Hi Jan,
This is a great question. Below are relevant studies from Ruth Litovsky's lab with Alan Kan and Matt Goupell (I am currently a postdoc in her lab). It turns out while mismatch in frequency "worsens" both ITD lateralization and ITD sensitivity in listeners with bilateral cochlear implants, it is quite remarkable how tolerant their lateralization responses and sensitivity are to those frequency mismatches. In my dissertation work I actually explored temporal mismatches by imposing binaural beats, using different pulse rates across the ears. I studied how this affects ITD lateralization and auditory object formation. In a nutshell: normally hearing adult listeners typically report hearing two sounds, while bilateral CI listeners are largely unable to dissociate a sound with large temporal mismatches (whereby different stimulation rates were presented across the ears) that have an imposed ITD, reporting the sound as being a single fused source--this isn't something we see with frequency mismatch in bilateral CI listeners. I am currently writing up the results but would be happy to share my ARO poster.
Best,
Tanvi Thakkar
Dear List,I am curious if you could recommend some reading for me. We have been increasingly interested in ITD coding with cochlear implants and have developed a nice little animal model which shows a surprisingly robust behavioral ITD sensitivity even if deafened in infancy and only implanted in young adulthood.One question we often get and which we would like to investigate is: how much does it matter if there is a bit of a mismatch between the frequency channels in the left and right ears? How badly do they have to be mismatched before ITD sensitivity disappears?I kind of assumed that there must have been a lot of psychoacoustics on this, at least in normally hearing human subjects. Of course at low frequencies, if you mismatch the left and right ears you get binaural beats, but what about envelope ITDs? You could deliver for example trains of short gabor clicks to each ear with a greater or lesser extent of carrier frequency mismatch, and see how the mismatch affects ITD thresholds. It seems like such an obvious thing to try, surely somebody must have done this or something similar? But a quick look on google scholar didn't yield very much. A modelling paper by Bonham and Lewis 1999 was the top hit. I haven't seen much in the way of data. Surely I must be missing something...? Any suggestions for relevant reading gratefully accepted.Best wishes,Jan---------------------------------------Prof Jan Schnupp
City University of Hong Kong
Dept. of Neuroscience