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Chapter 7: Visual Physiology
• We have an inverse retina, where light must pass through blood vessels, the optic nerve, and
other neurons before reaching the photoreceptors. Octopi have an everse eye, although it is
functionally similar to ours.
• Fundus: The back surface of the eye. The fovea has fewer blood vessels passing in front of it.
• We have 5 cell types organized into 3 layers.
• Outer nuclear layer: Photoreceptor cells, transduces light energy into neural signals
• Inner nuclear layer: Retinal circuitry of bipolar cells, amacrine cells, and horizontal cells connect
photoreceptors to ganglion cells.
• Ganglion cell layer: Fibres carry neural signals out of the eye, along the visual pathway to the
• 100 million photoreceptors to 1 million ganglion cells: convergence of signal achieved in the inner
• Plexiform Layer: Axons and dendrites connect between nuclear layers.
Outer Nuclear Layer
• Rods and cones are the 2 types of photoreceptor in
the outer nuclear layer. Rods are used for low- light scotopic
conditions, cones for high-light photopic conditions
(mesopic when both rods and cones are active).
• Rods have peak sensitivity at 510nm (blue/green). This
corresponds to behavioural spectral sensitivity in terms of this
wavelength having the lowest minimal detection threshold under
• Cones have peak sensitivity at 440nm (Short), 540nm (Med), and
570nm (Long). Behaviourally, under bright conditions maximal
sensitivity is at 555nm, so only M and L cones contribute to the perception of brightness.
• Purkinji Shift: During daylight, yellow looks brightest; in scotopic conditions, greens and blues look
• The outer segment of rods and cones consists of a stack of light-sensitive disks with photopigments,
and the inner segment contains the cell nucleus and synaptic terminals, housed in clefts.
• The clefts receive processes from bipolar cells and horizontal cells; each rod has one single cleft
(admits 2 horizontal processes and 2-5 bipolar processes) while each cone has several synaptic clefts.
• The tip of each photoreceptor abuts the pigment epithelium, which regenerate photopigments and
catch light not absorbed by the photoreceptor. In nocturnal animals, this epithelium is reflective so the
limited light can be used again.
• Stiles-Crawford effect: Photons arriving through the center of the pupil are most likely to be absorbed
(light parallel to long axis of receptor), whereas photons coming from nonparallel directions at the edge of
the pupil are less likely to be absorbed.
• The effect is more marked for cones than rods. Rods absorb 60% more from a dilate pupil than to
• If a photon has a frequency near the peak sensitivity of the receptor it strikes, it is more likely to be
absorbed. There are three distinct classes of cone photopigments, each having different spectral
sensitivities from each other, and from the rod photopigment. Page 2 of 7
• Only 2/3 of the photons absorbed by a photoreceptor result in a response, with the remainder
dissipated as heat.
• Each photopigment consists of a protein opsin, and a light-catching chromophore.
• Mammalian pigments use 11-cis retinal as the chromophore, and are known are rhodopsins. Rods
and cones contain different variants of rhodopsin.
• Photoisomerization occurs when an absorbed photon causes a change in the shape of the
chromophore molecule (from 11-cis retinal to all-trans retinal) which changes membrane potential and
leads to a visual response.
• Photoreceptors generate graded changes in membrane potential, and graded changes in
neurotransmitter release; the short distance to the ganglion cells does not require action potentials.
• Photoreceptors have a membrane potential of -40mV in darkness; increasing illumination
hyperpolarizes the cell to a saturation of -65mV – photoreceptors release less neurotransmitters in the
• Cones respond more quickly than rods, but require higher light levels (30-100 vs. a single photon)
• Principle of univariance states that when a photon is absorbed, its effect on a photoreceptor is the
same, regardless of the photon’s wavelength/frequency. Of course, photons of a particular frequency will
tend to be captured more by some receptors than others due to spectral sensitivity.
Inner Nuclear Layer
• Cones have a complex network of postsynaptic cells, whereas rods have simple connectivity; although
there are far fewer cones than rods, there are 8-10x as many cone-driven neurons.
• Bipolar cells transmit graded potentials “vertically” from photoreceptors to ganglion cells.
• ON bipolars are activated by an increase in the photon catch of receptors, and they depolarize
(increase rate of transmitter release).
• OFF bipolars are activated by a decrease in the photon catch of receptors, and they hyperpolarize
(decrease rate of transmitter release). This is the same as the action of the photoreceptors,
hyperpolarizing to light.
• Each cone bipolar connects to 1-10 cone receptors. Cone bipolars are divided into ON and OFF types
and connect with ganglion cells.
