Forty Flashcards
What 3 things does the retina do?
4) transduce the energy into neural messages (membrane conductance changes);
5) perform the initial processing of the neural information into spike codes of luminance, color contrast, spatial contrast or movement analysis.
6) transmit those neural messages to higher visual processing structures.
What 3 things does the cortex do?
7) receive thalamic relay, combine inputs from both eyes, initiate analysis of basic visual information.
8) create a visual percept based on comparison of learned visual experiences.
How sensitive to light is the eye? Why is vitamin A important? What is retinal? What are opsins? What is rhodopsin? What determines the wavelength that a photoreceptor responds to? What is a red cone photoreceptor sensitive to? What causes color blindness? What is the most common form?
- Photon Transduction. The eye is very sensitive to light, responding to as little as one
photon of light. A critical substrate to transduction is vitamin A, which becomes retinal
within the photo-sensitive proteins called opsins. Rhodopsin is in the rods and related
opsins are found in cones. It is the difference in the protein sequences that give the
different photoreceptors their specific sensitivity to different wavelengths of light. In the
case of rhodopsin, its protein confers the preferred wavelength of about 510 nm.
Note the confusing terminology: a red cone photoreceptor usually refers to a red-sensitive
cone, which is most sensitive to red light. However, since it maximally absorbs red light,
it may appear green-blue (when viewed under normal white light). Still we call it a red
cone to refer to its function, not its appearance. There are three different types of cones,
each responsive to a different wavelength of light (Blue-sensitive, 440 nm; Green-
sensitive, 535 nm; Red-sensitive, 570 nm).
Clinical Note: Color blindness is usually associated with the loss of one type of cone.
The most common form of color blindness is found in patients with a red-green
perceptual deficiency due to an error in the green cone’s pigment. Their defect is
inherited as a sex linked recessive trait in 8% of men and 0.5% of women.
What happens when light is absorbed by the retinal? What happens then in cones? In rods? What are the differences between the two?
When light is absorbed by the photopigment, retinal in the form of 11-cis-retinal absorbs
the light’s energy and changes its physical conformation to the isomer all-trans-retinal.
This conformation change results in the retinal molecule detaching from the protein yielding opsin and all-trans-retinal. Before the process of transduction can start again, all-trans-retinal has to re-isomerize back to 11-cis-retinal and reattached to the protein molecule.
Changes in the cone photo-sensitive proteins lead directly to changes in ion conductances
across the membrane to change the cell’s voltage. Communication by the rod’s photo-
sensitive proteins, rhodopsin, is less direct but allows for signal amplification.
Rhodopsin is found on membranous disks that have separated from the plasma membrane
and are floating as a long stack of pancakes in the outer segment. In order for the
conformational change of the rhodopsin molecule to change the membrane potential at
the synaptic terminal, the message has to jump the cytoplasmic gap. This is
accomplished using mechanisms similar to other forms of intracellular communications:
second messenger molecules. An activated rhodopsin molecule changes the shape of many molecules of a G-protein called transducin. Each activated transducin molecule changes the shape of many molecules of an enzyme phosphodiesterase. Each activated phosphodiesterase molecule converts many molecules of cyclic GMP into GMP. Since cyclic GMP opens Na+ channels in the plasma membrane, the rod membrane hyperpolarizes and the synaptic vesicle output decreases. This cascade thus creates a great deal of amplification (a single photon will cause many Na+ channels to close), yetthat response has a very long latency (tens of milliseconds).
What does white light cause on a cellular level? What about dark? How do photoreceptors relay signals?
Photoreceptors as Neurons. Although the photoreceptor outer segment is specialized to transduce light, the inner segment acts like any other sensory neuron. Note though that a white object does not open Na+are open, there is a constant sodium current flowing up the photoreceptor from the inner segment to the outer segment, and the cell is constantly depolarized at -20 mV. When the white light turns on, the sodium channels close, the sodium current turns off, the cell channels but closes them. Thus in the dark, the channels hyperpolarizes to -70 mV giving a “response”. In contrast, a black object causes an opening of a Na+ channel for a receptor cell with its membrane potential sitting at -70mV. Sodium ions rush in and depolarize the cell. Since there are no spike-generating channels in the membrane, the membrane remains depolarized as long as darkness is stimulating the photoreceptor leading to neurotransmitter release. Moreover, all retinal processing is mediated via synapses between neighboring retinal cells that communicate
without action potentials: changes in membrane voltage and changes in levels of
neurotransmitter release. Only the retinal ganglion cells fire spikes to relay the retinal
output over the great length of the optic nerve axons to reach the brain.
