in what two ways is frequency represented in the central nervous system?
-systematic organization of characteristic frequency is called tonotopy.
-tonotopic maps exist on the basilar membrane within each of the auditory relay nuclei, the MGN, and auditory cortex.
2. Phase locking
-recordings made from neurons in the auditory nerve show phase locking, the consistent firing of a cell at the same phase of a sound wave.
if a sound stimulus suddenly reaches your ears (within the horizontal plane), how do you localize the sound?
if a sound isn't coming from directly in front of you, there will be a delay between the time it takes the sound to reach one ear and the time it takes for the sound to reach the other ear. This is known as interaural time delay. This interaural time delay is what allows us to localize a sudden sound within the horizontal plane
let's say that (again in the horizontal plane) instead of hearing a sudden sound, you hear a continuous tone. The continuous tone has a very long wavelength, much greater than 20 cm. How would you be able to localize the sound?
in this case, the only thing that can be compared between continuous tones si the time at which the same phase of the sound wave reaches each ear.
Imagine you are exposed to a 200 Hz sound coming from the right. At this frequency, one cycle of the sound covers 172 cm, which is much more than the 20 cm distance between your ears. Afer a peak in the sound pressure wave passes the right ear, you must wait 0.6 msec, the time it takes sound to travel 20cm, before detecting a peak at the left ear.
let's say that (in the horizontal plane) you hear a continuous tone (as opposed to a sudden sound) that has a very high frequency (much less than 20 cm). How would you be able to localize this sound?
An interaural intensity difference exists between the two ears because your head effectively casts a sound shadow. There is a direct relationshop between the direction the sound comes from and the extent to which your head shadows the sound to one ear. If sound comes directly from the right, the left ear will hear a significantly lower intensity.
describe the cellular/nerval basis for the interaural time delay
one possibility is to use axons as delay lines and to measure small time differences precisely. A sound hitting the left ear triggers action potetnials in the left cochlear nucleus, which propagate along afferent axons into the superior olve. Within 0.6 msec of hitting the left ear, the sound reaches the right ear and triggers action potenials in axons from the right cochlear nucleus. However, because of the way the axons and neurons are arranged in the olive, the action potentials from each side take different lengths of time to arrive at the various postsynaptic neurons in the olive.
how is sound localized in the vertical plane?
the sweeping curves of the outer ear are essential for assessing elevation of a source of sound. the bumps and ridges apparently produce reflections of the entering sound. the delays between the direct path and the reflected path change as a sound source moves vertically.
how are the neurons in the primary auditory cortex arranged?
there are isofrequency bands running across A1. In other words, strips of neurons running acros A1 contain neurons that have fairly similar characteristic frequencies.
true or false? the structure of A1 and secondary auditory areas is similar to the corresponding visual cortex areas?
why is it that a bilateral cut of the auditory cortex leads to deafness, but a unilateral cut results in a surprising degree of retention of auditory function?
the reason for greater preservvation of function after lesions in auditory cortex is that both ears send output to cortex in both hemispheres.
the vestibular system evolved from what?
the lateral line organs
what are the two structures that compose the vestibular labyrinth?
the otolith organs, which detect the force of gravity and tilts of the head, and the semicircular canals, which are sensitive to head rotation.
what are the two otolith organs?
the utricle and the saccule
how many semicircular canals are there?
what is the sensory organ within each otolith organ called?
what do the saccule and utricle detect?
they detect changes of head angle as well as linear acceleration of the head
describe the structure of the macula
the macula contains hair cells, which lie among a bed of supporting cells with their cilia projecting into a gelatinous cap. Movements are transduced by hair cells in the maculae when the hair bundles are deflected.
what is located above the gelatinous cap of the macula?
otoliths. pieces of calcium carbonate
what is the especially tall cilium on a hair cell called?
bending of hairs toward the kinocilium results in a ________
bending of hairs away from the kinocilium results in a _________
the macula is oriented in which direction in the saccule?
the macula is oriented in which direction in the utricle?