Sound perception and itâ€™s neural coding

Sound perception and it’s neural coding 03/29/2012 Dr. Sorinel A. Oprisan PHYS 150

Sensations and perception “In psychology, sensation and perception are stages of processing of the senses in human and animal systems, such as vision, auditory, vestibular, and pain senses.” (http://en.wikipedia.org/wiki/Sensation_and_perception) Sensations are the first stages in the functioning of senses to represent stimuli from the environment, and perception is a higher brain function about interpreting events and objects in the world. (David G. Myers (2004). Exploring Psychology, Macmillan. pp. 140–141. ISBN 9780716786221) Review of concepts (6 min) http://www.sumanasinc.com/webcontent/animations/content/soundtransduction.html

Sensations and perception

Sensations and perception Sound introduced into the cochlea via the oval window flexes the basilar membrane and sets up traveling waves along its length. The taper of the membrane is such that these traveling waves are not of even amplitude the entire distance, but grow in amplitude to a certain point and then quickly fade out. The point of maximum amplitude depends on the frequency of the sound wave.

Each point on the basilar membrane oscillates up and down at the same frequency as the sound. What differs from point to point is the size of the oscillation.

Displacement of the basilar membrane in response to a pure tone

Zwislocki and Feldman, 1963

Sensations and perception The pinna and middle ear act as mechanical transformers and amplifiers, so that by the time sound waves reach the organ of Corti, their pressure amplitude is 22 times that of the air impinging on the pinna. Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear. Mammalian cochlear hair cells come in two anatomically and functionally distinct types: the outer and inner hair cells. Damage to these hair cells results in decreased hearing sensitivity, i.e. sensorineural hearing loss. The organ of Corti contains 15,000-20,000 auditory nerve receptors. Each receptor has its own hair cell. The shear on the hairs opens non-selective transduction ion channels that are permeable to potassium and calcium, leading to hair cell plasma membrane depolarization, activation of voltage-dependent calcium channels at the synaptic basolateral pole of the cells which triggers vesicle exocytosis and liberation of glutamate neurotransmitter to the synaptic cleft and electrical signaling to the auditory cortex via spiral ganglion neurons.

Confocal and scanning electron micrographs of inner and outer hair bundles (scale bar 1 mm) Eric A. Stauffer & Jeffrey R. Holt, J Neurophysiol, 98, 2007

Auditory nerve Each auditory nerve fiber responds to a narrow band of frequencies, with a phase locked response that increases in rate with sound intensity A tuning curve shows the amplitude of the input at each frequency required to produced an equal response in a device Frequency threshold curves (FTCs), or tuning curves, plot the minimum intensity of sound needed at a particular frequency to just stimulate an auditory nerve fibre above spontaneous activity. Damage to the cochlea easily abolishes the tip, and explains some features of Sensori-Neural Hearing Loss: raised thresholds and reduced frequency selectivity.

Characteristics of auditory nerve tuning curves • Band-pass in shape, about 1/3 octave wide. • Best or “characteristic” frequency. • Steep high frequency slope • Extended low frequency tail

Even when you just present a tone at a moderate intensity, more than one nerve fiber will respond. The brain has to look at the pattern of response across nerve fibers (sometimes called the “excitation pattern”).

Auditory nerve fibers respond at some rate even when no sound is presented = spontaneous activity.

A hair bundle = 20 to > 300 stereocilia (1mm – 100mm in length, a tip link (pink) is 150 nm long, a channel (yellow) is < 10 nm in diameter, and its gate (orange) moves by 4 nm) A robust stimulus (60 dB sound-pressure level) deflects the bundle only 10 nm, whereas a threshold stimulus moves the bundle less than 1 nm.

Applied force (green arrow) extends the tip link. When a channel opens (curved orange arrow), relax the tip link, causing the bundle to move still further (red arrow). A. J. Hudspeth, Y. Choe, A. D. Mehta, and P. Martin, PNAS 2000, 97(22):11765–11772

A stimulus force (green arrow) initially deflects the hair bundle, opening a transduction channel. A Ca2+ ion (red) that enters through the channel interacts with a molecular motor, probably myosin, and causes it to slip down the stereocilium’s actin cytoskeleton (orange arrow). Slackening of the tip link fosters a slow movement of the bundle in the positive direction (dashed red arrow). The reduced tension in the tip link then permits the channel to reclose.

