Ipsilateral Auditory Pathways Research Articles

Ipsilateral auditory cortex responses to the intact ear after unilateral noise trauma in juvenile rats

BackgroundWhile ear stimulation produces a robust response in the contralateral auditory cortex (AC), it produces only a weak response in the ipsilateral AC, known as interhemispheric asymmetry. Unilateral deafness can lead to AC plastic changes, resulting in reduced interhemispheric asymmetry and auditory perceptual consequences. However, the unilateral hearing loss-associated plastic changes are far from fully understood. The purpose of this study was to investigate AC responses to the ipsilateral unimpaired ear after noise injury to the contralateral ear in juvenile rats. MethodsRats (50 days) were monaurally exposed to an intense noise (10.0-12.5 kHz, 126 dB SPL) for 2 hours. The unexposed ear-induced ipsilateral AC responses were recorded 2 days and 4 months after exposure and compared between groups. ResultsThe noise exposure resulted in complete hearing loss in the exposed ear, but normal function in the other. Two days after exposure, the ipsilateral AC response induced by the intact ear was significantly enhanced and the threshold decreased (the early-onset effect). Four months after noise exposure, in addition to the increased response amplitude, the “slow-increasing” firing pattern of the neurons in the ipsilateral AC turned into the contralateral-AC-response-like “sharp-increasing” pattern (the late-onset effect) with shortened response latency. DiscussionThe early-onset effect can result from release of inhibition due to decreased contralateral input, while the late-onset effect may imply the formation of direct connections in the ipsilateral auditory pathway. The enhanced AC response may help maintain loudness perception and monaural sound localization after unilateral deafness.

The relevance of task-irrelevant sounds: hemispheric lateralization and interactions with task-relevant streams

The effect of unattended task-irrelevant auditory stimuli in the context of an auditory task is not well understood. Using human functional magnetic resonance imaging (fMRI) we compared blood oxygenation level dependent (BOLD) signal changes resulting from monotic task-irrelevant stimulation, monotic task-relevant stimulation and dichotic stimulation with an attended task-relevant stream to one ear and an unattended task-irrelevant stream to the other ear simultaneously. We found strong bilateral BOLD signal changes in the auditory cortex (AC) resulting from monotic stimulation in a passive listening condition. Consistent with previous work, these responses were largest on the side contralateral to stimulation. AC responses to the unattended (task-irrelevant) sounds were preferentially contralateral and strongest for the most difficult condition. Stronger bilateral AC responses occurred during monotic passive-listening than to an unattended stream presented in a dichotic condition, with attention focused on one ear. Additionally, the visual cortex showed negative responses compared to the baseline in all stimulus conditions including passive listening. Our results suggest that during dichotic listening, with attention focused on one ear, (1) the contralateral and the ipsilateral auditory pathways are suppressively interacting; and (2) cross-modal inhibition occurs during purely acoustic stimulation. These findings support the existence of response suppressions within and between modalities in the presence of competing interfering stimuli.

Functional activity mapping of rat auditory pathway after intratympanic manganese administration

In the present study, we report a new method of manganese enhanced magnetic resonance imaging (MEMRI) using intratympanic (IT) manganese administration. We explore Mn²⁺ uptake from the middle ear cavity into the cochlea through mechanically gated ion channels of the hair cell and also functional auditory tract tracing without the use of excessive auditory stimuli for a long time period outside the scanner. After manganese administration in animals with normal hearing and unilateral deafness, T1-weighted MR images were obtained for up to 48 h with a 3.0 T MR imager. In normal rats, the mean signal-to-noise ratio (SNR) at each region of interest on the auditory pathway was significantly higher in the IT injection group than in the intraperitoneal (IP) injection group (P<0.05). Furthermore, the cochlea showed Mn²⁺ signal enhancement only in the IT injection group. In unilateral deafness rats, the IT injection of Mn²⁺ into the deaf-side middle ear cavity demonstrated signal enhancement in the cochlea but not in other auditory structures without axonal transport of Mn²⁺ along the auditory pathway. On the other hand, the IT injection of Mn²⁺ into the normal-side middle ear cavity demonstrated that the mean SNRs at the cochlea, cochlear nucleus, superior olivary complex, lateral lemniscus and inferior colliculus were significantly higher in the ipsilateral auditory pathway than in the contralateral pathway (P<0.05). For the IP injection group, the mean SNRs at each auditory structure, except the cochlea, increased bilaterally. In conclusion, the present work demonstrated the potential advantages of a new IT MEMRI over conventional systemic injection strategies in that (i) the functional auditory tract tracing initiated by the hair cell function is possible and (ii) the axonal transport of Mn²⁺ ions by trans-synaptic activity is possible without auditory stimulation for a long time period outside MR scanner.

Contralateral White Noise Selectively Changes Left Human Auditory Cortex Activity in a Lexical Decision Task

In a previous study, we hypothesized that the approach of presenting information-bearing stimuli to one ear and noise to the other ear may be a general strategy to determine hemispheric specialization in auditory cortex (AC). In that study, we confirmed the dominant role of the right AC in directional categorization of frequency modulations by showing that fMRI activation of right but not left AC was sharply emphasized when masking noise was presented to the contralateral ear. Here, we tested this hypothesis using a lexical decision task supposed to be mainly processed in the left hemisphere. Subjects had to distinguish between pseudowords and natural words presented monaurally to the left or right ear either with or without white noise to the other ear. According to our hypothesis, we expected a strong effect of contralateral noise on fMRI activity in left AC. For the control conditions without noise, we found that activation in both auditory cortices was stronger on contralateral than on ipsilateral word stimulation consistent with a more influential contralateral than ipsilateral auditory pathway. Additional presentation of contralateral noise did not significantly change activation in right AC, whereas it led to a significant increase of activation in left AC compared with the condition without noise. This is consistent with a left hemispheric specialization for lexical decisions. Thus our results support the hypothesis that activation by ipsilateral information-bearing stimuli is upregulated mainly in the hemisphere specialized for a given task when noise is presented to the more influential contralateral ear.

