Video Module 5: Visual System Flashcards
How does light travel through the eye?
1) The light passes through the cornea
2) The light passes through the pupil, a hole in the iris
—The iris is a muscle that contracts to let more or less light into the eye
3) The light hits the lens, which focuses it onto the retina by bending
4) Light hits the surface of the retina, where it reaches photoreceptor cells (rods & cones)
rods
photoreceptor cells in the retina that are sensitive to low levels of light
- have only one kind of photopigment, therefore they do not detect color wavelengths
the fovea
an area in the middle of the retina with a high amount of cones and no rods
- area with the most detail
cones
photoreceptor cells in the retina that are specialized for detecting colour wavelengths of light
- have 1 of 3 kinds of photopigments
- highly concentrated in the fovea
bipolar cells
receive information from photoreceptor cells (rods and cones) and pass signals onto RGCs
- lateral inhibition: bipolar cells that detect stronger input will deliver higher inhibition to neighboring bipolar cells
retinal ganglion cells
receive information from bipolar cells
- often called “dot detectors”
- the optic nerve is composed of RGC axons
- the optic nerve exits the eye through the blind spot, an area of the retina with no rods or cones
- passes info to the LGN in the thalamus
receptive field (vision)
The region of the visual field and type of stimulus to which a neuron responds
- Receptive fields in the retina are small, but get larger as information flows through the visual system
- early stages: dots & edges
- later stages: complex patterns & objects
V1
a region of the cortex specialized for vision
- the first place where visual information arrives in the cortex after visual information leaves the LGN (lateral geniculate nucleus)
- neurons in V1 are often called “edge detectors”
ganglion cell: on-center, off-surround
a center-surround cell whose preferred stimulus is light in the center and shadow on the edges
ganglion cell: off-center, on-surround
a center-surround cell whose preferred stimulus is shadow in the center and light on the edges
What happens to the activity of a center-surround cell when its receptive field is completely covered in shadow, or completely covered in light?
The activity (firing of action potentials) remains the same. This is because the center-surround cell receives simultaneous inhibitory and excitatory input which cancel each other out.
- For example, in an off-surround cell, shining light on the edges of the cell’s receptive field will send inhibitory signals to the cell because it is the dispreferred stimulus. Shining light in the center of the RF will send excitatory signals to the cell.
- Center-surround cells are most active when the pattern of light in its RF matches its preference
V1 cells
often called “edge detectors” because they have a preference for line orientations
- When several LGN cells located next to each other are activated at once, they signal to V1 cells that there must be an edge of an object
tuning curve
a graph that shows the relative response of a V1 cell to a range of stimuli (line orientations in its receptive field)
- V1 cells will still be stimulated if the orientation of a line is somewhat close to its preferred orientation; the closer the line is, the higher activity the V1 cell will have
lateral inhibition
the process by which neighboring bipolar cells send inhibition signals laterally
- the perceived brightness of an object is proportional to the bipolar cell response
- edge enhancement is a result of lateral inhibition
How is the perceived brightness of an object related to bipolar cell activity?
The perceived brightness of an object is proportional to the bipolar cell response. Bipolar cells have a response that is the sum of the receptor response (from photoreceptors) and lateral inhibition from neighboring bipolar cells. For example, if a BC receives a response from a photoreceptor of 20mV, and lateral inhibition from a neighbor BC of -2mV, its response will be 18mV.
