title: Distortion products in auditory fMRI research: Measurements and solutions.
abstract: Nonlinearities in the cochlea can introduce audio frequencies that are not present in the sound signal entering the ear. Known as distortion products (DPs), these added frequencies complicate the interpretation of auditory experiments. Sound production systems also introduce distortion via nonlinearities, a particular concern for fMRI research because the Sensimetrics earphones widely used for sound presentation are less linear than most high-end audio devices (due to design constraints). Here we describe the acoustic and neural effects of cochlear and earphone distortion in the context of fMRI studies of pitch perception, and discuss how their effects can be minimized with appropriate stimuli and masking noise. The amplitude of cochlear and Sensimetrics earphone DPs were measured for a large collection of harmonic stimuli to assess effects of level, frequency, and waveform amplitude. Cochlear DP amplitudes were highly sensitive to the absolute frequency of the DP, and were most prominent at frequencies below 300 Hz. Cochlear DPs could thus be effectively masked by low-frequency noise, as expected. Earphone DP amplitudes, in contrast, were highly sensitive to both stimulus and DP frequency (due to prominent resonances in the earphone's transfer function), and their levels grew more rapidly with increasing stimulus level than did cochlear DP amplitudes. As a result, earphone DP amplitudes often exceeded those of cochlear DPs. Using fMRI, we found that earphone DPs had a substantial effect on the response of pitch-sensitive cortical regions. In contrast, cochlear DPs had a small effect on cortical fMRI responses that did not reach statistical significance, consistent with their lower amplitudes. Based on these findings, we designed a set of pitch stimuli optimized for identifying pitch-responsive brain regions using fMRI. These stimuli robustly drive pitch-responsive brain regions while producing minimal cochlear and earphone distortion, and will hopefully aid fMRI researchers in avoiding distortion confounds.
authors.affiliation: Department of Brain and Cognitive Sciences, MIT, United States. Electronic address: svnh@mit.edu.
authors.forename: Sam
authors.initials: S
authors.lastname: Norman-Haignere
authors.affiliation: Department of Brain and Cognitive Sciences, MIT, United States.
authors.forename: Josh H
authors.initials: JH
authors.lastname: McDermott
mesh_terms: D000161:Acoustic Stimulation; D000328:Adult; D016477:Artifacts; D003051:Cochlea / Q000502:physiology; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010898:Pitch Perception / Q000502:physiology*; D055815:Young Adult
references.citation: J Neurosci. 2013 Dec 11;33(50):19451-69
references.pmid: 24336712
references.citation: J Acoust Soc Am. 1980 Sep;68(3):825-9
references.pmid: 7419817
references.citation: Cereb Cortex. 2003 Jul;13(7):765-72
references.pmid: 12816892
references.citation: J Acoust Soc Am. 1974 May;55(5):1061-9
references.pmid: 4833699
references.citation: J Acoust Soc Am. 1965 Jun;37:1110-23
references.pmid: 14339721
references.citation: Neuroimage. 2004 Jun;22(2):755-66
references.pmid: 15193604
references.citation: Science. 1970 Jan 23;167(3917):392-3
references.pmid: 5409744
references.citation: Magn Reson Med. 2002 Jun;47(6):1202-10
references.pmid: 12111967
references.citation: Front Syst Neurosci. 2013 Oct 02;7:62
references.pmid: 24106464
references.citation: J Assoc Res Otolaryngol. 2011 Dec;12(6):767-82
references.pmid: 21826534
references.citation: Neuroimage. 2012 Jan 2;59(1):431-8
references.pmid: 21810475
references.citation: J Neurosci. 2004 Jul 28;24(30):6810-5
references.pmid: 15282286
references.citation: J Neurophysiol. 1996 Sep;76(3):1698-716
references.pmid: 8890286
references.citation: MAGMA. 2008 Sep;21(5):317-25
references.pmid: 18716815
references.citation: Hear Res. 1990 Aug 1;47(1-2):103-38
references.pmid: 2228789
references.citation: Br J Audiol. 2000 Aug;34(4):205-24
references.pmid: 10997450
references.citation: Cereb Cortex. 2006 Jun;16(6):835-48
references.pmid: 16151180
references.citation: Nature. 1991 Jan 31;349(6308):413-4
references.pmid: 1992342
references.citation: Nature. 2005 Aug 25;436(7054):1161-5
references.pmid: 16121182
references.citation: Neuroimage. 2010 Jan 15;49(2):1641-9
references.pmid: 19782757
references.citation: Nat Neurosci. 1998 Sep;1(5):422-7
references.pmid: 10196534
references.citation: J Acoust Soc Am. 1986 Aug;80(2):479-83
references.pmid: 3745680
references.citation: J Neurophysiol. 2004 Sep;92(3):1295-311
references.pmid: 15128750
references.citation: J Acoust Soc Am. 1967 Mar;41(3):676-89
references.pmid: 6045077
references.citation: J Acoust Soc Am. 1973 Dec;54(6):1496-516
references.pmid: 4780803
references.citation: Neuron. 2002 Nov 14;36(4):767-76
references.pmid: 12441063
references.citation: Neuroreport. 2011 Feb 16;22(3):111-5
references.pmid: 21178644
references.citation: J Neurophysiol. 2005 Nov;94(5):3181-91
references.pmid: 16014796
references.citation: Neuroimage. 2010 Oct 1;52(4):1410-9
references.pmid: 20483377
references.citation: J Acoust Soc Am. 2005 Jun;117(6):3816-31
references.pmid: 16018484
references.citation: J Acoust Soc Am. 1994 Jun;95(6):3529-40
references.pmid: 8046144
references.citation: Cereb Cortex. 2009 Mar;19(3):576-85
references.pmid: 18603609
references.citation: Cereb Cortex. 2012 Apr;22(4):745-53
references.pmid: 21709174