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.

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.

Auditory intensity processing: Effect of MRI background noise.

Studies on active auditory intensity discrimination in humans showed equivocal results regarding the lateralization of processing. Whereas experiments with a moderate background found evidence for right lateralized processing of intensity, functional magnetic resonance imaging (fMRI) studies with background scanner noise suggest more left lateralized processing. With the present fMRI study, we compared the task dependent lateralization of intensity processing between a conventional continuous echo planar imaging (EPI) sequence with a loud background scanner noise and a fast low-angle shot (FLASH) sequence with a soft background scanner noise. To determine the lateralization of the processing, we employed the contralateral noise procedure. Linearly frequency modulated (FM) tones were presented monaurally with and without contralateral noise. During both the EPI and the FLASH measurement, the left auditory cortex was more strongly involved than the right auditory cortex while participants categorized the intensity of FM tones. This was shown by a strong effect of the additional contralateral noise on the activity in the left auditory cortex. This means a massive reduction in background scanner noise still leads to a significant left lateralized effect. This suggests that the reversed lateralization in fMRI studies with loud background noise in contrast to studies with softer background cannot be fully explained by the MRI background noise.

Auditory intensity processing: Categorization versus comparison.

Intensity is an important parameter for the perception of complex auditory stimuli like speech. The results of previous studies on the processing of intensity are diverse since left-lateralized, right-lateralized and non-lateralized processing was suggested. A clear dependence of the lateralization on the kind of stimuli and/or task is not apparent. With the present functional magnetic resonance imaging (fMRI) study, we directly investigated the differences between a categorical and comparative task. To determine hemispheric involvement we used a method with contralateral noise presentation. Harmonic complexes were presented monaurally without and with contralateral noise. Both categorization and comparison of harmonic complexes according to their intensity more strongly involved the left than the right auditory cortex shown by a stronger effect of the additional noise on the activity in the left auditory cortex. Together with previous results, this suggests that left-lateralized processing of intensity in the auditory cortex can be observed independent of task and stimuli. The comparison task more strongly engaged the left auditory cortex than the categorization task probably due the additional need for sequential comparison and the right auditory cortex probably due to capacity reasons. Comparison also more strongly engaged areas associated with attentional processes and areas responsible for motor response selection. We suggest this to be caused by a more difficult response selection and by the need for continuous update of information in reference memory during the comparison task.

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

Intensity and duration are important parameters for the processing of speech and music. Neuroimaging results on the processing of these parameters in tasks involving the discrimination of stimuli based on these parameters are controversial. Depending on the experimental approach, varying hypotheses on the involvement of the left and right auditory cortices (ACs) have been put forward. The aim of the present functional magnetic resonance imaging (fMRI) study was to find differences and commonalities in location and strength of brain activity during the processing of intensity and duration when the same stimuli have to be actively categorized according to these two parameters. For this we used a recently introduced method to determine lateralized processing in the AC with contralateral noise. Harmonic frequency modulated (FM) tone complexes were presented monaurally without and with contralateral noise. During categorization of the tones according to their intensity, contralateral noise increased activity mainly in the left AC, suggesting a special role for the left AC in this task. During categorization of tones according to their duration, contralateral noise increased activity in both the left and the right AC. This suggests that active categorization of FM tones according to their duration does not involve only the left AC as has been suggested, but also the right AC to a substantial degree. The area around Heschl's sulcus seems to be the most strongly involved during both intensity and duration categorization, albeit with different lateralization. Altogether the results of the present study support the view that the lateralized processing of the same stimuli in the human AC is strongly modulated by the given task (top-down effect).

Interaction between bottom-up and top-down effects during the processing of pitch intervals in sequences of spoken and sung syllables.

The processing of pitch intervals may be differentially influenced when musical or speech stimuli carry the pitch information. Most insights into the neural basis of pitch interval processing come from studies on music perception. However, music, in contrast to speech, contains a stable set of pitch intervals. To converge the investigation of pitch interval processing in music and speech, we used sequences of the same spoken or sung syllables. The pitch of these syllables varied either by semitone steps like in music or by smaller intervals. Participants had to differentiate the sequences according to their different sizes of pitch intervals or to the direction of the last frequency step in the sequence. The results depended strongly on the specific task demands. Whereas the interval-size task itself recruited more regions in right lateralized fronto-parietal brain network, stronger activity on semitone than on non-semitone sequences was found in the left hemisphere (mainly in frontal cortex) during this task. These effects were also influenced by the speech mode (spoken or sung syllables). Our findings suggest that the processing of pitch intervals in sequences of syllables depends on an interaction between bottom-up (speech mode, pitch interval) and top-down effects (task).