Echolocation in Bats

auditory sensory receptors are located

in the organ of corti

organ of corti sits on

the basilar membrane

tonotopy

frequency is coded by its place on the basilar membrane

high frequency-base, low frequency-apex

mustached bat

CF-FM bat, 2nd harmonic is loudest at 61khz

doppler shift compensation of CF-FM mustached bat

used to determine velocity

where threshold is high, sensitivity is low; vice versa

the wider it is the less sharp

in this example, most sensitive to 61 khz (lowest threshold), most sharpness at 61 khz as well

bat emits call at 61khz until it meets an object coming towards them, the call frequency is lowered so the returned echo can be at their most sensitive frequency

adaptations of CF-FM bats auditory system

1. acoustic fovea
2. doppler shift compensation

acoustic fovea

overrepresentation of neurons sharply tuned to CF of call

horseshoe bat acoustic fovea

uses CF pulses, most sensitive to 83 khz

thickening and lengthening of basilar membrane where most sensitive (thickness to width ratio is proportional to stiffness of basilar membrane and thus frequency)

q10db

sharpness of 8th nerve tuning curves, characteristic frequency/width 10db

FM bat doesnt have

an acoustic fovea, broad band

little brown bat peripheral tuning

typical FM bat, broad range of units, no characteristic frequency

horseshoe bat tuning curves

large number of extremely narrowly tuned units

bats reduce their auditory sensitivity during

the emission of a call

FM bats reduce auditory sensitivity by

contracting middle ear muscles during call to lower loudness of the call signal (reduce movement of ossicles)

CF bats reduce auditory sensitivity by

using doppler shift compensation, this is because of the overlapping call-echo signals

neural processing of pulse-echo

cochlea→cochlear nucleus/medulla→superior olivary complex/pons→nucleus of lateral lemniscus/pons→inferior colliculus→medial geniculate→auditory cortex

Post stimulus time histograms(PSTH)

histograms of the time at which neurons fire

typical PSTH

respond faster w louder sounds

IC PSTH

no difference in timing of response (the reason for it is that they need to accurately encode the pulse and echo to calculate the delay)

IC neurons

low spiking threshold, very sharply tuned to a single frequency (time-locked) in an FM sweep, multiple measures of pulse-echo delay taken for distance estimation

delay tuned neuron in IC

pulse-echo pairs, no response or weak response to j an echo or j a pulse, but combined we get a strong response AND theyre tuned to a specific delay

auditory cortex

basilar membrane, auditory fovea, azimuthal axis, CF/CF area, FM/FM area

basilar membrane

tonotopically organized, base is high freq, apex is low freq

DSCF

acoustic fovea, extremely sensitive cells to the frequency of the second harmonic (not sure if this is always true but)

FM-FM area

cells are activated by 2 FM sweeps of a particular delay, measures distance

CF-CF area

cells are activated by 2 CF tones, measures velocity

azimumth

sound localization(wdym?)

distance coding in FM-FM area

pulse-echo delay tuned neurons used to measure distance,

FM1 paired with FM harmonic, combination-sensitive neurons and perpendicular is a delay axis, functional column organization

delay tuning in FM-FM area

dependent on position in cortex, FM-FM cortex has map of echo delay

absolute size

subtended angle (loudness) and distance (delay)

FM-FM neurons code for

a target of a particular size at a particular distance

medial geniculate coincidence detectors

pulse is delayed by axonal and synaptic delays AND inhibitory interneurons, echo is spread w no delay, if specific neuron is stimulated simultaneously by pulse and echo signal than that delay-sensitive neuron tells the distance

velocity coding in CF-CF area

CF component used for doppler shift analysis to compute velocity, combination-sensitive neurons selective for particular pulse-echo CFs to measure velocity

CF-CF area divided into 2 regions

CF1-CF2 and CF1-CF3, with relative velocity and CF1 frequency as an axis

CF-CF organization

help suga

doppler shift coding in DSCF

processes doppler shifted CF2 signals, tuned to the frequency AND amplitude of the ECHO, regardless of frequency of call, cortical counterpart of acoustic fovea, columnar arrangement = cells in each column are tuned to a particular CF2 and amplitude

why do bats emit a variety of harmonics?

so bats dont get confused with other bat calls, bats can only hear their fundamental and thus the harmonics of other bats do not trigger the combination-sensitive neurons