Biology Department, LMU, Germany
Wednesday 28 Wednesday 2009, 16:00
Seminar Room B10 (Basement)
Alexandra House, 17 Queen Square, London, WC1N 3AR
Population coding of azimuthal space in mammals and birds
In both mammals and birds, the dominant cues for the localization of low-frequency sounds in the azimuthal plane are interaural time differences (ITDs), the differences in the arrival time of a sound at the two ears. We used a decoding approach to compare the representation of ITDs in populations of neurons in gerbils and barn owls. We recorded the responses of neurons in the auditory mibrain to sounds with different ITDs, and decoded the responses based on two
strategies: a labeled-line strategy in which the identity of each neuron is preserved, and a summed population strategy in which the responses of all neurons are pooled together. For the barn owl, labeled-line decoding yielded far more information than summed population decoding, but for the gerbil, the two strategies were equivalent. This result is consistent with recent studies suggesting that azimuthal space is represented differently in the brains of mammals and birds. In a second study, we examined how the representation of ITDs in the gerbil is transformed in successive processing stages. Specifically, we compared ITD sensitivity in the first two stages of processing, the MSO and the DNLL. We found that the coding of ITDs is greatly improved in the DNLL, where, on average, ITD tuning curves have a larger dynamic range and less variability than in the MSO, and responses can contain up to twice the mutual information of those in the MSO.