Functional magnetic resonance imaging measurements of sound-level encodingin the absence of background scanner noise

Effects of sound level on auditory cortical activation are seen in neuroima ging data. However, factors such as the cortical response to the intense am bient scanner noise and to the bandwidth of the acoustic stimuli will both confound precise quantification and interpretation of such sound-level effe cts. The present study used temporally ''sparse'' imaging to reduce effects of scanner noise. To achieve control for stimulus bandwidth, three schemes were compared for sound-level matching across bandwidth: component level, root-mean-square power and loudness. The calculation of the loudness match was based on the model reported by Moore and Glasberg [Acta Acust. 82, 335- 345 (1996)]. Ten normally hearing volunteers were scanned using functional magnetic resonance imaging (fMRI) while listening to a 300-Hz tone presente d at six different sound levels between 66 and 91 dB SPL and a harmonic-com plex tone (F-0=186 Hz) presented at 65 and 85 dB SPL. This range of sound l evels encompassed all three bases of sound-level matching. Activation in th e superior temporal gyrus, induced by each of the eight tone conditions rel ative to a quiet baseline condition, was quantified as to extent and magnit ude. Sound level had a small, but significant, effect on the extent of acti vation for the pure tone, but not for the harmonic-complex tone, while it h ad a significant effect on the response magnitude for both types of stimulu s. Response magnitude increased linearly as a function of sound level for t he full range of levels for the pure tone. The harmonic-complex tone produc ed greater activation than the pure tone, irrespective of the matching sche me for sound level, indicating that bandwidth had a greater effect on the p attern of auditory activation than sound level. Nevertheless, when the data were collapsed across stimulus class, extent and magnitude were significan tly correlated with the loudness scale (measured in phons), but not with th e intensity scale (measured in SPL). We therefore recommend the loudness fo rmula as the most appropriate basis of matching sound level to control for loudness effects when cortical responses to other stimulus attributes, such as stimulus class, are the principal concern. (C) 2001 Acoustical Society of America.