Altered functional connectivity during speech perception in congenital amusia.

Individuals with congenital amusia have a lifelong history of unreliable pitch processing. Accordingly, they downweight pitch cues during speech perception and instead rely on other dimensions such as duration. We investigated the neural basis for this strategy. During fMRI, individuals with amusia (N = 15) and controls (N = 15) read sentences where a comma indicated a grammatical phrase boundary. They then heard two sentences spoken that differed only in pitch and/or duration cues and selected the best match for the written sentence. Prominent reductions in functional connectivity were detected in the amusia group between left prefrontal language-related regions and right hemisphere pitch-related regions, which reflected the between-group differences in cue weights in the same groups of listeners. Connectivity differences between these regions were not present during a control task. Our results indicate that the reliability of perceptual dimensions is linked with functional connectivity between frontal and perceptual regions and suggest a compensatory mechanism.

Spoken language is colored by fluctuations in pitch and rhythm. Rather than speaking in a flat monotone, we allow our sentences to rise and fall. We vary the length of syllables, drawing out some, and shortening others. These fluctuations, known as prosody, add emotion to speech and denote punctuation. In written language, we use a comma or a period to signal a boundary between phrases. In speech, we use changes in pitch ­ how deep or sharp a voice sounds ­ or in the length of syllables. Having more than one type of cue that can signal emotion or transitions between sentences has a number of advantages. It means that people can understand each other even when factors such as background noise obscure one set of cues. It also means that people with impaired sound perception can still understand speech. Those with a condition called congenital amusia, for example, struggle to perceive pitch, but they can compensate for this difficulty by placing greater emphasis on other aspects of speech. Jasmin et al. showed how the brain achieves this by comparing the brain activities of people with and without amusia. Participants were asked to read sentences on a screen where a comma indicated a boundary between two phrases. They then heard two spoken sentences, and had to choose the one that matched the written sentence. The spoken sentences used changes in pitch and/or syllable duration to signal the position of the comma. This provided listeners with the information needed to distinguish between "after John runs the race, ..." and "after John runs, the race...", for example. When two brain regions communicate, they tend to increase their activity at around the same time. The brain regions are then said to show functional connectivity. Jasmin et al. found that compared to healthy volunteers, people with amusia showed less functional connectivity between left hemisphere brain regions that process language and right hemisphere regions that process pitch. In other words, because pitch is a less reliable source of information for people with amusia, they recruit pitch-related brain regions less when processing speech. These results add to our understanding of how brains compensate for impaired perception. This may be useful for understanding the neural basis of compensation in other clinical conditions. It could also help us design bespoke hearing aids or other communication devices, such as computer programs that convert text into speech. Such programs could tailor the pitch and rhythm characteristics of the speech they produce to suit the perception of individual users.

The Multidimensional Battery of Prosody Perception (MBOPP).

Prosody can be defined as the rhythm and intonation patterns spanning words, phrases and sentences. Accurate perception of prosody is an important component of many aspects of language processing, such as parsing grammatical structures, recognizing words, and determining where emphasis may be placed. Prosody perception is important for language acquisition and can be impaired in language-related developmental disorders. However, existing assessments of prosodic perception suffer from some shortcomings.  These include being unsuitable for use with typically developing adults due to ceiling effects, or failing to allow the investigator to distinguish the unique contributions of individual acoustic features such as pitch and temporal cues. Here we present the Multi-Dimensional Battery of Prosody Perception (MBOPP), a novel tool for the assessment of prosody perception. It consists of two subtests: Linguistic Focus, which measures the ability to hear emphasis or sentential stress, and Phrase Boundaries, which measures the ability to hear where in a compound sentence one phrase ends, and another begins. Perception of individual acoustic dimensions (Pitch and Time) can be examined separately, and test difficulty can be precisely calibrated by the experimenter because stimuli were created using a continuous voice morph space. We present validation analyses from a sample of 57 individuals and discuss how the battery might be deployed to examine perception of prosody in various populations.

In vivo functional and myeloarchitectonic mapping of human primary auditory areas.

In contrast to vision, where retinotopic mapping alone can define areal borders, primary auditory areas such as A1 are best delineated by combining in vivo tonotopic mapping with postmortem cyto- or myeloarchitectonics from the same individual. We combined high-resolution (800 µm) quantitative T(1) mapping with phase-encoded tonotopic methods to map primary auditory areas (A1 and R) within the "auditory core" of human volunteers. We first quantitatively characterize the highly myelinated auditory core in terms of shape, area, cortical depth profile, and position, with our data showing considerable correspondence to postmortem myeloarchitectonic studies, both in cross-participant averages and in individuals. The core region contains two "mirror-image" tonotopic maps oriented along the same axis as observed in macaque and owl monkey. We suggest that these two maps within the core are the human analogs of primate auditory areas A1 and R. The core occupies a much smaller portion of tonotopically organized cortex on the superior temporal plane and gyrus than is generally supposed. The multimodal approach to defining the auditory core will facilitate investigations of structure-function relationships, comparative neuroanatomical studies, and promises new biomarkers for diagnosis and clinical studies.