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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