Owl Prey Localisation + Auditory Systems Flashcards
Describe the process of hearing
- Sound waves come in, cause tympanic membrane (ear drum) to move
- Movement transduced by 3 bones in mammals (single bone in birds)
- Transduced to cochlea
- Cochlea is filled with fluid that surrounds a basilar membrane that is innervated by neurons
- Sound waves enter through the round window, propagated through the vestibular duct
- Based on frequency of sound waves, there is an interference in the fluid that causes a distortion in the basilar membrane
- (High freq picked up near base of membrane, low freq higher)
- Distortion converted to neuronal signals
What is the function of the Organ of Corti
- Neurons that have on their tip sensory cilia
- These actually connect to part of the basilar membrane
- When the whole structure distorts the cilia physically move causing depolarisation
- They depolarise via MECHANOTRANSDUCTION (MET channels, mechanoelectrical transducer); a tiny proteinaceous thread couples channels in the cilia membrane, movement physically pulls gates open - ultrafast kinetics (microsecond response, no second messenger or chemical amplification)
- Most MET channels close within milliseconds allowing high frequency response to sound waves
What features of sound does the owl use to localise
a) elevation
b) azimuth
a) Intensity
b) timing
What is the interaural level difference (ILD) used to calculate for owls? How was this found out
- Elevation
- Owls have asymmetric ears
- Left ear downwards, right ear faces upwards
- Facial ruff greatly amplifies this directional symmetry
- Researchers blocked left ear - led to an overestimation in the height of prey
What is interaural time difference (ITD) used to calculate?
- Sound arrives at ears at 2 different times
- Difference in arrival is the temporal disparity
- ONGOING disparity is used to determine AZIMUTH
Describe the neural pathway of sound after it has entered the ear
- Arrives into brain through cranial nerve VIII to the Nucleus Angularis (NA) and Nucleus Magnocellularis (NM)
- NA goes directly to in inferior colliculus
- NM goes to the Nucleus Laminaris (bilaterally symmetrical)
- CRUCIAL - Input from contralateral NM enters the NL from the bottom, Input from the ipsilateral NM enters the NL from the top
- Coincidence cells predicted to be in the NL, determine the ITD
Knudsen and Konishi put electrodes in the inferior colliculus of owls and tests where input from sounds at different locations came from.
What did they find?
- Space-specific cells in the external nurcleus of the IC (ICX) are arranged topographically
- If you presented sound peripheral to the space-specific area of the IC, it INHIBITED that signal to allow tight contrast and accurate sound distinction
- These cells are biaural - need both ears
- Each responds to a unique combination of ITDs and interaural level differences (ILDs) which determine the neurons receptive field
- This topographical arrangement is a neural reconstruction of the owls auditory space
- to summarise, space-specific neurons in the IC receive interaural differences in time and intensity from pathways originating in the cochlear nuclei
Where is time information processed and where is intensity information processed?
- Time info processed by the Nucleus Magnocellularis
- Intensity info processed by the Nucleus Angularis
Discuss the neural mechanisms underlying the localisation of sound sources / the encoding of time differences between ears in the auditory system
- Jeffrey’s researched how the auditory system encodes non-coincident input from both ears to produce a coincident output
- He postulated that somewhere in the system, there must be a set of cells that are activated at the same time to produce this output
- He hypothesised coincidence detectors, that require input from both ears to be activated
- This required the presence of delay lines, that can prolong signals from the ipsilateral ear
- Mechanism: Input from the ears is relayed along a set of coincidence detector cells that neurons from both ears synapse with
- Only one that fires is the one with coincident input
- The anatomical substrate of this theory may be the Nucleus Laminaris due to the nature of the contralateral/ipsilateral input innovating it
How was plasticity discovered in auditory localisation of owls?
- Plugging one ear of the owl lead to systematic errors in sound localisation
- (Owls use intensity cues to determine elevation)
- Over weeks, young owls gradually learn to correctly locate the sound
- Adult animals did not show this correction
What role does vision play in this auditory plasticity?
- Visual input is required for plasticity of the auditory system
- Found by plugging, unplugging
- saw errors in the opposite direction
- Errors were corrected
- If blindfold on after ear unplugged, NO correction occurred
- Prism expt: When there is artificial mismatch between auditory and visual signals, owls recalibrate their auditory localisation to correspond with the altered visual world - vision wins
Where does auditory info intersect with visual information? What is the layout of receptive fields for each sense?
- Optic tectum
- Spatially restricted auditory and visual receptive fields
- These are aligned in space with one another
What is the neural pathway that links the auditory pathway and the visual pathway?
- Central nucleus of the Inferior Colliculus (ICC)
- External nucleus of the Inferior Colliculus (ICX)
- Optic Tectum
- Brainstem Tegmentum
- Motor nuclei for gaze control
What happens to receptive fields in the optic tectum of young owls wearing prisms?
- Visual receptive fields shift in correspondence with prism (but opposite direction)
- Over 8 weeks, auditory receptive fields shift into alignment with optically displaced visual receptive fields
- This anatomically allows the neurons receiving this information to pass the signal onto the next stage (head rotation)
The previous card stated that the auditory receptive fields of the optic tectum shift in response to an altered visual word.
How does this happen?
- ITDs are required to calculate azimuth (horizontal plane)
- Each OT neuron react preferentially to specific a specific ITD
- After wearing prisms, neurons ITDs CHANGE
- E.g., if a neuron reacted to stimuli directly in front of the owl, after wearing left-displacing prisms, the neuron would now react to sound arriving from the R ear first
