Vision Flashcards
Intro/basics
- visual system allows organisms to process visual detail as sight and enables the formation of several non-image photo response functions
- The system carries out several complex tasks such as identification and categorization of visual objects, assessing distances to and between objects, and guiding body movements in relation to the objects seen
Structural Overview
Retina
- retina consists of photoreceptor cells that contain two types rods cones
- opsin absorbs a photon and transmits a signal to the cell through a signal transduction pathway
—> hyperpolarization of the photoreceptor
Rods
- primarily found in the periphery of the retina- used to see at low levels of light
—> high sensitivity vision
Cones
- primarily in the center of the retina- three types of cones, which differ in wavelengths that they absorb
- used to distinguish color and other features at normal levels of light
—> provide high acuity vision
Bipolar cells
- cones synapse onto bipolar cells, which then synapse onto ganglion cells which can conduct AP to the brain
- a significant amount of visual processing arises from firing patterns between neurons in the retina
- bipolar cells have a central body from which two sets of processes arise: they can synapse with rods or cones and also accept synapses from horizontal cells
- bipolar cells transmit signals to ganglion cells- communicate via graded potentials, not action potentials
- have centre-surround receptive fields
Off bipolar cells
- photoreceptors hyperpolarize to light
- GLU is released in the dark
- OFF bipolar cells excited by GLU respond in the dark
ON bipolar cells
- are inhibited by GLU
- respond to light when GLU is reduced
—> both types transfer information from rods and cones to ganglion cells
Convergence of cells
- cones have little convergence
—> one cone connects to one bipolar cell
- rods have more convergence
—> can be up to 1000:1 (1000 cones for one bipolar cell) - allows increase in sensitivity to dim light
Bipolar Cells connect to ganglion cells
—> ganglion cells send visual information to the brain
—> ganglion cells exit retina through optic disc to the optic chiasm
—> nasal retina ganglion cells cross to opposite side of brain while temporal retina ganglion cells remain the same
—> information is processed in lateral geniculate nucleus which consists of 6 layers which receive input from the different types of ganglion cells
—> neurons of LGN then send information to the primary visual cortex
Types of ganglion cells
M ganglion cell
- large receptive field
- transient response
- rapid conduction
- achromatic
—> sensitive to depth, indifferent to color, rapidly adapt to stimulus
P ganglion cell
- small receptive field
- sustained response
- slow conduction
- colour opponent: red-green
—> sensitive to colour and shape
nonM-nonP ganglion cell
- very heterogeneous
- colour opponent blue-yellow
—> large centre-only receptive fields that are sensitive to color and indifferent to shape or depth
Processing in V1 (general)
- visual information is represented topographically from the retina to the visual cortex
—> ganglion cells that neighbor each other in the retina project to LGN neurons that neighbor each other and then to V1 neurons that also neighbor each other - V1 does not represent the retina proportionally
—> stimuli from central part of the visual field are processed in larger cortical space
Primary Visual cortical neurons respond to lines and edges
- receptive fields of LGN neurons resemble those of ganglion cells, the neurons in the V1 do not
Simple Cells
- most responsive to a bar of light with a specific orientation
- different simple cells will have distinct orientation preferences and sizes of their ON/OFF regions
- the OFF regions do not need to be symmetrical, and some only have one OFF region
- despite variations they all share common properties:
–> spatially restricted to a narrow area of the visual field - are best stimulated by bars of light - are highly sensitive to orientations
- have ON-OFF antagonism—> can be considered as line or edge detectors in specific regions of the visual field
Complex Cells
- have more complex and variable receptive fields than simple cells
- do not have mutually antagonistic ON and OFF regions
- are also highly orientation-selective and respond to light bars on a dark background or dark bars on a light background
- have larger receptive fields
- a bar with a specific orientation falling on any part of the receptive fields would excite them
—> more abstract line/edge detectors than simple cells because they generalize over space and respond to the line or edge ON or OFF anywhere in their receptive fields
Cells with similar properties are vertically organized in V1
- first level: specific parts of the visual cortex receive information from specific parts of the retina
—> represent specific parts of the visual field - second level - functional architecture: within an area, neurons that share similar properties are further organized along the axis perpendicular to the cortical surface
- Experiments from Hubel and Wiesel suggest that the V1 is organized as visual columns from the surface to the white matter underneath and cells within the same column share similar properties
- unlike LGN neurons who can only receive stimuli from of eye, individual neurons in V1 can respond to input from both eyes that correspond to the same spatial position, creating binocular vision which is important for depth
- BUT in input layer 4, neurons have a preference for either left or right eye, known as ocular dominance
—> literally can be seen as alternating stripes (one stripe right eye, one stripe left..)!
Information flows from layer 4 to layers 2/3 and then to 5 or 6 in the neocortex (including visual cortex)
- neocortex is divided into six layers based on cell density differences
- LGN axons terminate in layer 4
–> layer 4 neurons restrict their dendrites mostly within layer 4 so they can receive LGN input
–> send information to levels 2/3
- Layer 2/3 sends the info to neurons in layer 5 and through to other visual cortical areas
- layer 5 neurons send info to layer 6 and other cortical and subcortical areas e.g. basal ganglia
- layer 6 neurons send back info to layer 4 and then to LGN to provide feedback control on LGN input
—> simple cells are found in layer 4 which is consistent with them receiving info from LGN
—> complex cells are found in 2/3 and 5/6 which is consistent with them receiving input from simple cells