Which intensity levels do the photoreceptors respond to? What are these two types of vision called? What is lost in low light levels? Why? What is the intensity range of the eye? Name some ways in which it’s accomplished.
Light intensity. Rods respond in low light levels (scotopic vision) while cones response
in bright light conditions (photopic vision). A large amount of convergence from rods to
subsequent neurons further increase our sensitivity in low light levels, yet this increase
occurs at the expense of seeing color, form and fine detail. The photopic and scotopic
vision overlap in what is called the mesopic range. Thus, the visual system responds over
a very wide range. Unlike the somatosensory system that detects skin deflections over a
10-100 fold range, the visual system can operate for 10-13
UNITS, from 1 to 100,000,000,000,000 photons). Individual photoreceptors can shift
their intensity range by another 2-3 log units biochemically. Neural network mechanisms
add another few log units. Finally, combining rod and cone pathways greatly extend the
intensity range of the visual system.
Note: A common notion is that intensity to the eye is regulated by the iris. However,
simple geometry will indicate that a pupil size decrease of 2-4 fold will only reduce the
light flux by 4-16 fold. Thus, pupil size changes can only account for one log unit of
adaptation (although it does improve focus by increasing the depth of field).
- 10-16
range (or 13-16 LOG
Generally, what is the visual system interested in? What are the two different kinds of contrast? What is the receptive field of photoreceptors like?
Retinal Processing. The visual system is interested in light differences:
(A) spectral contrast in the same point in the visual field (detecting more light of one
wavelength relative to the other wavelengths of visible light) and
(B) spatial contrast in different parts of the visual field (detecting more light in one
place relative to the area surrounding it).
The receptive fields of Photoreceptors are very small because they can only absorb
photons that pass through their outer segments. Since the visual pigments are broad-band
transducers, they will absorb many different wavelengths of photons across the
spectrum. Therefore, a response from an individual photoreceptor will not reveal the
stimulus’s color or intensity yet the remaining retinal neurons compare the outputs of
individual photoreceptors to encode the stimulus color. How?
How is spectral contrast achieved? What is H-cell receptive field like? How do they transmit signals?
(A) spectral contrast – Horizontal cells extract spectral contrast. H-cells may receive
excitatory input from red cones yet inhibition from green cones. Such H-cells are more
spectrally selective than their broad-band photoreceptor input. In contrast, H-cells have
large dendrites and get inputs from many photoreceptors. Therefore, the H-cell receptive
field is very large so they are very non-selective for stimulus position. Like
photoreceptors and bipolar cells, H-cells do not spike; they transmit signals by graded
synaptic transmitter release.
How is spatial contrast achieved? What excites bipolar cells? What inhibits them? What happens with diffuse light? How and to where do bipolar cells transmit signals?
(B) spatial contrast – Photoreceptors excite bipolar cells
during center stimulation {1}. Horizontal cells inhibit
bipolar cells during any stimulation. The resulting output
from bipolar cells is Center-Surround antagonism or
‘spatial contrast detection’. Bipolar cells receive direct
excitation from overlying photoreceptors and relay direct
excitation to ganglion cells below. Horizontal cells are
strongly interconnected so have large receptive fields that
then inhibit the bipolar cells. The excitation from the
photoreceptors is stronger than the lateral inhibition from
the horizontal cells, so centered spots are relayed to the
ganglion cells via the bipolar cells. However, a diffuse
light, which activates both the overlying photoreceptors
and the interconnected horizontal cells, creates equal
excitation and inhibition onto bipolar cells, so that no
response occurs in either the bipolar cells or the ganglion cells {2}.
Bipolar cells are spatial contrast detectors, not detectors of
absolute changes in illumination. Like every other sensory
system, there is a center/surround antagonism due to lateral
inhibition. This spatial arrangement extracts input
information about the sensory world while it optimizes
transmission of information through minimal numbers of
axons in ascending pathways. Like photoreceptors and
horizontal cells, bipolar cells do not spike; they transmit
signals by graded synaptic transmitter release.
What is the only afferent connection between the inner and outer retina? What do amacrine cells do? How do ganglion cells transmit signals and to where?
Bipolar cells are the only afferent connection between the
outer and inner retina. Then amacrine cells begin the
process of detecting temporal contrast. The amacrine
cells are the inner retina’s interneurons that help form the
complex types of retinal ganglion cells with specific
stimulus requirements such as a preference for moving
stimulus within a narrow speed range or in a specific
direction. Because ganglion cell axons project a long
distance into the brainstem, these cells must fire action
potentials, which are relayed through the optic nerve to the
brain.
