Cognitive Neuroscience Lecture 10: Auditory Perception

*L10: Auditory*
Perception

The challenges of audition

Complex auditory scene analysis

o
How do we recognize identity and location of
objs in complex scenes?

Overview

Ear to brain

Neural representation of intensity, frequency,
and location

Stream segregation

a.
Grouping principles

Top-down processing

a.
Inference

Auditory object recognition

What (obj recognition) requires:

o
**Invariance**
(obj constancy): what each obj shares with other members of category (diff dogs
vs diff cows)

o
**Specificity**:
what is specific to each object (e.g. one dog bark vs another)

o
Have to solve both of these at the same time

Where (sound location is not given by geometry
of sensory surface like it is for vision)

o
Sound: audible variations in air pressure (mvt
of air molecs)

o
Frequency: number of compressed air molecs that
pass by ears/sec; pitch

o
Cycle: wavelength

o
Hertz: s-1
(humans respond to 20-20,000 Hz)

o
Amplitude: height of peak; volume

o
Complex sounds can be decomposed into
constituent freqs and amps

Challenge: extracting freqs and amps

o
Tympanic membraneà
ossiclesàoval
windowà
fluid in cochleaàbasilar
membraneàsensory
neurons

Basilar membrane coiled inside of cochlea

o
Base narrow and rigid: high freq

o
Apex wide and floppy: low freq

o
As frequency decreases, maximal deformation
moves toward apex

Tonotopic organization of basilar membrane

o
Tonotopic map: adj tones in adj locations of
neural surface

o
Bending basilar membraneàrelease NT

o
Activates spiral ganglion cells àaction potentials

§
Spiral ganglion cells: axons form auditory nerve
(VIII)

Auditory pathways

o
Multiple paths

§
Ascending converge on inferior colliculus

§
Splits

o
Bilateral connectivity

§
Superior olivary has info from both ears

§
All nuclei above cochlear nuclei receive input
from both ears (although stronger contralateral)

§
(cochlear nuclei are in SC)

o
Extensive feedback

§
A1 projects back to MGN and inf. Coll.

§
Considerable processing prior to cortex

Full pathway:

o   Auditory nerveàcochlear nucleusàsuperior oliveàinferior colliculusàMGNàA1

o   Tympanic membraneà ossiclesàoval windowà fluid in cochleaàbasilar membraneàsensory neuronsàauditory nerve fiber(spiral
ganglion cells)àipsilateral cochlear nucleusàsuperior olivary nucleusàinferior colliculus à medial geniculate nucleusà contralateral auditory cortex

**Auditory**
cortex: superior temporal gyrus

o
Heschl’s gyrus: primary auditory cortex (A1)
(core)

o
Planum temporale: 2ndary auditory cortex (belt)

o
Core, belt, parabeltà increasingly complex/associative
representation and processing

Neural representation of features of sound

Intensity (amplitude) (loudness)

o
Firing rate

§
Greater amplitude of vibration of basilar
membraneàgreater
number of AP in spiral ganglion cells

o
Number of active neurons

§
Greater amplitude of vibrations of basilar
membraneàlarger
area of basilar membrane affectedàgreater
number of SGC firing

Frequency

o
Local distortion of basilar membà activation of specific
spiral ganglion neurons

o
**Frequency**
tuned neurons**: each respond maximally to specific freqs; fairly sharp, not**
as sharp as visual

Auditory cortex: Tonotopy

Sylvian fissure

o
Apex of cochlea/lower frequency corresponds to
anterior A1

o
Base/high freq is posterior a1

Horizontal location ‘where’

Interaural time difference

o
Interaural time delay = difference between one
ear and another = 0.6 ms

o
Encoding difference = determining horizontal
location

Interaural intensity difference

o
Head casts a sound shadow creating differences
in intensity of the sound in each ear

o
ID location of a sound source

Computing ITD

o
Brainstem neurons ‘prefer’ (max firing) for
certain ITD

o
Key step: action potentials converge on an MSO
neuron that responds most strongly if their arrival is a certain ITD

o
Longer path from sound side to non-sound side
than from sound side to its own side

Computing IID

o
LSO

o
Different locations of sound sourceà different patterns of
excitation and inhibition; characteristic signals depending on location of
sound

o
Stronger stim to one side excites that side LSO,
and inhibits opposite LSO

o
Excitation from sound’s side greater than
inhibition from right side à
net excitation on sound side to higher centers

**ITD=MSO,**
IID=LSO

Auditory perception

o
Audition is not a simple mirroring of stim;
influenced by knowledge, and it influences other things

o
Reveals inner workings of prediction engine

Stream segregation

o
Auditory Inference: grouping by similarity and
proximity (Gestalt principles)

§
Group by similarity in frequency if it’s fast
enough

§
One melody for high freq, one melody for low
freq

§
“assumes” that similar sounds are more likely to
come from same object (esp if close in time)

o
Grouping by common fate

§
During modulation, hear separate sounds

§
After modulation, tones come back together

§
Auditory system assumes freq that fluctuate
together are more likely to come from same objs than those that do not

o
Object knowledge influences perception: top-down
processing

§
Perceptual filling in; “explanation for the gap”
if noise louder than original sound

§
Object knowledge exerts top-down effectàinfluences lower-level
processes (filling in)

§
Similar to vision amodal completion

ID freq, locations, intensities

Group into possible objs (e.g. similarity)

Auditory ID (requires obj knowledge (learned);
sound of something you already know)

Bottom up = 1, 2, 3

Top-down = 3, 2, 1