Lecture 7: Auditory Perception Flashcards
What is sound?
Molecules in the air get displaced when sound is played.
- darker areas are where they are close together and the other areas are where they are spread out.
- Sound is made up of a bunch of waves - jostling around
- Sound wave is the changes in air molecules
Anatomy of the ear
Outer Ear:
- Pinna
Middle Ear:
- Ossicles
- Ear canal
- Ear drum
Inner ear:
- Auditory nerve
- Cochlea
- Eustachian Tube
The role of these parts is to amplify, transduce and transmit sound to the brain.
What is the function of the ear?
- It is the transition of sound from the physical perception of the world into something that we can perceive.
- It changes mechanical sound energy into neural energy.
Pinna
- Cartilage structure
- Role is to catch sounds in the environments and they are going to amplify certain sounds and pass them on to deeper structures of the ear.
- Looks different across species because it adapts to capture sounds that are really relevant for that animal and to amplify those sounds to create adaptive behaviours (helps withs survival).
- Some animals can move their pinna. Structure of the ear is adapted for wheather you can move the pinnae or not.
Ear canal
- Like a funnel for sound
- major role is to amplify sounds
- Like a tube or chamber that sound will resonate in.
- amplifying certain frequencies over others that are particularly relevant for humans,
- Passes sound to the ear drum.
Ear drum
- Tissue that vibrates when sound waves hit it
- Where sound becomes transduced - the key point in transduction.
- vibrates in response to sound waves
- end of the line for the ear canal
Ossicles
- Chain of small bones → 3 bones
- Transmit sound to cochlea (pass on vibration from ear drum to cochlea)
- Malleus, Incus, Stapes.
Cochlea
- Cochlea is this coiled tube filled with fluid
- Very important membrane that will turn the signal into a neural signal
Why can’t sound waves just hit the cochlea? What do the ossicles do?
* They are needed to amplify the sound signal even more to get it to travel through thee fluid in the cochlea.
* If there were no ossicles and ear drums, the sound would go straight through the air and hit the cochlea → the signal will be damped down or lost. It will not be able to push the fluid in the cochlea the way it needs to.
* When the ear drum passes the sound to the ossicles, it ends up concentrating that energy into a really small space and that increases the pressure of the sound wave of the signal.
Basilar Membrane
- The cochlea has a basilar membrane
- The basilar membrane is the star of the show.
- You can see that at one end it is thick and narrow. At the other end, it is thin and wider.
- Given the structure of the basilar membrane, certain parts will vibrate more or less.
- The resistance is different along the basilar membrane = tonotopic map.
Tonotopic map
- Auditory equivalent to retinotopic map
- Different frequencies will make the basilar membrane vibrate more or less.
- At wide/thick end, you need high pitch frequencies.
- At thinner end, you need low pitch frequencies.
- Sounds that we hear in the environment are not usually just made up of one frequency, they are usually made up of many frequencies → generally when you perceive something, you wouldn’t just see generally one place on the basilar membrane oscillate in response to that signal, you would see many peaks along the basilar membrane.
- Early frequency mapping in the auditory system → frequencies of sound being seperated by one another.
*
Hair Cells
- Hair cells transduce mechanical signal into electrical (neural) signal
- It is still considered a mechanical signal when the basilar membrane vibrates so it needs to be transduced into the neural signal.
- At the top of the hair cells = stereocilia. These are tiny little hairs that are going to bend in response to the basilar membrane vibrating.
- When the stereo cilia bends, they will help transmit a signal to the auditory nerve.
Pink pathway (afferent) is the auditory nerve. Afferent = it is going to the CNS.
Blue patway (efferent). Efferent = information from the brain is being sent to the hair cells.
- Having these two pathways allows you to have a higher level of modulation of the sound.
Auditory Nerve
- Auditory nerve (comes out of hair cells) and projects to primary auditory cortex.
- Mostly to the contralateral side (ex: what you hear in your left ear will project to right hemisphere)
- There are some epsilateral projections.
- The signal will then travel up through some subcortical nuclei (some relay station it is getting processed in).
- The signal is continously tonotopically coded . Therefore, the coding is never lost in the auditory nerve and then it arrives at the auditory cortex. Note: auditory cortex is in the superior temporal lobe.
