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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 Is musical practice increasing the size of auditory cortex, or do people with large auditory cortex become musicians? What do we learn when we learn a new skill? Nat. Neurosci. 2006 Human psychoacoustical performance Frequency differences Pressure ratio between softest and loudest sounds… Hair motion at absolute threshold… Learning protocol 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 Nat. Rev. Neurosci. 2004 Fear conditioning… …changes cortical neurons Brain Research 2007 Environmental enrichment… Environmental enrichment… Environmental enrichment… Plasticity in auditory enriched environments Auditory plasticity requires stimuli but not interaction Just noticeable differences in azimuth at the center, tone stimuli Binaural Cues for Localising Sounds in Space time Interaural Time Differences (ITDs) Interaural Level Differences (ILDs) Interaural Time Difference (ITD) Cues ITD ITDs are powerful cues to sound source direction, but they are ambiguous (“cones of confusion”) Binaural disparities in humans ITD ILD 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 Spectral information about space The barn owl… 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 high frequencies in the barn owl C. Köppl, 1997 Processing of Interaural Time Differences To the Inferior Colliculus Sound on the ipsilateral side Contralateral side Medial superior olive Interaural time difference Preservation of Time Cues in AVCN spherical bushy cell endbulb of Held VIII nerve fiber • Auditory Nerve Fibers connect to spherical and globular bushy cells in the anteroventral 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. The coincidence detection model of Jeffress (1948) is the widely accepted model for low-frequency sound localisation Response 0 Interaural Time Difference Response 0 Interaural Time Difference 0 s Time Delay 0 s Cochlear Nucleus Left Ear Cochlear Nucleus Right Ear Se mi cir cul ar Can als Win dow MSO Auditory Nerve Activity Large calyx synaptic ending 0 s Time Delay Arrives at left ear 300 s later than at the right 300 s Cochlear Nucleus Left Ear Cochlear Nucleus Right Ear Se mi cir cul ar Can als Win dow MSO Auditory Nerve Activity Large calyx synaptic ending 300 s Time Delay Coincident spikes Arrives at left ear 300 s later than at the right 0 s Time Delay 300 s 0 s Cochlear Nucleus Left Ear Cochlear Nucleus Right Ear Se mi cir cul ar Can als Win dow MSO Auditory Nerve Activity Large calyx synaptic ending 300 s Time Delay Coincident spikes 0 s Time Delay Interaural Phase Sensitivity in the MSO to 1000 Hz 1 ms 1 ms Yin and Chan (1988) Processing of Interaural Level Differences To the Inferior Colliculus Sound on the ipsilateral side Lateral superior olive I>C Contralateral side C>I Interaural intensity difference 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. Ipsilateral Contralateral 100 20 20 Sound level (dB SPL) 100 0.125 32 0.125 32 Frequency (kHz) Caird and Klinke 1983 The Superior Olivary Nuclei – a Summary • Most neurons in the MSO respond best to sounds that occur earlier in the contralateral ear. • Most neurons in the LSO respond best to sounds that are louder in the ipsilateral ear. • Space representation is crossed, and therefore LSO projects mostly contralaterally and MSO ipsilaterally. Excitatory Connection Inhibitory Connection Midline IC LSO IC MNTB MSO CN CN Spatial hearing is plastic Plasticity in adults Nat. Neurosci. 1998 New ears… Sound localization by humans -30 0 30 Sound localization by humans Effect of modifying the ear Learning the new ears Knowing both ears Plasticity of the space map Knudsen, Nature 2002 Orientation responses to auditory and visual stimuli are congruent… Auditory orientation response Visual orientation response Prisms that shift the visual scene Auditory responses adapt to the visual shift The brain of the barn owl The ICC, ICX and the Superior Collicullus (Optic Tectum) Point-to-point correspondence between ICX and OT Neural correlate of the shift of auditory responses Shift in ITD sensitivity occurs first in ICX Axonal sprouting cause shift of ITD sensitivity in ICX Axonal sprouting cause shift of ITD sensitivity in ICX Time course of ITD shift Cellular mechanisms of ITD shift Anatomy of the instructive signal Visual activity in ICX uncovered by removing inhibition in OT Cellular mechanisms of ITD shift NMDA receptors are present at the transition stage… …but not when the shift is complete Cellular mechanisms of ITD shift GABA participates in the suppression of the normal responses Bicuculline Control Plasticity and age Old animals cannot change A sensitive period… During the sensitive period, plasticity potential is very large The normal map is robust and can be recovered at any age Recovery of the normal map requires rich environment Adult plasticity is possible after juvenile experience Adult plasticity is possible after juvenile experience Time course of adult adjustment