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Lecture

Chapter three


Department
Psychology
Course Code
PSYB51H3
Professor
Matthias Niemeier

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Chapter Three : Spatial Vision From Stars to Stripes
the iris regulate the number of light rays entering the eyeball ; and the cornea, lens, and aqueous
and vitreous humors focus these rays so that a clear image is formed on the retina
retinal translation help us perceive the pattern of light & dark areas in visual field regardless of
the overall light level
Visual Acuity
-contrast is the difference in luminance between objects in the background, or between lighter
and darker parts of the same object
Acuity – the smallest spatial detail that can be resolved ( eye doctors use specify in terms like
20/20 )
BUT - scientist like to talk about the smallest visual angle of a cycle of the grating that we can
perceive
oCyclefor a grating, a pair consisting of one dark bar and one bright bar (simply one
repetition of a black & white stripe)
visual angle - the angles subtended by an object in the retina
o is the angle that would be formed by lines going from the top & bottom (or left & right
depending on the orientation of the stripes ) of a cycle on the page through the center of the
lens & on to the retina
-this resolution acuity represents one of the fundamental elements of spatial vision: it is the finest
high
contrast detail that can be resolved, the limit is determined by the spacing of photoreceptors in the
retina
this resolution acuity represents one fundamental limits of spatial vision – it is the finest high-
contrast detail that can be resolved
othe limit is determined primarily by the spacing of photoreceptors in the retina
sine wave gratingsa grating with a sinusoidal luminance profile
olight intensity varies smoothly & continuously across each cycle if the receptors are
spaced such that whitest & blackest parts of the grating fall on separate cones, we should be
able to make out the grating
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BUT if entire cycle falls on a single cone , we will see nothing but gray field -- or
experience Aliasing (= misperception of the grating due to under sampling/ misperceive
the cycles to be longer than they actually are)
cones in the phobia have a center to center separation of about 0.5 minutes of arc [0.008°], which
fits nicely with the observed acuity limit of 1 minute of arc
oKeep in mind we need 2 cones per cycle to be able to perceive it properly
rods and cones in the periphery are packed less tightly BUT many receptors converge on each
ganglion cell - thus, poorer acuity in periphery than fovea
A Visit to the Eye Doctor
this method of reading large letter to small letter & assigning ratio - was by Dr. Hermann Snellen
(1862) constructed a set of block letters for which the letter as a whole was five times as
large as the strokes that formed the letter
He defined visual acuity as - the distance at which a person can just identify the letters/the
distance at which a person with normal vision can just identified letters
oIn later version – reviewer was positioned at a constant distance of 20 feet and the size of the
letters was altered normal vision would be defined as 20/20
In terms of visual angle a 20/20 letter is subtends at an angle of 5 arc minutes in the eye &
each stroke of 20/20 letter subtends an angle of 1 arc minute
If you have to be at 20 feet to read a letter that someone w/ normal vision can read at 40 feet -
20/40 vision
20/20 is gold standard BUT most healthy young adults have acuity closer to 20/15
Acuity for Low-Contrast Stripes
Sss
Why Sine Wave Gratings?
pure” sine wave grating may be rare in the real world - on a larger scale the visual system
appears to break down real-world images into a vast number of components each of which is
essentially a sine wave grating w/ particular spatial frequency
oThis method of processing is analogous to way in which auditory system deals w/ sound
Retinal Ganglion Cells & Stripes
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Each ganglion also respond well to certain types of stripes or gratings that specific spatial
frequency that matches its receptive field size Figure - e.g. ON ganglion cells respond to
grating of different spatial frequencies
oSo if spatial frequency is too lowthe ganglion cells responds weakly because part of the fat
bright bar of grating lands inhibitory surround damping the cells response (also occurs if
its frequency is high both bright & dark stripe fall into receptive field center )
Thus, need a specific frequency – bright bar fill the ON center & dark bar will the OFF
center
Cugell & Robson – 1st to record the responses of retinal ganglion cells to sinusoidal gratings ->
ganglion cells also responses depend on the PHASE of the grating - the system as whole is able
to see all 4 phase equally
oPhaseits position w/in the receptive field e.g. by shifting the phase by 90 degree
there will no net difference b/w light intensity in receptive field center & its surround 
response rates of cell dont change
oA second 90 degree shift produce negative response
oThird 90 degree shifts - bring the same situation after the 1st shift phase
The Lateral Geniculate Nucleus (LGN)
Axons of Ganglion cells synapse in the 2 LGN - which act as relay situations on way from the
retina to cortex ]
o6 layered structure ( bit like stack of pancakes that has been bent in the middle)
Neurons in bottom 2 layers are physically larger than those in the top 4 layers - Got their name for
this reason:
oBottom 2 layersMangocellular Layers receive input from M ganglion cells
Studies indicate this pathway responds to large, fast moving objects
oTop 4 layers Parvocellular Layers (small) receive input from P ganglion cells
Processing details of stationary targets
1stLeft LGN receives projections from Left sides of Retinas from Both eyes & Right LGN
receive projects from the Right sides of retinas
oEach layer of LGN receive inputs from one or the other eye from bottom to top :
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