The Journal of Neurophysiology Vol. 87 No. 1 January 2002, pp. 122-139
Copyright ©2002 by the American Physiological Society
Functional Role of Auditory Cortex in Frequency Processing and
Pitch Perception
Mark Jude
Tramo,1,2,3
Gaurav D.
Shah,1,2 and
Louis D.
Braida2
1Department of Neurology, Harvard Medical
School and Massachusetts General Hospital, Boston 02114-2696;
2Research Laboratory of Electronics, Department
of Electrical Engineering and Computer Science, Massachusetts Institute
of Technology, Cambridge 02139; and
3Eaton-Peabody Laboratory of Auditory
Physiology, Department of Otology and Laryngology, Harvard Medical
School and Massachusetts Eye and Ear Infirmary, Boston, Massachusetts
02114
Tramo, Mark Jude,
Gaurav D. Shah, and
Louis D. Braida.
Functional Role of Auditory Cortex in Frequency Processing and
Pitch Perception. J. Neurophysiol. 87: 122-139, 2002. Microelectrode studies in nonhuman
primates and other mammals have demonstrated that many neurons in
auditory cortex are excited by pure tone stimulation only when the
tone's frequency lies within a narrow range of the audible spectrum.
However, the effects of auditory cortex lesions in animals and humans
have been interpreted as evidence against the notion that neuronal
frequency selectivity is functionally relevant to frequency
discrimination. Here we report psychophysical and anatomical evidence
in favor of the hypothesis that fine-grained frequency resolution at
the perceptual level relies on neuronal frequency selectivity in
auditory cortex. An adaptive procedure was used to measure difference
thresholds for pure tone frequency discrimination in five humans with
focal brain lesions and eight normal controls. Only the patient with bilateral lesions of primary auditory cortex and surrounding areas showed markedly elevated frequency difference thresholds: Weber fractions for frequency direction discrimination
("higher"
"lower" pitch judgments) were about eightfold higher
than Weber fractions measured in patients with unilateral lesions of
auditory cortex, auditory midbrain, or dorsolateral frontal cortex;
Weber fractions for frequency change discrimination
("same""different" pitch judgments) were about seven times
higher. In contrast, pure-tone detection thresholds, difference
thresholds for pure tone duration discrimination centered at 500 ms,
difference thresholds for vibrotactile intensity discrimination, and
judgments of visual line orientation were within normal limits or only
mildly impaired following bilateral auditory cortex lesions. In light
of current knowledge about the physiology and anatomy of primate
auditory cortex and a review of previous lesion studies, we interpret
the present results as evidence that fine-grained frequency processing
at the perceptual level relies on the integrity of finely tuned neurons
in auditory cortex.
