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Lecture 7

PSYB51H3 Lecture Notes - Lecture 7: Homonymous Hemianopsia, Optic Chiasm, Visual Acuity


Department
Psychology
Course Code
PSYB51H3
Professor
Matthias Niemeier
Lecture
7

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Spatial Frequencies
o Visual images can be decomposed into sine waves with different spatial frequencies
Visual Acuity
o Acuity
o The smallest spatial detail that can be resolved
o The visual system codes images in terms of oriented stripes
o Limits of the visual system
o Narrowness of the stripes visual acuity
o Contrast the difference in illumination between a figure and its background
o Measuring visual acuity
o Ophthalmologists use distance (20/20, 20/30, 20/15)
o Vision scientists use the smallest visual angle of a cycle of a grating
Minimum visual angle of a cycle 0.017 degree
o Resolution acuity is limited by the spacing of photoreceptors in the retina
o The visual system “samples” the grating discretely
o Limitation at photoreceptor level activity of each photo receptor reflects just
one value of luminance
o Acuity is further limited by convergence (e.g. multiple photoreceptors projecting onto a
single bipolar cell etc.)
o We see gratings when their brightest and darkest parts fall on different cones but if the
entire cycle falls on a single cone, we see a uniform grey: aliasing
o Aliasing
o Refers to the artifacts the result when the signal reconstructed from samples is
different from the original continuous signal
o Contrast sensitivity function
o Describes the relationship between contrast and spatial frequencies
o Is based on many individually measured (absolute) contrast thresholds
o Sensitivity
o Inverse of a threshold
Functional Deficits of the Visual System
o Lesions in lower visual areas cause (retinotopic) visual field defects
o Heteronymous hemianopia
o Lesion of optic chiasm (e.g. tumour in hypothalamus)
o Homonymous hemianopia, quadrantanopia or scotoma
o Lesions after the optic chiasm
o Heteronymous
o Different visual field defects for the two eyes
o Homonymous
o Same defects for the two eyes
o Hemianopia
o Half the visual field is blind
o Quadrantanopia
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