Lecture 22 - Finishing Sounds in Space

Question 1

how we organize sounds in space: how we atribute sounds to particular locations and objects

Answer 1

binaural and monaural cues

Question 2

narrowly tuned ITD neurons

Answer 2

kind of a specificity code for determining the localization

these neurons looking for a particular difference in time at which sound reached one ear vs. another ear

if you had one or a very few neurons that responded to that difference = a specificity code

Question 3

broadly tuned ITD neurons

Answer 3

looking for a range of differences with more neurons responding= distributed code

Question 4

Jeffress Model

Answer 4

for narrowly tuned ITD neurons

main idea: you have a few set of neurons looking for the differences in signals as they reached one ear vs another ear

the system would have a number of coincidence detectors that only fire when it gets signals from both ears

if you have a set of coincidence detectors (1-9) and you get a signal that reaches both ears at the same time then the coincidence detector that codes for zero time difference will respond

different neuron responding to different time differences

Question 5

Coincidence detectors fire only when…

Answer 5

signals arrive from both ears

simultaneously.

Question 6

If signal reach both ears at the same time, then ITD is….

Answer 6

zero

Question 7

What is the first place where information from both ears is

combined?

Answer 7

superior olivary nucleus

Question 8

Physiological support for narrowly tuned ITD neurons

Answer 8

– Neurons in the inferior colliculus and superior olivary nuclei
respond to a narrow range of interaural time differences
(ITD).

– Single-cell recordings in the barn owl show a response to ITD from left and right.

– This is a specificity code.

Question 9

physiological evidence for Broadly-tuned ITD neurons

Answer 9

– Research on gerbils indicates that
neurons in the left hemisphere respond
best to sound from the right, and vice versa.
– Location of sound is indicated by the
ratio of responding to two types of ITD
neurons: left sensitive and right sensitive.
– This is a distributed coding system.

Question 10

opposite hemisphere advantage

Answer 10

when a signal comes into the right it gets to the left hemisphere first

Question 11

Physiological support

mammals vs. birds

Answer 11

Mammals (distributed coding) and birds (sparse coding) seem to use
different encoding schemes.

Question 12

Human echolocation (flash sonar)

can we see with sound?

Answer 12

for individuals that haven’t had sight for a very long time, they can use sound to help navigate

don’t just listen to passive sounds, but with flash sonar they produce a sound and listen to the frequency differences as the echo comes back: use auditory cues for a phenomenal sense of location

– Some blind individuals can train themselves to detect objects in the environment by producing
clicking sounds and listening to the echo = RE-MAP THEIR VISUAL AREAS!! = cross sensory mapping an area of the cortex because it’s not getting it’s usual input through vision = use auditory localization to make a spatial map instead of using retinotopic mapping

– They don’t see, but they can sense the location (and sometimes an identity) of an object.

– fMRI evidence suggests
echolocation experts, blind since very young, use striate cortex (V1)
to help represent the space.

– Extreme case of experience dependent
plasticity!!!