• Each rod bipolar connects to 30-50 rod photoreceptors. Rod bipolars only have ON responses, and
connect to a class of amacrine cells (AII), which then connect to either ON or OFF cone bipolars. So ON
signals from rods “piggyback” onto both the ON and OFF bipolar circuitry that connects cones to
• Stimulation of horizontal cells by photoreceptors is fed back to reduce the influence of photoreceptors
on bipolar cells. The result of this lateral inhibition is that activation of any one photoreceptor reduces
the activation of surrounding photoreceptors.
• Amacrine cells have extensive connections with bipolar cells, ganglion cells, and other amacrine cells,
and have many different functions. They outnumber both horizontal and ganglion cells.
Ganglion Cell Layer
• The ganglion cell layer contains 4 major classes of ganglion cells, and some amacrine cell bodies.
• Recent research has also uncovered another class of ganglions which sense light directly, and seem to
be involved in functions that require illumination level information, such as diurnal body rhythm and pupil
Bistratified Ganglion Midget Ganglion Parasol Ganglion Page 3 of 7
Incidence >10% 70-80% 8-10%
Luminance response Photopic only Photopic only Scotopic (low light
conditions) + photopic
Spatial response Nonopponent Opponent Opponent
Spectral response S versus (L + M) = blue- L versus M (central) = L + M = no opponency
yellow opponency red-green opponency
L + M (peripheral) = no
Spatial filtering Small receptive field Large receptive field
Temporal response Sustained, slow Transient, fast
Projection to lateral Konio LGN Parvo LGN Magno LGN
•Biplexiform ganglion cells connect directly to photoreceptors, depolarizing in response to increases
in photon catch. The precise role of these cells in visual processing is not clear
•They have processes in both the upper plexiform (between outer and inner nuclear layers), and the
lower plexiform (between inner nuclear and ganglion layers)
Response Properties of Ganglion Cells
•Spatial Opponency: Cell is excited when light falls in one part
of its receptive field, and inhibited when light falls in another part.
•These regions are arranged in a center-surround
configuration; the center response reflects bipolar cells, while the
opposing surround is a result of lateral inhibition form horizontal
•ON-center ganglion receptive fields have an excitatory center
and inhibitory surround – small light spots at the center produce
best response. OFF-center receptive fields have an inhibitory
center and excitatory surround – small dark spots at center
produce best response.
•Midget ganglion cells have smaller receptive fields than parasol
ganglion cells. The strength of inputs follow a bell-shaped
Gaussian distribution; the surround input is wider, but the center
input is more dominant.
•Spectral Response: Cones are divided into 3 classes based
on their spectral sensitivity: blue (short), green (medium), or red
(long wavelengths). Ganglion cell response depends on the cone
classes contributing to its receptive field.
•Spectral Opponency: Wavelengths from one portion of the spectrum excite a response, and other
wavelengths inhibit a response, due to imbalances in cone contributions to excitation or inhibition.
•There are blue-yellow and red-green opponencies. B-Y opponency is a property of bistratified
ganglion cells, excited by blue and inhibited by yellow. R-G opponency is mediated by midget ganglion
cells in retina centre.
•Short wavelength cones thus have their own class of ganglion cells, the bistratifieds. This likely has an
evolutionary origin, representing a very early mammalian colour coding circuit; L+M pathway is restricted
to Old World monkeys, associated with the more recent evolution or separate L and M cone types.
•R-G Opponency: Photoreceptors are arranged in a hexagonal lattice pattern. There seem to be no
special circuitry segregating L and M cone signals to ganglion cells; rather, opponency may be a
byproduct of receptive field sizes: midget receptive fields have a dominant input from one M or L in the
centre, and random inputs from M and L cones in the surround.
Information Processing by Ganglion Cells Page 4 of7
• Midget cells have a tonic or sustained temporal response, so the response is sustained at a high
level for the whole duration of the stimulus. Parasol ganglion cells have a phasic or transient temporal
response, so the change in response occurs only at the onset and offset of stimulation.
• Small receptive fields can resolve high levels of detail; large receptive fields filter out fine detail (neural
blur). Midget ganglion cells have smaller receptive fields than parasol ganglion cells.
• Chromatic opponency provides two chromatic channels of information processing: red versus green
(midget ganglion cells), and blue versus yellow (bistratified ganglion cells).
• Cells with spatial opponency provide an achromatic channel: dark versus light (parasol cells and
• The parvo (P) system has slow, sustained temporal responses from midget cells, and conveys
information about static form. The magno (M) system has fast, transient temporal responses from parasol
cells, and conveys information about motion.
• Separate ON and OFF channels enable the visual system to respond to both light increments and light
The Visual Pathway
• The fibers of retinal ganglion cells form the optic nerve, which terminates at