Asymmetrical enhancement of middle-latency auditory evoked fields with aging

We studied the effects of aging on middle-latency auditory evoked fields (P50m), and analyzed their interhemispheric differences. Magnetic responses following tone-burst stimuli to the right ear were measured in groups of 11 younger and 15 elderly subjects. The elderly subjects showed marked asymmetry in the P50m amplitudes. In the elderly group, the mean amplitude of the contralateral P50m was significantly larger ( P<0.0005) than that of the ipsilateral P50m, while no asymmetry was shown in the younger group. The amplitude enlargement in the contralateral P50m showed significant correlation with age ( R=0.60, P<0.005), while the ipsilateral P50m showed no correlation with age. These results suggest that the contralateral and ipsilateral auditory pathways are affected differently by ageing.

Efferent Gating of Human Auditory Attentional Processes

Brainstem evoked potentials were measured while subjects reduced either frontalis or lips/throat activity in response to visual information regarding EMG activity while attending to dichotically presented click stimuli. An inhibition of left ear activity occurred at the level of the cochlear nucleus (Wave 1) during articulatory muscle activity, suggesting that the right ear advantage is related to active inhibition of ipsilateral auditory pathways. Contralateral central inhibition of disattended information at the level of the brainstem (Wave 5) was also implicated. A neurophysiological model accounting for both attenuation of irrelevant stimuli and vigilance for high priority information is proposed.

Correlation between a unilateral midbrain-pontine lesion and abnormalities of brain-stem auditory evoked potential

Brain-stem auditory evoked potentials (BAEPs) were evaluated over a 39 day period in a patient with a unilateral pontine-midbrain lesion verified by CAT scan and autopsy. Waves I, II and III were present on the side of lesion, whereas all 5 waves were present on the side opposite the lesion. The findings suggest that the BAEPs may be obtained with only an intact ipsilateral auditory pathway. Crossing fibers in the trapezoid body also appear to make contributions to the normal generation of wave V.

Ipsilateral auditory extinction following frontal and basal ganglia lesions of the left hemisphere

When tested on a dichotic digits listening task, patients with unilateral lesions of the left frontal lobe and left anterior basal ganglia exhibited ipsilateral auditory extinction. This phenomenon has been documented before, but only in patients with lesions involving the region posterior and lateral to the posterior aspect of the left lateral ventricle. As patients with lesions in the equivalent region of the right hemisphere also exhibit extinction of verbal material arriving in their left ears it has been hypothesized that these posterior lesions disrupt an interhemispheric pathway connecting the two temporal lobes. As it appears that the ipsilateral auditory pathways are suppressed under conditions of dichotic stimulation, such an interhemispheric pathway would be the means whereby left-ear verbal stimuli reached the left (speech) hemisphere. Therefore a disruption of this pathway could result in left-ear extinction for verbal material. The finding that left anterior lesions also result in left-ear extinction poses problems for this hypothesis and in particular for the suggested posterior route of the interhimispheric pathway.

Ipsilateral auditory pathway suppression in dichotic CV speech perception: evidence from error analysis.

The exact nature of ipsilateral auditory pathway suppression under dichotic stimulation remains controversial. To help resolve this controversy, the distinctive feature confusions (blend vs nonblend errors) made on a dichotic listening CV test by two hemispherectomees were analyzed. Results support ipsilateral auditory pathway suppression under dichotic CV stimulation. Ipsilateral auditory pathway suppression is discussed in terms of stimulus specificity, subject, and task difference.

Dichotic listening in split brain subjects: Ipsilateral suppression reconsidered

One current view of the mechanism underlying the ear advantage in dichotic listening has two components. The first is the relative superiority of contraolateral over ipsilateral auditory pathways, the second, the supression of ipsilateral information. While the notion of superiority simply implies an advantage for contralateral information, supression suggests that the information is unavailable. Three commissurotomy patients were given dichotic tests using consonant—vowel syllables and digits, as well as a binaural fusion test, in order to evaluate their ability to use information presented to the ear ipsilateral to the language hemisphere. Consonant—vowel syllables produced complete or near complete extinction of left ear information, while the digits test revealed a substantial increase in the use of left ear information. The fusion task produced performance greater than what would be expected by probability summation of the performance of each ear alone. The data suggest that in the absence of the corpus callosum, information from contralateral and ipsilateral ears can be integrated. Furthermore, the inability of the subjects to use information presented to the ipsilateral ear appears to be a function of the degree to which the competing items are acoustically similar, rather than a general suptession mechanism.

The effect of dichotic, monaural and diotic verbal stimuli on auditory evoked potentials

To clarify the nature of the right ear preference in dichotic listening to speech stimuli, comparisons were made between evoked responses from the two hemispheres of normals when verbal input (dichotic, monaural, diotic) varied and when response accuracy varied. The positive component of the evoked response was significantly larger in the right hemisphere in all conditions. No significant asymmetry was seen in the negative component. Negative and positive components did not vary with response accuracy. However, the negative component was significantly larger in dichotic tasks than in all others. This difference may be related to complexity of dichotic input or to inhibition of the ipsilateral auditory pathway during dichotic stimulation.

The central auditory pathway.

The central auditory pathway.