Vision Study Guide

Stephen Allsop

• What are the two main classes of RGCs we discussed?

• What are the five major classes of retinal cells?

• There are several classes of RGCs, two of which are (a) the ON and OFF and (b) the Midget and Parasol.

• There are five major classes of retinal cells: photoreceptors (rods and cones), horizontal cells,bipolar cells, amacrine cells, and retinal ganglion cells (RGC).

• What type of organization do receptive fields of RGCs have?

• What is the response of photoreceptors to light?

• What about bipolar cells?

• What are the two types of cells in the retina that generate action potentials?

• The receptive fields of RGCs have antagonistic center/surround organization.

• All photoreceptors and horizontal cells hyperpolarize to light.

• There are both hyperpolarizing and depolarizing bipolar cells.

• Action potentials in the retina are generated only by amacrine and RGCs.

• What type of structure is the LGN?

• Where do the parvocellular and magnocellular layers receive input from?

• How are LGN cells and retinal ganglion cells similar?

• The lateral geniculate nucleus of the thalamus is a laminated structure. What is segregated in the laminae varies with species.

• The parvocellular layers receive input from the midget cells and the magnocellular layers from the parasol cells. Inputs from the left and right eyes are segregated in the laminae.

• The receptive field properties of LGN cells are similar to those of the retinal ganglion cells

• How is visual information represented in V1?

• What are the V1 transforms?

• What are the three models of V1 layout?

• The contralateral visual hemifield is laid out topographically in V1 of each hemisphere.

• V1 transforms are: orientation, direction, spatial frequency, binocularity, ON/OFF convergence and midget/parasol convergence.

• V1 is organized in a modular fashion. Three models of the layout of the modules are the ice cube, radial and swirl models.

• How many visual areas are there? How many connections do they make?

• What specialized function do extrastriate areas have?

• How do receptive fields change as you progress into higher visual areas?

• There are more than 30 visual areas that make more than 300 interconnections.

• Extrastriate areas do not specialize in any single function.

• The receptive field size of neurons increases greatly in progressively higher visual areas.

• What is MT involved in analyzing?

• What about area V4?

• What is a central role of IT?

• Area MT is involved in the analysis of motion , depth, and flicker.

• Area V4 engages in many aspects of analysis; not tuned for complex objects but can be tuned to objects of intermediate complexity such as geometric shapes. neurons in this region have dynamic properties.

• In inferotemporal cortex high level analysis takes place that includes object recognition.

• What is the relationship of APB to glutamate?

• How does APB affect the different responses of the retina?

• What response in the cortex does APB block?

• APB is a glutamate analog that blocks the ON bipolar.

• APB blocks the ON response in retinal ganglion cells; the OFF response and center/surround antagonism are unaffected.

• APB blocks light edge response in cortex but has no effect on orientation, direction and spatial frequency selectivity. APB reduces sensitivity for light increment.

• In primates how are ON/Off channels created?

• Why did On/Off channels emerge?

• Where do cone driven On/Off channels originate?

• In most primates there are only ON rod bipolars. The rod ON and OFF channels are created in the inner retina by amacrine cells.

• The ON and OFF channels have emerged in the course of evolution to enable organisms to process both light incremental and light decremental information rapidly and effectively.

• The cone driven ON and OFF channels originate at the level of the retinal bipolar cells; sign inversion in ON bipolars is provided by the mGluR6 receptor

• Where do the parasol and midget channels originate?

• Which channel presents the major input to MT?

• In what region do these two channels converge?

• How do receptive fields for these two channels differ?

• Two major channels originating in the retina are the midget and the parasol.

• The major input to MT is from the parasol cells

• The midget and parasol systems converge on some of the cells in V1.

• In central retina the receptive field center of midget RGC and parvocellular LGN cells is comprised of a single cone. Parasol cells have much larger receptive fields; the cone input is mixed in both the center and the surround.

• In which ways do the parasol and midget channels extend the range of vision?

• How does the midget and parasol cell ratio change as one moves from the center to the periphery?

• Which channel does V4 receive input from?

• The midget system extends the range of vision in the wavelength and high spatial frequency domains.

• The parasol system extends the range of vision in the high temporal frequency domain.

• The midget and parasol cell ratio from center to periphery changes from 8 to 1 to 1 to 1.

• V4 receives input from both the midget and parasol cells.

• What are the three qualities of color?

• Are cone photoreceptors broadly or finely tuned?

• What are the two cardinal color opponent axes?

• Where is color processed?

• There are three qualities of color: hue, brightness, and saturation.

• The three cone photoreceptors are broadly tuned.

• Color-opponent midget RGCs form two cardinal axes, red/green and blue/yellow.

• Color is processed in many cortical areas; lesion to any single extrastriate structure fails to eliminate the processing of chrominance information.

• Which System is important for color discrimination?

• How does isoluminance affect vision?

• Where do the most significant aspects of luminance adaptation occur?

• How are afterimages produced?

• parasol cells can perceive stimuli made visible by what attribute?

• The midget system is essential for color discrimination.

• Perception at isoluminance is compromised for all categories of vision.

• The most significant aspects of luminance adaptation occur in the photoreceptors.

• Afterimages are a product of photoreceptor adaptation and their subsequent response to incoming light.

• The parasol cells can perceive stimuli made visible by chrominance but cannot ascertain color attributes.

