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Lecture 3

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Psychology 2410A/B
Adam Cohen

WEEK 3: PERCEPTUAL DEVELOPMENT Basic Visual Capacities • Visual Acuity • How to determine visual acuity in newborns and young infants • Two of the most common methods involve using behaviour and electrical signals recorded form the scalp • The reason preferential looking works is that infants have spontaneous looking preferences • Measure the infant’s acuity by determining the narrowest stripe width that result in looking more to one side • Another method for measuring acuity involved measuring an electrical response called the visual evoked potential (VEP) • VEP, which is recorded by dick electrodes placed on the back of the infant’s head, over the visual cortex, is the pooled response of thousands of neurons that are near the electrode • If the stripes or checks are large enough to be detected by the visual system, the visual cortex generates an electrical response called the visual evoked potential • The VEP usually results in higher measurements of acuity, but both techniques indicate that visual acuity is poorly developed at birth • Acuity increases rapidly over the first 6-9 months • Full adult acuity is not reached until sometime after 1 year of age • The infants visual cortex is not fully developed • The visual cortex is only partially developed at birth and becomes more developed at 3 and 6 months, the time when significant improvements in visual acuity are occurring • If we look at newborn’s retina, we find that although the rod-dominated peripheral retina appears adultlike in the newborn, the all-cone fovea contains widely spaced and very poorly developed cone receptors • The newborn’s cones have far inner segments and very small outer segments • The small size of the outer segment means that the newborn’s cones contain less visual pigment and therefore do not absorb light as effectively as adult cones • The fat inner segment creates the coarse receptor lattice with large spaces between the outer segments • Most of the light-entering the newborn’s fovea is lost in the spaces between the cones and is therefore not useful for vision • Contrast Sensitivity • Contrast sensitivity is measured by determining the smallest difference between the dark and light bars of a grating at which an observer can still detect the bars • The spatial frequency of a grating is the number of cycles of the grating per degree of visual angle • Finer bars are generally associated with higher spatial frequencies • Moving closer to the grating decreases the spatial frequency • The infants ability to perceive contrast is restricted to low frequencies • Even at low frequencies, the infants contrast sensitivity is much lower than the adults • Infants can see little or nothing at frequencies above about 2 to 3 cycles/degree, the frequencies to which adults are most sensitive WEEK 3: PERCEPTUAL DEVELOPMENT • At 1 month, infants can see no fine details and can see only relatively large objects with high contrast • At 2 months, however, the infants contrast perception has improved so that the image becomes clearer • Perceiving Color • Research has shown that color vision develops early and that appreciable color vision is present within the first 3 to 4 months of life • One of the challenges in determining whether infants have color vision is that perception of a light stimulus can vary on at least two dimensions •its chromatic color •its brightness • When stimuli with different wavelengths are used to test color vision, their intensity should be adjusted so that they have the same brightness • A method called habituation to determine whether infants perceive the difference between these categories • 4 month old infants categorize colors the same way adult trichromats do • Perceiving Depth • Binocular disparity becomes functional early, and pictorial depth cues become functional later • Using Binocular Disparity •One requirement for the operation of binocular disparity is that the eyes must be able to binocularly fixate so that the two eyes are both looking directly at the object, and the two foveas are therefore directed to exactly the same place •Newborns have only a rudimentary, imprecise ability to fixate binocularly, especially on objects that are changing in depth •Although some divergence and convergence do occur in 1- and 2-month old infants, these eye movements do not reliably direct both eyes toward the target until about 3 months of age •Although binocular fixation may be present by 3 months of age, this does not guarantee that the infant can use the resulting disparity information to perceive depth •The beauty of random dot stereograms is that the binocular disparity information in the stereograms results in stereopsis •Thi
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