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Plasticity in sensory systems. Jan Schnupp on the monocycle. Activity and size of auditory cortex…. Schneider et al. Nat. Neurosci. 2003. …Are correlated…. …and correlated with musical abilities.
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Activity and size of auditory cortex… Schneider et al. Nat. Neurosci. 2003
Is musical practice increasing the size of auditory cortex, or do people with large auditory cortex become musicians?
Pressure ratio between softest and loudest sounds… • Frequency differences • Hair motion at absolute threshold… Human psychoacoustical performance
Perceptual learning • Partially non-specific • Playing tetris improves frequency discrimination • Partially due to passive exposure • But also to some extent requires active task performance
Animal models of auditory plasticity • Classical conditioning • Fear conditioning: associating a sound with a foot shock • Environmental enrichment and relatives • Manipulating the environment can have both beneficial and disruptive effects on the auditory system • Spatial hearing
Just noticeable differences in azimuthat the center, tone stimuli
Interaural Time Differences (ITDs) Interaural Level Differences (ILDs) Binaural Cues for Localising Sounds in Space amplitude time
ITD Interaural Time Difference (ITD) Cues ITDs are powerful cues to sound source direction, but they are ambiguous (“cones of confusion”)
ILD • ITD Binaural disparities in humans
Disambiguating the cone of confusion • Sounds on the median plane (azimuth 0, different elevations) have zero binaural disparities • This is a special case of the cone of confusion • Nevertheless, humans and other animals can determine the elevation of a sound source
Binaural Cues in the Barn Owl Barn owls have highly asymmetric outer ears, with one ear pointing up, the other down. Consequently, at high frequencies, barn owl ILDs vary with elevation, rather than with azimuth (D). Consequently ITD and ILD cues together form a grid specifying azimuth and elevation respectively.
Phase locking at highfrequencies in the barn owl C. Köppl, 1997
Processing of Interaural Time Differences To the Inferior Colliculus Contra- lateral side Sound on the ipsilateral side MSO neuron response Interaural time difference Medial superior olive
Preservation of Time Cues in AVCN • Auditory Nerve Fibers connect to spherical and globular bushy cells in the antero-ventral cochlear nucleus (AVCN) via large, fast and secure synapses known as “endbulbs of Held”. • Phase locking in bushy cells is even more precise than in the afferent nerve fibers. • Bushy cells project to the superior olivary complex. sphericalbushycell endbulbof Held VIII nervefiber
Left MSO The coincidence detection model of Jeffress (1948) is the widely accepted model for low-frequency sound localisation Right Ear Left Ear
Response 0 Interaural Time Difference Right CN Left MSO Left CN
Response 0 Interaural Time Difference Right CN MSO Left CN
1 ms 1 ms Interaural Phase Sensitivity in the MSO to 1000 Hz Yin and Chan (1988)
Processing of Interaural Level Differences To the Inferior Colliculus Sound on the ipsilateral side Contralateral side LSO neuron response I > C C > I Interaural intensity difference Lateral superior olive
The Calyx of Held • MNTB relay neurons receive their input via very large calyx of Held synapses. • These secure synapses would not be needed if the MNTB only fed into “ILD pathway” in the LSO. • MNTB also provides precisely timed inhibition to MSO.