The Neuroscience of Language

What is language? What is it for?

• Rapid efficient communication– (as such, other kinds of communication might be

called language for our purposes and might share underlying neural mechanisms)

• Two broad but interacting domains:– Comprehension– Production

Speech comprehension

• Is an auditory task – (but stay tuned for the McGurk Effect!)

• Is also a selective attention task– Auditory scene analysis

• Is a temporal task– We need a way to represent both frequency

(pitch) and time when talking about language -> the speech spectrogram

Speech comprehension

• Is also a selective attention task– Auditory scene analysis

• Which streams of sound constiutute speech?

• Which one stream constitutes the to-be-comprehended speech

• Not a trivial problem because sound waves combine prior to reaching the ear

Speech comprehension

• Is a temporal task– Speech is a time-varying signal

– It is meaningless to freeze a word in time (like you can do with an image)

– We need a way to consider both frequency (pitch) and time when talking about language -> the speech spectrogram

What forms the basis of spoken language?

• Phonemes

• Phonemes strung together over time with prosody

What forms the basis of spoken language?

• Phonemes = smallest perceptual unit of sound

• Phonemes strung together over time with prosody

What forms the basis of spoken language?

• Phonemes = smallest perceptual unit of sound

• Phonemes strung together over time with prosody = the variation of pitch and loudness over the time scale of a whole sentence

What forms the basis of spoken language?

• Phonemes = smallest perceptual unit of sound

• Phonemes strung together over time with prosody = the variation of pitch and loudness over the time scale of a whole sentence

To visualize these we need slick acoustic analysis software…which I’ve got

What forms the basis of spoken language?

• The auditory system is inherently tonotopic

Is speech comprehension therefore an image matching problem?

• If your brain could just match the picture on the basilar membrane with a lexical object in memory, speech would be comprehended

Problems facing the brain

•Acoustic - Phonetic invariance –says that phonemes should match one and only one pattern in the spectrogram

–This is not the case! For example /d/ followed by different vowels:

Problems facing the brain

• The Segmentation Problem:– The stream of acoustic input is not physically segmented into discrete

phonemes, words, phrases, etc.

– Silent gaps don’t always indicate (aren’t perceived as) interruptions in speech

Problems facing the brain

• The Segmentation Problem:– The stream of acoustic input is not physically segmented into discrete

phonemes, words, phrases, etc.

– Continuous speech stream is sometimes perceived as having gaps

How (where) does the brain solve these problems?

– Note that the brain can’t know that incoming sound is speech until it first figures out that it isn’t !?

– Signal chain goes from non-specific -> specific

– Neuroimaging has to take the same approach to track down speech-specific regions

Functional Anatomy of Speech Comprehension

• low-level auditory pathway is not specialized for speech sounds

• Both speech and non-speech sounds activate primary auditory cortex (bilateral Heschl’s Gyrus) on the top of the superior temporal gyrus

Functional Anatomy of Speech Comprehension

• Which parts of the auditory pathway are specialized for speech?

• Binder et al. (2000)– fMRI– Presented several kinds of stimuli:

• white noise• pure tones• non-words• reversed words• real words

These have non-word-like acoustical properties

These have word-like acoustical properties but no lexical associations

word-like acoustical properties and lexical associations

Functional Anatomy of Speech Comprehension

• Relative to “baseline” scanner noise

– Widespread auditory cortex activation (bilaterally) for all stimuli

– Why isn’t this surprising?

Functional Anatomy of Speech Comprehension

• Statistical contrasts reveal specialization for speech-like sounds– superior temporal gyrus– Somewhat more prominent on left side

Functional Anatomy of Speech Comprehension

• Further highly sensitive contrasts to identify specialization for words relative to other speech-like sounds revealed only a few small clusters of voxels

• Brodmann areas– Area 39– 20, 21 and 37– 46 and 10

Next time we’ll discuss

• Speech production• Aphasia• Lateralization