How General Are Neural Codes In

Sensory Systems?

Presenter: 유승범(Department of Brain and Cognitive Sciences)

1. Inter- aural Level Difference(ILD)

ILD is the detecting the difference of

intensity reaching each sides of the ear.

Since head works like low frequency

filter, ILD is more related with

distinguishing spatial difference of

higher frequency.

[ Shorter width: higher cut-off]

2. Inter- aural Time Difference(ITD)

* ITD is the time difference of signal reaching to both ears.

* Since ITD is works recognizing the spatial difference by temporal

information, then how can it be re-perceived as spatial information?

* Place of coincidence detectors transforms the temporal to spatial information.

ITD and Coincidence Detection

3. Inter- aural Phase Difference(IPD)

When lagging signal’s phase is equivalent to ITD±T of leading signal and ITD±T

exist within physiological range, it causes phase ambiguity.

(T is cycle of stimuli tone)

IPD and Phase Ambiguity(Phantom Phenomena)

4. Inhibitory Input

There are systematic shifts of post-synaptic pulses due to IPSP gradient.

As inhibition increases(GI max increase), the ITD shifts into the leftward.

1. Tonotopic arrange difference

Avians have ITD map running orthogonal to

frequency tonotopic map. Thus,within iso-frequency

layer, full representation of ITD is available!

However, mammals have parallel ITD maps to

primary frequency tonotopic map.

2. ITD Utilizing Strategies

Avians has steady ITD peak with their function.

This is Characteristic Delay and it closely

stays within physiological range.

Characteristic Delay

Mammal’s neuron, however, has different

ITD peaks according to the frequencies.

[Each Neurons have Characteristic Frequencies]

Mammals utilize steepest slope of ITD functions.

1. Mammal uses glycinergic pathways to inhibit Medial Superior Olive(MSO).

2. Avian utilize GABAergic pathways to inhibit Nuclei Laminaris(NL) .

➤ Within this difference, mammal utilize hyper-polarizing mechanism in

inhibiting whereas avian applies depolarizing mechanism.

1. Mammal’s MNTB implies feed-forward inhibition.

2. Avian’s SON implies feed-backward inhibition.

• Mammal’s inhibition is related with time-window shift.

• Avian’s inhibition is characterized as de-correlation.

1. (No temporal shift, but gain control)

3. Different Patterns of Inhibitory Input

New Model Suggestion

[New Model Suggestion]

Jeffress-like model with the avians.

New model for mammals are so

called “two-hemispheric channel” model.

➤ Azimuthal change of position in

one channel accompanied by

channel in other side by opposite sign.