#1448
Functional asymmetry for auditory processing in human primary auditory cortex
J. T. Devlin1, J. Raley1, E. Tunbridge1, K. Lanary1, A. Floyer-Lea1, C. Narain1, P. Jezzard1, P. M. Matthews1, D. R. Moore2
1Centre
for Functional Magnetic Resonance Imaging of the Brain, University of Oxford, U. K.
2MRC Institute of Hearing Research, Nottingham, U. K.
Summary
Structural asymmetries in the supratemporal
plane of the human brain are often cited as the
anatomical basis for the lateralisation of language
predominantly to the left hemisphere1,2. However,
similar asymmetries also are found for structures
mediating earlier events in the auditory processing
stream3-7, suggesting that functional lateralisation
may occur even at the level of primary auditory
cortex. We tested this hypothesis using functional
magnetic resonance imaging to evaluate human
auditory cortex responses to monaurally presented
tones. Relative to silence, tones presented
separately to either ear produced greater activation
in left than right Heschl’s gyrus, the location of
primary auditory cortex. In contrast, in nonprimary auditory cortex a contralateral dominance
was observed. This functional lateralisation for
primary auditory cortex is distinct from the
contralateral dominance reported for other
mammals8, including non-human primates, and may
have contributed to development of the unique role
for the left hemisphere in language processing.
 To assess relative laterality of auditory cortex
responses, we computed a Laterality Index (LI):
(Contralateral - Ipsilateral BOLD signal change)
× 100
LI =
(Contralateral + Ispilateral BOLD signal change)
+100 indicates completely contralateral activation
-100 indicates completely ipsilateral activation
 Using mean signal change avoided biasing the
laterality calculation through
(1) an arbitrary statistical threshold for counting
“active” voxels, or
(2) differences in the volume of the ROIs across
hemispheres (i.e. partial volume effects).
Primary auditory cortex
 The majority of primary auditory cortex (PAC) is located on
Heschl’s gyrus and was therefore identified on each
participant’s structural scan as an anatomic correlate of PAC.
E.g.:
 The current study used sparse sampling9 to measure
cortical auditory responses to monaurally presented
tones
Figure 3: Laterality indices in PAC
Left ear stimulation
Figure 1: Sparse sampling paradigm
Laterality index
0
10
15
20
25
Time (seconds)
Scanner
Scanner
5
Tone
30
60
60
20
20
R
-20
-20
-60
*
(p<0.01)
-60
-100
Individual subjects
Group mean (±SEM)
 Within PAC, monaurally presented tones led to greater
BOLD signal change in the left hemisphere regardless of
which ear was stimulated
35
 Participants discriminated between high (4000Hz) and
low (250Hz) frequency tones (90db SPL) by pressing
one of two buttons as quickly as possible after the tone
onset.
(p<0.001)
*
-100
Tone
Right ear stimulation
100
100
Expected
BOLD signal
Results
 Monaural tones relative to silence activated Heschl’s
gyrus and adjacent non-primary areas bilaterally
 Defined as areas adjacent to PAC which were activated by the
tone vs. silence comparison in the group
HG
removed
Masked
1
2
3
Step 1: Tones vs. silence in RFX (cluster stats: Z>3.5, p<0.05)
Step 2: Anatomically masked to include coordinates of human
non-primary areas10
Step 3: Removed individual subject’s HG
Figure 2: Auditory cortex activations
Right hemisphere
Figure 4: Laterality indices in non-primary areas
Left hemisphere
PreCS
CS
SMG
STG
PreCS
PreCS IFS
PT
HG Insula
PP
Left ear stimulation
CS
IPS
SMG
PTr
PTr
STS
MTG
TP
PT
Insula PP HG STG
STS
MTG
TP
Left ear stimulation
30
Laterality index
IPS
Right ear stimulation
30
(p<0.05)
(p<0.07)
10
*
10
-10
-10
-30
-30
*
 In non-primary auditory cortex, monaurally presented
tones led to a small contralateral advantage regardless of
which ear was stimulated, and the pattern was consistent
across a range of statistical thresholds for identifying
group activations (Table)
Right ear stimulation
p<0.01
p<10-15
The upper panels display lateral views of the inflated left and right
hemisphere surfaces with sulci and gyri shown in dark and light grey,
respectively. The middle and bottom panels show activation in cortical
auditory fields due to left and right ear stimulation.
 The N1m signal in MEG is taken as an indicator of PAC but
may instead reflect responses in non-primary regions lateral
to PAC16,17 and thus would be consistent with the current
findings.
 fMRI studies have considered aggregate signal from both
primary and non-primary regions. In our study, when these
regions were combined, a small contralateral advantage also
emerged due to the larger volume of non-primary auditory
cortex.
Our results demonstrate that within primary auditory
cortex there is a left hemisphere dominance for processing
simple auditory stimuli which is not found in surrounding,
non-primary regions. If speech specific operations do not
begin until the signal reaches the cerebral cortex18, then one
function of PAC may be to determine whether an incoming
signal has sufficient spectral and temporal complexity to be
treated as speech. A left hemisphere advantage would then
facilitate the rapid temporal processing in adjacent left
hemisphere auditory areas19. If correct, this may explain why
this pattern of functional organisation is different from the
contralateral dominance seen in other species, including other
primates8. On the other hand, like humans, at least two
species of great apes, chimpanzees (Pan troglodyte) and
gorillas (Gorilla gorilla), show structural auditory cortex
asymmetries which are not seen macaques (Macaca
mulatto)20-22. Thus, there appears to be a hierarchy in the
degree of hemispheric lateralisation in primates, with monkeys
showing the least asymmetry and humans the most. Whether
the relative difference in structural asymmetry are matched by
a corresponding functional shift from a contralateral auditory
dominance to a left hemisphere dominance in chimpanzees
and gorillas or whether left functional lateralisation for
auditory processing is unique to humans remains to be
demonstrated.
Non-primary auditory cortex
 Half of all trials had a silent stimulus.
 The purpose of the task was simply to control attention
by forcing participants to attend to the tones throughout
the scanning.
 Previous MEG and fMRI studies have reported a
contralateral dominance for monaural processing11-15, but
have not distinguished between primary and non-primary
regions.
Discussion
Current Study
Stimulation
Paradigm
Relation to previous studies
Laterality calculations
Table: Laterality indices in non-primary regions.
Mean laterality Indices
Masking threshold Left ear stimulation Right ear stimulation
Z > 4.0
8±3
4±3
Z > 3.5
7±3
9±3
Z > 3.1
6±3
10 ± 3
Z > 2.3
7±4
11 ± 5
Italics indicates non-significant effect.
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