Visual System: Photons to memory
-- Each sensory system responds with some specificity to a stimulus
and each employs specialized cells - the peripheral receptors - to
translate the stimulus into a signal that all neurons can use.
-- Precision requires that labor be divided among neurons so that
not only different stimulus energies (light vs sound vs mechanical
deformation of skin or hair) but also different stimulus qualities
(e.g., color vs motion) are analyzed by separate groups of neurons.
-- Organization along labeled lines, comparison of events that occur
simultaneously at different receptors serves recognition of stimulus
strength or contrast.
Perception of a sensory experience can change even though the input
remains the same.
Receptors are specific for a narrow range of input.
Sensory transduction - all receptors transduce the energy to which
they are sensitive into a change in membrane voltage.
Neuronal signaling is accomplished by a combination of rate and
temporal codes.
Center/surround organizations brought about by lateral inhibition
serve to sharpen responses over that which would be achieved by
excitation alone.
Sekuler, R., and Blake, R., Perception, 2nd edition. New York: McGraw-Hill, Inc., 1990. © 1990 by
McGraw-Hill, Inc. Adapted by permission of McGraw-Hill, Inc.
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Visual Field: Monocular and Binocular Zones
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Upper half of visual field projects onto inferior (ventral) half of retina and lower half of visual field projects
onto superior (dorsal) half of retina
Left visual hemifield projects to nasal hemiretina of left eye and temporal hemiretina of right eye
Ganglion cells’ axons from temporal hemiretinas do not cross.
Thus nasal retina axons innervate contralateral LGN and temporal retinal axons innervate ipsilateral LGN.
Thus the left visual field is processed by right LGN and V1; and right visual field is processed by left LGN
and left V1.
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Left LGN
The LGN from monkey
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LGN
• The LGN consists of six layers.
• Each layer receive input from one eye.
• Magnocelluar
– inner two layers ( 1, 2 layer)
– input from M ganglion cells
• Pavocelluar
– outer four layers ( 3, 4, 5, 6 layer)
– input from P ganglion cells
• Visual system
– divided into two or more streams of information,
– different aspects of visual perceptions are processed separately
• The division is first evident at the ganglion cells
– M class, P class
• P cells (80%) - P pathway
– selective for wavelength & high spatial frequencies
– slow sustained responses
• M cells (10 %) - M pathway
– sensitive to low spatial frequencies
– transient responses
LGN
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P-retinal ganglion cells (abundant in retina; small receptive
fields; sensitive to color contrast; and high sensitivity to
spatial frequency; thus their pathway will encode form and
color) project to the Parvocellular Layers (3, 4, 5, 6) of the
LGN.
M-retinal ganglion cells (fewer in retina; larger receptive
fields; not sensitive to color contrast; lower spatial but higher
temporal frequency sensitivity; thus their pathway will
encode gross features and movement) project to the
Magnocellular Layers (1, 2) of the LGN.
Reflective of their inputs, Parvocellular (parvo=small) layers
have smaller cells, smaller RFs, are slow to respond, and are
color sensitive. Thus form and color are encoded by these
layers. Magnocellular Layers (magno=large) have larger
cells, large RFs, respond faster, and are not sensitive to color.
Thus these layers encode gross features and movement.
LGN RFs: Similar concentric RFs to those of ganglion cells,
but with stronger (larger) surround. This translates to more
sensitivity to contrast than ganglion cells.
If you were to lower an electrode straight dorsoventrally from
Layer 6 until 1, what would you see? Retinotopy is preserved,
so you’d see one part of the retina present in all layers and the
orientations in each layer would be in register.
If you were to close the right eye and just allow light to hit
the one eye, then you would be able to glean which layers are
ipsilateral (Layers 2, 3, 5) and contralateral (Layers 1, 4, and
6).
Hubel and Wiesel (1977)
These recordings are from
LGN.
LGN neurons have concentric
receptive fields with either an
on center-off surround or off
center-on surround
organization. This on centeroff surround cell fires rapidly
when the light encompasses
the center region, and is
inhibited when light is
positioned over the surround.
