97:2230-2238, 2007. First published Jan 3, 2007; doi:10.1152/jn.00788.2006 J NeurophysiolGutschalk and Andrew J. Oxenham E. Courtenay Wilson, Jennifer R. Melcher, Christophe Micheyl, AlexanderRate and Streaming Alternating in Frequency: Relationship to Perceived Cortical fMRI Activation to Sequences of Tones You might find this additional information useful...32 articles, 12 of which you can access free at: This article cites http://jn.physiology.org/cgi/content/full/97/3/2230#BIBLincluding high-resolution figures, can be found at: Updated information and services http://jn.physiology.org/cgi/content/full/97/3/2230 can be found at: Journal of Neurophysiologyabout Additional material and information http://www.the-aps.org/publications/jnThis information is current as of March 19, 2007 . http://www.the-aps.org/.American Physiological Society. ISSN: 0022-3077, ESSN: 1522-1598. Visit our website at (monthly) by the American Physiological Society, 9650 Rockville Pike, Bethesda MD 20814-3991. Copyright © 2005 by the publishes original articles on the function of the nervous system. It is published 12 times a yearJournal of Neurophysiology on March 19, 2007 jn.physiology.orgDownloaded fromCortical fMRI Activation to Sequences of Tones Alternating in Frequency:Relationship to Perceived Rate and StreamingE. Courtenay Wilson,1,2,3Jennifer R. Melcher,1,3,4Christophe Micheyl,2Alexander Gutschalk,2,3,4and Andrew J. Oxenham1,21Speech and Hearing Bioscience and Technology Program, Harvard–MIT Division of Health Sciences and Technology;2ResearchLaboratory of Electronics, Massachusetts Institute of Technology, Cambridge;3Eaton–Peabody Laboratory, Massachusetts Eye and EarInfirmary; and4Department of Otology and Laryngology, Harvard Medical School, Boston, MassachusettsSubmitted 31 July 2006; accepted in final form 19 December 2006Wilson EC, Melcher JR, Micheyl C, Gutschalk A, Oxenham AJ. Cor-tical fMRI activation to sequences of tones alternating in frequency: relation-ship to perceived rate and streaming. J Neurophysiol 97: 2230 –2238, 2007.First published January 10, 2007; doi:10.1152/jn.00788.2006. Human lis-teners were functionally imaged while reporting their perception ofsequences of alternating-frequency tone bursts separated by 0, 1/8, 1,or 20 semitones. Our goal was to determine whether functionalmagnetic resonance imaging (fMRI) activation of auditory cortexchanges with frequency separation in a manner predictable from theperceived rate of the stimulus. At the null and small separations, thetones were generally heard as a single stream with a perceived rateequal to the physical tone presentation rate. fMRI activation inauditory cortex was appreciably phasic, showing prominent peaks atthe sequence onset and offset. At larger-frequency separations, thehigher- and lower-frequency tones perceptually separated into twostreams, each with a rate equal to half the overall tone presentationrate. Under those conditions, fMRI activation in auditory cortex wasmore sustained throughout the sequence duration and was larger inmagnitude and extent. Phasic to sustained changes in fMRI activationwith changes in frequency separation and perceived rate are compa-rable to, and consistent with, those produced by changes in thephysical rate of a sequence and are far greater than the effectsproduced by changing other physical stimulus variables, such assound level or bandwidth. We suggest that the neural activity under-lying the changes in fMRI activation with frequency separationcontribute to the coding of the co-occurring changes in perceived rateand perceptual organization of the sound sequences into auditorystreams.INTRODUCTIONA brief sound, repeated many times, changes its perceptualcharacter depending on the rate at which it is repeated. At slowrepetition rates, each individual repetition is heard as a distinctperceptual event. At higher rates (⬎10 –12 times/s), the indi-vidual sounds blur into a single, rough-sounding percept. Ateven higher rates (⬎30–50 times/s), the sound begins to havea tonal quality, with a pitch corresponding to the rate ofrepetition (e.g., Warren 1999).The effect of repetition rate on activation in human auditorycortex was examined in functional magnetic resonance imag-ing (fMRI) studies using prolonged (e.g., 30 s) sequences ofvarious stimuli, including broadband and narrowband noisebursts, tone bursts, clicks, and speech (Binder et al. 1994;Harms and Melcher 2002; Harms et al. 2005; Tanaka et al.2000). In response to sequences with long gaps between eachsound (ⲏ200 ms), activation amplitude generally increases asthe rate of sound presentation is increased. However, when thesilent gap between successive sounds is ⱗ200 ms, activationhas a different dependency on sound presentation rate. First,the overall activation amplitude (averaged over sound dura-tion) begins to decrease with increasing rate. Second, the timecourse of activation is profoundly affected: at low rates acti-vation is mainly sustained throughout the sequence presenta-tion, whereas at high rates it becomes more phasic, dominatedby prominent response peaks just after sequence onset andoffset (Harms and Melcher 2002; Harms et al. 2005).The fMRI rate studies undertaken so far have mainly con-sidered sequences consisting of repetitions of the same orsimilar sounds rather than, for instance, sequences of tonesalternating in frequency (i.e., sequences of the form AB-ABAB. . ., where A and B are tones of different frequency). Asequence of alternating tones has the interesting property thatit can be readily manipulated to form very different perceptssimply by changing the frequency separation between tones.When there is no frequency separation (i.e., all tones have thesame frequency), the tones are heard as a coherent sequencewith a perceived rate that corresponds to the physical presen-tation rate of the tones. However, when a sufficiently largefrequency difference is introduced, the A and B tones segregateperceptually into two independent sequences or “streams”(e.g., Miller and Heise 1950), in a phenomenon commonlyreferred to as “auditory stream segregation” (Bregman 1990).When this happens, the perceived rate of each stream equalsthat of the A- (or B-) tones alone; i.e., the perceived rate is halfthe physical rate of the overall ABAB sequence.The changes in percept with increasing frequency separationof ABAB tone sequences raise the following question: Doesthe cortical fMRI activation