Lecture Notes 5 - psyc203 PDF

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PSYC211 – Machado – 5

Auditory System: Sense of Hearing

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Audition How are sounds from our environment translated into neural signals? Hearing commences with the capture of sound waves by the ear. Within the ear, auditory receptors convert the mechanical energy of sounds into an electrical signal, and then that information is transmitted to the brain.

Sound Waves • Sound is caused by small areas of high and low pressure propagating outward from the source. The Auditory Apparatus The ear has three parts: 1. Outer ear 2. Middle ear 3. Inner ear (Image from Neuroscience: Exploring the Brain, p. 276, Figure 11.3)

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Outer Ear • Pinna – a prominent fold of cartilage-supported skin; captures sound and focuses it into the auditory canal. • Auditory Canal – ends at the eardrum Middle Ear • Ear drum (or tympanic membrane) • Ossicles – middle ear bones

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PSYC211 – Machado – 5 Middle Ear When a sound wave reaches the middle ear, a series of high and low pressure regions impinge upon the eardrum. The arrival of a high pressure region pushes the eardrum inward; the arrival of a low pressure region pulls the eardrum outward. The continuous arrival of high and low pressure regions causes the eardrum to vibrate. Because the eardrum is attached to the bones in the middle ear, the bones begin vibrating as well. Thus, the sound signal is transformed into the mechanical vibrations of the bones in the middle ear. These vibrations are then transmitted to the fluid of the inner ear via vibration of the membrane at the oval window.

Inner Ear • Cochlea – spiral-shaped fluid-filled tube; contains the hair cells that serve as the receptors for audition. • Within the cochlea, the vibrations produce waves in the fluid that cause the hair cells to move. • The hair cells respond by converting the mechanical signal into an electrical signal. • Within the cochlea, hair cells synapse on spiral ganglion cells. (Image from Neuroscience: Exploring the Brain, p. 289, Figure 11.15)

Tuning curve of a spiral ganglion cell • Spiral ganglion cells are tuned to specific frequencies (e.g., a spiral ganglion cell may be maximally sensitive to a sound of 1600 Hz, with the firing rate dropping off rapidly for lower- and higherfrequency sounds).

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Tinnitus With tinnitus, a person hears noises in the absence of any sound stimulus. The subjective sensation can take many forms, including buzzing, humming, and whistling. You may have experienced temporary tinnitus after listening to loud music. Tinnitus is typically caused by disease processes affecting the cochlea or auditory nerve, but it can also be caused by spontaneous activity. Inner Ear → CNS Your inner ear contains structures for both the sense of hearing and the sense of balance. The axons of the spiral ganglion cells exit the cochlea and converge with the axons of vestibular neurons to form the vestibulocochlear nerve. The vestibulocochlear nerve carries nerve impulses for both balance (vestibular nerve) and hearing (auditory nerve) from the ear to the brain. Thus, auditory information travels via the vestibulocochlear nerve to the brain stem.

from the cochlea into the brain stem, where the spiral ganglion cells synapse on neurons in the cochlear nuclei, which are located at the level of the lower pons-upper medulla. Auditory Pathways • From the cochlear nuclei, auditory information ascends bilaterally to the inferior colliculi, which are part of the midbrain. • Neurons in the inferior colliculus synapse on neurons in the medial geniculate nucleus of the thalamus. • Neurons in the medial geniculate nucleus synapse on neurons in primary auditory cortex.

Primary Auditory Cortex • The first region of cortex to process sound • Located in the superior temporal lobe and buried within the lateral sulcus. • Primary auditory cortex = Heschl’s gyri = A1 • In primary auditory cortex, the frequency tuning properties of the cells define a tonotopic map. • The tonotopic organization of the cells within the cochlea is maintained through to primary auditory cortex.

(Image adapted from Cognitive Neuroscience: The Biology of the Mind, page 110, Figure 4.9)

Vestibulocochlear Nerve

• The vestibulocochlear nerve carries the signal

(Image adapted from Cognitive Neuroscience: The Biology of the Mind, page 165, Figure 5.1)

PSYC211 – Machado – 5 Overview: Sound Waves from the Environment to the Brain • Outer Ear: Sound waves are captured by the pinna and focused into the auditory canal. • Middle Ear: The sound waves strike the ear drum and cause it to vibrate, which then vibrates the bones in the middle ear, which then vibrates the membrane at the oval window. • Inner Ear → Brain: The vibrations cause waves in the fluid within the cochlea. These waves move the hair cells, which convert the mechanical signal into an electrical signal. The electrical signal from hair cells is transmitted to spiral ganglion cells, whose axons exit the cochlea, join the vestibulocochlear nerve, and then synapse on cells in the brain stem.

PSYC211 – Machado – 5

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Sound Localization The tonotopic map in primary auditory cortex explains how we can identify different tones. How do we identify the location of a sound source? Sounds can be localized based on slight asymmetries in the arrival time at the two ears (e.g., sound sources on your left arrive at your left ear first). The difference in the arrival time of a sound at each ear is called the interaural time. Sound localization along the vertical plane is not as good in humans. To determine the elevation of sound sources, humans depend on the bumps and ridges of the outer ear, which produce reflections of the entering sound. Our ability to determine the elevation of a sound source is seriously impaired if the pinnae are covered.

Overview: Sound from the environment to primary auditory cortex • External Ear: Sound waves are captured by the pinna and focused into the auditory canal. • Middle Ear: The sound waves strike the ear drum and the vibrations pass through the ossicles to the cochlea. • Inner Ear: Hair cells within the cochlea transduce the vibrations into a neural signal, which is sent to spiral ganglion cells, whose axons form the cochlear nerve, which carries info to the brain stem via the vestibulocochlear nerve. • Brain Stem: Auditory information is received by the cochlear nuclei (in the pons/medulla) and then is sent bilaterally to the inferior colliculi. • Thalamus: From the inferior colliculus, auditory information is sent to the medial geniculate nucleus (MGN) of the thalamus. • Cortex: From the MGN, auditory information is sent to primary auditory cortex in the superior temporal lobe. Auditory Pathways • Not all auditory information crosses the midline: Each of your ears projects auditory information to both hemispheres of your brain. • Whether sensory information is transmitted unilaterally or bilaterally helps determine the extent to which damage to a sensory system affects our sensory experience. • Do you think the outcome is better if you damage a sensory system for which information is transmitted unilaterally or bilaterally?

Reference List Bear, M. F., Connors, B. W., & Paradiso, M. A. (1996). Neuroscience: Exploring the Brain. Baltimore, MD: Williams & Wilkins. ISBN 0-683-00488-3 (Figure 11.3 from page 276 and Figure 11.15 from page 289) Gazzaniga, M. S., Ivry, R. B., & Mangun, G. R. (2002). Cognitive Neuroscience: The Biology of the Mind (Second ed.). New York, NY: W. W. Norton & Company. ISBN 0-393-97777-3 (Figure 4.9 from page 110) Gazzaniga, M. S., Ivry, R. B., & Mangun, G. R. (2009). Cognitive Neuroscience: The Biology of the Mind (Third ed.). New York, NY: W. W. Norton & Company. ISBN 978-0-393-92795-5 (Figure 5.1 from page 165)...