The cat's meow: A high-field fMRI assessment of cortical activity in response to vocalizations and complex auditory stimuli

A population of neurons selective for human voice in the monkey brain

Many animals can extract useful information from the vocalizations of other species. Neuroimaging studies have evidenced areas sensitive to conspecific vocalizations in the cerebral cortex of primates, but how these areas process heterospecific vocalizations remains unclear. Using fMRI-guided electrophysiology, we recorded the spiking activity of individual neurons in the anterior temporal voice patches of two macaques while they listened to complex sounds including vocalizations from several species. In addition to cells selective for conspecific macaque vocalizations, we identified an unsuspected subpopulation of neurons with strong selectivity for human voice, not merely explained by spectral or temporal structure of the sounds. The auditory representational geometry implemented by these neurons was strongly related to that measured in the human voice areas with neuroimaging and only weakly to low-level acoustical structure. These findings provide new insights into the neural mechanisms involved in auditory expertise and the evolution of communication systems in primates.

Dual Representation of the Auditory Space

Auditory spatial cues contribute to two distinct functions, of which one leads to explicit localization of sound sources and the other provides a location-linked representation of sound objects. Behavioral and imaging studies demonstrated right-hemispheric dominance for explicit sound localization. An early clinical case study documented the dissociation between the explicit sound localizations, which was heavily impaired, and fully preserved use of spatial cues for sound object segregation. The latter involves location-linked encoding of sound objects. We review here evidence pertaining to brain regions involved in location-linked representation of sound objects. Auditory evoked potential (AEP) and functional magnetic resonance imaging (fMRI) studies investigated this aspect by comparing encoding of individual sound objects, which changed their locations or remained stationary. Systematic search identified 1 AEP and 12 fMRI studies. Together with studies of anatomical correlates of impaired of spatial-cue-based sound object segregation after focal brain lesions, the present evidence indicates that the location-linked representation of sound objects involves strongly the left hemisphere and to a lesser degree the right hemisphere. Location-linked encoding of sound objects is present in several early-stage auditory areas and in the specialized temporal voice area. In these regions, emotional valence benefits from location-linked encoding as well.

A comparison of multisensory features of two auditory cortical areas: primary (A1) and higher-order dorsal zone (DZ)

From myriads of ongoing stimuli, the brain creates a fused percept of the environment. This process, which culminates in perceptual binding, is presumed to occur through the operations of multisensory neurons that occur throughout the brain. However, because different brain areas receive different inputs and have different cytoarchitechtonics, it would be expected that local multisensory features would also vary across regions. The present study investigated that hypothesis using multiple single-unit recordings from anesthetized cats in response to controlled, electronically-generated separate and combined auditory, visual, and somatosensory stimulation. These results were used to compare the multisensory features of neurons in cat primary auditory cortex (A1) with those identified in the nearby higher-order auditory region, the Dorsal Zone (DZ). Both regions exhibited the same forms of multisensory neurons, albeit in different proportions. Multisensory neurons exhibiting excitatory or inhibitory properties occurred in similar proportions in both areas. Also, multisensory neurons in both areas expressed similar levels of multisensory integration. Because responses to auditory cues alone were so similar to those that included non-auditory stimuli, it is proposed that this effect represents a mechanism by which multisensory neurons subserve the process of perceptual binding.

Keeping track of sound objects in space: The contribution of early-stage auditory areas

The influential dual-stream model of auditory processing stipulates that information pertaining to the meaning and to the position of a given sound object is processed in parallel along two distinct pathways, the ventral and dorsal auditory streams. Functional independence of the two processing pathways is well documented by conscious experience of patients with focal hemispheric lesions. On the other hand there is growing evidence that the meaning and the position of a sound are combined early in the processing pathway, possibly already at the level of early-stage auditory areas. Here, we investigated how early auditory areas integrate sound object meaning and space (simulated by interaural time differences) using a repetition suppression fMRI paradigm at 7 T. Subjects listen passively to environmental sounds presented in blocks of repetitions of the same sound object (same category) or different sounds objects (different categories), perceived either in the left or right space (no change within block) or shifted left-to-right or right-to-left halfway in the block (change within block). Environmental sounds activated bilaterally the superior temporal gyrus, middle temporal gyrus, inferior frontal gyrus, and right precentral cortex. Repetitions suppression effects were measured within bilateral early-stage auditory areas in the lateral portion of the Heschl's gyrus and posterior superior temporal plane. Left lateral early-stages areas showed significant effects for position and change, interactions Category x Initial Position and Category x Change in Position, while right lateral areas showed main effect of category and interaction Category x Change in Position. The combined evidence from our study and from previous studies speaks in favour of a position-linked representation of sound objects, which is independent from semantic encoding within the ventral stream and from spatial encoding within the dorsal stream. We argue for a third auditory stream, which has its origin in lateral belt areas and tracks sound objects across space.

Reduction of sound-evoked midbrain responses observed by functional magnetic resonance imaging following acute acoustic noise exposure

Short duration and high intensity acoustic exposures can lead to temporary hearing loss and auditory nerve degeneration. This study investigates central auditory system function following such acute exposures after hearing loss recedes. Adult rats were exposed to 100 dB sound pressure level noise for 15 min. Auditory brainstem responses (ABRs) were recorded with click sounds to check hearing thresholds. Functional magnetic resonance imaging (fMRI) was performed with tonal stimulation at 12 and 20 kHz to investigate central auditory changes. Measurements were performed before exposure (0D), 7 days after (7D), and 14 days after (14D). ABRs show an ∼6 dB threshold shift shortly after exposure, but no significant threshold differences between 0D, 7D, and 14D. fMRI responses are observed in the lateral lemniscus (LL) and inferior colliculus (IC) of the midbrain. In the IC, responses to 12 kHz are 3.1 ± 0.3% (0D), 1.9 ± 0.3% (7D), and 2.9 ± 0.3% (14D) above the baseline magnetic resonance imaging signal. Responses to 20 kHz are 2.0 ± 0.2% (0D), 1.4 ± 0.2% (7D), and 2.1 ± 0.2% (14D). For both tones, responses at 7D are less than those at 0D (p < 0.01) and 14D (p < 0.05). In the LL, similar trends are observed. Acute exposure leads to functional changes in the auditory midbrain with timescale of weeks.

A Hitchhiker's Guide to Functional Magnetic Resonance Imaging

Functional Magnetic Resonance Imaging (fMRI) studies have become increasingly popular both with clinicians and researchers as they are capable of providing unique insights into brain functions. However, multiple technical considerations (ranging from specifics of paradigm design to imaging artifacts, complex protocol definition, and multitude of processing and methods of analysis, as well as intrinsic methodological limitations) must be considered and addressed in order to optimize fMRI analysis and to arrive at the most accurate and grounded interpretation of the data. In practice, the researcher/clinician must choose, from many available options, the most suitable software tool for each stage of the fMRI analysis pipeline. Herein we provide a straightforward guide designed to address, for each of the major stages, the techniques, and tools involved in the process. We have developed this guide both to help those new to the technique to overcome the most critical difficulties in its use, as well as to serve as a resource for the neuroimaging community.