SPATIAL VISION: FROM SPOTS TO STRIPES
VISUAL ACUITY: OH SAY, CAN YOU SEE?
Contrast: difference in luminance between an object and the back ground, or between lighter
and darker parts of the same object.
Acuity: the smallest spatial detail that can be resolved.
Cycle: for a grating, a pair consisting of one dark bar and one bright bar.
Visual angle: the angle subtended by an object at the retina
To calculate visual angle of your resolution acuity divide size of the cycle by the viewing
distance at which you could just barely make out the orientation of the gratings, then take the
arctangent of this ratio.
This resolution acuity represents one of the fundamental limits of spatial vision: it’s the finest
high-contrast detail that can be resolved
The limit is determined by the spacing of photoreceptors in the retina.
Sine wave grating: a grating with a sinusoidal luminance profile. Light intensity in such gratings
varies smoothly and continuously across each cycle.
Aliasing: misperception of a grating due to undersampling.
A VISIT TO THE EYE DOCTOR
The method for designating visual acuity was inventedin 1862 by a Dutch eye doctor, Herman
Although 20/20 vision is often considered the gold standard, most healthy young adults have an
acuity level closer to 20/15.
ACUITY FOR LOW-CONTRAST STRIPES
Spatial frequency: number of times a pattern, such as a sine wave grating, repeats in a given
unit of space.
o Example, if you view your book from 120 cm away, the visual angle between each pair
of white stripes is about 0.25 degree, so the spatial frequency of this grating is 1/0.25 =
4 cycles per degree.
Cycles per degree: the number of pair of dark and bright bars per degree of visual angle.
You might think that the wider the stripes (the lower the spatial frequency), the easier it would
be to distinguish the light stripes from dark stripes but this isn’t true. Schade, Fergus Campbell
and Dan Green demonstrated that the human contrast sensitivity function (CSF) is shaped like
an upside down U.
Contrast threshold: smallest amount of contrast required to detect a pattern.
For a 1 cycle/degree grating to be just distinguishable from uniform gray, the dark stripes must
be about 1% darker than the light stripes o Example if a light stripe reflects 1000 photons, dark stripe should reflect 990 photons).
o The reciprocal of this threshold is 1/0.01 = 100.
A contrast of 100% corresponds to a sensitivity value of 1. The CSF value reaches this value at
about 60 cycles/degree
RETINAL GANGLION CELLS AND STRIPES
Retinal ganglion cells respond vigorously to spots of light. They also respond well to certain
types of stripes or gratings.
When the spatial frequency of the grating is too low, the ganglion cell responds weakly because
of the fat, bright bar of the grating lands in the inhibitory surround, damping the cell’s response.
When the spatial frequency is too high, the ganglion cell responds weakly because both dark
and bright stripes fall within the receptive field center, washing out the response.
When spatial frequency is just right, with bright bar filling the center and dark bar filling the
surround, the cell responds vigorously.
Thus, retinal ganglion cells are “tuned” to spatial frequency: 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 first to record responses of retinal ganglion cells to
In addition to showing that these cells respond vigorously to gratings of just the right size they
discovered that the responses depend on the phase of the grating, its position within the
In figure 3.10, when the grating has a light bar filling the receptive-field center and dark bars
filling the surround, this ON- center cell responds vigorously, increasing the firing rate.
If the grating phase is shifted by 90 degrees, half the receptive field center will be filled by a light
bar and half by a dark bar, and similarly for the surround. There won’t be a net difference
between the light intensity in the receptive field’s center and its surround so the cell’s response
rate doesn’t change from its resting rate when the grating is turned on.
A second 90 degree shift puts the dark bar in the centre and the light bars in the surround,
producing a negative response
A third phase shift returns us to the situation after the first shift and the cell will be blind to the
grating (other ganglion cells would respond to the 90 and 270 degree phases but not to the 0
and 180 degree phases, which is why the visual system as a whole is able to see all 4 phases
THE LATERAL GENICULATE NUCLEUS
The axons of retinal ganglion cells synapse in the 2 lateral geniculate nuclei (LGNs), one in each
These act as relay stations on the way from the retina to the cortex. LGN of primates is a 6 layered structure, like a stack of pancakes that has been bent in the
middle (geniculate means bent).
The neurons in the bottom 2 layers are larger than those in the top 4 layers, the bottom 2 are
called magnocelular layers and the top 4 are called parvocellular layers
Another way the 2 types of layers differ is that the magnocellularlayers receive input from M
ganglion cells in the retina, and parvocellular layers receive input from P ganglion cells.
Magnocellular pathway responds to large, fast moving objects, and the parvocellular pathway is
responsible for processing details of stationary targets.
Even more splitting takes between magno and parvo layers. Between these 2 layers we find
layers consisting of koniocellular cells. Each layer is involved in different aspects of processing.
o Example, one layer is specialized for relaying signals from the S cones and may be part
of a “primordial” blue-yellow pathway.
