Different roles of similarity and predictability in auditory stream segregation

Sound sources often emit trains of discrete sounds, such as a series of footsteps. Previously, two dif¬ferent principles have been suggested for how the human auditory system binds discrete sounds to¬gether into perceptual units. The feature similarity principle is based on linking sounds with similar characteristics over time. The predictability principle is based on linking sounds that follow each other in a predictable manner. The present study compared the effects of these two principles. Participants were presented with tone sequences and instructed to continuously indicate whether they perceived a single coherent sequence or two concurrent streams of sound. We investigated the influence of separate manipulations of similarity and predictability on these perceptual reports. Both grouping principles affected perception of the tone sequences, albeit with different characteristics. In particular, results suggest that whereas predictability is only analyzed for the currently perceived sound organization, feature similarity is also analyzed for alternative groupings of sound. Moreover, changing similarity or predictability within an ongoing sound sequence led to markedly different dynamic effects. Taken together, these results provide evidence for different roles of similarity and predictability in auditory scene analysis, suggesting that forming auditory stream representations and competition between alter¬natives rely on partly different processes

Magnetoencephalographic Correlates of Perceptual State During Auditory Bistability

Bistability occurs when two alternative percepts can be derived from the same physical stimulus. To identify the neural correlates of specifc subjective experiences we used a bistable auditory stimulus and determined whether the two perceptual states could be distinguished electrophysiologically. Fourteen participants underwent magnetoencephalography while reporting their perceptual experience while listening to a continuous bistable stream of auditory tones. Participants reported bistability with a similar overall proportion of the two alternative percepts (52% vs 48%). At the individual level, sensor space electrophysiological discrimination between the percepts was possible in 9/14 participants with canonical variate analysis (CVA) or linear support vector machine (SVM) analysis over space and time dimensions. Classifcation was possible in 14/14 subjects with non-linear SVM. Similar efects were noted in an unconstrained source space CVA analysis (classifying 10/14 participants), linear SVM (classifying 9/14 subjects) and non-linear SVM (classifying 13/14 participants). Source space analysis restricted to a priori ROIs showed discrimination was possible in the right and left auditory cortex with each classifcation approach but in the right intraparietal sulcus this was only apparent with non-linear SVM and only in a minority of particpants. Magnetoencephalography can be used to objectively classify auditory experiences from individual subjects

Neurophysiological Mechanisms Involved in Auditory Perceptual Organization

In our complex acoustic environment, we are confronted with a mixture of sounds produced by several simultaneous sources. However, we rarely perceive these sounds as incomprehensible noise. Our brain uses perceptual organization processes to independently follow the emission of each sound source over time. If the acoustic properties exploited in these processes are well-established, the neurophysiological mechanisms involved in auditory scene analysis remain unclear and have recently raised more interest. Here, we review the studies investigating these mechanisms using electrophysiological recordings from the cochlear nucleus to the auditory cortex, in animals and humans. Their findings reveal that basic mechanisms such as frequency selectivity, forward suppression and multi-second habituation shape the automatic brain responses to sounds in a way that can account for several important characteristics of perceptual organization of both simultaneous and successive sounds. One challenging question remains unresolved: how are the resulting activity patterns integrated to yield the corresponding conscious percepts

Pattern of BOLD signal in auditory cortex relates acoustic response to perceptual streaming

<p>Abstract</p> <p>Background</p> <p>Segregating auditory scenes into distinct objects or streams is one of our brain's greatest perceptual challenges. Streaming has classically been studied with bistable sound stimuli, perceived alternately as a single group or two separate groups. Throughout the last decade different methodologies have yielded inconsistent evidence about the role of auditory cortex in the maintenance of streams. In particular, studies using functional magnetic resonance imaging (fMRI) have been unable to show persistent activity within auditory cortex (AC) that distinguishes between perceptual states.</p> <p>Results</p> <p>We use bistable stimuli, an explicit perceptual categorization task, and a focused region of interest (ROI) analysis to demonstrate an effect of perceptual state within AC. We find that AC has more activity when listeners perceive the split percept rather than the grouped percept. In addition, within this ROI the pattern of acoustic response across voxels is significantly correlated with the pattern of perceptual modulation. In a whole-brain exploratory test, we corroborate previous work showing an effect of perceptual state in the intraparietal sulcus.</p> <p>Conclusions</p> <p>Our results show that the maintenance of auditory streams is reflected in AC activity, directly relating sound responses to perception, and that perceptual state is further represented in multiple, higher level cortical regions.</p

Event-related potential correlates of sound organization: Early sensory and late cognitive effects

We tested whether incoming sounds are processed differently depending on how the preceding sound sequence has been interpreted by the brain. Sequences of a regularly repeating three-tone pattern, the perceived organization of which spontaneously switched back and forth between two alternative interpretations, were delivered to listeners. Occasionally, a regular tone was exchanged for a slightly or moderately lower one (deviants). The electroencephalogram (EEG) was recorded while listeners continuously marked their perception of the sound sequence. We found that for both the regular and the deviant tones, the early exogenous P1 and N1 amplitudes varied together with the perceived sound organization. Percept dependent effects on the late endogenous N2 and P3a amplitudes were only found for deviant tones. These results suggest that the perceived sound organization affects sound processing both by modulating what information is extracted from incoming sounds as well as by influencing how deviant sound events are evaluated for further processing

