Effects of task-switching on neural representations of ambiguous sound input

Abstract

The ability to perceive discrete sound streams in the presence of competing sound sources relies on multiple mechanisms that organize the mixture of the auditory input entering the ears. Many studies have focused on mechanisms that contribute to integrating sounds that belong together into one perceptual stream (integration) and segregating those that come from different sound sources (segregation). However, little is known about mechanisms that allow us to perceive individual sound sources within a dynamically changing auditory scene, when the input may be ambiguous, and heard as either integrated or segregated. This study tested the question of whether focusing on one of two possible sound organizations suppressed representation of the alternative organization. We presented listeners with ambiguous input and cued them to switch between tasks that used either the integrated or the segregated percept. Electrophysiological measures indicated which organization was currently maintained in memory. If mutual exclusivity at the neural level was the rule, attention to one of two possible organizations would preclude neural representation of the other. However, significant MMNs were elicited to both the target organization and the unattended, alternative organization, along with the target-related P3b component elicited only to the designated target organization. Results thus indicate that both organizations (integrated and segregated) were simultaneously maintained in memory regardless of which task was performed. Focusing attention to one aspect of the sounds did not abolish the alternative, unattended organization when the stimulus input was ambiguous. In noisy environments, such as walking on a city street, rapid and flexible adaptive processes are needed to help facilitate rapid switching to different sound sources in the environment. Having multiple representations available to the attentive system would allow for such flexibility, needed in everyday situations to maintain stable auditory percepts, and to allow rapid scanning of interesting events in a busy environment.

Selective entrainment of brain oscillations drives auditory perceptual organization

“…However, the task-irrelevant responses were weaker and fading over the course of the sequence. The overall pattern of results fits with previous research revealing that the auditory system is able to hold parallel acoustic regularities in multiple time-scales (Costa-Faidella et al 2011; Horvath et al 2001; Pannese et al 2015; Sussman et al 2014; Ulanovsky et al 2004; Winkler et al 2009). Our results support the idea that auditory streaming builds-up over time (Bregman 1978; Sussman et al 2007; Sussman-Fort and Sussman 2014; cf., Deike et al 2012).…”

“…The goal of the auditory system is to provide veridical representations of the sound sources in the environment, sorting out the individual auditory objects from the intricate sound mixtures, a feat known as auditory scene analysis (Bregman 1990). Holding multiple representations of the environment simultaneously available would therefore enable the flexible identification of sound sources (Sussman et al 2014). According to our data, proto-objects that may compete to describe the acoustic scene are encoded in the activity of the auditory system (Bregman 1990; Winkler et al 2012).…”

“…Our subjective experience therefore conforms to the Gestalt principle of exclusive allocation (Kohler 1947), which states that any sensory element should not be used in more than one description of the natural scene at a time. Whether this principle also applies at the neural level, specifying memory representations of the stimulus input (i.e., whether multiple internal models are held simultaneously or only the current attended one) is still a matter of intense debate (Sussman et al 2014; Denham et al 2014; Grossberg et al 2004). …”

“…Behavioral evidence on auditory spontaneous perceptual switches suggests that multiple alternative organizations are held simultaneously and compete to describe the acoustic scene (Denham et al 2014; Pressnitzer and Hupe 2006; Sterzer et al 2009; Sussman et al 2014). Electrophysiological studies in humans have traditionally embedded violations of established regularities within the acoustic streams in order to use change detection auditory evoked potentials, such as the mismatch negativity (MMN) (Näätänen et al 1978), as an index of sound organization (Sussman et al 1998, 1999).…”

“…Electrophysiological studies in humans have traditionally embedded violations of established regularities within the acoustic streams in order to use change detection auditory evoked potentials, such as the mismatch negativity (MMN) (Näätänen et al 1978), as an index of sound organization (Sussman et al 1998, 1999). However, besides yielding conflicting results, with some studies showing simultaneous encoding of alternative organizations (Pannese et al 2015; Sussman et al 2014) while others suggesting that only the currently perceived organization is represented (Sussman et al 2002; Sussman 2013; Winkler et al 2006), evidence of this nature is intrinsically indirect and does not inform about the neural mechanisms underlying the representation of sound organization.…”