Where does the optic nerve enter the cranium? What happens then? What are the 5 terminal zones for retinal axons and what are they each responsible for?
- Retinal Targets. The optic nerve enters the cranium through the optic foramen, where it meets with the optic nerve from the other eye at the optic chiasm. There,
reorganization takes place so that neurons sensitive to the left visual field project to the right side of the brain and neurons sensitive to the right visual field project to the left side of the brain. There are 5 terminal zones for retinal axons:
hypothalamus (superchiasmatic nucleus) to entrain our circadian rhythm. Here, our biological clock runs a little slow at times, but gets a message to keep to a 24 hour cycle (in concert with other sensory cues).
pretectal nuclei to serve accommodative and pupillary reflexes (in addition to autonomic changes).
accessory terminal nuclei to mediate eye movement reflexes that stabilize large visual field motion on the retina (in concert with vestibular ocular reflexes).
superior colliculus (SC) that controls orienting behaviors such as foveation and fixation (in concert with cortical control).
thalamus (mainly the lateral geniculate nucleus, LGN) to relay information to the cerebral cortex for visual perception (in concert with indirect path from SC).
Describe the layers of the superior colliculus and what input they receive? What is its role?
SC has overlapping maps of visual, auditory & somatosensory world
In the superficial layers of the superior colliculus, there is a full representation of the
external world with overlapping inputs from retina and visual cortex. The middle layers
get position-dependent inputs from somatosensory and auditory cortex and the deep
layers send an output to head and eye muscles. These multi-sensory maps enable
coordinated motor responses to stimuli in extracellular space.
Electrical microstimulation in the superficial layers results in a saccadic eye movement
toward the appropriately positioned retinotopic area. These results have supported a
hypothesis that the role of the superior colliculus is for specialized sensory-motor
integration. Stimulation of the deep layers has effects on both eye movements and
orienting head movements (and vibrissa and pinna movements in appropriate animals).
A second hypothesis suggests that the SC is a pure sensory area that controls attention,
which relays its output to appropriate motor nuclei controlling eye and head position.
What are the input and outputs of the lateral geniculate nucleus? How is it organized?
The LGN has a six-layered structure whose
main input is retina and main output in
visual cortex. There is also a prominent
feedback path from the visual cortex and an
input from the brain stem. The latter may
serve to regulate the strength of the relay
during different states of attention, sleep or
eye/head movements. Although both retinas
provide input to the LGN, it is surprising
that there are no neurons in the LGN that are
binocular (responding to both eyes).
In the LGN, each eye segregates its input
into its own layer. However, since both eye inputs represent the same part of visual
space, the inputs terminate according to a retinotopic map. Consequently, if you send an
electrode perpendicular through the LGN layers, and record from neurons to analyze their
receptive field positions, the electrode would encounter cells with receptive fields in the
same part of visual space. Irrespective of the peripheral sensory surface, the sensory
allotment on the cortical surface is distorted. Like the lips and finger tips take up more
somatosensory cortical surface than the skin on your back, so too the foveal visual
processing in visual cortex takes up more relative space than that devoted to the far visual
periphery. The distortion is due to difference in peripheral receptor densities.
Generally, what are the roles of layer 2-6 of the cerebral cortex?
- General Cortical Anatomy. All cerebral cortex has six layers above the white matter. Layer 4 receives input from thalamus and layer 6 sends back information to thalamus. Layer 5 communicates with subthalamic structures (like the SC) and can even send output as far as the spinal cord (e.g., from motor cortex). The remaining layers 2 and 3 do complex local processing.
Explain the input received from the LGN in the cortex including how processing occurs with simple and complex cells.
Hierarchical Visual Processing. LGN axons relay concentric visual receptive fields to
the primary visual cortex. These inputs are weakly responsive to diffuse light (weak at
least compared to photoreceptors). Several LGN inputs sum to create the first cortical
cell, called a “simple” cell. The simple cell has a receptive field that responds best to an
oriented bar of a specific location. This property can be predicted based on the elongated
receptive field evaluated by flashing small spots. Several simple cells of like orientation
converge onto a “complex cell”, which has a larger receptive field whose properties
cannot be predicted based on flashing stimuli within its receptive field. In a single
cortical perpendicular region, all these cell types respond to the same position in visual
space and the same preferred orientation sensitivity.
