Abstract
The encoding of auditory spatial acuity (measured as the precision to distinguish between two spatially distinct stimuli) by neural circuits in both auditory cortices is a matter of ongoing research. Here, the event-related potential (ERP) mismatch negativity (MMN), a sensitive indicator of preattentive auditory change detection, was used to tap into the underlying mechanism of cortical representation of auditory spatial information. We characterized the MMN response affected by the degree of spatial deviance in lateral acoustic space using a passive oddball paradigm. Two stimulation conditions (SCs)—specifically focusing on the investigation of the mid- and far-lateral acoustic space—were considered: (1) 65° left standard position with deviant positions at 70, 75, and 80°; and (2) 95° left standard position with deviant positions at 90, 85, and 80°. Additionally, behavioral data on the minimum audible angle (MAA) were acquired for the respective standard positions (65, 95° left) to quantify spatial discrimination in separating distinct sound sources. The two measurements disclosed the linkage between the (preattentive) MMN response and the (attentive) behavioral threshold. At 65° spatial deviations as small as 5° reliably elicited MMNs. Thereby, the MMN amplitudes monotonously increased as a function of spatial deviation. At 95°, spatial deviations of 15° were necessary to elicit a valid MMN. The behavioral data, however, yielded no difference in mean MAA thresholds for position 65 and 95°. The different effects of laterality on MMN responses and MAA thresholds suggest a role of spatial selective attention mechanisms particularly relevant in active discrimination of neighboring sound sources, especially in the lateral acoustic space.
Highlights
The accurate localization of single sound sources in everyday complex acoustic environments is a difficult task
The present study aims at scrutinizing the resolution of lateral acoustic space as reflected by location-mismatch negativity (MMN), focussing on possible magnitude of deviation (MoD)-effects and laterality-effects in MMN amplitude and latency
The MMN component was identified as a negative deflection with a reversed polarity at mastoid sites within a time window of 200 ± 50 ms after stimulus onset as exemplary demonstrated in Figure 3 for the Fz electrode and 15◦-deviation in SC [two levels: mid-lateral position at 65◦ (SC65) and SC95
Summary
The accurate localization of single sound sources in everyday complex acoustic environments is a difficult task. Binaural cues are interaural time (ITD) and intensity differences (IID) caused by different sound arrival times and intensities at the two ears varying with spatial position of the sound sources (Middlebrooks and Green, 1991; Blauert, 1996). Location cues (ITDs and IIDs) for sound source processing are extracted in nuclei of the superior olivary complex which send converging information to the inferior colliculus in the midbrain (reviewed in Grothe et al, 2010). To date, it is unclear in which way location information is encoded in the auditory cortex and how spatial acuity finds a neuronal representation or in which way the evoked activity for two distinct sound sources differs. Further consideration suggest the involvement of four channels: two—a contralateral and ipsilateral—for each hemisphere (Stecker et al, 2005), whereas the balance in activation of these channels may differ between hemispheres (Krumbholz et al, 2005; discussed in Magezi and Krumbholz, 2010)
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