Sound Localisation and Sensory Interaction

Flashcards covering the mechanisms of sound localisation (ILDs and ITDs), the neural pathways (LSO and MSO), and the developmental role of sensory interaction based on barn owl experiments.

What are the two primary methods mammals use to localise sounds in the horizontal plane?

The detection of Interaural Level Differences (ILDs) and Interaural Time Differences (ITDs).

What is the physical cause of Interaural Level Differences (ILDs)?

The head acts as a barrier that reflects or absorbs sound waves, making the sound louder in the near ear and quieter in the far ear.

For which type of sound frequencies are ILDs mainly used?

Higher frequency sounds.

What is the smallest Interaural Level Difference (ILD) humans can detect?

As small as $1-2\,dB$.

What is the definition of Interaural Time Difference (ITD)?

The difference in the arrival time of the same sound at the two ears; sound from one side reaches the near ear before the far ear.

For which type of sound frequencies are ITDs mainly used?

Lower frequency sounds.

What is the smallest Interaural Time Difference (ITD) humans can detect?

As small as $10\,\mu s$.

Which brainstem centers are primarily involved in detecting ILDs and ITDs?

The Lateral Superior Olive (LSO) and the Medial Superior Olive (MSO).

Where do all neurons from the ear first enter before reaching the LSO and MSO?

The Cochlear Nucleus (CN).

Describe the Excitatory-Inhibitory (EI) pathway utilized by the LSO.

LSO neurons receive an excitatory input ($+$) from the near ear and an inhibitory input ($-$) from the far ear; the inputs summate to determine position.

At what position is the population output of the LSO near maximal?

When sound is from the same side of the head (the near ear), where the excitatory input is much larger than the inhibitory input.

In the ILD circuit, what is the LSO output when sound is on the centreline?

The population output is around half maximal because the loudness is the same in both ears and the inputs balance ($ILD = 0$).

Describe the Excitatory-Excitatory (EE) pathway utilized by the MSO.

MSO neurons receive two excitatory inputs (one from each ear) and become maximally active only when both inputs arrive at the same time.

How does the MSO circuit compensate for the time delay of sound reaching the far ear?

The nerve from the far ear is longer than the nerve from the near ear, allowing the circuit to detect simultaneous arrival for specific sound positions.

To which side of the head is the output of each MSO channel broadly tuned?

The opposite side of the head.

What are the two distinct phases of sound localisation circuit development?

1. Initial connections formed by genetic programmes early in development. 2. Calibration and refinement using alignment with the visual map.

In the experiments by Knudsen and Knudsen, how was the barn owl's visual field shifted?

The visual field was shifted $20^{\circ}$ to the left using prisms placed in front of the eyes.

How did the juvenile barn owls respond to the shifted visual field after 42 days?

The auditory map shifted to align with the modified visual field, even though the auditory field itself was not modified.

What happened to the barn owl's auditory response immediately after the prisms were removed on day 49?

The visual response quickly re-aligned to the stimulus, but the auditory response remained shifted, having adapted to the prisms.

Why does visual information dominate the development of auditory space perception?

Because the visual world is mapped directly onto photoreceptors, providing a template to teach the brain how to interpret auditory cues.

What is the nature of the LSO circuit output compared to the MSO circuit output in mammals?

The Left LSO is tuned to sound from the left, while the Left MSO is tuned to sound from the right.