Auditory Brain PDF

Sound Localization Cochlear nucleus: structure on each side of the brain that receives signals via Type 1 auditory nerve fibers from inner hair cells in the ipsilateral ear Spatial Hearing ◦ We use spherical coordinates: position of a source relative to the observe is given as the: ◦ AZIMUTH: a...

Sound Localization Cochlear nucleus: structure on each side of the brain that receives signals via Type 1 auditory nerve fibers from inner hair cells in the ipsilateral ear Spatial Hearing ◦ We use spherical coordinates: position of a source relative to the observe is given as the: ◦ AZIMUTH: angle in the horizontal plane Minimum audible angle: min. angular separation between a reference sound source and a difference sound source emitting a tone of the same frequency that yields 75% correct judgements ◦ ELEVATION: angle in the vertical plane ◦ DISTANCE Hearing Accuracy ◦ Better at accurately locating sounds in the horizontal plane than the vertical Brainstem processing ◦ Interaural level Difference: difference in the sound level of the same sound at 2 ears ◦ processed in the lateral superior olive > inferior colliculus ◦ Acoustic shadow: area on the other side of the head from a 2nd source in which the loudness of the sound is reduced because the sound waves are partially blocked by the head ◦ Interaural Time Difference: difference in arrival time of the same sound at the 2 ears ◦ processed in the medial superior olive > inferior colliculus ◦ ITD peaks at 90 degrees Types of cues 1. BINAURAL CUES: provide information about the azimuth (horizontal) of sound ◦ ILD = for higher frequency sounds ◦ ITD = for lower frequency sounds Neural code for lower frequency sound is phase-locked which allows us to perceive with precision ITD as low frequency Less precise as frequency increases Cone of confusion: hypothetical cone-shaped surface in auditory space ◦ When 2 equally distant sound sources are located on a cone of confusion, their locations are confusable because they have highly similar ILD and ITD ◦ SOLUTION: head motion ◦ Turning head to one side changes sound ILD and ITD and eliminates ambiguity 2. MONAURAL CUES: provide information about the elevation (vertical) of the sound and resolve ambiguity in binaural cues ◦ Sound reflecting off the ridges of the pinna causes direction-specific frequency distortions ◦ spectral cues: determine the location of a sound in the vertical plane and resolve front/back ambiguities ◦ Unique for everyone because we all have different shaped pinnae Distance Perception Types of cues to estimate the distance of a sound: 1. Intensity: inverse square law relationship ◦ 2x as far away = 4x lower intensity 2. Relative intensity of direct vs reflected/echo of the sound Doppler effect Doppler effect: the frequency of a sound wave changes if its moving relative to an observer ◦ Towards you = higher frequency/pitch ◦ Away from you = lower frequency/pitch Ascending Pathway: from Ear to brain Auditory pathway Sound > Cochlea > Cochlear nucleus (brain stem) > bilaterally superior olivary complexes > inferior colliculus > medial geniculate nucleus > primary auditory cortex (A1) ◦ cochlear nucleus: in the brain stem (one on each side of the brain); it receives signals via Type I auditory nerve fibers from inner hair cells in the ipsilateral ear. ◦ superior olivary complex: in the brain stem/pons (one on each side of the brain); a stop on the ascending auditory pathway receiving signals from both cochlear nuclei. ◦ inferior colliculus: in the midbrain (one on each side of the brain); a stop on the ascending auditory pathway. ◦ medial geniculate body (MGB): in the thalamus (one on each side of the brain); the next stop on the ascending auditory pathway after the inferior colliculus. Descending Pathway: from Brain to Ear ◦ Numerous pathways carry signals between auditory cortex, subcortical auditory structures, and the ears ◦ Reduction in motile response of the OHCs ◦ Help protect the ear from damage by activating the acoustic reflex Acoustic reflex: contraction of tiny muscles attached to ossicles that limit their movements, preventing overstimulation of the cochlea Processing occurs in the brainstem Substantial processing of auditory signals occurs in the brainstem: ◦ Localization based on time and intensity = SUPERIOR OLIVE > COCHLEA (cochlear amplifier) ◦ Spectral cues = COCHLEAR NUCLEUS ◦ Unification of location cues = INFERIOR COLLICULUS > SUPERIOR COLLICULUS ◦ SC: Has spatial map of sound Primary Auditory cortex (A1) ◦ Located in superior temporal gyrus (STG) in lateral fissure ◦ Core region: primary auditory cortex (A1) + belt and parabelt regions ◦ Tonotopic organization that begins at the basilar membrane is maintained in A1 and core region ◦ Neurons that code for lower frequencies are in the anterior/rostral part of A1 ◦ Neurons that code for high frequencies at posterior/caudal end of A1 ◦ Frequency tuning neurons in A1 can be narrow or broad ◦ Will fire most to their preferred frequency and less to others ◦ Broadly-tuned neurons may play a role in integrating components of complex