cognitive models and the neural basis of speech perception

Question 1

Motor theory of speech perception:

Answer 1

• Two components of the theory:
  
  1. Speech perception is the result of a specialised speech module that operates separately from the mechanisms involved in perceiving non-speech sounds and is uniquely human (“speech is special”)
     - Evidence: Speech (but not other sounds) are perceived categorically
  2. The objects of speech perception are intended articulatory events rather acoustic events
     - Evidence: Speech sounds are highly variable (articulations less so)

Question 2

Perceiving intended articulatory events:

Answer 2

· Motor theory
· Alternative to motor theory

Question 3

Evidence for the motor theory of speech perception:

Answer 3

· Passive listening to meaningless monosyllables activates auditory cortex
· But motor and premotor areas are also activated
· TMS over premotor areas interferes with phoneme discrimination in noise but not colour discrimination

Question 4

Evidence against the motor theory of speech perception:

Answer 4

· Categorical perception can also be demonstrated for non-speech sounds (e.g. musical intervals; Burns & Ward 1978 JASA)
- So not the result of a specialised speech module
· With training, chinchillas shows the same phoneme boundary for a /da/-/ta/ continuum as humans (Kuhl & Miller 1978 JASA)
- So not unique to humans

Question 5

Brain basis of speech perception - classic model:

Answer 5

· “Classic” model from 19th century neurologists (Broca, Wernicke, Lichtheim)
- Superior temporal gyrus for speech perception (Wernicke’s area)
- Inferior frontal gyrus for speech production (Broca’s area)
- Left hemisphere dominant Karl Wernicke (1848-1905)

Question 6

Brain basis of speech perception - dorsal and ventral streams

Answer 6

Dorsal stream:
- mapping speech sounds onto articulatory representations
- activated for tasks focussing on perception of speech sounds
- left hemisphere dominant
- Broca’s area part of dorsal stressm also involved in attention
Ventral stream:
- mapping speech sounds onto lexical representations
- activated for tasks focussing on comprehension
- bilateral

Question 7

evidence for ventral stream processing

Answer 7

• anterior temporal damage associated with semantic impairment
• inferior temporal damage associated with comprehension deficits

Question 8

evidence for dorsal stream processing

Answer 8

• listening to syllables activates motor and premotor areas
• TMS over premotor areas interferes with phoneme discrimination in noise but not colour discrimination

Question 9

cohort model

Answer 9

• uniqueness point (UP): time-point in the speech signal when only one word becomes consistent with the speech input
• word is recognised at the UP, even before the whole word has been heard
  Key Features:
• words are activated immediately upon minimal input
• multiple words are activated
• words compete for recognition

Question 10

cohort model - evidence from shadowing task

Answer 10

• average response latency is approx 250ms
• average duration of words 375 ms
• implies that listeners recognise words even before they have heard the ends of the words
• consistent with Cohort model

Question 11

cohort model 2

Answer 11

• learning new words slows down recognition od existing words
• consistent with cohort model

Question 12

cohort model - shortcoming

Answer 12

· The Cohort model is still very influential in the field
· However, it is an example of a ‘verbal model’, which makes it difficult to evaluate
· A better way of being sure what a model or theory predicts, is to implement it as a computer program (‘computational model’)

Question 13

trace model - explaining context effects

Answer 13

• word-level activity ‘feeds back’ to influence lower-level phoneme activity
• although this explanation has been strongly challenged