Abstract
Acoustic signals pass through numerous transforms in the auditory system before perceptual attributes such as loudness and pitch are derived. However, relatively little is known as to exactly when these transformations happen, and where, cortically or sub-cortically, they occur. In an effort to examine this, we investigated the latencies and locations of cortical entrainment to two transforms predicted by a model of loudness perception for time-varying sounds: the transforms were instantaneous loudness and short-term loudness, where the latter is hypothesized to be derived from the former and therefore should occur later in time. Entrainment of cortical activity was estimated from electro- and magneto-encephalographic (EMEG) activity, recorded while healthy subjects listened to continuous speech. There was entrainment to instantaneous loudness bilaterally at 45, 100, and 165 ms, in Heschl's gyrus, dorsal lateral sulcus, and Heschl's gyrus, respectively. Entrainment to short-term loudness was found in both the dorsal lateral sulcus and superior temporal sulcus at 275 ms. These results suggest that short-term loudness is derived from instantaneous loudness, and that this derivation occurs after processing in sub-cortical structures.
Highlights
The loudness of a sound corresponds to the subjective impression of its magnitude
This leads to the following questions: (1) At what stage or stages in the auditory pathway are the transforms leading to short-term and long-term loudness taking place? (2) Of those transformations taking place in the cortex, where do these transformations take place? Some authors have put the first question differently, by asking: “at what stage or stages along the auditory pathway is sound intensity transformed into its perceptual correlate?” (Behler and Uppenkamp, 2016)
The locations are approximate in all cases, and especially in the 100-ms case, since expression was found in neighboring cortical regions as well
Summary
The loudness of a sound corresponds to the subjective impression of its magnitude. While loudness is partly determined by the physical intensity of a sound, it is strongly affected by frequency content (spectrum) and by fluctuations in the sound over time, as well as by the way that sound is transformed and processed in the auditory system. The long-term loudness corresponds to the overall loudness of a relatively long segment of sound, such as a whole sentence or a musical phrase They proposed a model in Loudness Representations in Human Cortex which transformations and processes that are assumed to occur at relatively peripheral levels in the auditory system (i.e., the outer, middle, and inner ear) are used to construct a quantity called “instantaneous loudness” that is not available to conscious perception. At later stages in the auditory system the neural representation of the instantaneous loudness is transformed into the short-term loudness and long-term loudness, via processes of temporal integration This leads to the following questions: (1) At what stage or stages in the auditory pathway are the transforms leading to short-term and long-term loudness taking place? Even at the most peripheral level of auditory neural coding (the auditory nerve), substantial transformations of the sound have already occurred
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