NATURE 
THURSDAY, MAY 22, 1902. 
SPACE PERCEPTION. 
Studies in Auditory and Visual Space Perception. By 
Arthur Henry Pierce, Ph.D. Pp. vi + 361. (New 
York, London and Bombay: Longmans, Green and 
Co., 1901.) Price 6s. 6d. net. 
HE larger and more interesting part of this book 
deals with the problem of the localisation of sound. 
We all know that we, in common with other animals 
having the sense of hearing, can, with’ considerable 
accuracy, determine the direction from which a sound 
comes to us. We hear a lark, and after a little feeling 
about, if we may use such an expression, we are certain 
that the bird is not far from a place in the sky to which 
we can confidently point, and examining with our eyes 
the region near that point we soon see the lark. 
In all ordinary efforts to find out where the source of 
sound is, we move our head, either alone or with our 
body. Ifthe necessary movement is angularly small, we 
may move the head without moving the body, if it is large 
we must move the body; so far as the result is con- 
cerned it is indifferent how the head is moved, with or 
without the movement of the body. 
What we do is to turn to the side from which the 
sound comes and continue this movement until the | 
median plane of the head is in such a position that the 
slightest movement of the median plane will put the 
source of sound into the right or left hemisphere ; we 
then know that the source of sound is in the median | 
plane and in front. Having thus found a vertical plane 
containing the source of sound, we have next (and experi- 
ence seems to show that this is really the order followed) 
to determine the place of the source of sound in the 
semicircle in front from zenith to nadir. This cow/d be 
done in a precisely similar way. We might turn our 
head so that its vertical axis became horizontal and our 
median plane coincided with the horizontal plane, and 
now rotating the head about its vertical axis (now hori- 
zontal) we could get the source of sound into the median 
plane of the head. The intersection of the two planes 
each of which contains the source of sound would, of 
course, be the line passing through the head and the 
source of sound. Put more generally, by inclining the 
head we could find two positions of the median plane of 
the head each containing the source of sound, and the 
intersection fof these planes is the direction sought. 
Birds seem to use this method, and it is worthy of note 
that birds have no concha, but human beings find what 
may be called the altitude of the source of sound in 
another way. 
If we look at a source of sound, such as a splashing 
fall of water or a fizzing steam-pipe, the more complex 
the sound the better, and rotate the head about its right 
‘and left axis, so as to look now up, now down, now for- 
ward, we find that a very notable change of sound takes 
place just at the position of the head when weare looking 
Straight at the source of sound. This rather abrupt 
‘change of quality of the sound seems to be caused by the 
‘acoustic shadow of the traygus. This shadow is, of 
course, not analogous to the black shadow to which we 
‘are accustomed when comparatively large bodies inter- 
NO. 1699, VOL. 66] 
1, 
cept a beam of light, but rather to the coloured shadow 
due to diffraction, and therefore does not diminish the 
intensity of the sound, but changes its quality. It may 
be noted that the tip of the tragus is almost exactly in 
front of the external meatus. Whether this explanation 
of the mechanism is correct or not, it seems certain that 
in locating sounds we do really turn the head (with or 
without the body) about a vertical axis until we find the 
source of sound in front, and then look up and down 
until we are looking at the source. 
Now what has just been described is not at all what 
Prof. Pierce, and most of the authors whose experi- 
ments and speculations he discusses, mean by the 
localisation of sound. What they investigate is the 
question how far we can, wzthout moving the head, de- 
termine the position of the source of asound. All are 
agreed that we can tell with certainty whether the sound 
comes from the right or from the left or is in the median 
plane, but some think this is all, while some, including 
Prof. Pierce, think a good deal more than this can be 
made out without moving the head. 
One defect in the account of many of these experiments 
is that no indication is given of how errors arising from 
involuntary and unconscious movements of the head are 
guarded against. In the experiments described it is 
found that the accuracy of localisation is greatest when 
the source of sound is nearly in front or nearly behind 
the observer. But these are exactly the positions in 
which a slight movement of the head gives the greatest 
| help, so that unless care is taken to avoid any, however 
slight, movement of the head, we can gather little from 
the experiments as to the accuracy of localisation with 
the head fixed. There are three ways in which this 
source of error can be eliminated. First, by making the 
sound of suck short duration that there is no time to 
turn the head during its continuance. This was the plan 
adopted by the present writer when he in 1874, at the 
meeting of the British Association in Belfast, recom- 
mended the snapping of two coins as the source of sound, 
and he is pleased to learn from Prof. Pierce that this form 
of the experiment is still used as a parlour amusement in 
America. Second, by mechanically fixing the head. It 
is difficult, though not impossible, to accomplish this 
without the introduction of apparatus which will interfere 
with the uninterrupted access of sound to the ears. 
Third, by recording any movement of the head which 
may take place by means of tapes placed round the head, 
the ends of the tapes being connected with a recording 
apparatus, so that the movement of the head may be 
noted. Experiments in which such movement occurred 
might then be excluded. It is well known that such 
involuntary and unconscious movements do occur. Most 
of us have heard of the device by means of which a 
famous French army surgeon used to detect feigned 
deafness in unwilling conscripts. He led the supposed 
deaf man along a stone-paved passage and secretly 
dropped a coin. The conscript jerked his head a little, 
on which the surgeon said, “ My friend, you are not very 
deaf, you heard that franc fall.” So unless we have 
some means of ensuring fixity of the head we cannot be 
certain that the greater accuracy of localisation in some 
positions is not, partly at least, due to involuntary and 
unconscious movement of the head. 
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