BISC 421 1st Edition Lecture 22 Outline of Current LectureI. Central Auditory SystemCurrent LectureCentral Auditory SystemAuditory System: Central Pathways-need to know about the names of the nucleiParallel Pathways from the Brainstem to the Cortex: Overview-Parallel pathways, each analyzes particular features•Block diagram of what we just saw•Theres an equivalent of where and what pathway in the auditory system•We will concentrate on the posteroventral cochlear nucleus and the anteroventralcochlear nucleus •The auditory system is a little bit delayed in figuring out what a sound is versus where it came fromStreams separate in cochlear nucleus: different cell typesproject to specific nuclei in a manner somewhat reminiscent of “what” and “where” pathways in vision.Cochlear NucleiVIII nerve: branches to innervate the 3 cochlear nuclei in a tonoptopic pattern: Dorsal Cochlear Nucleus Posteroventral Cochlear Nucleus Anteroventral Cochlear Nucleus Start of feature processing.Distinct cell classes in the ventral nuclei: Stellate Cells preserve stimulusintensity (AVCN & PVCN).•Anatomical picture of the cochlear nuclei (where the division of labor occurs)•Innervates 3 cochlear nuclei in a tonotopic pattern•Two anatomical kinds of cells – stellate cells fire harder and harder the more current(amplification of sound-‐ encodes the loudness or intensity of sound)•Auditory complex is very sensitive to timing•Bushy cells in the AVCN are capable of preserving timing•Stellate cells innervate lateral superior olive•Bushy cells innervate medial superior olive•Bushy cells don't fire very fast but they are blocked on to a particular phase of the stimulus but stellate cells fire very fastCircles and Sine Waves phase-lock movie again•Watch phase lock move on black board•Idea is that a particular hair cell will only fire when a stimulus is around 90 degrees•This is called phase locking•This is what hair cells and bushy cells do•This is how we encode timing and timing differencesSound Localization: : Interaural time-difference – ITDITDs, given by the difference in arrival times of waveform features at the two ears, are useful localization cues only for long wavelengths. In (a), the signal comes from the right, and waveform features such as the peak numbered 1 arrive at the right ear before arriving at the left. Because the wavelength is greater than twice the head diameter, no confusion is caused by other peaks of the waveform, such as peaks 0 or 2. In (b), the signal again comes from the right, but the wavelength is shorter than twice the head diameter. As a result, every feature of cycle 2 arriving at the right ear is immediately preceded by a corresponding feature from cycle 1 at the left ear. The listener cannot locate the source of the sound based on timing.If the length of the sound wave is longer than the diameter of the head (that is, if the frequency is low enough), then differences in interaural phase provide information about the location of a sound source.•Takes about 0.6 ms to get from one side of the head to another•We can use this to localize the sound and where it is coming from•If there is an ongoing sound we use ITDs•If the wavelength is longer than the head there will be a unique phase angle at each ear•However if the wavelength is very high we won't be able to tell•The wavelength has to be longer than the head for this to be effective-‐ know the concept in the box.The coincidence detection model of Jeffress (1948)A model for low-frequency sound localization. The coincidence detecting circuits are in the MSO.•There is a way to take these time differences and relay them to the medial superiorolive•If sound is coming from the right and I want neuron to fire when it gets input from the left and want neuron to sense sound at same time then the axon coming from the right is short and the left ear long•Coming from straight ahead they have the same length delay lines.Interaural Intensity Differences Help Localize High Frequency Sounds.- At high frequencies, the head acts as a sound shadow.-Interaural level differences, calculated for a source in the azimuthal (horizontal) plane.The is located at an azimuthal angle of 10° (green curve), 45° (red), or 90° (blue) with respect to the listener’s forward direction.•Myelin helps with conduction velocity as well as size•We also have sound shadows cast to localize the source on the azimuthal plane•If it is a little bit off center, the sound difference between the ear starts to creep up•At 90 degrees there is a very big difference in the sound•Can use these differences in loudness to localize the sound.Circuits Sensitive to Interaural Intensity Difference: the LSOOne branch innervates the lef LS0 and evokes a spike that travels to higher centers. The other branch crosses the midline and excites a cell in the right MNTB. Cells in the MNTB are inhibitory and synapse with targets in the LSO.Here, the activated cell in the MNTB inhibits a neuron in the right LSO.•This comparison happens in the lateral superior olive•Slice at the MNTB•Idea is that input coming in from both ears and one excites the left LSO and theother crosses the midline and excites cell in right MNTB which inhibits a neuron inthe right LSO•A sound is louder in the left than the right ear is the bottom line-‐ left LSO fires hard•So far have only been talking about binaural sound in the horizontal planeThe pinna is used for monaural sound localizationThe pinna changes the acoustic spectrum that reaches the middle ear as a function of the position of the sound source. We use our pinnas to detect elevation.•But we also need to be able to localize sound in elevation•We use our pinnas to detect elevation monaural•This is spectral content•Take any sound and decompose it into a series of many frequenciesTo Cortex via the Inferior Colliculus & Medial Geniculate Nucleus of the Thalamus•In the inferior coliculus we start to put together all of the cues we’ve been talking about•Cells there can actually form a complete map of space•Relays information to the medial geniculate to overlap with regions of visual space as wellOwls have a map of space in their inferior colliculus.•This was discovered in the owl•Put an owl in the middle of a hoop and found that there were cells that responded to specific regions in auditory space•Owls