Enhanced representation of spectral contrasts in the primary auditory cortex

Abstract

The role of early auditory processing may be to extract some elementary features from an acoustic mixture in order to organize the auditory scene. To accomplish this task, the central auditory system may rely on the fact that sensory objects are often composed of spectral edges, i.e., regions where the stimulus energy changes abruptly over frequency. The processing of acoustic stimuli may benefit from a mechanism enhancing the internal representation of spectral edges. While the visual system is thought to rely heavily on this mechanism (enhancing spatial edges), it is still unclear whether a related process plays a significant role in audition. We investigated the cortical representation of spectral edges, using acoustic stimuli composed of multi-tone pips whose time-averaged spectral envelope contained suppressed or enhanced regions. Importantly, the stimuli were designed such that neural responses properties could be assessed as a function of stimulus frequency during stimulus presentation. Our results suggest that the representation of acoustic spectral edges is enhanced in the auditory cortex, and that this enhancement is sensitive to the characteristics of the spectral contrast profile, such as depth, sharpness and width. Spectral edges are maximally enhanced for sharp contrast and large depth. Cortical activity was also suppressed at frequencies within the suppressed region. To note, the suppression of firing was larger at frequencies nearby the lower edge of the suppressed region than at the upper edge. Overall, the present study gives critical insights into the processing of spectral contrasts in the auditory system.

Dynamic representation of spectral edges in guinea pig primary auditory cortex

“…STRFs were obtained from 180-s multitone pip stimuli (Blake and Merzenich 2002;Catz and Noreña 2013;deCharms et al 1998;Gourévitch et al 2009;Noreña et al 2008;Valentine and Eggermont 2004). Tone pips (49 frequencies, 8 frequencies per octave covering 6 octaves between 500 Hz and 32 kHz) were presented randomly over time (independent Poisson process for each frequency with a rate of 2 Hz and a 50-ms dead time designed to prevent tones of the same frequency from overlapping in time).…”

“…First, we further investigated the effects of spectral notch depth to determine whether edge enhancement could be seen for spectral contrasts as small as 10 dB. Second, the edge enhancements described in our previous study (Catz and Noreña 2013) were observed after a 180-s presentation of the AFB stimulus. This is relatively fast but still too slow to dynamically shape the representation of real acoustic environments, which can vary on much smaller (fractions of a second) timescales (Singh and Theunissen 2003).…”

“…This process is particularly challenging because sounds in the natural environment (such as conspecific vocalizations) have complex spectra that change rapidly with time and are often embedded in background noise (Palmer and Shamma 2004;Shamma and Micheyl 2010;Singh and Theunissen 2003;Theunissen and Elie 2014). Although we are far from a complete understanding of how complex sounds are represented in the central auditory system, it appears that the representation of many sound features is context dependent, i.e., it depends on the spectrotemporal past and present of the acoustic mixture (Blake and Merzenich 2002;Brosch et al 1999;Brosch and Scheich 2008;Brosch and Schreiner 2000;Catz and Noreña 2013;Gourévitch et al 2009;Noreña et al 2008). …”

“…Such studies showed that neural responses to the target tone could be strongly suppressed, or in some cases enhanced, by the prior presentation of the modulating tone; that is, they could be strongly context dependent. Other studies derived neural properties, such as spectrotemporal receptive fields (STRFs), from a variety of more complex stimuli, such as white noise, spectrotemporally modulated ("rippled") noise, ensembles of random tone bursts, and conspecific vocalizations (Blake and Merzenich, 2002;Catz and Noreña 2013;deCharms et al 1998;Depireux et al 2001;Eggermont et al 1983;Escabí and Read 2005;Gourévitch et al, 2009;Noreña et al 2006Noreña et al , 2008Rabinowitz et al 2011Rabinowitz et al , 2012Theunissen et al 2001; Theunissen and Elie 2014; Valentine and Eggermont 2004). It was shown that although the shapes of STRFs are relatively insensitive to some stimulus characteristics (i.e., SPL; Pienkowski and Eggermont 2010;Valentine and Eggermont 2004), they are sensitive to others (i.e., stimulus spectrotemporal density; Blake and Merzenich 2002;Noreña et al 2008).…”

“…We recently investigated the auditory cortical representation of spectral edges or contours, and how this representation varies with their physical properties (i.e., width, depth, and slope) (Catz and Noreña 2013). To investigate the respective representation of frequencies near to and remote from the spectral edges, we devised a stimulus ensemble composed of random tone bursts (between 500 Hz and 32 kHz, 1 ⁄8 octave step) with or without an attenuated frequency band (AFB), within which tone levels are reduced to create edge contrasts.…”