L&C Lecture 6 - Speech

• This lecture discusses the genetic basis for speech, neural and anatomical mechanisms, and theories of speech perception.

Genetic Basis for Speech

• Humans globally are speech users, making us a speech-dominant species.
• Speech is a primary communication mode across cultures, often accompanied by gestures.
• Research explores the genetic basis of speech, investigating the effects of gene mutations/disruptions.
• Chromosome 7 has implications for speech, as several genes there link to speech and language abilities; mutations can cause language issues.

FOXP2 and the KE Family

• Fisher et al. (1998) analyzed the KE family genes and discovered that half of the members showed severe speech and language disorder.
• Inheritance patterns suggested a single gene mutation (autosomal dominant inheritance).
• Individuals with the mutation had unintelligible speech, struggled with lip/tongue control, and failed to produce words or understand grammar.
• IQ remains within normal range.
• Simon Fisher and colleagues demonstrated that affected KE family members shared a mutation of the FOXP2 gene on chromosome 7.
• Minor gene mutations can profoundly affect language (KE family).
• Deletion of FOXP2 can leave children unable to verbally communicate.
• FOXP2 is a regulator gene that controls multiple genes' activity with involvement in neural development, synaptic plasticity, and motor coordination.
• FOXP2 is important for developing brain regions including the basal ganglia (motor learning/sequencing, speech production) and Broca's area (language processing/speech production).
• Versions of FOXP2 exist in many animals/birds (like Broca’s and Wernicke’s area in apes).
• FOXP2 is important for learning parts of speech and vocal learning and has functions such as lung/gut development, and communication (even bird song).

Evolution of FOXP2

• FOXP2 is evolutionarily ancient and present in phylogenetically different animals, showing convergent evolution across species/lineages.
• It supports vocal learning and motor control in species like humans, songbirds, and bats.
• There are only 3 amino acid changes in the FOXP2 gene between humans and mice, and 2 between humans and chimpanzees, but these have massive effects on speech and language evolution.
• FOXP2 is in chromosome 7.
• FOXP2 is important in regulating the genetic network of speech and language.
• Its long evolutionary history, as present in many animals, suggests speech/language builds on ancient evolutionary systems.
• FOXP2 is only part of the genetic puzzle of the language, as neuro-geneticists are looking to map out the complete picture.

Neural & Anatomical Mechanisms for Speech

• Speech processing is highly distributed across the brain, even if areas such as Broca's and Wernicke's are important.
• The brain uses different neural networks for encoding language semantics.
• The entire brain is essential for language.
• Brains are complex, as is language, and language use works the brain hard involving many areas.
• Speech is a complex motor activity, not just an auditory one.
• Systems used to speak move dynamically, precisely, and in a coordinated way.
• A complex set of machinery is needed: mouth, nose, lips, larynx, vocal tract, teeth, tongue, and vocal folds.
• This requires control that animals struggle with.
• Humans evolved a lower larynx (voice box) as a key anatomical adaptation that helps them speak.
• The position of the larynx is crucial for humans' ability to produce clear/varied speech sounds.
• Other primates/most animals have higher larynx positions compared to those of humans.
• Vocal tract anatomy moves dynamically and the evolution of the extended larynx allows for a lot of vocal flexibility.
• Nonhuman primates aren't great vocal learners and have limited sounding production.
• Primates generally have much less control over vocal apparatuses than humans.
• Humans and other vocal-learning species have direct neural connections between vocal chords and brain areas for speech.
• Vowels require lots of vocal control, consonants less (which may have different evolutionary basis).

Evolution of Vocal Control

• Evidence of some vocal control in some primate case studies indicates that mechanisms for speech may have deep evolutionary roots.
• Orangutans can learn to play the Kazoo and use sophisticated control over their vocal apparatus.
• Orangutans can show control over their vocal apparatus to produce new sounds but in unnatural contexts like whistling, suggesting that it takes unique circumstances to learn this type of vocal control.

Theories of Speech Perception

• Key research question: How do we come to understand speech?

Motor Theory of Speech Perception

• First developed by Lieberman and colleagues in the 1960s.
• Claims speech perception is innate (nativist) and human-unique.
• Humans understand and categorize speech, especially vocal tract gestures versus identifying sound patterns, for communication.

Auditory Theory of Speech Perception

• Speech perception is based primarily on the acoustic properties of speech.
• Claims perception is dependent on auditory mechanisms and we understand through training and experience with the processing sounds from speakers (pitch, duration, and frequency).
• Speech perception is not uniquely human.
• Speech perception can be shaped through genetics and learning.
• Tests can determine whether speech perception is innate.
• Categorical speech perception in infants (Eimas et al 1971) has high-amplitude sucking study.
• Test discrimination of speech-sounds (e.g. phonemes - BA and PA) and note when they change their sucking reflex, suggesting can sounds can be discriminated.
• The motor theory supports this perception that is driven by innate knowledge of production.
• Auditory theory supports that perception is driven by innate auditory perception based on sounds, not vocal apparatus.
• Testing determined if speech perception is unique.
• One study showed avoidance conditioning using chinchillas. They were taught to avoid shocks when speech sounds were played.
• Primate cases, such as Panzee, can discriminate synthetic & degraded speech.
• Experience is critical for speech perception.
• General auditory abilities back auditory theory.

McGurk Effect

• Interaction between hearing and vision that causes a perceptual illusion.
• Demonstrates how the brain combines visual and auditory information to create a unified understanding of speech.
• Phoneme categorisation is influenced by visual information of mouth movements.
• Hearing BA + but seeing GA = perceive DA.
• It supports the motor theory and is inconsistent with auditory theory, but learning could play a role.

Speech Perception Summary

• Motor theory claims: Speech perception is partly innate, human-unique, and involves perception of vocal tract gestures.
• Auditory Theory claims: Speech perception is dependent on auditory mechanisms and can be shaped through genetics and learning.

Summary

• Research shows genetic basis for speech, amino acid changes in FOXP2 support human unique speech and language.
• FOXP2 mutations can lead to speech disorders (KE Family).
• Anatomical basis: human speech apparatus & evolution of vocal control
• Speech perception combines innate and learned mechanisms, lending support to both Motor & Auditory Theories.