sounds ◦ People with damage to A1 show poor performance in tasks related to pitch perception ◦ In monkeys: Tasks requiring pitch activate areas equivalent to the core area in humans Tasks requiring recognition of complex stimuli activate areas equivalent to parabelt area in humans WHAT AND WHERE What (sound sources) pathway: core > belt & parabelt > the anterior temporal cortex Where (location of sound) pathway: core > posterior auditory cortex > posterior parietal cortex Hearing disorders Measuring ability to hear ◦ Pure tone audiometry: instrument that presents pure tone with known frequency and amplitude to the right or left ear ◦ Used in estimating the listener's absolute threshold for specific frequencies ◦ Pure tone audiometry: most common way ◦ Audiogram: measures perceptual thresholds at different frequencies Hearing impairments Hearing impairment: decrease in the ability to detect or discriminate sounds Tinnitus: persistent perception of sound not caused by any actual stimulus Hearing impairments can be either conductive or sensorineural: Conductive: caused by a problem where the sound is not transmitted to the cochlea ◦ May be caused by ◦ Blockage of ear canal (earwax) ◦ Damage to tympanic membrane ◦ Something interfering with the conduction and amplification by the ossicles Sensorineural: caused by damage to the cochlea, the auditory nerve, or the auditory areas or pathways of the brain; or due to overexposure to loud sounds and results in signal transduction or neural impairment 1. AGE RELATED IMPAIRMENTS ◦ Presbycusis: age-related hearing loss that happens as a result of exposure to noise over the lifetime, and other lifestyle and genetic factors Affect high-frequency hearing first 2. NOISE-INDUCED IMPAIRMENTS ◦ Can be temporary and reversible or permanent ◦ Vibrations of the basilar membrane that are too large (above 85dB) can break the tip-links between hair cells Can manifest as a notch in the audiogram 3. HAIR CELL DEATH ◦ Excitotoxicity: excess glutamate causes swelling and damage to auditory neurons ◦ Reduced blood flow to cochlea ◦ Oxygen-based free radicals can damage hearing From certain types of antibiotics over prolonged period Cochlear implants ◦ Linear array of electrodes implanted in the cochlea to stimulate auditory neurons along the basilar membrane ◦ Effective at restoring hearing ◦ Good for understanding speech ◦ Reproduction of pitch is not perfect ◦ For children born deaf, cochlear implants tend to work better when implanted at an earlier age, during the critical period for acquiring language External components: ◦ Microphone ◦ sound processor ◦ Transmitter Internal components: ◦ receiver–stimulator ◦ electrode system that spirals around the cochlea and stimulates auditory nerve fibers, using both place coding and temporal coding. Echolocation Echolocation: Sound localization based on emitting sounds and then processing the echoes to determine the nature and location of the object that produced the echoes. Precedence effect: localization of a sound as originating from a source in the direction from which the sound first arrives; minimizes the effect of echoes on sound localization. Vision and Sound Localization ventriloquism effect: tendency to localize sound on the basis of visual cues when visual and auditory cues provide conflicting information. Vision can bias the perceived location of a sound depending on 3 factors: 1. Visual and auditory events must be reasonably close together in time (contiguity) 2. The 2 events must be plausibly linked 3. The 2 events must be plausibly close together in space (contingency) Auditory Scene Analysis auditory scene: All the sound entering the ears during the current interval of time. auditory scene analysis: process of extracting and grouping together the frequencies emitted by specific sound sources from among the complex mixture of frequencies emitted by multiple sound sources within the auditory scene. ◦ Organize the auditory scene perceptually into a set of distinct auditory streams ◦ Auditory streams: assortment of frequencies occurring over time that were all emitted by the same sound source or related sound sources. ◦ Auditory stream segregation: process of perceptual organization of the auditory scene into a set of distinct auditory streams Simultaneous grouping HARMONIC COHERENCE Frequencies that are harmonics of the same fundamental frequency tend to be grouped together SYNCHRONY Sounds that begin, end, or change at the same time tend to be grouped together Sequential Grouping Frequency Similarity: The degree to which 2+ sounds have similar pitches, influencing how they are grouped and perceived as related or part of the same auditory stream. Temporal Proximity: The closeness in time between successive sounds, which affects how they are grouped together; sounds occurring close together in time are more likely to be perceived as part of the same auditory sequence. Seeing by Hearing sensory substitution device (SSD): Any artificial aid in the process of acquiring information via one sense that is usually acquired Åvia another sense.