Pitch Perception Chapter 9 Hearing and Language Pages 270 excluding language First to respond to stimulus from cochlea basic sound perception Primary auditory cortex Forebrain Second auditory cortex Complex aspects of sound Auditory cortex is topographically organized Looks like a map of basilar membrane Adjacent neurons in the cortex receive info from adjacent receptor locations in the basilar membrane Base of cochlea near where it emerges from the inner ear other end is known as the apex Stimulate base shows up in corresponding part of auditory cortex Reminder experience of the frequency of sound Frequency Theory basilar membrane vibrates in synchrony with a sound auditory nerve axons fire at the same frequency Problem individual neurons can fire at no more than about 1 000 Hz but we can hear up to 16 000 Hz due to the absolute refractory period of neurons Explains at best the lower pitched sounds Volley Theory groups of neurons can follow the frequency of a sound where a single neuron cannot take turns picture in textbook This does happen in auditory system but there aren t really enough neurons in auditory nerve to keep up with very high frequencies Place Theory sounds with different frequencies induce peaks of maximal vibration in different places on the basilar membrane Base narrow and stiff tend to see peaks of high frequency sound Apex wide and floppy tend to see peaks of low frequency sound Because the structure of basilar membrane is different frequency theory further disproved because it s unlikely the entire structure would vibrate as one with different structure 16 kHz 200 Hz tonotopic organization of basilar membrane Primary auditory cortex contains a tonotopic map of the basilar membrane further back you go the higher the pitch goes Problem Can hear lower than 200 Hz gets down to 20 or 30 Hz run out of basilar membrane what happens is that the lower frequencies cause the entire basilar membrane to vibrate in synchrony frequency theory Frequency Place Theory combination of frequency and place theory Synchrony of firing rate of auditory nerve axons with sound frequency pitch perception of sounds up to about 200 Hz Perception of Complex Sounds Fourier harmonic analysis Cocktail party effect Sound localization Place of maximal vibration on basilar membrane pitch perception of sounds 200 Hz Currently accepted by most scientists Pure tones Any complex sound can be broken down into pure tones aka component frequencies One fundamental frequency and multiple overtones to create the complex sound The basilar membrane of the cochlea acts as the Fourier harmonic analyzer tone spreads out across the membrane and the different tones vibrate different areas thus serving as an analyzer Ability to sort out meaningful auditory signals from a complex background of noises Attention is involved not reflexive like walking Possible role played by outer hair cells which work as effectors rather than receptors receive signal from brain act as muscles to shrink or grow causing tectorial membrane to alter stimulates basilar membrane Altering membrane via outer hair cells allows you to filter out or reduce the intensity of certain frequencies and therefore pay attention to other frequencies better Poorly understood a lot of computation in brain regions but know little about the neural systems that govern it Three types of binaural i e involving use of both ears cues brain locates the source of a sound based on differences between the sound at the two ears don t work if the source is aligned with the midline of body has to be right or left Phase difference sound arriving from one side of the body is at a different phase point of the wave at each ear works for low frequency sounds there s a picture in the textbook Superior olive has pathways that perform quick trigonometry to figure out where the sound is Animals head cocking doing the side head tilt behavioral way to adjust the wave input and better locate the source of the sound more for those that rely on hearing Predatory birds head cock as a way to improve depth perception Never stop at the surface when studying behavior Intensity difference the head creates a sound shadow near ear receives a slightly more intense sound than far ear Neural networks reflexive that are able to detect this subtle difference in loudness