Lecture 2: Pre-natal sensation and perception
(continued)
Prenatal chemosensation
•
Week 12
– Taste buds present in mouth
– Synapses between buds and afferent nerve fibres reach maximum at 15th week.
– Have achieved a fully developed function by week 15.
–
Witt & Reutter (1996). Developmental Biology, 1996.
•
Flavors from the mother’s diet during pregnancy are transmitted to amniotic fluid and
swallowed by the foetus.
•
Evidence for prenatal taste perception
Prenatal taste perception
– Infants show preference for foods introduced in utero
• Carrot juice preferred by infants of mothers who consumed juice whilst pregnant
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•
Menella, Jagnow & Beauchamp (2001). PEDIATRICS Vol. 107 No. 6
Olfactory system also highly developed at birth
– Preferences for pleasant odours (chocolate, banana) over unpleasant odours (rotten eggs) seem to
mirror adult preferences
• Steiner, 1979
•
Breastfeeding newborns can discriminate mothers from strangers using smell alone
• Porter, Makin, Davis & Christensen, 1992
Foetal auditory system
•
Foetal sound environment
– High frequency sounds and intensity attenuated
• Amniotic fluid etc acts as low-pass filter
– Maternal sounds
• Heartbeat, digestion, movement, voice
– Foetal sounds
• Heartbeat, movement
– External sounds
• Voices, environment sounds, music
–
Vocal prosidy, rythym, accentuation and pitch preserved
• Subject to attenuation of intensity and maybe complete masking
–
Perhaps explaining recognition of mother’s rather than fathers voice
Foetal auditory system
• Cochlea processes sounds between 15- 20 weeks
• i.e. before it is anatomically mature
• The cochlear structures of the ear appear to function by the 20th week
• Mature synapses have been found between the 24th and 28th weeks
• (Pujol et al. 1991).
Foetal auditory system
•
Sensitivity range of frequencies small
– 200 to 1000 Hz
• (adult 20 - 20,000 Hz)
– As frequency range broadens so does perceptual abilities
• Discriminate simultaneously presented frequencies
• Differentiate speech sounds
• Discriminate quiet sounds
–
•
Busnel & Granier-Deferre, 1983 ; Lecanuet, 1996)
Foetus can ‘hear’ and respond to a sound pulse starting about 16 weeks of age (Shahidullah &
Hepper, 1992)
• this is even before the ear construction is complete.
• If exposed to loud music, and their heart beat will accelerate.
•
Prenatal maternal speech can influences newborns' perception of speech sounds.
–
DeCasper, A. & Spence (1986). Infant Behavior and Development, 9, 133-150.
Foetal auditory perception
•
Perception of sound localisation
– Important for survival
•
How are sounds localised?
– Depends on interaural temporal and intensity differences for left/right discrimination
– Depends on acoustic ‘shadow’ cast by shape of pinna from front/back discrimination
– Depends on learned associations between distance and loudness
•
Developed in newborn
– Can discriminate left/ right sounds
•
Is sound localisation present in foetus?
•
Sound localisation in foetus?
Foetal auditory perception
– Anecdotal reports of foetal orienting to sounds
– But, unlikely to develop in utero
•
•
•
•
Low frequency sounds pass through uterus
No acoustic shadow from pinna for low frequency sounds
Restricted frequencies means interaural frequency differences not large enough for detection
Interaural time differences too minimal for detection
–
–
Small head of foetus
Faster speed of sound in fluid
Visual system
•
Ontogenesis of two visual pathways emerge early
– Primary visual system
• Retina - thalamus - cortex
– Second visual system
• Retina - superior colliculus - thalamus
• Phylogenetically older system
• More developed at birth
•
Eye structure and retinae develop rapidly in 1st and 2nd trimesters
•
Development of fine retinal structures occurs - fovea- after birth
• Banks & Shannon, 1993
•
Cell size, myelination and neural connectivity in optic nerve, LGN, cortex also develop after
birth
Summary
•
All senses develop during prenatal period
•
Some senses more developed than others
•
Sensory ontogeny at birth linked to postnatal experience-dependent development
– Sensory capacities to taste, smell, see, hear, touch progress so that infant is in a high state of
sensory preparedness
•
Sensory systems not fully developed at birth are highly plastic – Can adapt and compensate
Neural plasticity and perceptual development
(part 1)
Neonatal brain
• Basic blueprint of nervous system in place by birth
– By six months of gestation most neurons have been produced
• Newborn’s brain continues to undergo rapid changes during first two years of
development
– Synaptogenesis (~10 fold increase)
– Synaptic pruning
– Myelination
Synaptogenesis and pruning
•
First synapses generally appear about week 23
•
Spontaneous overproduction
– Peaks during first year of life
•
Post-natal pruning is experience dependent
•
•
continues into adulthood
Synaptic pruning varies by cortical region
•
•
Occipital lobe synapses peak at 4 to 8 months and are pruned to adult level by age 5 to 10
Prefontal cortex peaks at 1 to 2 years and pruned to adult levels in late adolescence
–
Huttenlocher, 1979
Myelination
• Appears within the 4th month of gestation and first year of life
• Onset varies with brain region
– Structures supporting basic functions first myelinated
• Spinal chord, medulla
– First post-natal year
• basic sensory and motor structures are myelinated
– Later in childhood
• Integrative structures myelinated; association cortex, cortico-subcortical connections, cortico-cortico
connections
Myelination
•
Postnatal myelination in humans
•
Continues until late teenage years for some structures
–
E.g. Paus T, Zijdenbos A, Worsley K, Collins DL, Blumenthal J, Giedd JN, Rapoport JL, Evans AC. (1999). Structural maturation of neural pathways in children and adolescents: in
vivo study. Science 283:1908-1911.
•
•
111 children
Internal capsule
•
Arcuate fascicularus
–
–
•
Fibre tract involved in motor function
Subserves speech function
Nerve conduction velocity increases with increase myelination
•
Salamy, 1978
Plasticity throughout lifespan
• Some plasticity needs to be retained in order for
– Acquisition of new knowledge
– Integration of new information with stored knowledge
– Reorganise prior knowledge
• Plasticity retained through
– synaptogenesis
– changes in strength of synapse
• Hebbian synapse
– “Neurons that fire together, wire together”
Role of experience
• Experience influences neuroconnectivity
– Experience-expectant systems
• Common to all individuals
–
–
–
–
E.g language system in place for acquisition
Final brain organisation develops by linguistic environment
Linguistic experience required during critical period
Language acquisition develops to same level across individuals
– Experience-dependent systems
•
•
•
•
•
Vary across individuals
Based on unique experiences
E.g. musical abilities
Can develop at any stage during lifetime
Skill acquisition related to amount of exposure or practice