Audition and Vision

Exam curve High score – 94 (average of highest and second highest scores. 90% (A) = 84.6 80% (B) = 75.2 70% (C) = 65.8 60% (D) = 56.4

Organization of sensory systems Peripheral before central Relatively more developed peripherally than centrally earlier in development Infants are not blind or deaf at birth By 4 to six months of age, vision and hearing are close to adultlike

Auditory system

Maturity of auditory system External ear is small at birth, but this does not affect hearing Middle ear cavity continues to grow during early childhood Chain of ossicles have reached adult dimensions and rigidity by birth Dimensions of cochlea at adult levels by 5 months gestational age. Outermost hair cells (auditory receptors) continue to develop after birth Auditory nerve well myelinated at birth.

Studies that show that infants can hear at (and even before) birth Heart rate changes Respiration changes Newborns startle in response to loud noises Cortical and brainstem ERPs Preferential listening Mom’s voice Familiar song Familiar stories

There are still significant differences in infants hearing from adults In what way is infants’ hearing different from adults? To what aspects of auditory stimuli do newborns respond differently than adults?

Dimensions of sound Intensity Frequency Loudness Frequency Pitch Intensity and frequency are related Both contribute to how easily heard a stimulus is Intensity and Frequency interact If you hold either constant and vary the other, the sound will seem louder/quieter or higher/lower

Limits to the human auditory system Relations between intensity and frequency are not linear Audition is a very complicated system It is possible to explore the limits of intensity and frequency

Intensity threshold Intensity measured in decibels Hold frequency constant, vary intensity Measure the intensity needed to be detected

Examples of decibel levels 20 dB Rustling leaves 40 dB Quiet room 80dB Lecturer 110dB Rock concert (you can feel vibrations, short term hearing loss) 180 dB Spacecraft launch

Intensity Thresholds Adults’ thresholds are lower than infants 5 dB Measurements might depend on methods Evoked potentials give lower thresholds than heartrate variability measures

Frequency threshold Again, depends on measures Evoked potentials and heart rate Reduced sensitivity in infants to high and low frequencies Behavioral measures Infants more responsive to high than low frequencies Infants may respond differently to high and low frequencies High – Orienting Low – Arousal With any measure, infants seem to respond ideally to the middle range, where the human voice is

Development in the visual system

Components of the visual system Lens Retina Optic Nerve Visual Cortex

Lens Fully Developed at birth Function: Accommodation Ability to shift focus to different distances Visual system can only focus on one distance at a time To adjust to seeing at a new distance, the shape of the lens is changes Accomplished by expanding and contracting the muscles around the eye to change the thickness of the lens Infants do not see distance as well as adults Originally thought to be because of immature lens Newer studies show that they can accommodate, but they are slow and innaccurate until after about 2 months of age

Retina Composed of rods and cones Many more rods than cones Most cones are in the fovea Fovea is less mature than periphery Cones are shorter and fatter in infants than adults until 11 months Infants have better peripheral than foveal vision

Fovea development

Cone density in the fovea

Optic nerve Thinner and shorter in infants, not fully myelinated Myelination complete by about 3 months Peripheral-central progression Begins at 24 weeks gestation Length myelinated by 1 month postnatal Dramatic increase between1 and 3 months

Connections between eye, lateral geniculate, visual cortex Depends on spontaneous activity prenatally Preventing such activity leads to abnormal connections between LGN and cortex

Striate (visual) cortex Full complement of neurons and denrites are short at birth, few project Dendritic spines proliferate after birth, peak at about 6 months Ocular dominance columns are probably formed during this time where spines and synapses are growing then being pruned Myelination complete after about 3 months

Acuity Measure of the level of detail transmitted by visual system Optic systems work by input (stimulus) and output (vision) with a filter between Acuity only as good as the filter The filter is the whole visual system How good is it?

Development of acuity