FUNCTIONAL SPECIALIZATION OF HUMAN AUDITORY CORTICAL FIELDS

1
FUNCTIONAL SPECIALIZATION OF HUMAN AUDITORY
CORTICAL FIELDS G. C. Stecker1,2, I. Liao1,
T.J.Herron1, X.Kang1, E.W.Yund1, T. J. Rinne1,3,
D.L.Woods1 1. HCNLAB, Neurology, UC Davis and
VANCHCS, Martínez, CA. 2. Speech and Hearing
Sciences, Univ. of Washington, Seattle, WA, 3.
Dept. of Psychology, Univ. of Helsinki, Helsinki,
Finland
282.14  

• INTRODUCTION. We examined fMRI BOLD activations
  in auditory cortex using cortical surface mapping
  techniques to examine the influence of sound
  frequency, location, and intensity and attention
  on activation maps.
• Identical experiments were performed using
  continuous an sparse sampling imaging parameters
  to examine the effects of scanner-generated
  masking noise.
• Alterations in activation patterns as a
  consequence of tone frequency, location, and
  intensity were examined during difficult visual
  attention conditions to control for the potential
  differences in the attention-attracting
  properties of different stimuli.
• Repeated studies were performed in each subject
  with the goal of visualizing the individual
  patterns of auditory cortical fields (ACF)
  organization.
• The results were analyzed using surface-based
  tools that provide improved precision in
  comparing with 3D analyses and that permit the
  visualization of activation patterns

CORTICAL INFLATION AND ALIGNMENT
AC
Methods. The cortex was segmented with FreeSurfer
(Fischl, Sereno, Tootell and Dale, 1999) to
obtain the gray/white boundary (left, yellow).
The cortex was then inflated (shown for the left
hemisphere of one subject) and colored to show
local curvature values (green gyrus, red
sulcus) at each point. Each hemisphere was
further inflated into a sphere and aligned with
anatomical templates of the same hemisphere using
FreeSurfer and projected using a Mollweide
equal-area projection. Because we were primarily
interested in auditory cortex (dashed box) we
chose a projection where Heschls gyrus lay near
the equator to minimize distortion.
STIMULI AND TASKS. Subjects performed a one-back
matching task, attending to the auditory or
visual modality in alternating 23.2s blocks.
Auditory stimuli were three-tone patterns (tone
durations 250 ms, 10 ms rise/fall, total duration
750 ms) presented once every 1.5 s. Tone
intensities were set at 70 or 90 dB SPL in
different blocks and tones were presented over
broadband masking noise (70 dB SPL). Each tone
pattern contained three tones different equally
spaced over a one-half octave range that was
centered at 225, 900 or 3600 Hz in different
blocks. Visual stimuli were faces or words in
different blocks. Visual stimuli were presented
at central fixation for 700 ms every 1.5 s.
During visual attention conditions subjects
performed a one-back matching task, identifying
facial identity (across expressions) or word
category.
Mollweide Projection of Cortical structure.
Averaged over 62 subjects (124 hemispheres).
Hemisphere alignment based on surface structure
is more precise than volumetric co-registration
(Kang et al, 2004). Here, the precision of
alignment is reflected in the clarity of sulcal
structure. Lobes are coded by color.
Anatomical Structures (white labels) AG,
angular gyrus CC, corpus callosum CG, cingulate
gyrus CalcS, calcarine sulcus ColS, collateral
sulcus Cun, cuneate CS, central sulcus FG,
fusiform gyrus HG, Heschls gyrus, IFG, inferior
frontal gyrus IPL, inferior parietal lobule
IPS, intraparietal sulcus ITG, inferior temporal
gyrus ITS, inferior temporal sulcus LG, lingual
gyrus LGofin, long gyrus of the insula LOS,
lateral occipital sulcus MedFG, medial frontal
gyrus MidFG, mid-frontal gyrus, MidTP, middle
temporal gyrus PCL, paracentral lobule PHG,
parahippocampal gyrus POS, parieto-occipital
sulcus PoCG, postcentral gyrus, PoCS,
postcentral sulcus PreCG, precentral gyrus
PreCun, precuneus PTO, parietal/temporal/occipita
l point SF, Sylvian fissure SFG, superior
frontal gyrus SMG, supramarginal gyrus SPL,
superior parietal lobule TOS, transverse
occipital sulcus. See http//www.ebire.org/hcnlab
/cortical-mapping for details.
SEMI-RANDOM BLOCK DESIGN. Stimuli were presented
in 23.2s blocks. On 25 of the visual attention
blocks, visual stimuli were presented alone and
on 25 of the auditory attention blocks, auditory
stimuli were presented alone. On the remaining
blocks, visual and auditory stimuli were
presented concurrently and subjects attended to
either the visual stimuli or the tones. Auditory
and visual conditions were randomly selected
(with the constraint that auditory and visual
attention conditions alternated) and paired for
each experiment using Presentation. IMAGING.
Whole brain (29-slices, 92x92 voxels, 4-mm
thickness, 1-mm gap) echo-planar images (TR2.9s
or 11.6s, TE 40 ms) were obtained. Seven
subjects were studied. Each participated on six
experiments on separate days three with sparse
and three with continuous sampling. Each
experiment lasted approximately 75 min. BEHAVIOR.
Hits rates averaged 54.7 (mean RT 670 ms) in
visual attention conditions and 44.0/ (mean RT
450 ms) in auditory attention conditions. Mean
RTs to auditory targets slowed during continuous
sampling, but no significant changes were seen in
accuracy. Accuracy was enhanced and mean RTs
shorter for louder targets.
Modeling human auditory cortical fields (ACFs).
Kaas et als (2000) model of primate ACFs can
projected onto areas showing auditory activations
in the human brain. Apparent tonotopic extrema
(e.g., mesial-lateral HG) may reflect summed
activations from adjacent ACFs with similar
frequency sensitivity. Regional differences in
feature processing are evident in population
responses from different cortical regions.

• CONCLUSIONS.
• Different features are differentially processed
  in different regions of auditory cortex.
• Multiple tonotopic regions are evident in
  population averages, but smaller discrete regions
  seen in individual subjects.
• Individual subjects show reliable but distinct
  activation patterns with incomplete sampling of
  the auditory cortical surface. This differences
  likely reflect variations in venous anatomy, and
  complicate inferences about ACF organization.
• Attention does not merely enhance responses
  similar to increase loudness but recruits
  processing in additional cortical regions.

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