Neuroanatomical and perceptual deficits in auditory agnosia : a study of an auditory agnosia patient with inferior colliculus damage

Auditory agnosia is a rare disorder in which individuals lose the ability to understand sounds. In this thesis, I examine an auditory agnosia patient with brainstem damage, but intact cortex. The patient was severely impaired when instructed to type the names of sounds. The patient, however, was only mildly impaired when instructed to choose the correct sound out of four written alternatives, which implies partial auditory perception. In two fMRI scans, conducted a year apart, passive listening to sounds resulted with a unique activation pattern in her auditory cortices. In particular, her anterior primary and associative auditory fields were much less responsive to sounds than more posterior primary and associative auditory fields. The functional dissociation between these regions suggests connections between the anterior primary and associative regions, and between the posterior primary and associative regions. Hitherto, these connections were only reported in monkeys. An EEG study that examined mismatch negativity for frequency, duration, and intensity of sounds, demonstrated that the patient’s ability of detecting changes to frequency and duration of sounds is bilaterally impaired, whereas the detection of changes to sound’s intensity is impaired in the left hemisphere but intact in the right hemisphere. Behavioral studies also show that the patient’s auditory perceptual deficit is partially due to impaired perception of the duration of sounds. For instance, when the patient heard two subsequent clicks, she was impaired at discriminating these sounds by the duration of their intervening interval. In a spoken word discrimination task, she was also impaired at discriminating words that could only be distinguished by their temporal properties (voice onset-time). Based on these findings, I argue that the patient experiences auditory agnosia be- cause the brain stem injury prevents the transmission of critical auditory information to the auditory cortex. As a result of this absence, the auditory fields responsible for sound recognition, the anterior auditory fields, are not recruited. In a dichotic listening task, the patient extinguished sounds presented to the right ear, and in a sound localization task she perceived sounds as emerging from the left auditory hemi-field. Given cumulative evidence that associates the posterior auditory cortex with sound localization and phonological-acoustic analysis of verbal material from the contra-lateral hemi-field, the patient’s performance in these tasks suggest that her spared auditory abilities is due to processing in her right posterior auditory cortex. This role of the patient’s right posterior auditory cortex is consistent with both the fMRI study, in which the right posterior auditory cortex was consistently responsive to sounds, and the EEG study, in which detection of changes to sound intensity was restricted to the right hemisphere

Neuromodulation of Right Auditory Cortex Selectively Increases Activation in Speech-Related Brain Areas in Brainstem Auditory Agnosia

Auditory agnosia is an inability to make sense of sound that cannot be explained by deficits in low-level hearing. In view of recent promising results in the area of neurorehabilitation of language disorders after stroke, we examined the effect of transcranial direct current stimulation (tDCS) in a young woman with general auditory agnosia caused by traumatic injury to the left inferior colliculus. Specifically, we studied activations to sound embedded in a block design using functional magnetic resonance imaging before and after application of anodal tDCS to the right auditory cortex. Before tDCS, auditory discrimination deficits were associated with abnormally reduced activations of the auditory cortex and bilateral unresponsiveness of the anterior superior temporal sulci and gyri. This session replicated a previous functional scan with the same paradigm a year before the current experiment. We then applied anodal tDCS over right auditory cortex for 20 min-utes and immediately re-scanned the patient. We found increased activation of bilateral auditory cortices and, for speech sounds, selectively increased activation in Broca’s and Wernicke’s areas. Future research might consider the long-term behavioral effects after neurostimulation in auditory agnosia and its potential use in the neurorehabilitation of more general auditory disorders

From where to what: a neuroanatomically based evolutionary model of the emergence of speech in humans [version 3; referees: 1 approved, 2 approved with reservations]

