A positron emission tomographic study of auditory localization in the congenitally blind

Abstract

We have used positron emission tomography (PET) to measure regional cerebral blood flow (rCBF) in sighted and congenitally blind subjects performing auditory localization tasks. During scanning, the spectral and binaural cues of localized sound were reproduced by a sound system and delivered via headphones. During tasks that required auditory localization both the sighted and blind subjects strongly activated posterior parietal areas. In addition, the blind subjects activated association areas in the right occipital cortex, the foci of which were similar to areas previously identified in visual location and motion detection experiments in sighted subjects. The blind subjects, therefore, demonstrated visual to auditory cross-modal plasticity with auditory localization activating occipital association areas originally intended for dorsal-stream visual processing. To determine the functional connectivity of pre-selected brain regions in primary and non-primary auditory and posterior parietal cortex in the two cohorts, we performed an inter-regional correlation analysis on the rCBF data set. During auditory localization in the blind subjects, rCBF activity in the right posterior parietal cortex was positively correlated with that in the right occipital region, whereas in sighted subjects correlations were generally negative. There were no significant positive occipital correlations in either cohort when reference regions in temporal or left parietal cortex were chosen. This indicates that in congenitally blind subjects the right occipital cortex participates in a functional network for auditory localization and that occipital activity is more likely to arise from connections with posterior parietal cortex.

Fig. 1.

Statistical parametric maps of increased regional cerebral blood flow in the blind subjects compared to rest. All areas shown are significant at Z > 4.2 after Bonferroni correction for multiple comparisons (see “Subtraction analysis” in Materials and Methods). A illustrates the within-group comparison of auditory localization and delayed-matching compared to rest, and B shows the comparison of auditory localization with movement versus rest.

Fig. 2.

Statistical parametric maps from the between-group contrasts showing regions of relatively increased activation in the blind and regions of enhanced activation in the sighted cohort.A shows relatively increased activation in the blind cohort compared to the sighted in the auditory localization and delayed matching compared to rest contrast. The rCBF increases in the sighted subjects compared to the blind in the same task are shown inB.

Fig. 3.

Inter-regional correlations for the right IPL region (Talairach coordinates, 42, −48, 32) in the delayed-matching auditory-localization condition for sighted (A) and blind (B) subjects. Correlations are displayed as statistical maps in standard stereotactic space (left side of the image corresponds to the left side of the brain). The values of the correlation coefficients are given by a gray scale, with positive correlations in white and negative correlations in black; significant positive correlations (p < 0.01) are shown inred, and significant negative correlations are shown inblue. Nine representative horizontal sections are shown, from left to right, at −16, −12, −8, −4, 0, 4, 8, 12, and 16 mm relative to the AC-PC line, covering the majority of the occipital lobe. C shows regions with significantly different correlation coefficients (p < 0.05) between the sighted and blind cohorts. Correlations significantly larger in sighted subjects are displayed in red, and those greater in blind subjects are displayed in yellow.