Harvard-MIT Division of Health Sciences and Technology
HST.723: Neural Coding and perception of Sound
Theme Paper 5 Topic & References
Topic: Neural Maps and Plasticity
*References include items to note when reading the texts.
Background
Brugge JF, and Reale RA (1985). Auditory cortex. In Cerebral Vortex, Vol. 4, A.
Peters and E.G. Jones (eds). Plenum, pp. 229-271.
Suga N, Xiao Z, Ma X, and Ji W (2002). Plasticity and corticofugal modulation for
hearing in adult animals. Neuron 36: 9-18.
Cacace AT, Tasciyan T, and Cousins JP (2000). Principles of functional
magnetic resonance imaging: Application to auditory neuroscience. J. Am. Acad.
Audiol. 11: 239-272.
Functional Imaging and Psychophysics
Belin P, Zatorre RJ, Lafaille P, Ahad P, and Pike B. (2000). Voice-selective areas
in human auditory cortex. Nature 403: 309-12.
This study looked for brain areas involved in the analysis of human voices.
The brain activity produced by vocal and non-vocal sounds was compared
using fMRI. "Voice-selective" areas were identified in non-primary auditory
cortex.
The authors suggest that these voice-selective areas may be analogous to
face-selective areas in the visual system. They also suggest that there
could be similar voice-selective areas in non-human species. Do you agree?
You can find related papers and download the stimuli for this paper (if using
Windows) from www.zlab.mcgill.ca.
Zatorre RJ and Belin P. (2001). Spectral and temporal processing in human
auditory cortex. Cerebral Cortex 11: 946-953.
Lesion and imaging data suggest that temporal and spectral processing of
acoustic signals may be performed differentially in the left vs. right auditory
cortex. The purpose of this paper is to test this hypothesis using PET
applied to human listeners.
The approach involved imaging subjects in different conditions of acoustic
stimulation. The temporal and spectral properties of the stimuli were varied
systematically and independently across conditions. Unlike many imaging
studies, the analyses did not simply involve a pair-wise comparison of
conditions. Instead, they included a regression analysis across multiple
conditions that looked for brain areas showing a covariation between
activity and temporal or spectral stimulus properties.
The activity of some auditory cortical areas showed a covariation with
temporal stimulus properties whereas others showed a covariation with
spectral stimulus properties. Covariation with temporal properties was
stronger on the left hemisphere whereas the covariation with spectral
properties was stronger on the right.
Carefully consider the stimuli, analysis methods, and results. Critically
evaluate the authors' conclusion that their findings confirm a lefthemispheric specialization for rapid temporal processing and a
complementary right-hemispheric specialization for spectral processing.
Hofman, van Riswick, and van Opstal (1998). Relearning sound localization with
new ears. Nature Neurosci. 1: 417-421.
Spatial elevation cues are contained mainly in spectral changes caused by
the direction-dependent filtering of the pinnae, head and shoulders.
Everyone's ears are different and it has been shown that listening through
someone else's ears often results in a severe deterioration of elevation
judgments. In other words, some learning or adjustment to one's own ears
must go on. This study shows that such learning can take place over a
matter of days or weeks. By inserting molds into the subjects' ears, the
authors were able to disrupt the normal elevation cues, resulting in initially
very poor elevation acuity. However, after wearing the molds continuously
for (in some cases) up to 6 weeks (!), performance approached pre-mold
levels. What does the observation that good performance was again
achieved immediately after removal of the molds tell us.
Neurophysiology
Kamke MR, Brown M, and Irvine DRF (2003). Plasticity in the tonotopic
organization of the medial geniculate body in adult cats following restricted
unilateral cochlear lesions. J. Comp. Neurol. 459: 355-367.
This study explores the tonotopic mapping of the ventral division of the
medial geniculate body (MGv) in the thalamus. The experiments are
performed in normal cats and in other cats that have had high-frequency
hearing loss caused by damage to the cochlea.
After hearing loss, there was a large change in the tonotopic map, with an
expanded region of the MGv devoted to characteristic frequencies near the
edge of the lesion. This re-organization is usually called “plasticity” because
something that is plastic can be re-shaped. Note that the expanded region
had near-normal thresholds.
Analogous plasticity has been observed in previous studies of the auditory
cortex. Since there are extensive “descending” connections from the cortex
to the thalamus, the plasticity seen in this study could result from a “topdown” process. Alternatively, the plasticity in thalamus could cause the
plasticity in cortex via a “bottom-up” process.
Kilgard MP, and Merzenich MM (1998). Cortical map reorganization enabled by
nucleus basalis activity. Science 279: 1714-1718.
The tonotopic mapping of single units in auditory cortex is explored. Normal
rats are compared to rats that have undergone a several week period in
which tones of specific frequency are presented to the animal and are paired
with electrical stimulation of nucleus basalis (NB). NB is a forebrain nucleus
that projects widely into the cortex.
The rats that were electrically stimulated showed a large increase in cortical
area in which single units were tuned to the frequency of the tonal stimulus.
Just one week of stimulation showed some effect, but not as large as 3-4
weeks of stimulation, indicating that substantial time is necessary for the
reorganization.
Some of the reorganization may depend on the neurotransmitter
acetylcholine. Other studies have determined that many of the NB neurons
are cholinergic and that NB stimulation causes acetylcholine release in the
cortex.
One suggestion of the study is that NB may mediate cortical reorganization
that follows hearing or learning of behaviorally important sounds.
Yan W, and Suga N. (1998). Corticofugal modulation of the midbrain frequency
map in the bat auditory system. Nature Neurosci. 1: 54-58.
The paper explores the effects of the “corticofugal” system on responses of
inferior colliculus neurons. The corticofugal system is any descending
system that begins in the cortex and projects to a lower center.
Electrical stimulation of cortex (accompanied by simultaneous tone
presentations) changes the tonotopic mapping of the IC. In particular,
electrical stimulation of a CF region in cortex decreases CFs of some IC
neurons in such a way as to increase the number of neurons in the IC that
are tuned to that cortical CF.
The time course of these changes is on the order of hours or minutes, too
long for the re-growth of new connections.
Further Reading
Dale AM, and Halgren E (2001). Spatiotemporal mapping of brain activity by
integration of multiple imaging modalities. Current Opinion in Neurobiology 11:
202–208. Describes how fMRI might be combined with evoked potentials for
better characterization of human brain function.
Robertson D, and Irvine DRF (1989). Plasticity of frequency organization in
auditory cortex of guinea pigs with partial unilateral deafness. J. Comp. Neurol.
82: 456-471. A precursor to the assigned Kamke et al. thalamus study describing
plasticity in the auditory cortex.
Bakin JS, and Weinberger N (1990). Classical conditioning induces CS-specific
receptive field plasticity in the auditory cortex of the guinea pig. Brain Res. 536:
271-286. A classic study of rapid cortical plasticity associated with learning.
Related to the assigned Kilgard and Merzenich study.
Melcher JR, Sigalovsky IS, Guinan JJ Jr, and Levine RA (2000). Lateralized
tinnitus studied with functional magnetic resonance imaging: Abnormal inferior
colliculus activation.J. Neurophysiol. 83: 1058-1072. Highlights the work of the
discussion leader for this theme.