How Does the Brain Interpret Sound Waves?

[saltine]

The Question

I want to understand how a sound wave changes in the form of its signal into the brain. I don't know the terminologies. I don't know what this subject is called. If you know the right terms or know where I should look, please fill me in.

My question is: What is the form of the signal that pertains to the sensation of sound?


My Understanding

Here I show you the way I view the question so you know what level of understanding I have. Since I have never been tested on this knowledge, you can assume that anything I say below could be wrong:

First I know that sound wave is a transmitted in the form of changing pressure over time. It can be measured and plotted on a graph with force as the y-axis and time as the x-axis. A pure tone will appear as a sinewave on such a graph.

Second I know that the ear is an organ that tranduces this pressure through a curved tube inside the ear. The lower the frequency of the incoming tone, the further it travels. There are receptors along the tube to detect vibration. These detectors transduce vibration into action potentials. Action potentials can be plotted on a 2D graph. The y-axis is voltage. The x-axis is time. The intensity of vibration translates into the frequency of occurance ot the spikes. A burst of spikes is loud. Along the tube, each detector is only responsible for detecting a certain frequency. Therefore, to send the signal corresponding to a song into the brain, it takes a bundle of nerves. If you order and label each nerve and plot the signal, it is a stack of 2D plots f_i(x). x-axis is time. y-axis is the voltage. The index i is the identification number of a nerve. In some sense the ear is a spectrum analyzer.

Third. . . What happens after that? I assume that the nerve bundle gets separated and nerve cells interact with each other as they are activated to form relays. Certain features of sounds are formed and detected in these relays. (What is the list of features that the pathway detects?) However, from here on the signals are still transmitted in the form of spikes. Meaning is formed and encoded in the connectivity of the nerve cells. For example, if nerve cell 1 is can excite nerve cell 2, then 1 means something to 2. Inhibition can also occur. Memory is a phenomenon cause by this connectivity. So, where is the song that I remember? It is in the connection in the brain. You remember it not because it is store somewhere in a certain format, but that the que you have in thinking about it reactivates the connections that you activated when you first heard that song.

So to answer the question, "what is the form of the signal that pertains to the sensation of sound?" The answer is that, "by the time it gets to the level of sensation, sound is no long represented by a signal, but by the connectivity among the cells. It is not a signal. It is a map."


Is this right?

 

[Reality_Patrol]

Your question pertains to the physiology of the cochlea. The cochlea is the organ in the brain responsible for transducing acoustic pressure waves into neural impulses.

Each cochlea has about 3000 "inner" hair cells which produce impulses when excited. The inner hair cells are "tonotopically" mapped, which means each inner hair cell is excited by a specific narrow range of frequencies, basically a pure tone. In other words each inner hair cell responds to a unique pure tone.

So, there are around 3000 signals (per ear) that get sent to the brain which encode sound. How the brain translates these neural impulses into "perceived sound" is not well understood but is an area of very active research.

Each cochlea also has around 12000 outer hair cells that receive signals from the brain to help process the incoming pressure waves. They act mostly as biological amplifiers, which is one reason humans have such a wide dynamic range. People who are "severe to profoundly" hearing impaired have lost the amplification provided by these outer hair cells for the most part.

When the inner hair cells are stop working total deafness results.

 

[saltine]

Thanks, do you happen to know where the preception of sound inside the brain is studied? I want to know about how they go about studying it and what they have found.

 

[Math Is Hard]

Psychophysics:
http://en.wikipedia.org/wiki/Psychophysics

And more generally, neuroscience.

 

[atyy]

http://www.ncbi.nlm.nih.gov/pubmed/...med_ResultsPanel.Pubmed_RVDocSum&ordinalpos=1

Annu Rev Psychol. 2008;59:119-42.
The biological basis of audition.

Recanzone GH, Sutter ML.

Center for Neuroscience and Section of Neurobiology, Physiology & Behavior, University of California at Davis, California 95618, USA. ghrecanzone@ucdavis.edu

Interest has recently surged in the neural mechanisms of audition, particularly with regard to functional imaging studies in human subjects. This review emphasizes recent work on two aspects of auditory processing. The first explores auditory spatial processing and the role of the auditory cortex in both nonhuman primates and human subjects. The interactions with visual stimuli, the ventriloquism effect, and the ventriloquism aftereffect are also reviewed. The second aspect is temporal processing. Studies investigating temporal integration, forward masking, and gap detection are reviewed, as well as examples from the birdsong system and echolocating bats.

 

[atyy]

This is also very cool: http://www.tc.umn.edu/~cmicheyl/demos.html

 

[saltine]

Re:
http://www.tc.umn.edu/~cmicheyl/demos.html
I listened to the three samples. I could clearly hear two tones in each.

Re:
http://www.ncbi.nlm.nih.gov/pubmed/1...m&ordinalpos=1


Auditory System
o Its functions
o Its components
o Its component interactions

Functions

o To detect things (enemy, friend, food)
o To locate things (enemy, friend, food) (called Auditory Spatial processing)
o To communicate meaning

Components

o Ears (Periphery, Left and Right Cochlea),
o Parts of the brain:
- Brainstem: Cochlear nucleir (left and right)
- Brainstem: Superior olivary complex
- Midbrain: Inferior colliculus (left and right)
- Thalamus: Medial geniculate (left and right)
- Cerebral Cortex: Auditory cortex (left and right)

Component Interactions

Ears:
o Converts sound waves into tonotopically mapped action potentials for the brain

Brain:
o Associate differences in volume (from the two ears) with location (related term: Binaural Cues)
o Recognize feature patterns of sound, including temporal features
o Associate feature patterns as objects (objects are further associated with threats or resources)
o Associate feature patterns with semantic symbols


Types of Experiments:
o Lesion Experiments
o Physiology Experiments
o Imaging Experiments

Known Influences:
o Visual influence on auditory spatial processing
o Ventriloquism Aftereffect

Terms related to the study of temporal processing:
o Temporal Integration
o Forward Masking
o Neuroethological Studies
o Gap Detection