A. J. Hudspeth, Y. Choe, A. D. Mehta, and P. Martin, PNAS 2000, 97(22):11765–11772

If the internal-energy content of a two-state channel is EO in the open state and EC in the closed state, the equilibrium probabilities of these two configurations, respectively pO and pC, are related by the Boltzmann equation A. J. Hudspeth, Y. Choe, A. D. Mehta, and P. Martin, PNAS 2000, 97(22):11765–11772 (Fettiplace& Mackney, 2006

http://www.leica-microsystems.com/science-lab/the-patch-clamp-technique-an-introduction/http://www.leica-microsystems.com/science-lab/the-patch-clamp-technique-an-introduction/

A virtuous loop. Sound evoked perturbation of the organ of Corti elicits a motile response from outer hair cells, which feeds back onto the organ of Corti amplifying the basilar membrane motion.

Two-tone suppression If a tone at a fiber's CF is played just above threshold for that fiber, the fiber will fire. But if a second tone is also played, at a frequency and level in the shaded area of the next diagram, then the firing rate will be reduced. This two-tone suppression demonstrates that the normal auditory system is non-linear. Two-tone inhibition is the result of mechanical processes inthe cochlea.

Place Code Theory: Helmholtz's theory Sensation of a low frequency pitch derives exclusively from the motion of a particular group of hair cells, while the sensation of a high pitch derives from the motion of a different group of hair cells

Temporal Code Theory: According to temporal code theory, the location of activity along the basilar membrane is irrelevant. Rather, pitch is coded by the firing rates of nerve cells in the audotry nerve. But a single nerve cell can not signal at a rate of 20,000 Hz

Cochlear Microphonic: The cochlear microphonic is a discovery that cast doubt on Helmholtz's place code and supports the temporal code theory. It was discovered by Wever.

Phase Locking is an empirical observation that supports the volley principle.

Sensations and perception

Tonotopic organization: The spatial layout of frequencies in the cochlea along the basilar membrane is repeated in other auditory areas in the brain. http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html

http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.htmlhttp://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html

Auditory cortex Located in the temporal lobe, the auditory cortex is the primary receptive area for sound information. The auditory cortex is composed of Brodmann areas 41 and 42, also known as the anterior transverse temporal area 41 and the posterior transverse temporal area 42, respectively. Both areas act similarly and are integral in receiving and processing the signals transmitted from auditory receptors. (http://en.wikipedia.org/wiki/Sensation_and_perception) There are between 30,000 and 40,000 nerve fibres in the cochlea of a normal hearing adult (Tylstedt, 2003).

The number of myelinated axons per millimetre length of the cochlear duct: • -300 fibres per millimetre at the basal end of the cochlea • reaches a maximum in the lower second turn with 1400 fibres per millimetre- • decreases again towards the cochlear apex to about 400 fibres per millimetre. • This corresponds to an average of 15 nerve fibres per inner hair cell in the lower second turn and 3-4 nerve fibres per inner hair cell at the base and apex (Spoendlin and Schrott, 1989). • Average interneural distance of 700 nm in the lower second turn of the cochlea and 3300 nm at the basal end.

Temporal response patterns of a low-frequency axon in the auditory nerve. The stimulus waveform is indicated beneath the histograms, which show the phase-locked responses to a 50-ms tone pulse of 260 Hz. Note that the spikes are all timed to the same phase of the sinusoidal stimulus. (After Kiang, 1984.)

Each hair cell has about 10-20 auditory nerve fibers connected to it. These fibers have different thresholds. Inner hair cells stimulate the afferent auditory nerve, outer hair cells generally do not, but are innervated by the efferent auditory nerve. Efferent activity may influence the mechanical response of the basilar membrane via the outer hair cells.

Sound perception and itâ€™s neural coding