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Chapter 3

PSYB51H3 Chapter 3: Chapter 3

Course Code
Matthias Niemeier

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Chapter 3: Spatial Vision: From Spots to Stripes
The process of getting an image of the world in front of us to an interpretation of that world starts in the
eyeball, where the postreceptor layers of the retina translate the raw light array captured by the
photoreceptors into the patterns of spots surrounded by darkness, or vice versa, detected by the
ganglion cells
This retinal translation helps us perceive the pattern of light and dark areas in the visual field,
regardless of the overall light level, i.e., it enables us to see almost as well at dusk as at noon
Whereas ganglion cells in the retina respond preferentially to spots of light, neurons in the cerebral
cortex prefer lines, edges, and stripes
Visual Acuity: Oh Say, Can You See?
The visual system codes images in terms of oriented stripes
Acuity: the smallest spatial detail that can be resolved
Cycle: for a grating, a pair consisting of one dark bar and one bright bar; one repetition of a black and
white stripe
Visual angle: the angle subtended by an object at the retina
Under ideal conditions, humans with very good vision can resolve gratings like those in Figure 3.2 page
57 when one cycle subtends an angle of about 1 minute of arc (0.017 degree)
This resolution acuity represents one of the fundamental limits of spatial vision: it is the finest high-
contrast detail that can be resolved. The limit is determined primarily by the spacing of
photoreceptors in the retina
Sine wave grating: a grating with a sinusoidal luminance profile, e.g., Figure 3.4 page 58
The light intensity in such gratings varies smoothly and continuously across each cycle
High-contrast sine wave gratings can be distinguished from a uniform gray field, as long as adjacent
pairs of light and dark stripes are separated by at least 1 arc minute of visual angle
Aliasing: misperception of a grating due to undersampling
We misperceive the cycles to be longer than they are in a sine wave grating if the entire cycle falls
on a single cones, rather than the whitest and blackest parts falling on separate cones
Cones in fovea have a center-to-center separation of about 0.5 minutes of arc (0.008 degree), which fits
nicely with the observed acuity limit of 1 minute of arc, since two cones per cycle are required to be
able to perceive the grating accurately
Rods and cones in the periphery are packed together less tightly, but here many receptors converge on
each ganglion cell. As a result, visual acuity is much poorer in the periphery than in the fovea
A Visit to the Eye Doctor
Herman Snellen in 1862 invented the method for designating visual acuity that is used by eye doctors
by constructing a set of block letters for which the letter as a whole was five times as large as the
strokes that formed the letter (Figure 3.5)
Distance at which a person can just identify the letters divided by the distance at which a person
with “normal” vision can just identify the letters
Later, changes were made and the person was kept at 20 feet in front of the letters, and the letters
themselves were made smaller rather than moving the person—so normal vision came to be defined as
A 20/20 letter is designed to subtend an angle of 5 arc minutes (0.083 degree) at the eye, and each
stroke of a 20/20 letter subtends an angle of 1 arc minute (the familiar 0.017 degree)
Thus, if you can read a 20/20 letter, you can discern detail that subtends 1 minute of arc
If you have to be at 20 feet to read a letter the normal eye can see at 40 feet, you have 20/40 vision
(worse than normal). Most healthy young adults have an acuity level closer to 20/15
Acuity for Low-Contrast Stripes
Otto Schade in 1956 wondered, “What happens when the contrast of the stripes is reduced—if the light
stripes are made darker and the dark stripes lighter?”
Spatial frequency: the number of grating cycles in a given unit of space

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Cycles per degree: the number of pairs of dark and bright bars per degree of visual angle
The lower the spatial frequency, the wider the stripes. You may think that the lower the spatial
frequency, the easier it would be to distinguish the light stripes from the dark stripes
Contrast sensitivity function (CSF): a function describing how the sensitivity to contrast (defined as
the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus
For humans, it is shaped like an upside down U (Figure 3.7)
We obtain the units for the y-axis in this graph by taking the reciprocal of the contrast threshold
(smallest amount of contrast required to detect a pattern)
A contrast of 100% corresponds to a sensitivity value of 1. The CSF reaches this value at about 60
cycles/degree, which corresponds to a cycle width of 1 minute of arc, the resolution limit we measured
previously for high-contrast stripes (which is determined primarily by cone spacing). However, the falloff
in the CSF on the other side of the curve must be due to neural factors
Why Sine Wave Gratings?
Although “pure” sine wave gratings may be rare in the real world, patterns of stripes with more or less
fuzzy boundaries are quite common
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 with a particular spatial frequency
Retinal Ganglion Cells and Stripes
In addition to spots of light, each ganglion cell also responds well to certain types of stripes or gratings
Figure 3.9 shows how an ON retinal ganglion cell responds to gratings of different spatial frequencies
A grating with low spatial frequency gets a weak response from the ganglion cell because the fat
bright bar of the grating lands in the inhibitory surround
When the spatial frequency is too high, a weak response is given because both dark and bright
stripes fall within the receptive-field center
Each cell responds best to a specific spatial frequency that matches its receptive-field size, and it
responds less to both higher and lower spatial frequencies
Christina Enroth-Cugell and John Robson were the first to record the responses of retinal ganglion cells
to sinusoidal gratings, and discovered also that responses depend on the phase of the grating—its
position within the receptive field
Figure 3.10: 0 degrees would mean that a bright bar of just the right size is filling the receptive-field
center of the ON-center cell
Shifting the grating phase by 90 degrees would allow half the receptive-field center to be filled with
a bright bar, and the other half with a dark bar, and similarly for the surround. So, there would be no
net difference between the light intensity of the center and the surround. The cell’s response rate
does not change from its resting rate
Shifting another 90 degrees would cause the center to be filled with a dark bar rather than bright,
producing a negative response
Another 90 degrees shift brings us back to what happened after the first 90 degree shift
Note that other ganglion cells would respond to the 90-degree and 270-degree phases but not to the 0-
degree and 180-degree responses, and that is why the visual system is able to see all four phases
The Lateral Geniculate Nucleus
The axons of the retinal ganglion cells synapse in the two lateral geniculate nuclei (LGN): structure in
the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and
output connections to the visual cortex
In primates, the LGN is made of 6 layers. The bottom two layers have cells that are physically larger
than the upper layers, and therefore are called the magnocellular layers. The top four layers are called
parvocellular layers
The magnocellular layers receive input from M ganglion cells in the retina, and the parvocellular layers
receive input from the P ganglion cells
The magnocellular pathway responds to large, fast-moving objects and the parvocellular pathway
processes details of stationary targets
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