[EvaH (OP)]

Lionel got in touch with us to ask:

I installed a sensor that switches on a light when it detects a sound. However, when I clap my hands, the light appears to come on fractions of a second before I hear the sound. Does it take my brain longer to process the sound than the light?

We'll be answering this on next week's show, but what do you think?

[evan_au]

Your brain instinctively knows that light travels faster than sound.

In cases where the sound and picture on a TV are not perfectly synchronized, your brain finds that sound arriving before the picture is more noticeable than sound arriving after the picture.
See: https://en.wikipedia.org/wiki/Audio-to-video_synchronization#Recommendations

There are mechanisms in the brain that synchronize the arrival of sight and sound, despite any differences in brain processing delay that are queried by the OP.

I heard an anecdote that one older person in a household complained that the TV was out of sync, with the audio arriving considerably before the picture. However, everyone else thought it was ok.
- I don't want to scare the OP, but in this case, it was apparently an early symptom of brain degeneration, where the patient could still sense both sight and sound, but the synchronisation mechanism in the brain was impaired.
- From this anecdote, it would suggest that sound can be processed by the brain faster than images, despite the slower speed for the sound to reach your ears.

The brain's P300 signal on an electroencephalogram indicates when something novel or unexpected has been experienced.
- The "300" comes from the roughly 300ms delay between presenting the stimulus and detecting that it is novel.
- Experiments showed a similar 300ms delay for stimuli presented by either sight or sound.
- However, this time may be dominated by the time for the brain to consciously process the novelty aspect rather than the processing time of the raw sensation
See: https://en.wikipedia.org/wiki/P300_(neuroscience)#History
 
Where a sensation has a deeply ingrained association with a life-threatening event, it can be processed in deeper layers of the brain (eg amygdala), before there is time for a conscious assessment, triggering an immediate surge in hormones, and a "fight or flight" response.

A very chatty description of some other factors here: https://theconversation.com/it-feels-instantaneous-but-how-long-does-it-really-take-to-think-a-thought-42392

[Halc]

I installed a sensor that switches on a light when it detects a sound. However, when I clap my hands, the light appears to come on fractions of a second before I hear the sound. Does it take my brain longer to process the sound than the light?

For one, the sound of the clap is anticipated since it is self-generated.  The light on the other hand really is delayed.  The clapper device has to parse the sound out of the stream of ambient noises and then throw a switch when it hears the clap. It takes time to throw an electrical switch and then a bit more time for the light to come on, all fractions of a second of course.  It adds up. The sound really does arrive well before the light in this case, and that stands out as strange since, as Evan points out, you're used to getting it the other way around.

[evan_au]

There is another effect at work here - your ear has a volume control, managed by a tiny muscle in the inner ear.

Since the clap is anticipated, the volume control may turn down before the clap, and turn up after the clap.

So what you are hearing may be the residual echoes in the room after the clap, not the initial clap that turns on the light(?)
See: https://en.wikipedia.org/wiki/Tensor_tympani_muscle#Involuntary_control_(tympanic_reflex)

[alancalverd]

There are different aspects to "processing speed". There is quite a bit of research on voluntary response to sound and light stimuli and the bit that interests me is the time taken to respond to an unexpected red light or shouted "stop". It turns out that it takes longer to translate the light signal into action than the audible warning.

This is significant in morse communication.  Audio morse copying at 20 - 40 words/minute is considered competent, but it is extremely difficult to exceed 10 wpm  with light signals.   

[evan_au]

I heard some neuroscientists had traced sensory nerve pathways into the brain. As I recall:
- For one path they studied, the signal had to pass through about 12 neurons to reach the brain.
- But for the sense of smell, there were only 2 neurones from the olfactory bulb in the nose to the brain (helped by the close proximity of nose and brain...)

Of course, for a smell to reach your nose in detectable concentration, it must slowly diffuse outwards from the source, at an average speed much slower than light or sound...

[Bored chemist]

It would be interesting  to get someone else to clap.

[alancalverd]

It would be interesting  to get someone else to clap.

As an occasional musician, I can vouch for the delay between cause and effect being at least 50 years.

[EvaH (OP)]

Asked and answered! Thanks for the great question Lionel

https://www.thenakedscientists.com/podcasts/question-week/do-our-brains-process-sound-or-light-faster