The Journal of Neurophysiology Vol. 83 No. 2 February 2000, pp. 1058-1072
Copyright ©2000 by the American Physiological Society
Lateralized Tinnitus Studied With Functional Magnetic Resonance
Imaging: Abnormal Inferior Colliculus Activation
J. R.
Melcher,1,2,3
I. S.
Sigalovsky,1,3
J. J.
Guinan Jr.,1,2,3 and
R. A.
Levine1,2,3,4
1Eaton-Peabody Laboratory, Massachusetts Eye
and Ear Infirmary, Boston 02114; 2Department of
Otology and Laryngology, Harvard Medical School, Boston 02114;
3Speech and Hearing Sciences Program,
Harvard-MIT Division of Health Sciences and Technology, Massachusetts
Institute of Technology, Cambridge 02139; and
4Neurology Service, Massachusetts General
Hospital, Boston, Massachusetts 02114
Melcher, J. R.,
I. S. Sigalovsky,
J. J. Guinan Jr., and
R. A. Levine.
Lateralized Tinnitus Studied With Functional Magnetic Resonance
Imaging: Abnormal Inferior Colliculus Activation. J. Neurophysiol. 83: 1058-1072, 2000. Tinnitus,
the perception of sound in the absence of external stimuli, is a common
and often disturbing symptom that is not understood physiologically.
This paper presents an approach for using functional magnetic resonance
imaging (fMRI) to investigate the physiology of tinnitus and
demonstrates that the approach is effective in revealing
tinnitus-related abnormalities in brain function. Our approach as
applied here included 1) using a masking noise stimulus to
change tinnitus loudness and examining the inferior colliculus (IC) for
corresponding changes in activity, 2) separately considering
subpopulations with particular tinnitus characteristics, in this case
tinnitus lateralized to one ear, 3) controlling for intersubject differences in hearing loss by considering only subjects with normal or near-normal audiograms, and 4) tailoring the
experimental design to the characteristics of the tinnitus
subpopulation under study. For lateralized tinnitus subjects, we
hypothesized that sound-evoked activation would be abnormally
asymmetric because of the asymmetry of the tinnitus percept. This was
tested using two reference groups for comparison: nontinnitus subjects
and nonlateralized tinnitus subjects. Binaural noise produced
abnormally asymmetric IC activation in every lateralized tinnitus
subject (n = 4). In reference subjects
(n = 9), activation (i.e., percent change in image
signal) in the right versus left IC did not differ significantly.
Compared with reference subjects, lateralized tinnitus subjects showed
abnormally low percent signal change in the IC contralateral, but not
ipsilateral, to the tinnitus percept. Consequently, activation
asymmetry (i.e., the ratio of percent signal change in the IC
ipsilateral versus contralateral to the tinnitus percept) was
significantly greater in lateralized tinnitus subjects as compared with
reference subjects. Monaural noise also produced abnormally asymmetric
IC activation in lateralized tinnitus subjects. Two possible models are
presented to explain why IC activation was abnormally low contralateral
to the tinnitus percept in lateralized tinnitus subjects. Both assume
that the percept is associated with abnormally high
("tinnitus-related") neural activity in the contralateral IC.
Additionally, they assume that either 1) additional activity
evoked by sound was limited by saturation or 2) sound stimulation reduced the level of tinnitus-related activity as it
reduced the loudness of (i.e., masked) the tinnitus percept. In
summary, this work demonstrates that fMRI can provide objective measures of lateralized tinnitus and tinnitus-related activation can be
interpreted at a neural level.
