Auditory Spatial Perception
The brain regions that combine the acoustic cues from both the ears to estimate the spatial position in the soundscape is well known. However, the brain basis underlying auditory spatial motion is not well understood. In our neuroimaging study, an acoustic stimulus with virtual motion was employed to understand auditory spatial motion.
So in this project, I aimed to validate the percept induced by a virtual motion of auditory stimuli. I characterised the stimuli psychophysically in humans.
Virtual moving sound synthesis
This project synthesizes stereo audio signals that simulate moving sound sources for a specified angular speed and path at a given angular resolution.
It first extracts the HRIR from the Wisconsin HRTF database, computes the equalization required for diffuse field response, interpolates linearly in frequency domain for the positions requested, resamples to the output sampling rate required, convolves with input of user choice and applies equalization together with global scale factor and then concatenates the responses to stimulate movement in the virtual soundscape and finally saves the output as wav file.
Moving sound source setup
I built an apparatus capable of delivering static or moving sound stimuli in free field in a soundproof chamber. This used an electric motor with adjustable speed with an attached rotor arm to which a small speaker was fixed, to achieve sound-source rotatory movement in the azimuthal plane through the subject’s ear canal.
I have replicated in 3 participants the intra-auricular recording approach using amplitude-modulated noise stimulus i.e. I recorded from the ear canal when static sounds were delivered from azimuthal positions recorded in 10° intervals from zero (midline, front); I also recorded motion stimuli from the ear canal when a speaker moved around the head with an angular motion of 100°/s or 50°/s. Static recordings from adjacent positions were then concatenated to create stimuli virtually moving at speeds of 100°/s or 50°/s.
Here is the visual summary of the project
I tested each participant’s perception of these stimuli using criterion-free psychophysics: AXB paradigm, where X was always a moving stimulus and A and B were either a moving or a concatenated stimulus. Participants were asked to identify which of stimuli A or B was different from X.
The results confirmed that none of the three participants were able to distinguish concatenated from motion stimuli at 100°/s (performance was at chance level in each participant). However, at a 50°/s speed, two of three participants were able to discriminate concatenated stimuli from motion stimuli. I concluded that the percept of virtual motion is speed dependent - at high speeds (greater than 100°/s) virtual motion is indistinguishable from true motion while at lower speeds (lesser than 50°/s) it can be distinguished
Peer reviewed publication
Poirier C, Baumann S, Pradeep D, Joly O, Hunter D, Balezeau F, Sun L, Rees A, Petkov CI, Thiele A, Griffiths TD. Auditory motion-specific mechanisms in the primate brain. PLoS Biology 2017, 15(5), 1-24.