EDHS 10 Speech Perception

Speech perception

-is an active process

• As soon as the acoustic information reaches the cortex, the brain actively starts to try to interpret, to make sense of the information.

Infant speech perception

Habituation technique:

if the same stimulus is repeatedly used- the response wears off

• A new stimulus- if perceived as different - will revive the response intensity

The pattern play back (Haskins Laboratories)

• The first widely used speech synthesizer for perceptual experiments. It converted visual spectrogram patterns into sound — essentially a reverse sound spectrograph. Researchers could hand-paint and simplify these patterns, allowing precise control over the sounds they produced for experiments.

Categorical Perception

• the ability to discriminate only as well as one can identify

• The tendency to assign what you perceive to an existing language category

• A change in some variable along a continuum is perceived, not as gradual but as instances of discrete categories.

Other categorical acoustic cues

• Variations in the time rise of the intensity of frication from slow to rapid causes listeners to change from sh [shop]- perception to ch [ch] perception

Phenomic restoration

When a phoneme in a word is removed or masked by noise (like a cough or static),people often don’t notice the gap and still perceive the full word naturally. The brain uses context and prior knowledge to mentally “fill in” the missing sound, making the speech seem continous and complete.

Categorical perception and learning

• Listeners can be trained to become more sensitive to differences within a phonemic category

• Adult bilinguals are often quite insensitive to perceptual distinctions in their non native language ( even if they can produce them)

Localization

the attempts to identify regions in the brain that are responsible for specific parts of psychological functions.

Lateralization

the process during which one side of the brain becomes dominant in performing specific (parts of ) functions

• [lateralization is often interpreted in function of the need for parallel processing]

Cerebral lateralization

Role of left hemisphere in speech processing (production & perception) stems from data on:

• patients with aphasia

• studies of split brain patients

• Right - ear advantage (REA) in dichotic listening

• Evoked potential studies

Broca’s Aphasia

motor programming problems- comprehension can be intact

• difficulty producing speech ( slow, effortful, broken sentences, but usually makes sense)

• comprehension is preserved mostly

Wernicke’s aphasia

(sensory aphasia - trouble with linguistically decoding auditory information)

• Speech is fluent and grammatical correct, but often nonsense or irrelevant words

• Comprehension is poor - they struggle to understand others

Dichotic Listening

• Different messages are displayed to left and right ear

• Stimuli presented to the right ear are often better remembered than those presented to the left ear

• The better performance of the right war is often explained by the fact that the right ear has more direct connection with the left hemisphere

• Hence: if there is a conflict between left and right, the right ear information gets “ priority treatment”

EEG recordings

• Indications that auditory processing occurs in both hemispheres but that phoneme identification is lateralized to the left hemisphere.

  -The more analytical work is done on the left side

Neurophysiological development and perception

• The critical period of acquisition (lenneberg, 1968):

  -Flexibility and plasticity of brain tissues/functions for specialization.

  - (diminishes with time)

• Duration of critical period (debatable) relevant to:

  -First and second language acquisition

• -production and perception

Theories of speech perception

• Any speech perception theory must account for:

  -variability (intra- + inter-speaker)

  -Categorization of stream of co-articulated speech sounds

• Two major groups:

  -Motor theories

  -Auditory theories

The motor theory of speech perception

• perception of speech is linked to an internal motor recognition:

• critic of general validity of theory: babies can classify sounds (chinchillas too!)

• The theory has gained new interest since the discovery of mirror neurons.

Auditory theories

• The processing of raw acoustic input by the auditory system can account for speech perception;

• In this framework, there is no reference to production;

• Based on sensitivity to acoustic patterns:

  -Template matching

  -Feature detection

Multimodality of speech

• Speech perception is not only an auditory activity- it is also a visual activity

• The McGurk effect