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