Division of labor between left and right human auditory cortices during the processing of intensity and duration

Effect of age on lateralized auditory processing

The lateralization of processing in the auditory cortex for different acoustic parameters differs depending on stimuli and tasks. Thus, processing complex auditory stimuli requires an efficient hemispheric interaction. Anatomical connectivity decreases with aging and consequently affects the functional interaction between the left and right auditory cortex and lateralization of auditory processing. Here we studied with magnetic resonance imaging the effect of aging on the lateralization of processing and hemispheric interaction during two tasks utilizing the contralateral noise procedure. Categorization of tones according to their direction of frequency modulations (FM) is known to be processed mainly in the right auditory cortex. Sequential comparison of the same tones according to their FM direction strongly involves additionally the left auditory cortex and therefore a stronger hemispheric interaction than the categorization task. The results showed that older adults more strongly recruit the auditory cortex especially during the comparison task that requires stronger hemispheric interaction. This was the case although the task difficulty was adapted to achieve similar performance as the younger adults. Additionally, functional connectivity from auditory cortex to other brain areas was stronger in older than younger adults especially during the comparison task. Diffusion tensor imaging data showed a reduction in fractional anisotropy and an increase in mean diffusivity in the corpus callosum of older adults compared to younger adults. These changes indicate a reduction of anatomical interhemispheric connections in older adults that makes larger processing capacity necessary when tasks require functional hemispheric interaction.

Acoustics versus linguistics? Context is Part and Parcel to lateralized processing of the parts and parcels of speech

The purpose of this review is to provide an accessible exploration of key considerations of lateralization in speech and non-speech perception using clear and defined language. From these considerations, the primary arguments for each side of the linguistics versus acoustics debate are outlined and explored in context of emerging integrative theories. This theoretical approach entails a perspective that linguistic and acoustic features differentially contribute to leftward bias, depending on the given context. Such contextual factors include stimulus parameters and variables of stimulus presentation (e.g., noise/silence and monaural/binaural) and variances in individuals (sex, handedness, age, and behavioural ability). Discussion of these factors and their interaction is also aimed towards providing an outline of variables that require consideration when developing and reviewing methodology of acoustic and linguistic processing laterality studies. Thus, there are three primary aims in the present paper: (1) to provide the reader with key theoretical perspectives from the acoustics/linguistics debate and a synthesis of the two viewpoints, (2) to highlight key caveats for generalizing findings regarding predominant models of speech laterality, and (3) to provide a practical guide for methodological control using predominant behavioural measures (i.e., gap detection and dichotic listening tasks) and/or neurophysiological measures (i.e., mismatch negativity) of speech laterality.

Effect of Contralateral Noise on Speech Intelligibility

In patients with strong asymmetric hearing loss, standard clinical practice involves testing speech intelligibility in the ear with the higher hearing threshold by simultaneously presenting noise to the other ear. However, psychoacoustic and functional magnetic resonance imaging (fMRI) studies indicate that this approach may be problematic as contralateral noise has a disruptive effect on task processing. Furthermore, fMRI studies have revealed that the effect of contralateral noise on brain activity depends on the lateralization of task processing. The effect of contralateral noise is stronger when task-relevant stimuli are presented ipsilaterally to the hemisphere that is processing the task. In the present study, we tested the effect of four different levels of contralateral noise on speech intelligibility using the Oldenburg sentence test (OLSA). Cortical lateralization of speech processing was assessed upfront by using a visual speech test with fMRI. Contralateral OLSA noise of 65 or 80 dB SPL significantly reduced word intelligibility irrespective of which ear the speech was presented to. In participants with left-lateralized speech processing, 50 dB SPL contralateral OLSA noise led to a significant reduction in speech intelligibility when speech was presented to the left ear, i.e. when speech was presented ipsilaterally to the hemisphere that is mainly processing speech. Thus, contralateral noise, as used in standard clinical practice, not only prevents listeners from using the information in the better-hearing ear but may also have the unintended effect of hampering central processing of speech.

Brain Lateralization: A Comparative Perspective

Comparative studies on brain asymmetry date back to the 19th century but then largely disappeared due to the assumption that lateralization is uniquely human. Since the reemergence of this field in the 1970s, we learned that left-right differences of brain and behavior exist throughout the animal kingdom and pay off in terms of sensory, cognitive, and motor efficiency. Ontogenetically, lateralization starts in many species with asymmetrical expression patterns of genes within the Nodal cascade that set up the scene for later complex interactions of genetic, environmental, and epigenetic factors. These take effect during different time points of ontogeny and create asymmetries of neural networks in diverse species. As a result, depending on task demands, left- or right-hemispheric loops of feedforward or feedback projections are then activated and can temporarily dominate a neural process. In addition, asymmetries of commissural transfer can shape lateralized processes in each hemisphere. It is still unclear if interhemispheric interactions depend on an inhibition/excitation dichotomy or instead adjust the contralateral temporal neural structure to delay the other hemisphere or synchronize with it during joint action. As outlined in our review, novel animal models and approaches could be established in the last decades, and they already produced a substantial increase of knowledge. Since there is practically no realm of human perception, cognition, emotion, or action that is not affected by our lateralized neural organization, insights from these comparative studies are crucial to understand the functions and pathologies of our asymmetric brain.

