Auditory neurons
Humans can accurately localize where a sound is coming from around us. This good localization ability can occur even in unfavourable conditions where there is noise in the background, except when the noise is very loud.
One major cue that we use to locate the position of a source of a sound around us is the difference in sound levels at the two ears – the inter-aural level difference (ILD).
Imagine a sound coming from different locations around you. When the sound comes from one side of the head, it will sound louder in the ear on that side of the head and softer in the other ear. When the sound comes from the other side of the head, these perceptions are reversed for the two ears. Finally, when the sound comes from directly in front of you, it will sound equally loud in the two ears.
This occurs because the head and body cast an acoustic "shadow" and block some sound from getting from one side of the body to the other side. Thus, when the sound is coming from directly in front of you, this block doesn't occur, and the sound seems (and is) equally loud in the two ears. Otherwise, when it comes from one side or the other of the midline, it seems (and is) louder in the ear nearer the sound and softer in the other ear. Thus, ILDs are important for sound localization in azimuth.
Instructions and experimental setup
In this simulation you will "record" from a neuron in auditory cortex.
In the Spatial tuning tab, the left panel represents the auditory stimulus that the neuron is responding to, in this case a tone playing at a certain azimuth relative to the head. You can control the location of the sound source by dragging the circle with the sound symbol along the semicircle around the head. When you press "Play sound", the tone will play for one second.
The oscilloscope on the top right shows the amplified voltage trace recorded extracellularly from an electrode close to the cell body of the neuron. Each of the near-vertical lines represents an action potential. If your speakers are on, you will also be able to hear the tone and the neuron's activity (you will need headphones to hear the location of the tone, though). Below the oscilloscope is a graph that automatically plots the number of spikes fired by the neuron on each trial. Which sound location do your neurons respond best to?
In the Population analysis tab, you will be able to categorize tuning functions for randomly chosen neurons to get an idea of what the naturally occurring distribution is in the brain.
- Contra-selective neurons respond more to sounds in the contra-lateral ear.
- Ipsi-selective neurons respond better to sounds in the ipsilateral ear.
- Midline-selective neurons respond best to sounds that are straight ahead.
- Off-midline selective neurons respond best to a specific location that is off to one side of the head.
- Insensitive neurons respond the same way regardless of the location of the sound.
Mapping the neuronal tuning function
Once you've collected 5 samples in each direction, you'll be able to download the experiment data.
Population analysis of sound source tuning
Categorize the following neuron by clicking on the labels of the chart on the right:
Metrics of spatial tuning:
| Contra selective | Ipsi selective | Midline selective | Off-midline selective |
|---|---|---|---|
| Azimuth at 50% of maximum response | Azimuth at function's peak | ||