Modeling the auditory scene: predictive regularity representations and perceptual objects

Predictive processing of information is essential for goal directed behavior. We offer an account of auditory perception suggesting that representations of predictable patterns, or ‘regularities’, extracted from the incoming sounds serve as auditory perceptual objects. The auditory system continuously searches for regularities within the acoustic signal. Primitive regularities may be encoded by neurons adapting their response to specific sounds. Such neurons have been observed in many parts of the auditory system. Representations of the detected regularities produce predictions of upcoming sounds as well as alternative solutions for parsing the composite input into coherent sequences potentially emitted by putative sound sources. Accuracy of the predictions can be utilized for selecting the most likely interpretation of the auditory input. Thus in our view, perception generates hypotheses about the causal structure of the world

Investigating how neural entrainment relates to beat perception by disentangling the stimulus-driven response

Beat perception – the ability to perceive a steady pulse in music – is nearly ubiquitous in humans, but the neural mechanisms underlying this ability are unknown. A growing number of electroencephalography (EEG) studies suggest that beat perception is related to neural entrainment, a phenomenon in which cyclic changes in the excitability of populations of neurons synchronize with a rhythmic stimulus. However, the relationship between acoustically-driven and entrainment-driven neural activity is unclear. This thesis presents EEG research that extends our understanding neural entrainment is related to beat perception by characterizing, equating, and finally removing the stimulus-driven response in the neural signal isolating the entrainment-driven responses. Chapter 1 presents a general overview of how neural entrainment may relate to beat perception, the common methods of measuring neural entrainment, and current debates in the literature about how best to account for the stimulus-driven response in the neural signal and also what the neural power spectrum reflects. Chapter 2 presents research on how perceptual and acoustic factors in auditory stimuli influence neural spectral power in a series of experiments in which beat strength, tone duration, and onset/offset ramp duration were manipulated. The results suggest that both perceptual and acoustic factors influence neural spectral power, and that accounting for the stimulus-driven response in the neural spectrum is more complicated than previously assumed. Chapter 3 presents research on how power and phase of the neural signal relate to beat strength and beat location while controlling the stimulus-driven response. The results indicated a relationship between neural entrainment and beat strength, and also, between oscillatory phase and beat location. Chapter 4 presents research on the potential neural mechanisms of beat perception by examining neural activity during a silent immediately after rhythm perception for testing for ongoing, oscillatory activity. The results, although not statistically robust, suggest that entrained activity continues into silence, indicating a relationship between neural entrainment and beat perception. Chapter 5 presents a general discussion of Chapters 2-4 in the context of the existing literature, limitations, and broader interpretations of how these results relate to future directions in the field

Neural Correlates of Auditory Perceptual Awareness under Informational Masking

Our ability to detect target sounds in complex acoustic backgrounds is often limited not by the ear's resolution, but by the brain's information-processing capacity. The neural mechanisms and loci of this “informational masking” are unknown. We combined magnetoencephalography with simultaneous behavioral measures in humans to investigate neural correlates of informational masking and auditory perceptual awareness in the auditory cortex. Cortical responses were sorted according to whether or not target sounds were detected by the listener in a complex, randomly varying multi-tone background known to produce informational masking. Detected target sounds elicited a prominent, long-latency response (50–250 ms), whereas undetected targets did not. In contrast, both detected and undetected targets produced equally robust auditory middle-latency, steady-state responses, presumably from the primary auditory cortex. These findings indicate that neural correlates of auditory awareness in informational masking emerge between early and late stages of processing within the auditory cortex

Maturation of auditory temporal integration and inhibition assessed with event-related potentials (ERPs)

Background: We examined development of auditory temporal integration and inhibition by assessing electrophysiological responses to tone pairs separated by interstimulus intervals (ISIs) of 25, 50, 100, 200, 400, and 800 ms in 28 children aged 7 to 9 years, and 15 adults.Results: In adults a distinct neural response was elicited to tones presented at ISIs of 25 ms or longer, whereas in children this was only seen in response to tones presented at ISIs above 100 ms. In adults, late N1 amplitude was larger for the second tone of the tone pair when separated by ISIs as short as 100 ms, consistent with the perceptual integration of successive stimuli within the temporal window of integration. In contrast, children showed enhanced negativity only when tone pairs were separated by ISIs of 200 ms. In children, the amplitude of the P1 component was attenuated at ISIs below 200 ms, consistent with a refractory process.Conclusions: These results indicate that adults integrate sequential auditory information into smaller temporal segments than children. These results suggest that there are marked maturational changes from childhood to adulthood in the perceptual processes underpinning the grouping of incoming auditory sensory information, and that electrophysiological measures provide a sensitive, non-invasive method allowing further examination of these changes