• What are some mechanisms for analyzing depth?

• Where and how is stereopsis processed?

• Utilizing motion parallax for depth processing necessitates neurons specific for what three things?

• Numerous mechanisms for analyzing depth have been identified that include vergence and accommodation, stereopsis, parallax, shading, and perspective.

• Several cortical structures process stereopsis utilizing disparity information; the number of disparities represented is limited as in the case of color coding.

• Utilizing motion parallax for depth processing necessitates neurons specific for direction, velocity and differential velocity; several areas, including V1 and MT process motion parallax.

• Area MT contributes to analysis of _____ ?

• How is shading and perspective analyzed?

• In some brain areas, what type of information is integrated to lead to more accurate and quicker assessment of depth?

• Area MT contributes to the analysis of motion, motion parallax, depth, and flicker; however, these analyses are also carried out by several other structures.

• Little is known at present about the manner in which information about shading and perspective is analyzed by the brain.

• In some brain areas information processed on the basis of such cues as stereopsis, motion parallax and shading, is integrated leading to more accurate and quicker assessment of depth.

• Where have neurons that respond to subjective contours been found?

• What is the result of V4 and IT lesions?

• How selective are IT neurons?

• What are the three theories of form processing?

• Neurons responding to subjective contours have been found in V2.

• Recognition of objects transformed in various ways is compromised by V4 and IT lesions. V4 lesions also produce major deficits in visual learning and in selecting "lesser" stimuli.

• Some IT neurons are selective for objects including faces, but most respond to a variety of objects whose recognition is based on the differential activity of a great many neurons.

• Three theories of form processing in the brain are (a) analysis by orientation of line segments, (b) spatial mapping onto a topographically organized brain region and (c) Fourier analysis. Areas V2, V4 and IT play important roles in intermediate vision.

• Classes of eye movements?

• How are eye movements produced?

• How does neuronal firing relate to the degree of eye movement?

• What does the superior colliculus code?

• Retinal input to the SC comes from what type of cells?

• Classes of eye movements are vergence and conjugate, with the latter comprised of two types, saccadic and smooth pursuit.

• Eye movements are produced by 6 extraocular muscles that are innervated by axons of the 3rd, 4th and 6th cranial nerves.

• The discharge rate in neurons of the final common path is proportional to the angular deviation of the eye. Saccade size is a function of the duration of the high-frequency burst in these neurons.

• The superior colliculus codes saccadic vectors whose amplitude and direction is laid out in an orderly fashion and is in register with the visual receptive fields.

• The retinal input to the SC comes predominantly from w-like cells. The cortical downflow from V1 is from layer 5 complex cells driven by the parasol system.

• What are the two major cortical systems controlling saccadic eye movement?

• How do the two systems differ in their access to the brainstem?

• Which system is important for producing express saccades?

• In which process does the FEF play a central role?

• Two major cortical systems control visually guided saccadic eye movements: The anterior (MEF,FEF) and the posterior(parietal, occipital, SC).

• The anterior system has direct access to the brainstem whereas the posterior system passes through the colliculus.

• The posterior system is essential for producing express saccades.

• The FEF plays a central role in the planning of saccadic sequences.

• What are some proposed roles of the medial eye fields?

• What is the result of paired ablation of FEF and SC?

• Which area carry vector codes vs place codes?

• How do the posterior system, LIP, FEF, and MEF contribute to “looking”?

• How do inhibitory circuits contribute to eye movements?

• The role of the medial eye fields remains a puzzle. It may be involved in hand-eye coordination, in establishing spatial relationships and in visuo-motor learning.

• Paired ablation of the FEF and SC eliminates visually guided saccadic eye movements.

• Areas V1, V2, FEF, LIP and SC carry a vector code. MEF carries a place code.

• The posterior system is important for object identification, for deciding where to look and where not to look. LIP in addition is important for deciding when to look. The FEF and MEF contribute to where to look.

• Inhibitory circuits, as from the substantia nigra and in the frontal eye fields, are essential for generating properly directed saccadic eye-movements.

• How has motion been classified?

• What are the majority of V1 and MT cells selective for?

• What is the function of the AOS?

• Motion has been classified into several different types that includes planar, circular, radial as well as differential for parallax.

• The majority of V1 cells and most MT cells are directional and velocity selective. Some cells are sensitive to differential velocities of movement.

• The AOS, that begins with RGCs that form three axes of direction selectivity that correspond to the three axes of the semicircular canals, is involved in generating pursuit eye movements for image stabililization.

• How is apparent motion induced?

• The notion that motion cues can provide important information for object recognition is referred to as?

• What does motion parallax provide information about?

• Stationary stimuli that flicker with various temporal asynchronies induce apparent motion.

• Motion cues can provide important information for object recognition often referred to as "structure from motion."

• One of the most important tasks of motion analysis is motion parallax as it serves to provide vital information about depth.

• What is a “phosphene”?

• What was the “size” of the percept produced by electrical stimulation?

• Detailed examination of electrical stimulation has lead to what other siginificant effects other than generating phosphenes?

• A phosphene is a visual impression created by electrical stimulation of a particular brain area.

• the “size” of the percept produced by electrical stimulation was 16 min of visual angle diameter.

• - eliciting saccadic eye movements - the threshold for eliciting saccadic movements changes as a function of the depth of the electrode - it can produce interference effects