In the brain of primates, the auditory cortex connects with the frontal lobe via the temporal pole (auditory ventral stream; AVS) and via the inferior parietal lobe (auditory dorsal stream; ADS). The AVS is responsible for sound recognition, and the ADS for sound-localization, voice detection and integration of calls with faces. I propose that the primary role of the ADS in non-human primates is the detection and response to contact calls. These calls are exchanged between tribe members (e.g., mother-offspring) and are used for monitoring location. Detection of contact calls occurs by the ADS identifying a voice, localizing it, and verifying that the corresponding face is out of sight. Once a contact call is detected, the primate produces a contact call in return via descending connections from the frontal lobe to a network of limbic and brainstem regions. Because the ADS of present day humans also performs speech production, I further propose an evolutionary course for the transition from contact call exchange to an early form of speech. In accordance with this model, structural changes to the ADS endowed early members of the genus Homo with partial vocal control. This development was beneficial as it enabled offspring to modify their contact calls with intonations for signaling high or low levels of distress to their mother. Eventually, individuals were capable of participating in yes-no question-answer conversations. In these conversations the offspring emitted a low-level distress call for inquiring about the safety of objects (e.g., food), and his/her mother responded with a high- or low-level distress call to signal approval or disapproval of the interaction. Gradually, the ADS and its connections with brainstem motor regions became more robust and vocal control became more volitional. Speech emerged once vocal control was sufficient for inventing novel calls

Neuroanatomical and perceptual deficits in auditory agnosia : a study of an auditory agnosia patient with inferior colliculus damage

Auditory agnosia is a rare disorder in which individuals lose the ability to understand sounds. In this thesis, I examine an auditory agnosia patient with brainstem damage, but intact cortex. The patient was severely impaired when instructed to type the names of sounds. The patient, however, was only mildly impaired when instructed to choose the correct sound out of four written alternatives, which implies partial auditory perception. In two fMRI scans, conducted a year apart, passive listening to sounds resulted with a unique activation pattern in her auditory cortices. In particular, her anterior primary and associative auditory fields were much less responsive to sounds than more posterior primary and associative auditory fields. The functional dissociation between these regions suggests connections between the anterior primary and associative regions, and between the posterior primary and associative regions. Hitherto, these connections were only reported in monkeys. An EEG study that examined mismatch negativity for frequency, duration, and intensity of sounds, demonstrated that the patient’s ability of detecting changes to frequency and duration of sounds is bilaterally impaired, whereas the detection of changes to sound’s intensity is impaired in the left hemisphere but intact in the right hemisphere. Behavioral studies also show that the patient’s auditory perceptual deficit is partially due to impaired perception of the duration of sounds. For instance, when the patient heard two subsequent clicks, she was impaired at discriminating these sounds by the duration of their intervening interval. In a spoken word discrimination task, she was also impaired at discriminating words that could only be distinguished by their temporal properties (voice onset-time). Based on these findings, I argue that the patient experiences auditory agnosia be- cause the brain stem injury prevents the transmission of critical auditory information to the auditory cortex. As a result of this absence, the auditory fields responsible for sound recognition, the anterior auditory fields, are not recruited. In a dichotic listening task, the patient extinguished sounds presented to the right ear, and in a sound localization task she perceived sounds as emerging from the left auditory hemi-field. Given cumulative evidence that associates the posterior auditory cortex with sound localization and phonological-acoustic analysis of verbal material from the contra-lateral hemi-field, the patient’s performance in these tasks suggest that her spared auditory abilities is due to processing in her right posterior auditory cortex. This role of the patient’s right posterior auditory cortex is consistent with both the fMRI study, in which the right posterior auditory cortex was consistently responsive to sounds, and the EEG study, in which detection of changes to sound intensity was restricted to the right hemisphere

Additive effects of mild head trauma, blast exposure, and aging within white matter tracts: A novel DTI analysis approach.

