28 FEBRUARY 2014 VOL 343 SCIENCE www.sciencemag.org 978
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Speech provides a fascinating window into brain processes. It is understood effortlessly, and despite a huge variability, manifests both within and across speakers. It is also a stable and reliable carrier of
linguistic meaning, complex and intricate as it
may be. How speech is encoded and decoded
has puzzled those seeking to understand how
the brain extracts sense from an ambiguous,
noisy environment (see the figure). On page
1006 in this issue, Mesgarani et al. (1) demonstrate the neural basis of speech perception
by combining linguistic, electrophysiological, clinical, and computational approaches.
How do brains use the pattern of pressure
waves in the air that is speech (“
speech-as-sound”) and extract meaning (“
speech-as-speech”) from it reliably, despite huge variability between speakers and background
noise? Studies dating as far back as the 1950s
showed that natural speech is highly redundant—speech sounds convey their identity
by a large number of disparate acoustic cues
(2). However, to ensure stable cue-to-speech
translation by brains, an invariant code—
something like a dictionary of speech units—
seems necessary. What, then, is the nature of
the representation of speech units in the brain,
and how do they combine into larger, mean-ing-bearing pieces?
In the 1930s, linguists Roman Jakobson
and Nikolai Trubetzkoy classified conso-
nants and vowels along articulatory dimen-
sions: Their description of the basic units of
speech recognition referred to elements such
as the place in the oral cavity where air is
compressed on its way out (“labial,” “dental,”
“velar,” etc.), the manner of air release (“plo-
sive,” “sonorant,” etc.), and whether the vocal
cords vibrate or not (“voiced,” “unvoiced”)
(3). For example, the sound /p/ is a composite
of features—[+labial, –voiced, +plosive]—
distinguishable from /b/ [+labial, +voiced,
+plosive] and from /t/ [+alveolar, –voiced,
+plosive]. Distinctive features, then, help to
characterize the nature of invariance, while
systematically grouping speech units in clus-
ters. These features have therefore played a
central role in speech recognition research.
But what actually happens in human brains
during speech perception, and where? It may
be that invariance is expressed in
terms of articulation-related dis-
tinctive features (as proposed by
linguists). Invariance may also be
reflected already in sensory areas;
alternatively, brain processes may
achieve invariant representations
of speech sounds only outside
the auditory system proper. One
extreme possibility is that distinc-
tive features correlate with acous-
tic ones, in which case the invari-
ant coding of sounds may already
occur in sensory areas. At the other
extreme, as suggested by the influ-
ential motor theory of speech per-
ception, speech sounds may well
be represented by the articulatory
gestures used to produce them (4).
A recent form of this view actually
posits mirror neurons in the brain
that do precisely that—map sounds
onto motor actions. In that case, the invariant
representation of speech would by necessity
occur in motor areas, outside of the auditory
Mesgarani et al. recorded responses to
speech sounds in the brains of human patients
who were about to undergo brain surgery for
clinical reasons. These recordings give a more
detailed view of the electrical activity in the
human brain than noninvasive methods such
as electroencephalograms or functional magnetic resonance imaging, although they still
reflect the average responses of large neuronal populations. Using these electrical signals, the authors demonstrate a high degree of
invariance of speech representation as early
as in the human auditory cortex by showing
that speech sounds of different speakers and
The Neural Code That
Makes Us Human
Yosef Grodzinsky1,3 and Israel Nelken2,3
How does a certain pattern of vibration in
the air reliably represent a meaningful speech
1Department of Linguistics, McGill University, 1085 Dr. Pen-field Avenue Montréal, Québec H3A1A7, Canada. 2
Department of Neurobiology, The Alexander Silberman Institute of
Life Sciences, Hebrew University, Jerusalem, 91904 Israel.
3The Edmond and Lily Safra Center for Brain Sciences, The
Hebrew University of Jerusalem, Givat Ram, Jerusalem
91904, Israel. E-mail: email@example.com
Speech perception. How highly variable speech sounds (
vowels and consonants) are represented as stable phonetic units in
the brain has not been clear. Acoustic-to-phonetic transformation may involve processing in the superior temporal gyrus of the
human brain (1). The illustration shows phonetic symbols from
the International Phonetic Alphabet superimposed on the language regions of the left cerebral hemisphere.