- The auditory cortex is not the terminus for auditory perception. It will send the information to other regions. For example, it has connections with brocas area and wernickes area (for language) and the primary motor cortex (movement- making music, dancing, motor activity with respect to sound).
What is the role of auditory cortex in auditory perception?
- It has a major role in creating a complex auditory percept.
- If you destroy the auditory cortex, you don’t lose all if your auditory perception. There is some processing going on at the lower subcortical levels
Dorsal vs Ventral pathway
Dorsal: sound localisation (where are things)
- ability to figure out where sound is in space around us
Ventral: sound properties (what is sound?)
- what the sound source is
- what does it correspond to in the world.
Frequency
- Frequency (Hz)
- Perceptual property of frequency is pitch
- Hertz = cycle per second
- one cycle is from the top of one wave to the top of another. Each cycle is = 60Hz.
- the two waves below are taken place in the same amount of time, but there are more cycles in the high frequency wave (top) vs the low frequency wave (bottom).
- The human ear shows optimal tuning. It captures and amplifies sounds that are between about 1000 and 4000Hz. This tends to correspond to the range of frequencies that really show up in human speech.
High pitch = high frquency
Low pitch = low frequency
Amplitude
- Amplitude is measured in decibels
- The perceptual property is loudness
- High amplitude = louder.
Complex sound waves
- In reality, sounds that we hear are generally complex versus just one single sound wave.
- Each complex sound wave is made up of a combinantion of simple sound waves.
- You can deconstruct the sign wave
- You sum the waves together to get the complex one
- The auditory system can do the deconstruction and reconstruction.
Equal loudness contours
- Pitch and frequency determine loudness and perception.
- The two interact to generate your perception of loudness. Not all frequencies that we hear are perceived at the same loudnesss.
- Phons: perceptual measurement of loudness → perceptual scale →”how loud is that?”
- Red curve: demonstrates how much decibel power we need to play the sound so that it is as loud as another frequency. How loud do you have to play a sound at 1000Hz so that it sounds like a sound of 10K Hz.
- Different steepness of curve depending on loudness perception (soft/quiet sound = steeper).
- **Your perception of loudness is a function of both the frequency and the amplitude of the wave. **
What are the two cues we use for locating sounds in space?
- Internaural time difference: when a sound is made on the right side of your body, the sound will arrive sooner to your right ear than its going to arrive to your left ear. Your auditory system is going to compute that difference to figure out that the sound is on the right side of your body.
- Interaural level difference: It will be louder in your right ear then left and your auditory system will compute that difference.
Relation of anatomy to function
* Not all animals have ears on the side of their head.
* Because our ears are on each side of our head, they are good at detecting horizontal sounds. Our ears are well situated to detect sounds that deviate from right in front of us.
* Vertical souds is done by pinna. The way that sounds hit your pinna from above or below iss going to be different. Reflections and amplifications will be different which will help you figure out if something is above or below you.
Temporal grouping
Sequential integration: Auditory streams arise
→ Process of connecting sounds together in time
→ Connecting sounds together in time, we end up creating distinct auditory streams (temporal proximity).
→ Important for when you are listening to different people talking
→ Important for making music: you need to be able to parse out or group together different pitches and music.
→ Essentialy, getting these auditory streams based on the relationships and time.
Physical cue: Temporal Proximity
When do you need to do this?
Pitch/Harmonic grouping
- Remember: most sound waves that you are hearing are complex sound waves that are composed of a bunch of simple sound waves. When you add all of them together, you get the complex sound wave that you hear.
- Relationship between sound waves is really important for hearing pitch (harmonics).
- Fundamental frequency: Lowest frequency component (lowest pitch you might hear in that sound.
- ** Harmonics:** Multiples of the fundamental frequency
* harmonic structure influences what we hear
* Systematic, predicatable relationship to the fundamental frequency.
Challenge: Do a bunch of frequencies come from the same source?
- this will help us say if they come from the same source
Pitch/ harmonic grouping
Target frequency
You can shift the fundamental frequency and harmonics around the target frequency.
* You can keep a sound at exactly the same pitch and just move all of the sine waves around it up and down. You will go from hearing one sound to hearing 2 sounds. When you bring them back togetther you end up hearing just one sound again = breaking thee harmonics.
