Evidence for cue-independent spatial representation in the human auditory cortex during active listening

Abstract

Few auditory functions are as important or as universal as the capacity for auditory spatial awareness (e.g., sound localization). That ability relies on sensitivity to acoustical cues-particularly interaural time and level differences (ITD and ILD)-that correlate with sound-source locations. Under nonspatial listening conditions, cortical sensitivity to ITD and ILD takes the form of broad contralaterally dominated response functions. It is unknown, however, whether that sensitivity reflects representations of the specific physical cues or a higher-order representation of auditory space (i.e., integrated cue processing), nor is it known whether responses to spatial cues are modulated by active spatial listening. To investigate, sensitivity to parametrically varied ITD or ILD cues was measured using fMRI during spatial and nonspatial listening tasks. Task type varied across blocks where targets were presented in one of three dimensions: auditory location, pitch, or visual brightness. Task effects were localized primarily to lateral posterior superior temporal gyrus (pSTG) and modulated binaural-cue response functions differently in the two hemispheres. Active spatial listening (location tasks) enhanced both contralateral and ipsilateral responses in the right hemisphere but maintained or enhanced contralateral dominance in the left hemisphere. Two observations suggest integrated processing of ITD and ILD. First, overlapping regions in medial pSTG exhibited significant sensitivity to both cues. Second, successful classification of multivoxel patterns was observed for both cue types and-criticallyfor cross-cue classification. Together, these results suggest a higherorder representation of auditory space in the human auditory cortex that at least partly integrates the specific underlying cues.auditory space | binaural cues | human auditory cortex | fMRI | sound localization A cross species, one of the most important functions of hearing is the capacity to localize and orient to sound sources throughout 360°of extrapersonal space. This function relies on the neural processing of a variety of acoustical cues, but particularly on interaural time (ITD) and level differences (ILD) that convey sound-source directions in the horizontal dimension (azimuth). The acoustical basis of these cues is well understood: sound arrives earlier and with greater intensity at the ear nearest the source. The magnitudes of both cues depend systematically on azimuth angle, sound frequency, and head size/shape. Acoustic reflections from surfaces of the outer ear and of the listening room further alter these cues in a frequency-and cuespecific manner.In the absence of echoes and reverberation, ITD and ILD cues vary more or less in parallel (i.e., redundantly) and human listeners appear to use the cues interchangeably. However, ITD is the psychophysically dominant cue for localization when sounds contain energy below ∼1.5 kHz (1-3). For higher-frequency sounds, ILD tends to dominate, although ITD carried by fluctuations in the sound envelope a...

Cortical auditory distance representation based on direct-to-reverberant energy ratio

“…However, in the present Varying Frontal Distance condition, the stimuli contained both DRR and intensity-related distance cues, i.e., all possible available cues. Our previous behavioral studies suggest that in cases like this, auditory distance-discrimination performance is based on an integrated representation of source distance cues instead of any individual distance cue alone (level, ILD, or DRR) (Kopco et al, 2012), consistent with recent fMRI findings regarding horizontal direction cue processing (Higgins et al, 2017). The areas activated more strongly to frontal distance changes than intensity changes could thus involve both DRR specific populations and networks that assemble independent features to a more integrated spatial representation.…”

“…Interestingly, roughly the same areas showed overlapping activations to individually presented ILD and ITD cues in a recent fMRI study (Higgins et al, 2017). It is thus possible that the posterior AC areas that were activated by frontal distance cues include neuron populations that process representations of the auditory space instead of individual acoustic features only (Higgins et al, 2017;Palomäki et al, 2005), although these representations may not have an orderly topographical organization comparable to cortical representations of the visual space (Salminen et al, 2009;Stecker and Middlebrooks, 2003).…”

“…However, this evidence was obtained for sources simulated to originate at the side of the head that varied in distance along the interaural axis, for which both DRR and ILD cues are available. It is thus possible that the findings are an epiphenomenon of activations of direction-encoding neurons that are sensitive to ILD (Imig et al, 1990;Johnson and Hautus, 2010;Lehmann et al, 2007;Tardif et al, 2006;Zimmer et al, 2006), which have been later shown to activate areas overlapping with the putative distance representations (Higgins et al, 2017; 6 Stecker et al, 2015). Further studies are, therefore, needed to verify the existence of auditory cortex distance representations that do not involve cues shared with directional hearing.…”

Representation of auditory motion directions and sound source locations in the human planum temporale

“…Previous studies argued that successful cross-condition classification reflects an abstract representation of stimuli conditions (Fairhall and Caramazza, 2013;Higgins et al, 2017;Hong et al, 2012).…”

“…Significant cross-condition classification has typically been considered as a demonstration that the region implements a partly common and abstracted representation of the tested conditions (Fairhall and Caramazza, 2013;Higgins et al, 2017;Hong et al, 2012). For instance, a recent study elegantly demonstrated that the human auditory cortex at least partly integrates interaural time and level differences (ITD and ILD) into a higher-order representation of auditory space based on significant for cross-cue classification (training on ITD and classifying ILD, and reversely).…”