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

You have seen song selective neurons in pre-motor HVc of birds

Anesthetics?

You have seen song selective neurons in frontal forebrain

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

Monkey representation of species specific calls similar to bats

Foreshadow

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_