Auditory motion processing in early blind subjects

Abstract

Although the neural substrates of visual motion processing have been extensively researched for several decades (for a review, see Culham et al. 2001), little is known about auditory motion processing. The few neuroimaging studies investigating auditory motion show the involvement of inferior and posterior parietal lobules, the dorsal and ventral pre-motor cortex (Bremmer et al. 2001; Griffiths et al. 1998, 2000; Lewis et al. 2000) and occasionally the additional involvement of the planum temporale (Baumgart et al. 1999; Bremmer et al. 2001; Lewis et al. 2000; Warren et al. 2002). These studies were conducted only on sighted subjects. In blind people, further functional neuroimaging has shown the involvement of striate and extra-striate cortices during Braille reading (Buchel 1998; Burton et al. 2002a; Sadato et al. 1996, 1998) and during language tasks (Burton et al. 2002b, 2003; Roder et al. 2002). However, fewer studies have investigated simpler auditory tasks (Alho et al. 1993; Kujala et al. 1995; Leclerc et al. 2000; Weeks et al. 2000). The present study investigated whether auditory motion perception could induce an occipital activation in early blind subjects (but not in sighted controls), indicating the developmental cross-modal reorganisation of their occipital cortex. Six early blind subjects and six blindfolded sighted controls matched in gender and age and who gave their informed consent participated in the study. The fMRI data were acquired by a 2 Tesla Brucker Imager. The experimental protocol was divided into 40 blocks each lasting 24 s, which were distributed over two sessions. The repetition time was 4.8 s, which corresponded to five scans per block. Two active conditions (one condition per block) were recorded with a rest period inbetween. The first condition was a fixed stimuli condition and the second one a motion stimuli condition. Auditory stimuli were composed of pure tones, or complex sounds, either fixed at various positions around the subject (fixed stimuli condition), or animated by a transverse movement in the horizontal plane (motion stimuli condition). In both conditions the auditory stimuli were composed of 10% pure tones (one sine wave) and 90% complex sounds (six sine waves). The presence of two different kinds of sound (pure tones and complex sounds) within each block allowed auditory recognition by the subjects both in the fixed stimuli and in the motion stimuli conditions. After having heard an individual stimulus, subjects were requested to determine the nature (i.e., ‘‘is it a pure tone or a complex sound ?‘’) and detect any movement of the sound. If movement was detected, the subjects were asked to determine the direction of the movement (i.e. from right to left ear or the reverse). Their answer was given by pressing buttons held in each hand. When the stimulus was identified as a fixed pure tone or as a pure tone moving towards the right, subjects had to press the button in their right hand. When the stimulus was identified as a fixed complex sound or as a complex sound moving towards the left, subjects had to press the button in their left hand. In all other cases (i.e. pure tone moving towards the left or complex sound moving towards the right), subjects had to press both buttons simultaneously. All subjects underwent a training period to learn this system before taking part in the study. Within a block, the same stimulus was repeated until subjects gave an answer (with a maximum of three repetitions); thereafter the next stimulus was given. Behavioural results showed no significant difference in the percentage of correct responses between early C. Poirier AE O. Collignon AE A. De Volder (&) Neural Rehabilitation Engineering Laboratory, Universite Catholique de Louvain, Ave. Hippocrate, 54, UCL-54.46, 1200 Brussels, Belgium E-mail: devolder@gren.ucl.ac.be Tel.: +32-2-7645482 Fax: +32-2-7649422