VA Flashcards
Types of VA
Detection - minimum visible
Resolution -minimum resolvable, optics limitations of eye and phr spacing
Isolated identification - min recognizable
Crowded identification - min recognizable with horizontal contour interaction
Hyperacuity (vernier) - min discriminable
Cardiff Acuity Test
Vanishing optotypes
FPL Vernier Acuity
- Not reliably measurable before 10 weeks
- Rapid emergence shown in longitudinal study
- Different developmental time course than grating acuity
Vernier Acuity vs. Resolution Acuity
- Vernier acuity developed in parallel to grating acuity between birth & 6 mos
- Vernier acuity initially worse than grating acuity then dramatically improves between 2 to 8 mos
- Vernier acuity of very young infants remains controversial
- Data from monkeys parallel Shimojo results
- Attainment of adult levels of vernier acuity is delayed compared to resolution acuity
Sweep VEP Vernier and Grating Acuity
Infants n = 57 Adults n = 4
Isolate pattern specific responses from those due to motion in the stimulus
VEP grating acuity reaches adult levels earlier than Vernier acuity
relationship between VEP Vernier acuity and grating acuity follows which trend?
acuity follows the same developmental trajectory established by previous psychophysical studies of humans and monkeys
* reduction of neural blur, possibly associated with synapse elimination during later development
* hyperacuity-level visual processing can be achieved only after postnatal development at the level of extra-striate cortex
Contrast threshold
lowest contrast detectable for a given size stimulus
Contrast = (Lmax-Lmin)/(Lmax+Lmin)
contrast sensitvity
reciprocal of the contrast threshold
reason for high and low frequency roll-off
High frequency roll-off due to optical blur-imperfect optics of the human eye
Low frequency roll-off due to lateral inhibition-center surround organization of retinal ganglion cells
Importance of contrast sensitivity
Low to midrange SF important for facial recognition, recognition of real world targets and mobility
Contrast Sensitivity Development in Monkeys
- Spatial CSFs measured longitudinally on a single infant monkey at 6 ages (between 10 and 38 weeks)
– Shift upward in sensitivity and rightward in spatial scale
– Changes of scale = changes in spatial filtering or integration properties of the visual system
– Changes of sensitivity = efficiency with which visual system can process targets
Human Infant PL Contrast Sensitivity
- Infant CSFs are shifted down and to the left compared to adults
- Low frequency roll-off not observed at 1 month suggesting mechanisms for lateral inhibition are immature
- Peak of the CSF moves to higher spatial frequencies with age
Limitations of the Infant Eye on Vision
- Optics
- Pupil
- Accommodation
- Foveal cones
– Cone dimensions
– Cone spacing - Grating acuity is worse than predicted by cone spacing
- Post-receptoral limitation on acuity in young infants
- Spatial scale change caused by cone migration.
- Cone mosaic of an infant which provides input to a cortical unit.
- Single cones superimposed on the luminance profile of a cosine grating
- Increased cone spacing produces a spatial scale shift toward lower spatial frequencies.
how does cone packing and phr maturation explain contrast sensitvity?
- Adult-like band pass filtering appears by 2-3 months post-natal (FPL)
- Sensitivity to high spatial frequencies continues to develop beyond 6 months of age
- Photoreceptor maturation may explain the rise in contrast sensitivity with age
– Reduced quantal absorption by short, immature
cone outer segments - Cone packing density may explain the shift toward higher spatial frequencies
– Decrease in cone diameter with development of the fovea
– Higher packing density of cones, decreases size of receptive field of bipolar cells
Clinical measures of spatial vision
- Visual acuity- ability to see fine details
- Contrast sensitivity- ability to see shades of grey
