Reexamining the Evidence for a Pitch-Sensitive Region: A Human fMRI Study Using Iterated Ripple Noise

Abstract

Human neuroimaging studies have identified a region of auditory cortex, lateral Heschl's gyrus (HG), that shows a greater response to iterated ripple noise (IRN) than to a Gaussian noise control. Based in part on results using IRN as a pitch-evoking stimulus, it has been argued that lateral HG is a general "pitch center." However, IRN contains slowly varying spectrotemporal modulations, unrelated to pitch, that are not found in the control stimulus. Hence, it is possible that the cortical response to IRN is driven in part by these modulations. The current study reports the first attempt to control for these modulations. This was achieved using a novel type of stimulus that was generated by processing IRN to remove the fine temporal structure (and thus the pitch) but leave the slowly varying modulations. This "no-pitch IRN" stimulus is referred to as IRNo. Results showed a widespread response to the spectrotemporal modulations across auditory cortex. When IRN was contrasted with IRNo rather than with Gaussian noise, the apparent effect of pitch was no longer statistically significant. Our findings raise the possibility that a cortical response unrelated to pitch could previously have been errantly attributed to pitch coding.

Cortical Pitch Regions in Humans Respond Primarily to Resolved Harmonics and Are Located in Specific Tonotopic Regions of Anterior Auditory Cortex

“…Finally, although our findings indicate the importance of resolved spectral cues, we nonetheless observed a small but significant response for unresolved harmonics relative to noise, which is consistent with some role for temporal pitch cues Barker et al, 2011). Because our harmonic complexes were simply combinations of pure tones and because we observed similar pitch responses for stimuli with and without frequency variation, our results are not susceptible to the concerns regarding spectrotemporal modulation that have been raised for "iterated-ripple-noise" stimuli Bendor, 2012) (although our results do not rule out the possibility that pitchsensitive regions may also respond to slow spectrotemporal modulations as suggested by Barker et al, 2013).…”

“…3). Our results thus help to explain why many recent studies have failed to observe robust pitch responses using unresolved pitch stimuli (Hall and Plack, 2007;Garcia et al, 2010;Barker et al, 2011. Earlier reports of localized responses in anterolateral HG to temporal pitch cues can be reconciled with our findings by the fact that many such studies used stimuli containing some resolved harmonics, similar to those that produced a large neural response in our study Gutschalk et al, 2002;Patterson et al, 2002;Hall et al, 2005;Barrett and Hall, 2006).…”

“…Although many neuroimaging studies have investigated pitch responses Gutschalk et al, 2002Gutschalk et al, , 2004Patterson et al, 2002;Krumbholz et al, 2003;Penagos et al, 2004;Hall et al, 2005Hall et al, , 2006Chait et al, 2006), the anatomy and response properties of pitch-sensitive regions have remained controversial Plack, 2007, 2009;Griffiths et al, 2010;Puschmann et al, 2010;Barker et al, 2011Griffiths and Hall, 2012;Sedley et al, 2012). One potential reason for this lack of consensus is that most studies have focused on pitch stimuli with unresolved harmonics (but see Schönwiesner and Zatorre, 2008;Hall and Plack, 2009;Steinmann and Gutschalk, 2012).…”

“…In contrast, the large majority of neuroimaging studies have focused on temporal pitch cues conveyed by unresolved pitch stimuli Patterson et al, 2002;Hall et al, 2005;Barker et al, 2011). As a consequence, two questions remain unanswered.…”

“…A central finding in this debate is that sounds with perfect temporal periodicity, but with harmonics that are spaced too closely to be resolved by the cochlea, produce only a weak pitch percept (Houtsma and Smurzinski, 1990; Shackleton and Carlyon, 1994). This finding has been taken as evidence for the importance of spectral cues in pitch perception.In contrast, the large majority of neuroimaging studies have focused on temporal pitch cues conveyed by unresolved pitch stimuli Patterson et al, 2002;Hall et al, 2005;Barker et al, 2011). As a consequence, two questions remain unanswered.…”