The stimulus produces little
change when crossing center
and surround. These neurons
are ideal for signaling changes
in illumination from edges.
Flow of Visual Information: retina  (pretectum; superior colliculus) LGN 
V1  V2  What/Where Pathways
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Zeki, S., A Vision of the Brain. Oxford, UK: Blackwell Scientific, 1993.
The primary visual cortex : V1
• The LGN neurons mainly project to the primary
visual cortex.
• V1 consists of six layers and several sub layers are
arranged in bands parallel to the surface of the
cortex.
• The axons from the LGN terminate on cortical
neurons in layer 4 of V1.
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These neurons are from V1 and are simple cells.
Neural Computation
Simple Cells of V1
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For a cell that has separated and elongated on and off regions (simple RF), you need the following effective
stimulus:
It must excite the specific segment of the retina innervated by receptors in the excitatory zone (specific position
on the retina and also a specific (excitatory) position in the RF);
It should have the correct linear properties (either bar or edge);
It should have a specific axis of orientation;
Note: some simple cells are directionally sensitive; and diffuse light is not effective since the firing rate elicited
in the on-region cancels that of the off-region.
Its on region is made from input from many on-center LGN cells lining up at a particular orientation, and its off
region is made from the input of off-center LGN cells lining up at a particular orientation.
They tend to be segregated in layers 4 and 6 of V1.
Complex Cells of V1
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From a group of simple cells with same axis of orientation but with slightly offset RF positions
(but overlapping), you can produce complex cells.
Thus for a complex cell, the axis of orientation is still important (responds well to bars or edges),
but you no longer are sensitive to the position of that stimulus in the RF (since you no longer have
clear on or off regions).
Note: some can be direction selective, and some can be end-inhibited or end-stopped.
They tend to be all over (outside of layer 4).
P layer and M layer in V1
• P layer
– The P layer neurons send their axons to neurons in
the sub-layer 4C
• M layer
– The M layer neurons send their information to
neurons in sub layer 4C  4B  V2  V5.
– Cells in layer 4B
• selective for the direction of movement
• some of these neurons are binocular and sensitive to
retinal disparity
Splits of P pathway in V1
• The P pathway splits to produce two new path
ways in the upper layers of V1.
• P-B pathway (blobs)
– primarily deal with color
• P-I pathway (interblob region)
– sensitive to the orientation of the stimulus
Columnar Organization to Modules in V1
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Ocular dominance and orientation columns
Orientation columns: Identical axis of
orientation in all layers
Geometric vs. Pinwheel (more likely as
measured by voltage-sensitive dyes that
flouresce when particular orientation columns
light up during presentation of a bar of light
with a specific axis of orientation.)
A hypercolumn/module (1mm by 1mm by
2mm) consists of an axis of all (0 to 180
degrees) orientations (orientation columns);
alternating columns process separate inputs
from each eye (ocular dominance columns); and
blobs related to color.
Visual area 2
• Thick stripes
– sensitive to orientation and movement
– sensitive to retinal disparity
• Thin stripes
– not orientation selective
– color sensitive
• Interstripes
– orientation selective
– not selective to direction and color
Visual area 4
• Both subdivisions of the P pathway, the thick
stripes(color) and the inter-stripes(form), project
to V4.
• Some V4 cells respond not to the wavelength of
light but to its ‘color’(color constancy).
• Damage to this area of V4 in humans impairs
the ability to distinguish color(achromatopsia).
• V4 is important for object discrimination.
• V4 projects primarily to the temporal visual
cortex
Visual Areas 3 and 5
• The M pathway projects to V3 and V5.
• Cells in V3
– orientation selective
– concerned with processing dynamic form
• Cells in V5(MT)
– process information on motion and
stereoscopic depth
Fates of Different V1 Layers
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Area MT
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Zeki, S., A Vision f the Brain. Oxford, UK: Blackwell Scientific, 1993.
Zeki, S., A Vision f the Brain. Oxford, UK: Blackwell Scientific, 1993.
Different Areas Analyze Different Aspects of A Stimulus