area ofinhibition. This pattern of activity is basic to all sen- sory systems and constitutes a neural unit. The concept of a neural unit furnishes a unifying explanation for if the size of the inhibitory area and the magnitude of inhibition are assumed to increase with the intensity of the stimu- lus. He also demonstrates that the pro- posed model is consistent with the occurrence of inhibition at one level or at successive levels of the nervous system, and that the dimensions of the neural unit determined for the skin and for the eye account for the differences between Mach bands on the skin and in the eye. The funneling effects found in directional hearing as a function of the inequality of the magnitude or time of arrival of two sounds also has its counterparts in other sense organs. A difference of 1 millisecond is sufficient to localize a sensation of vibrations, taste, or smell as coming from the point from which stimulation arrives first. Moreover, the localization of vi- bration on the body may be shifted by introducing delays in activating two vibrators analogous to that which occurs in the dichotic localization of a sound, though the cutaneous receptors, unlike the auditory receptors, possess “local signs” that indicate spatial position. Even when two vibrators are placed in a vertical position on the same side of the body, it is possible to localize the sensation as coming from a point from which stimulation arrives first. Thus localization is not tied to interactions between the hemispheres. Only in experiments on warmth and pain has localization not appeared in time intervals as short as 1 millisecond. Von Békésy attributes this failure to his procedure, which produced a slow onset time for both these sensations. He shows auditory and cutaneous localization to be best when the onset of a stimulus is abrupt. Speed of Transmission Von Békésy’s study of funneling expands into related topics. Since we do not perceive a large area over which traveling waves move when the body is stimulated by a vibrator, the funneling process in localization must be rapid and in the millisecond range, _ Change in localization as a result ‘of SCIENCE, VOL. 158