Is there a Â‘retinotopicÂ’ representation of echo locations in the calcarine cortex of the blind brain?
63.428, Wednesday, May 15, 8:30 am - 12:30 pm, Orchid Ballroom
Jennifer L. Milne1,2, Melvyn A. Goodale1,2,3, Lore Thaler4; 1The Brain and Mind Institute, The University of Western Ontario, London, Canada, 2Neuroscience Program, The University of Western Ontario, London, Canada, 3Department of Psychology, The University of Western Ontario, London, Canada, 4Department of Psychology, Durham University, Durham, United Kingdom
We have shown previously that activity in calcarine cortex of an early blind echolocation expert is greater for mouth-click echoes reflected from objects located in contralateral space. Here, we used fMRI to investigate in more detail whether or not the mapping of echoes in the calcarine cortex of an early blind echolocation expert resembles retinotopic mapping in the sighted brain. We also investigated the mapping of sound sources. An early blind expert echolocator listened to binaural recordings of echolocation and source sounds in the fMRI scanner. The recordings were made earlier as the echolocator made clicks in the presence of a sound reflecting surface (or simply listened to a sound emitting loudspeaker) located at azimuth angles from -60°, -20°, 0°, +20°, to +60°. In addition, at -20° and +20° azimuth, recordings were made separately at 0°, -20° and +20° elevation. For each location in echo and source conditions, the participant had pressed a response key with a separate finger. For echolocation, we were able to map -20° azimuth in medial right calcarine, +20° azimuth in medial left calcarine, and 0° azimuth towards the apex of the occipital pole. For source hearing, we were able to map only -60° azimuth in medial right calcarine, anterior to the -20° azimuth echo representation. We were unable to map elevation for either echoes or sources. We were able to confirm the validity of our analysis by mapping the motor representation of the fingers used to press the response keys. In their entirety, the data are consistent with the idea that there is a representation of azimuth of echo locations in calcarine cortex that resembles the representation of azimuth of visual locations in retinotopic coordinates. The data suggest that this mapping is more pronounced for echolocation than source hearing.