1. Left LGN receives projections from the left side of the retina in both eyes, and the right LGN
receives projctions from the right side of both retinas.
2. Each layer of the LGN receives input from one or the other eye.
Layers 1, 4, and 6 of the right LGN receive input from the left (contralateral) eye, while layers 2,
3, and 5 get their input from the right (ipsilateral) eye
o Contralateral means opposite side of the body (or brain)
o Ipsilateral means same side of the body (or brain)
Each LGN layer contains a highly organized map of a complete half of the visual field. Figure 3.12
shows how objects in the right visual field are mappedonto different layers of the left LGN (the
right side of the world falls on the left side of the retina, whose ganglion cells project to the left
This ordered mapping of the world onto the visual nervous system, known as topographic
mapping, provides us with a neural basis for knowing where things are in space.
LGN neurons have concentric receptive fields similar to retinal ganglion cells: they respond well
to spots and gratings.
Then why don’t the ganglion cell axons simply travel directly back to the cerebral cortex?
o LGN isn’t merely a stop on the line from retina to cortex, there are many connections
between other parts of the brain and the LGN
o There are more feedback connections from the visual cortex to the LGN than from the
LGN to the cortex
o LGN is a location where various parts of the brain can modulate input from the eyes.
Example, LGN is part of a larger brain structure called the thalamus. When you
go to sleep, the entire thalamus is inhibited by circuitry elsewherein the brain
that works to keep you asleep
If you were sleeping with your eyes open you wouldn’t see anything in a dimly
lit room because input would go from your retinas to your LGNs, but the neural
signals would stop there before reaching the cortex.
Bright lights and alarms will be perceived and cause you to wake up because
thalamic inhibition isn’t complete. STRIATE CORTEX
The receiving area for LGN inputs in the cerebral cortex lies below the inion.
This area’s also named: primary visual cortex (V1), area 17, or striate cortex: area of the
cerebral cortex that receives direct inputs from the LGN, as well as feedback from other brain
areas, and is responsible for processing visual information.
Striate cortex consists of 6 major layers, some of which have sublayers.
Fibers from LGN project mainly to layer 4, with magnocellular axons coming in to sublayer 4C
alpha and parvocellular axons projecting to sublayer 4c beta.
Striate cortex contains on the order of 200 million cells(more than 100x as LGN).
Figure 3.14 illustrates 2 important features of the visual cortex: topography and magnification.
o The fact that the image of the woman’s right eye brow is mapped onto regions
corresponding to the numbers 3 and 4 in the striate cortex, tells the visual system that
the eyebrow must be in positions 3 and 4 of the visual field. This is topographical
o Objects imaged on or near the fovea are processed by neurons in a large part of the
striate cortex, but objects imaged in the far right or left periphery are allocated only a
tiny portion of the striate cortex.
o This distortion of the visual-field map on the cortex is known as cortical magnification
because the cortical representation of the fovea is greatly magnified compared to the
cortical representation of the peripheral vision.
TOPOGRAPHY OF THE HUMAN CORTEX
Much of what we know about cortical topography and magnification comes from anatomical
and physiological studiesin animals.
Earliest studies in humans were based on correlating visual field defects with cortical lesions.
MRI is useful for anatomical imaging of soft tissues, including the brain.
MRI lets us see the structure of the brain.
fMRI is a noninvasive technique for measuring and localizing brain activity
fMRI doesn’t measure neural activity directly, it measures changes in blood oxygen level that
reflect neural activity.
Blood oxygen level-dependent (BOLD) signals reflect a range of metabolically demanding neural
By comparing blood flow when visual stimuli are presented in one portion of the visual field to
blood flow when the field is blank, you can find portions of the brain that respond specifically to
that stimulation of that portion of the field.
This makes it possible to map the topography of VI in the living human brain.
SOME PERCEPTUAL CONSEQUENCES OF CORTICAL MAGNIFICATION
One consequence of cortical magnification is that visual acuity declines in an orderly fashion
with eccentricity. This was demonstrated by Hermann Rudolf Aubert. High resolution requires a great number of resources: a dense array of photoreceptors, one-to-
one lines from photoreceptors to retinal ganglion cells,and a large chunk of striate cortex.
To see the entire visual field with such high resolution, we’d need eyes and brains too large to fit
in our heads. Thus we evolved to have a visual system that provides high resolution in the center
and lower resolution in the periphery.
Although visual acuity falls off rapidly in peripheral vision, it’s not the major obstacle to reading
or object recognition in the visual periphery, the real problem is visual crowding: the
deleterious effect of clutter on peripheral object recognition. Objects that can be easily
identified in isolation seem indistinct and jumbled in clutter.
Crowding impairs not only the discrimination of object features and contours but the ability to
recognize and respond appropriately to objects in clutter.
Crowding simplifies the appearance of the peripheral array by promoting consistent appearance
among adjacent objects at the expense of an ability to pick out individual objects.
RECEPTIVE FIELDS IN STRIATE CORTEX
David Hubel and Torsten Wiesel tried to map the recept