Flashcards in Visual System/Ophthalmology Deck (14):
What are the major steps in phototransduction in the outer segments of rods?
1. Photon is absorbed by a molecule retinal; a pigment in rhodopsin
2. Absorption of photon changes the conformation of cis-retinal to trans-retinal, inducing a conformational change in rhodopsin to to metarhodopsin
3. Metarhodopsin activates a GTP-binding protein called transducin
4. Transducis binds and activates a cGMP phophodiesterase (PDE)
5. cGMP PDE molecule remains active as long as the transducin bound to it remains active
6. A decrease in the concentration of cGMP closes non-seletive cation channels in the surface membrane
7. Cell becomes hyperpolarized
What are the receptive field properties of retinal ganglion cells?
Retinal ganglion cells have donut-shaped receptive fields and come in two opposite types:
•“ON” center ganglion cells – excited by light shining in their centers and inhibited by light in the periphery
•“OFF” center ganglion cells → excited by light shining in their periphery and inhibited by light shining in their centers
The receptive field surround is mediated by horizontal cells.
•Horizontal cells behave as though they have excitatory receptors for glutamate released from photoreceptors
•They make inhibitory synapses on the neighboring photoreceptors in the field center
•When excited by glutamate, they release GABA to inhibit neighboring photoreceptors in the receptive field
What are color-opponent ganglion cells?
Cones of different color preferences converge in the retina to produce ganglion cells with receptive fields that are partial to particular colors.
In the fovea, where color discrimination is best because all of the photoreceptors there are cones, most of the bipolar cells are connected directly to one kind of cone in the field center and indirectly (via horizontal cells) to cones with a different color preference in the field surround.
This creates a RED ON-center and GREEN OFF-surround receptive field, which is passed along to the ganglion cells.
All combinations of red-green on-off opposing fields exist. In addition to red-green opponents, there are blue-yellow opponent cells, thereby spanning the entire spectrum (the yellow selectivity is created by converging both red and green cones).
Where is color processed in the cortex?
The geniculostriate is the primary visual pathway for the processing of form and color. It extends from the retina through the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex (V1). The ganglion cells, which carry information from the retina to the LGN are of two types, each named for the LGN layer to which they project
Color processing is handled in central regions of the hypercolumns called “blobs”. They care ONLY about color; They receive input from many color-opponent neurons. The positions of all these fields overlap entirely, so there is no spatial information in the color-only blob cell’s response.
What are the receptive field characteristics of cortical simple and complex cells? How are these receptive field properties achieved by synaptic inputs from lower order cells?
Simple: Cortical cells which respond to more complex stimuli and receptor fields than ganglion cells. Their receptive fields are created from converging LGN neurons with center-surround receptive fields. This creates a receptive field shape that is a straight line, and the orientation of the line is crucial. Several cells with similar but spatially offset receptive fields converge on a higher order cell to create an altogether new type of receptive field.
Complex: Have receptive fields like simple cells, with one big exception: they abstract for position. Major inputs are a convergence of several simple cells whose positions are slightly offset. The converging simple cells make excitatory synapses on the complex cell, and any single simple cell can cause the complex cell to fire
Draw the major features of a hypercolumn
Can't draw on here, but here is the basic structure:
• Layered from layer 1 (the top, at the surface of the brain) to layer 6 (the bottom, at the border with the white matter). LGN axons terminate in layer 4.
• Viewed from the surface of the cortex, each hypercolumn is divided in two parts: one half for each eye (labeled C for contralateral and I for ipsilateral). These are the ocular dominance columns.
• About half of the cortical cells—the ones in each hypercolumn near the border between the two eyes—become binocular (receive inputs from both eyes).
Describe the meaning of a sensitive period in the development of the cortex. Indicate the importance of this concept in diagnosing and treating abnormal development.
Sensitive period (or critical period): period of time when connections can be altered by visual experience.
• For humans, the critical period is the first 2 -3 years
• Connections, while genetically determined, are not immutable, at least for a while after birth.
• It is important to diagnose and treat pathologies which may cause abnormal development of the visual cortex before connections are no longer immutable.
Explain the meaning of “ocular dominance” in cells of the visual cortex and indicate its physical basis.
A measure of the relative synaptic input to a cell from each eye.
Discuss the changes in ocular dominance caused by monocular deprivation, binocular deprivation, and alternating monocular deprivation.
Experiments in baby animals have shown that if one eye is closed for even a few days, the cortical cells lose virtually all connections to the deprived eye. The longer the period of monocular deprivation during the sensitive period, the worse the outcome. Even if the eye is then reopened, it does not recover. The connections, once lost, are lost for good. HOWEVER, the same experiment performed in an adult results in virtually no effect on the vision.
• Conclusion: while connections are genetically determined, they are not immutable, at least for a while after birth (sensitive period). These results suggest the explanation known as “disuse” atrophy, or “use it or lose it”.
Indicate the conditions under which the effects of abnormal developmental experiences can be reversed.
If the abnormal development is detected and treated within the sensitive period, the effects of abnormal development up until that point can be reversed. For example, as mentioned above, infants born with congenital strabismus or cataracts can end up with relatively normal vision, provided these defects are detected and repaired soon after birth. However, the longer you wait, the greater the visual deficit will be. If not repaired by 2 years of age, there will be substantial permanent visual deficits
Develop a differential diagnosis and treatment options for
common complaints such as red eyes and eye pain.
Pathway of color processing
Color information from the blob cells then travels ventrally from V1 (primary visual cortex) to the temporal lobe, coursing through regions of V2 and then onto area V4.
The stripe regions of V2 receive inputs from the color blobs in V1.
In area V4, anterior and inferior to the primary visual cortex (V1), cells have relatively large receptive fields in the central areas of the retina and respond only to fairly narrow bands of wavelengths over the visible spectrum, some as narrow as 10 mm. Given a little lateral inhibition at a higher level of color processing, color cells with bandwidth selectivities as narrow as our perceptions could easily exist.
Discuss the changes in ocular dominance caused by binocular deprivation
Depriving a baby animal of sight out of both eyes does results in a primary cortex that is, for the most part, normal. In this experiment, there were more non-responsive cells, but most cells could be driven, and most were binocular. Altogether, about half of the cells had normal receptive fields. The adult animals, however, were behaviorally blind, which means that higher order visual cells were completely disrupted.
• Conclusion: something is happening that involves an interaction—perhaps competition—between the two eyes.