Short- and long-term memory for pitch and non-pitch contours: Insights from congenital amusia

Congenital amusia is a neurodevelopmental disorder characterized by deficits in music perception, including discriminating and remembering melodies and melodic contours. As non-amusic listeners can perceive contours in dimensions other than pitch, such as loudness and brightness, our present study investigated whether amusics' pitch contour deficits also extend to these other auditory dimensions. Amusic and control participants performed an identification task for ten familiar melodies and a short-term memory task requiring the discrimination of changes in the contour of novel four-tone melodies. For both tasks, melodic contour was defined by pitch, brightness, or loudness. Amusic participants showed some ability to extract contours in all three dimensions. For familiar melodies, amusic participants showed impairment in all conditions, perhaps reflecting the fact that the long-term memory representations of the familiar melodies were defined in pitch. In the contour discrimination task with novel melodies, amusic participants exhibited less impairment for loudness-based melodies than for pitch- or brightness-based melodies, suggesting some specificity of the deficit for spectral changes, if not for pitch alone. The results suggest pitch and brightness may not be processed by the same mechanisms as loudness, and that short-term memory for loudness contours may be spared to some degree in congenital amusia.

The impact of task difficulty on the lateralization of processing in the human auditory cortex

Perception of complex auditory stimuli like speech requires the simultaneous processing of different fundamental acoustic parameters. The contribution of left and right auditory cortex (AC) in the processing of these parameters differs. In addition, activity within the AC can vary positively or negatively with task performance depending on the type of task. This might affect the allocation of processing to the left and right AC. Here we studied with functional magnetic resonance imaging the impact of task difficulty on the degree of involvement of the left and right AC in two tasks that have previously been shown to differ in hemispheric involvement: categorization and sequential comparison of the direction of frequency modulations (FM). Task difficulty was manipulated by changing the speed of modulation and by that the frequency range covered by the FM. To study the impact of task‐difficulty despite covarying the stimulus parameters, we utilized the contralateral noise procedure that allows comparing AC activation unconfounded by bottom‐up driven activity. The easiest conditions confirmed the known right AC involvement during the categorization task and the left AC involvement during the comparison task. The involvement of the right AC increased with increasing task difficulty for both tasks presumably due to the common task component of categorizing FM direction. The involvement of left AC varied with task difficulty depending on the task. Thus, task difficulty has a strong impact on lateralized processing in AC. This connection must be taken into account when interpreting future results on lateralized processing in the AC.

Effect of sequential comparison on active processing of sound duration

Previous studies on active duration processing on sounds showed opposing results regarding the predominant involvement of the left or right hemisphere. Duration of an acoustic event is normally judged relative to other sounds. This requires sequential comparison as auditory events unfold over time. We hypothesized that increasing the demand on sequential comparison in a task increases the involvement of the left auditory cortex. With the current fMRI study, we investigated the effect of sequential comparison in active duration discrimination by comparing a categorical with a comparative task. During the categorical task, the participant had to categorize the tones according to their duration (short vs long). During the comparative task, they had to decide for each tone whether its length matched the tone presented before. We used the contralateral noise procedure to reveal the degree of participation of the left and right auditory cortex during these tasks. We found that both tasks more strongly involve the left than the right auditory cortex. Furthermore, the left auditory cortex was more strongly involved during comparison than during categorization. Together with previous studies, this suggests that additional demand for sequential comparison during processing of different basic acoustic parameters leads to an increased recruitment of the left auditory cortex. In addition, the comparison task more strongly involved several brain areas outside the auditory cortex, which may also be related to the demand for additional cognitive resources as compared to the more efficient categorization of sounds. Hum Brain Mapp 38:4459-4469, 2017. © 2017 Wiley Periodicals, Inc.

Auditory processing disorders with and without central auditory discrimination deficits

Auditory processing disorder (APD) is defined as a processing deficit in the auditory modality and spans multiple processes. To date, APD diagnosis is mostly based on the utilization of speech material. Adequate nonspeech tests that allow differentiation between an actual central hearing disorder and related disorders such as specific language impairments are still not adequately available. In the present study, 84 children between 6 and 17 years of age (clinical group), referred to three audiological centers for APD diagnosis, were evaluated with standard audiological tests and additional auditory discrimination tests. Latter tests assessed the processing of basic acoustic features at two different stages of the ascending central auditory system: (1) auditory brainstem processing was evaluated by quantifying interaural frequency, level, and signal duration discrimination (interaural tests). (2) Diencephalic/telencephalic processing was assessed by varying the same acoustic parameters (plus signals with sinusoidal amplitude modulation), but presenting the test signals in conjunction with noise pulses to the contralateral ear (dichotic(signal/noise) tests). Data of children in the clinical group were referenced to normative data obtained from more than 300 normally developing healthy school children. The results in the audiological and the discrimination tests diverged widely. Of the 39 children that were diagnosed with APD in the audiological clinic, 30 had deficits in auditory performance. Even more alarming was the fact that of the 45 children with a negative APD diagnosis, 32 showed clear signs of a central hearing deficit. Based on these results, we suggest revising current diagnostic procedure to evaluate APD in order to more clearly differentiate between central auditory processing deficits and higher-order (cognitive and/or language) processing deficits.

Neuronal basis of speech comprehension

Verbal communication does not rely only on the simple perception of auditory signals. It is rather a parallel and integrative processing of linguistic and non-linguistic information, involving temporal and frontal areas in particular. This review describes the inherent complexity of auditory speech comprehension from a functional-neuroanatomical perspective. The review is divided into two parts. In the first part, structural and functional asymmetry of language relevant structures will be discus. The second part of the review will discuss recent neuroimaging studies, which coherently demonstrate that speech comprehension processes rely on a hierarchical network involving the temporal, parietal, and frontal lobes. Further, the results support the dual-stream model for speech comprehension, with a dorsal stream for auditory-motor integration, and a ventral stream for extracting meaning but also the processing of sentences and narratives. Specific patterns of functional asymmetry between the left and right hemisphere can also be demonstrated. The review article concludes with a discussion on interactions between the dorsal and ventral streams, particularly the involvement of motor related areas in speech perception processes, and outlines some remaining unresolved issues. This article is part of a Special Issue entitled Human Auditory Neuroimaging.