25 - More on sound, the earS, & the auditory system (10.22.25)

More on sound, the earS, & the auditory system

10.22.25

Announcements

• Exam 1
• Curved exam grades now posted
• Exam 2
• postponed until Monday, November 3rd
• mark your calendars!

Today's Agenda

• Finish up "Sound: Physical & Perceptual Dimensions" (Monday's lecture)

• Move onto "The Auditory System" (today & Friday's lectures)

1. Sound (cont'd)

CYMATICS VIDEO

How Loudness Varies Across Frequency

• Decibels measure the physical sound pressure level, which is an objective measurement of sound energy.

• Phons measure the subjective perception of loudness by the human ear

• Phon - unit of loudness equal to the amplitude of a standard (1,000 Hz tone at 10 dB)

Deconstructing A Waveform

• Fourier transform - a way of separating a waveform into components, each of which has a distinct frequency

• Fundamental frequency - the lowest frequency component, which determines perceived pitch

• Other higher frequencies are called harmonics

Deconstructing A Waveform (cont'd)

2. the auditory system (finally)

After the ear, where to next?

Ascending Pathways:From the Ear to the Brain

• Axons from the inner and outer hair cells = the auditory nerve.
• The first stop is the cochlear nucleus of the medulla.
• Some go to the dorsal nucleus while others go to the ventral nucleus.
• Next is the superior olivary nucleus then the inferior colliculus
• Alt route: bypass the superior olivary nucleus & go directly to the inferior colliculus.
• From the inferior colliculus, the neurons travel to the medial geniculate nucleus of the thalamus
• Finally, from there to the auditory cortex (areas 41 and 42).

From the Ear to the Brain

Auditory processing begins in the brainstem

• whereas visual processing in the thalamus

There is a single overarching pathway (midbrain and thalamus connected) Less strongly lateralized than vision

Tonotopic Representation in Auditory Cortex

Similar principle to the organization of visual cortex

Tuning Curves in Primary Auditory Cortex (A1)

• Similar principle to the representation of orientation in visual cortex
• Considerable variety in the shape of the tuning function
• As with orientation, frequency is represented in the population code (cannot be resolved from a single neuron)

Dorsal and Ventral Pathways in Cortical Processing of Auditory Information

• Similar organization to visual information processing
• Strongly supported by selective deficits seen in patients with brain damage
• Note the neuronal populations that respond to both modalities

Localizing Sound

Thinking about sound in head-centered coordinates

Azimuth: side-to-side dimension (left or right of median plane) Elevation: up-down dimension (above or below horizontal plane) Distance: from center of head (total, any direction)

Localizing Sound

Measuring the accuracy of sound localization

Same/different judgment Minimum audible angle: 75% correct threshold < 10 degrees, can be as low as 1 degree How? Not directly represented in the cochlea (tonotopically organized)

How Sound is Influenced by the Head

• Acoustic shadow cast by the head
• Frequency-modulated
• less severe for lower-frequency sound
• Causes differences in the intensity (dB) of sound between the two ears for sounds off of the median plane

Intraural Level Differences

Interaural level difference caused by the acoustic shadow peaks at 90 degrees azimuth

Intraural Time Differences

• Very small difference in arrival time to each ear due to the speed of sound (on the order of microseconds)
• Normal human sensitivity, as measured in laboratory experiments using artificial stimuli, is very high (< 100 μs)

Ambiguity in Sound Localization

• Sounds from corresponding points in front of and behind a person will produce essentially identical intraural differences
• Insufficient information to resolve the source of the difference in neural activity

Resolving Ambiguity with Head Motion

• By turning the head, intraural differences emerge immediately
• Often done without thinking (unconscious)

Perceiving Elevation

• The pinna causes incoming sounds to reverberate (echo)
• Whether these reverberations amplify or attenuate a sound depends on frequency and elevation (in addition to azimuth)
• You can learn to interpret these differences in terms of elevation

Perceiving Elevation

• Spectral shape cue: the pinna-induced modification of the sound spectrum
• This ability to learn how to interpret the spectral shape cue is important because each person’s pinnae (plural) are unique, like a fingerprint

Perceiving Elevation

• Spectral shape cue: the pinna-induced modification of the sound spectrum
• This ability to learn how to interpret the spectral shape cue is important because each person’s pinnae (plural) are unique, like a fingerprint

Distance Cues for Sound

• Comparing loudness to familiar sounds
• very assumption-heavy
• Sound quality and distance
• reduction in sound level is greater for higher frequencies
• creates a progressive “blurring” effect
• Sound and movement
• moving towards or away
• Doppler effect: the frequency of a moving sound source is higher in front of the source

Human Echolocation

Daniel Kish- a blind man who taught himself to ”see” the world using echolocation

Separating Echoes from the Source

Precedence effect: Sound will arrive first and more intensely from the source, distinguishing it from echoes

Thanks for listening!

Misc resources below:

Interactive 3D model of the ear

Music Is Sound (mnemonic for the ossicles)

Exit ticket