Veterans of recent military conflicts have experienced a high rate of mild traumatic brain injuries from blast exposure (BE). Difficulty detecting the neuroanatomical effects of BE using standard imaging protocols, including diffusion tensor imaging (DTI), has hindered the development of evi-dence-based treatments. A possible reason for this challenge is that many past DTI studies attempt-ing to identify neuroanatomical markers of BE have ignored the broad range of cumulative BE among Veterans, and therefore potentially reduced sensitivity to associations between BE and DTI metrics. Here, we compare commonly used DTI metrics: fractional anisotropy and mean, axial, and radial diffusivity (FA, MD, AD, RD) in U.S. Military Veterans with and without a history of BE using both the traditional method of dividing participants into two equally weighted groups, and an alternative method, wherein each participant is weighted by their BE quantity, severity, and recen-cy. While no differences in FA, MD, RD and AD were detected using the traditional method, the alternative method revealed diffuse and extensive changes in FA and RD (and minimal in MD and AD) associated with BE. These effects were quantified within 80 anatomically-defined white mat-ter tracts as the percentage of voxels with significant changes, which identified the acoustic and op-tic radiations, fornix, uncinate fasciculus, inferior occipito-frontal fasciculus, cingulum, and the an-terior commissure as the pathways most affected by BE. Moreover, additive effects of aging were present in many of the same tracts suggesting that the neuroanatomical effects of BE may com-pound with age

Additive effects of aging and blast induced mild traumatic brain injury within white matter tracts: A novel DTI analysis approach.

Veterans of recent military conflicts have experienced a high rate of mild traumatic brain injuries from exposure to blasts (bTBI). Difficulty detecting the neuroanatomical effects of bTBI using standard imaging protocols, including diffusion tensor imaging (DTI), has hindered the development of evidence-based treatments. A possible reason for this challenge is that many past DTI studies attempting to identify neuroanatomical markers of bTBI have ignored the broad range of cumulative blast exposure among Veterans, and therefore potentially reduced sensitivity to associations between bTBI and DTI metrics. Here, we compare commonly used DTI metrics: fractional anisotropy and mean, axial, and radial diffusivity (FA, MD, AD, RD) in U.S. Military Veterans with and without a history of blast exposure using both the traditional method of dividing participants into two equally weighted groups, and an alternative method, wherein each participant is weighted by their blast exposure quantity, severity, and recency. While no differences in FA, MD, and AD (and minimal in RD) were detected using the traditional method, the alternative method revealed diffuse and extensive changes in all four DTI metrics associated with bTBI. These effects were quantified within 80 anatomically-defined white matter tracts as the percentage of voxels with significant changes, which identified the acoustic and optic radiations, fornix, uncinate fasciculus, inferior occipito-frontal fasciculus, cingulum, and the anterior commissure as the pathways most affected by bTBI. Moreover, additive effects of aging were present in many of the same tracts suggesting that the neuroanatomical effects of bTBI may compound with age

Functional mapping of the human auditory cortex

Objective: To use functional magnetic resonance imaging to map the auditory cortical fields that are activated, or nonreactive, to sounds in patient M.L., who has auditory agnosia caused by trauma to the inferior colliculi. Background: The patient cannot recognize speech or environmental sounds. Her discrimination is greatly facilitated by context and visibility of the speaker’s facial movements, and under forced-choice testing. Her auditory temporal resolution is severely compromised. Her discrimination is more impaired for words differing in voice onset time than place of articulation. Words presented to her right ear are extinguished with dichotic presentation; auditory stimuli in the right hemifield are mislocalized to the left. Methods: We used functional magnetic resonance imaging to examine cortical activations to different categories of meaningful sounds embedded in a block design. Results: Sounds activated the caudal sub-area of M.L.’s primary auditory cortex (hA1) bilaterally and her right posterior superior temporal gyrus (auditory dorsal stream), but not the rostral sub-area (hR) of her primary auditory cortex or the anterior superior temporal gyrus in either hemisphere (auditory ventral stream). Conclusions: Auditory agnosia reflects dysfunction of the auditory ventral stream. The ventral and dorsal auditory streams are already segregated as early as the primary auditory cortex, with the ventral stream projecting from hR and the dorsal stream from hA1. M.L.’s leftward localization bias, preserved audiovisual integration, and phoneme perception are explained by preserved processing in her right auditory dorsal stream