Reading assignmentEffect vision on auditory processing in the barn owl. Knudsen (2002)

Brain stem nucleiCochlear nucleus

•Nucleus angularis•Nucleus magnocellularis

Nucleus laminaris

Hair cell

Cochlear nerve(8th cranial nerve)

Nucleus angularisprocesses ILD (elevation)

Nucleus MagnocellularisProcess ITD (azimuth)

Nucleus laminaris processes ITD

Brainstem circuits and processing:•NM and NA: Primary auditory processing in CNS•Have unique response characteristics:

•Disruption of NA disrupts owls ability to calculate elevation•Disruption of NM disrupts owls ability to calculate azimuth

Behavior pharmacology

Cochlear nucleus

Delay line slows transmission of early signal line to sync it with phase lagged signal

Encoding interaural timing differences (ITD) as a spatial code: Delay lines and coincidence detectors

Recall: behavior studies indicate that owls cue on ongoing disparity (linear phase lag)

Different delays can be spatially encoded with different delay lengths: phase miss matches= no output Physiological evidence: Simplified

Coincidence detectors respond best to simultaneous input

Lengthening axon delays signal coincidence will occur

As electrode moves down cells respond to changing delay preferences

NMaxons

Nucleus Magnocellularis neurons project bilaterally and asymetrically to Nucleus Laminaris

Key anatomical features:•Each cell ramifies NL on both sides•Each cell has multiple collaterals •Collaterals form discrete tufts of axon terminals •Collaterals from both sides “en-mesh” •Tufts fall in an orderly and linear fashion across NL•A perfect substrate for a series of delay lines

Two stained cells from left NM

Left (back of head)

Right (back of head)

Ipsilateral and cotralateral inputs from NM to both NL’s on right and left side

Delay line

NL output heads to lateral lemniscus then to inferior colliculus

Inferior colliculus is composed of :•Central nucleus of the inferior colliculus (ICC)

•Recieves input from nucleus laminaris•Input is topographically organized by frequency • ITD is represented within each frequency channel•Outputs to ICX in a delay specific manner (frequency information is spatially merged).

•External nucleus of the inferior colliculus (ICX) •Arranged so that different delays are spatially (topographically) organized•Also recieves ILD information topographically organized by elevation (from NA)

External nucleus of the Inferior colliculus (ICX):•Integrates the spatial code for time delay (ITD; azimuth) with interaural loudness differences (ILD; elevation) •Forms a spatial map of auditory signal position•Uses a classic “center+ surround-” (lateral inhibition) to contrast enhance signal.•Projects to optic tectum

Audio

visual

Optic tectum integrates auditory representation from ICX with visual information.

Optic prisms: behavioral experiments •Auditory response adjust (sync) to match optic experience

Auditory visual (biased)

Prisms removed

Pre prismsDay 1

Day 42

These experiments tell us that visual information guide auditory development•Only works in juviniles 60+ day old.

Optic tectum: integration site for auditory and visual information•If visual input is biased (with prisms) the spatial location of visual information shifts linearly with the amount of bias.•Auditory input from ICX initially does not.•@ 8 weeks both auditory and visual information is realigned in biased position.•This is reversible•Again, only works during the appropriate developmental stage of the owl (juveniles between 60-200 days old).

Optic prisms: physiological experiments

Disparity in audio visual sync lead to rewiring from ICC to ICX to bring audio system into sync with visual system

•Rewiring is permanent: once treated the animal can switch even as adults.•This means that there have been changes in the physical ramification of axon terminals & synapses from ICC into the ICX.