A Method to Study Adaptation to Left-Right Reversed Audition

An unusual sensory space is one of the effective tools to uncover the mechanism of adaptability of humans to a novel environment. Although most of the ...

Keio University

Tokyo Denki University

Brain activity was measured by magnetoencephalography (MEG) to investigate whether the early auditory system can detect changes in audio-visual patterns when the visual part is presented earlier. We hypothesized that a template underlying the mismatch field (MMF) phenomenon, which is usually formed by past sound regularities, is also used in visually based sound prediction. Activity similar to the MMF may be elicited by comparing an incoming sound with the template. The stimulus was modeled after a keyboard: an animation in which one of two keys was depressed was accompanied by either a lower or higher tone. Congruent audio-visual pairs were designed to be frequent and incongruent pairs to be infrequent. Subjects were instructed to predict an incoming sound based on key movement in two sets of trials (prediction condition), whereas they were instructed not to do so in the other two sets (non-prediction condition). For each condition, the movement took 50 ms in one set (Delta = 50 ms) and 300 ms in the other (Delta = 300 ms) to reach the bottom, at which time a tone was delivered. As a result, only under the prediction condition with Delta = 300 ms was additional activity for incongruent pairs observed bilaterally in the supratemporal area within 100-200 ms of the auditory stimulus onset; this activity had spatio-temporal properties similar to those of MMF. We concluded that a template is created by the visually based sound prediction only after the visual discriminative and sound prediction processes have already been performed.

Modulation of early auditory processing by visually based sound prediction.

Brain activity was measured by magnetoencephalography to investigate the spatiotemporal stage of visual processing at which predictive and sensory integration begins. We examined the consequences of a visual mismatch between preliminary prediction and incoming stimulus. Following auditory cues (1000- and 1250-Hz tones) for prediction, congruent and incongruent images, pictures of two musical keys, were presented to volunteers. When they predicted visual inputs on the basis of preceding auditory cues, we detected a mismatch signal for predictive-sensory incongruities in the striate and extrastriate areas for 100-200 ms after image presentation. As this signal reflects a compatibility analysis, we propose that the integration process begins in these areas approximately 100 ms after image presentation.

Predictive and sensory integration begins at an early stage of visual processing.

Unconscious monitoring of multimodal stimulus changes enables humans to effectively sense the external environment. Such automatic change detection is thought to be reflected in auditory and visual mismatch negativity (MMN) and mismatch negativity fields (MMFs). These are event-related potentials and magnetic fields, respectively, evoked by deviant stimuli within a sequence of standard stimuli, and both are typically studied during irrelevant visual tasks that cause the stimuli to be ignored. Due to the sensitivity of MMN/MMF to potential effects of explicit attention to vision, however, it is unclear whether multisensory co-occurring changes can purely facilitate early sensory change detection reciprocally across modalities. We adopted a tactile task involving the reading of Braille patterns as a neutral ignore condition, while measuring magnetoencephalographic responses to concurrent audiovisual stimuli that were infrequently deviated either in auditory, visual, or audiovisual dimensions; 1000-Hz standard tones were switched to 1050-Hz deviant tones and/or two-by-two standard check patterns displayed on both sides of visual fields were switched to deviant reversed patterns. The check patterns were set to be faint enough so that the reversals could be easily ignored even during Braille reading. While visual MMFs were virtually undetectable even for visual and audiovisual deviants, significant auditory MMFs were observed for auditory and audiovisual deviants, originating from bilateral supratemporal auditory areas. Notably, auditory MMFs were significantly enhanced for audiovisual deviants from about 100 ms post-stimulus, as compared with the summation responses for auditory and visual deviants or for each of the unisensory deviants recorded in separate sessions. Evidenced by high tactile task performance with unawareness of visual changes, we conclude that Braille reading can successfully suppress explicit attention and that simultaneous multisensory changes can implicitly strengthen automatic change detection from an early stage in a cross-sensory manner, at least in the vision to audition direction. 

Early auditory change detection implicitly facilitated by ignored concurrent visual change during a Braille reading task.

Sensory gating is a preattentional mechanism to filter irrelevant information from the environment. It is typically reflected as a suppression of the event-related P50 component for successive sounds in the auditory modality. Although stress-induced sympathetic activation has been reported to disrupt P50 suppression, little is known about the modulatory effect of parasympathetic activation on early auditory sensory gating. We determined the parasympathetic effect on the magnetic P50 (P50m) suppression by controlling the respiratory rhythm and recording data simultaneously with magnetoencephalography and electrocardiography, using three successive click sounds as stimulus and ten normal individuals as study participants. The respiratory rhythm was guided by visual cues and set at 0.3, 0.25, or 0.2 Hz for distinct auditory stimulus sequence blocks. Heart rate variability analysis showed that slow respiration leads to significantly large high-frequency power, which is known as the parasympathetic index, whereas low-frequency/high-frequency ratio, known as the sympathetic index, did not differ with the respiratory rhythm. Although P50m suppression was observed in the left and right primary auditory areas for every respiratory condition, the left P50m intensity for the first sound was significantly decreased in the case of slow respiration, thereby indicating disruption of the left P50m suppression. Since background alpha oscillatory power, reflecting the arousal level, was similar for every respiratory rhythm, it is concluded that parasympathetic activation enhanced by slow respiration modulates P50m gating by reducing the initial neural sensitivity for an auditory input. Not only sympathetic but also parasympathetic effects should be considered in the evaluation of P50/P50m biomarkers.

Atsushi Aoyama

Faculty of Environment and Information Studies,
Shonan Fujisawa Campus (SFC),
Faculty of Environment and Information Studies, Shonan Fujisawa Campus (SFC)

Keio University

A Method to Study Adaptation to Left-Right Reversed Audition

Atsushi Aoyama

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Keio University

A Method to Study Adaptation to Left-Right Reversed Audition

Faculty of Environment and Information Studies,
Shonan Fujisawa Campus (SFC),
Faculty of Environment and Information Studies, Shonan Fujisawa Campus (SFC)