When and Where of Auditory Spatial Processing in Cortex: A Novel Approach Using Electrotomography

The modulation of brain activity as a function of auditory location was investigated using electro-encephalography in combination with standardized low-resolution brain electromagnetic tomography. Auditory stimuli were presented at various positions under anechoic conditions in free-field space, thus providing the complete set of natural spatial cues. Variation of electrical activity in cortical areas depending on sound location was analyzed by contrasts between sound locations at the time of the N1 and P2 responses of the auditory evoked potential. A clear-cut double dissociation with respect to the cortical locations and the points in time was found, indicating spatial processing (1) in the primary auditory cortex and posterodorsal auditory cortical pathway at the time of the N1, and (2) in the anteroventral pathway regions about 100 ms later at the time of the P2. Thus, it seems as if both auditory pathways are involved in spatial analysis but at different points in time. It is possible that the late processing in the anteroventral auditory network reflected the sharing of this region by analysis of object-feature information and spectral localization cues or even the integration of spatial and non-spatial sound features

Peak activations of brain regions for all sound locations, as revealed by sLORETA analysis.

<p>Activations at the time of the N1 and P2 components of the responses to the sound onset were contrasted with a 40-ms prestimulus period of silence. Colour coding shows <i>t</i>-values, with statistically significant activations (<i>p</i><0.05) at <i>t</i>≥4.1 for N1 and <i>t</i>≥3.6 for P2. Data from all subjects were projected onto a single anatomical image (T2 MNI-template “Colin 27” of sLORETA). Horizontal and coronal slices were positioned at MNI <i>Z</i> and <i>X</i> coordinates as given in the figure (A, anterior; L, left; P, posterior; R, right). Data are as given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0025146#pone-0025146-t002" target="_blank">Table 2</a>.</p

ROI-analysis of primary auditory cortex (BA 41) at the time of the N1 and P2.

<p>Data were collapsed for four adjacent loudspeaker positions, resulting in four data sets, each covering a range of 40 degrees (LE, left eccentric; LC, left central; RC, right central; RE, right eccentric; black arcs in the schematic view of the set-up). In the coronal and horizontal slices (MNI-template as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0025146#pone-0025146-g002" target="_blank">Fig. 2</a>), voxels of the ROI are marked in white. The plots show <i>t</i>-values as a function of sound location (error bars, standard errors across subjects), resulting from contrasts of activations with a 40-ms prestimulus period of silence for the whole ROI.</p

Processing of left, right, central, and eccentric sound locations (all <i>p</i><0.05).

<p>Processing of left, right, central, and eccentric sound locations (all <i>p</i><0.05).</p

Locations of peak <i>t</i>-values for N1 and P2 responses to all sound positions vs. silence as revealed by sLORETA (all <i>p</i><0.01).

<p>Locations of peak <i>t</i>-values for N1 and P2 responses to all sound positions vs. silence as revealed by sLORETA (all <i>p</i><0.01).</p

Allocentric or craniocentric representation of acoustic space: an electrotomography study using mismatch negativity.

The world around us appears stable in spite of our constantly moving head, eyes, and body. How this is achieved by our brain is hardly understood and even less so in the auditory domain. Using electroencephalography and the so-called mismatch negativity, we investigated whether auditory space is encoded in an allocentric (referenced to the environment) or craniocentric representation (referenced to the head). Fourteen subjects were presented with noise bursts from loudspeakers in an anechoic environment. Occasionally, subjects were cued to rotate their heads and a deviant sound burst occurred, that deviated from the preceding standard stimulus either in terms of an allocentric or craniocentric frame of reference. We observed a significant mismatch negativity, i.e., a more negative response to deviants with reference to standard stimuli from about 136 to 188 ms after stimulus onset in the craniocentric deviant condition only. Distributed source modeling with sLORETA revealed an involvement of lateral superior temporal gyrus and inferior parietal lobule in the underlying neural processes. These findings suggested a craniocentric, rather than allocentric, representation of auditory space at the level of the mismatch negativity

Auditory-evoked potentials.

<p>(<b>A</b>) Grand-average AEPs with N1 and P2 components at a left (C3), vertex (Cz), and right (C4) electrode position, plotted as a function of time relative to sound onset for left-eccentric (LE), left-central (LC), right-central (RC), and right-eccentric (RE) ranges of sound locations. Black horizontal bars indicate stimulus duration. (<b>B</b>) Topographies for the four ranges of sound locations (LE, LC, RE, RC) at the time of N1 and P2. (<b>C</b>) Difference topographies of N1 and P2, comparing right and left sound positions, and central and eccentric sound positions. Filled circles indicate electrodes with significant differences in amplitude values (significant <i>t</i>-values according to permutation tests, all <i>p</i><0.05).</p