Chapter 8 Flashcards
Hearing is More Than Detection of Sounds
It involves constructing a model of the world:
(1) what objects do the sounds correspond to, (2) where are they?, (3) what do they mean?
The nature of sound
Changes in (air) pressure that have characteristic properties such as amplitude and frequency. Pure tones are sinusoids in a pressure/time plot. More complex sounds can be described as a sum of sinusoids. Frequency relates to pitch; amplitude relates to loudness.
From ear to brain
4–5 synapses from ear to cortex. Medial geniculate nucleus projects to primary auditory cortex (also called “core”). Core area is surrounded by secondary auditory cortex (including belt and parabelt regions). Information ascends and descends in the pathway.
The auditory nerve and auditory cortex have a tonotopic organization.
Locating sounds
Mechanism 1: Inter-aural differences
- Time differences
- Intensity differences
- Heschl’s gyrus (A1) & belt region
Mechanism 2: distortions of sound by head and outer ear
- head-related transfer function
- Planum temporale (posterior to A1)
Auditory stream segregation
Separating out a single stream of different sounds into different objects and locations (cocktail party problem). Auditory memory is implicated (according to Naatanen et al.). Two brain regions strongly implicated: (1) auditory cortex (e.g. mismatch negativity effect), (2) parietal cortex (due to role in spatial processing).
MMN occurs when a sound is unexpected relative to preceding sounds. MMN in schizophrenia: a marker for deficits in pre-attentive information processing.
Music perception
Music, like vision or language, may be decomposed into different mechanisms, including melody vs rhythm, and pitch vs temporal.
Amusia: auditory agnosia in which music perception (as a whole, or one of its mechanisms) is affected more than the perception of other sounds.
Tone-deafness or congenital amusia: developmental difficulty in perceiving pitch relationships.
Congenital amusia
Linked to increased grey matter in auditory cortex and frontal regions.
Absolute pitch
Smaller volume of regions in right superior temporal cortex & planum temporale.
Voice perception
Voices convey socially important information (age, sex, gender, emotion)
Evidence for a “voice selective” region in the superior temporal lobe (part of “what” stream). Responds more to vocal than non-vocal sounds. Effect of TMS on voice perception. Responds to changes in identity of stimuli more than other acoustic changes.
Speech perception
Gaps don’t occur between words but occur with certain consonants (that restrict flow of air). Problem of segmenting continuous input (“ice-cream” vs. “I scream”). Problem of inter-speaker differences; pitch affected by age and sex; different dialects, talking speeds etc.
Co-articulation = consecutive speech sounds blend into each other due to mechanical constraints on articulators.
Speech perception in the brain:
- Primary auditory cortex responds equally to speech and other sounds in both left and right hemispheres.
- Areas more anterior to this in left hemisphere respond more to intelligible speech relative to unintelligible speech of similar acoustic complexity (Scott et al. 2000).
- Left hemisphere particularly important for speech: damage can result in a type of auditory agnosia (pure word deafness) in which environmental sounds and music are identified but not speech – speech appears “too fast” or is “distorted”.
Dual routes for speech perception
As with vision, it has been suggested that speech perception has two functionally distinct pathways: what and how route.
What” route:
- Ventral route along temporal lobe.
- Recognizes speech acoustically.
- The discrete percepts are mapped onto abstract representations that specify nature of acoustic signal (e.g. voicing, timing, phonemes, syllables).
- Important for speech comprehension (i.e. makes contact with semantic knowledge).
“How” route…
- Dorsal route involving parieto-frontal circuit, connection through arcuate fasciculus (white-matter tract).
- Recognizes speech motorically (i.e. motor theory of speech perception).
- Motor theory revived after discovery of mirror neurons in premotor and inferior frontal cortices: respond with motor movement, but also when seeing/hearing gestures in other people.
- Discrete percepts mapped onto units of articulation.
- Used to say and learn unfamiliar words
(pre)motor regions role with difficult speech signals.
Deficits in repeating and learning new phonology
= deficit in “how” route, intact “what”?
Patients with deep dysphasia cannot repeat nonwords and make semantic errors in repetition (e.g. hear “cat”, say “dog”)
= deficit in “how” route, rely on impoverished “what”?
Explanations of the McGurk illusion
- Multisensory perception. Left pSTS (cf. TMS reduces illusion).
- Illusion arises from activating the motor system for speech production (including inferior frontal).
Potential role of these regions (STS & inferior frontal) in so-called categorical perception of ambiguous syllables.
Categorical perception = continuous changes in input are mapped on to discrete percepts
Pitch
The perceived property of sounds that enables them to be ordered from low to high.
Pure tones
Sounds with a sinusoid waveform (when pressure change is plotted against time).
Loudness
The perceived intensity of sound.
