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Why vocal communication is hard to study physiologically
Categorical components
Complex signals vary along multiple parameters
For example, frequency, duration, temporal envelope, etc.
Therefore hard to map onto continuous variables like most sensory studies
This displays idea of Song Selectivity
Neurons specialized for song
Neurons don’t respond well to any other sounds
You have seen bat echolocation
Fre
quen
cy(k
Hz)
120
90
60
30
Time (msec)
-- Because of Doppler shift
delayed in time
-- Doppler shifted echo frequency target (moth) velocity
-- Shifted in frequency
Because speed of sound information on target (e.g., moth) distance
CF-CF neurons show selectivity form pulse echo pairs
Fre
quen
cy (
kHz)
Res
pons
e(s
pike
s/se
c)
Pulse alone Echo alone Pulse + Echo
Weak or no response Strong responseWeak or no response
30
60
90
120
time time time
You have seen neurons whose responses are selective for specific pulse echo combos
This reinforces idea of combination selectivity
Neurons specialized for specific Doppler shifts or delaysDifferent brain areas organized to analyze Doppler shift and delay
How are species specific communications sounds analyzed by mustached bats?
The same neurons that encode echolocation respond to these social callsBut not selectively
These responses seen in DSCF, CF-CF, FM-FM
But that’s not all
Neurons responding to both echolocation and social calls found in frontal cortex
So we need a different way (not selective neurons) to account for brain encoding
Population code:- neurons respond to subset of of vocalizations- partial selectivity- combined activity pattern tells you which vocalization
Temporal code
- not just how many spikes but when they fire- using a population- combined with temporal pattern much better reliability
Monkey representation of species specific calls similar to bats
Primates: Generalist or Specialist
A Generalist would analyze all sounds of which vocalizations are just 1 A specialist would have a specialized auditory system
What not to do as a scientist Chapter 1 of Primate Audition: “The species –general view of primate audition is incorrect …”
Primates are not pure generalist
They have some specialization for important sounds like social calls
But there auditory systems do not show the degree of specialization of bats
Evolution and selection has had a large influence on primate auditory processing
How primates use vocal communication
To avoid predators For example in vervet monkeys predation accounts for ~ 70% of deathsAlarm calls help to prevent predationBut be careful some predators can detect alarm calls
Vervets have different predatorsEagles, leopards and snakesEach predator type needs to evoke a different behavioral response
For leopard they must get to high ground (but beware of eagles)
For eagle get low to ground (but that would get you eaten by a snake or leopard)
Therefore vervets have developed different calls for each predator type
Some primate species can detect and respond to alarm calls of different species of animals (e.g birds or deer) hunted by a common predator
How primates use vocal communication
To find food Calls convey information aboutLocation andCharacteristics of food sourceCan convey quantity and qualityFailure to call when food is encountered punihsment
To find a mate
Aggression
Affiliative
Coo
Grunt
Primate Song
Primates that string together multiple different notes in song a rareIn general they just call
Song/Call distinction
Also important because bird song is acoustically closer to human speech than primate calls
Some primates like the Gibbon siamong sing.
However no relationship between the few singers and closeness to humans.
Anatomy
Cochlea Multiple brain stem areas Inferior colliculus MGB
Various parallel and hierarchical auditory pathways not shown
cortex
Neural coding of primate vocalizations
Population code:
Temporal code
In some species they have shown preference for (but not selectivity) forward over reverse calls
Preference – responds stronger to forward but responds to bothSelectivity – almost no response to reverse
This is still unresolved though as in some species very contradictory results exist (in macaques one study shows forward preference, one shows slight reverse preference_