Light hitting the eye passes through the {cornea}, then the {lens}, and then hits the {retina} (the tissue lining the back of the eye).
The primary role of the cornea and lens is {to focus light}.
Q. How does the eye’s lens focus?
A. There’s a band of muscles around the lens which can change its shape.
The primary role of the {iris} is {to control how much light reaches the retina}.
The human eye has two types of {photoreceptors}, located on the {retina}: {rods} and {cones}.
Q. What can the rods do which cones can’t?
A. Sensitive to very low levels of light.
Q. What can the cones do which rods can’t? (two key functions)
A. Sense color; discern fine detail
Q. Which has greater acuity—rods or cones?
A. Cones.
Q. How do cones perceive color?
A. By comparing the activations of three subtypes, each of which is sensitive to a different frequency.
Visible light range from ~{360 nm} (violet) to ~{750 nm} (red).
Cones are distributed {almost entirely at the fovea} (i.e. {the center of the eye}), while rods are distributed {with no concentration at the fovea, a peak at 20°, and a slow drop at further angles}.
Q. Why do we point our eyes at a target when we want to perceive it in detail?
A. Cones have greater acuity than rods, and they’re almost entirely at the fovea.
Q. What trick can you use to improve your perception of an object in a very dark room?
A. Look at it about 20° from straight-on, to maximize the number of light-sensitive rods sensing that data.
Photoreceptors stimulate {bipolar cells}, which in turn excite {ganglion cells}, whose axons converge to form {the optic nerve}. That tract first reaches the brain in a part of the {thalamus} called the {lateral geniculate nucleus (LGN)}; it’s then transmitted from there to the {occipital lobe}.
Q. In what sense is the optic nerve not just a “dumb cable”?
A. The cells in the optic nerve process the input as it is transmitted.
{Lateral inhibition}: {when stimulated cells inhibit the activity of neighboring cells}. Important in vision because {this increases the effective contrast at edges (“edge enhancement”)}.
Q. What is the “Mach band” optical illusion?
A. A strip of gray appears darker at an edge abutting a lighter strip, and lighter at an edge abutting a darker strip.
Q. Why does “Mach banding” occur?
A. Lateral inhibition effectively suppresses photoreceptors at the edges next to lighter regions and enhances those next to darker regions.
Q. What’s the common significance of Mach banding in computer graphics?
A. Gradients appear to have bands in them when the derivative changes sharply.
{Single-cell recording}: {a procedure which records a voltage time series for a single neuron}
Q. How is single-cell recording used to establish a cell’s receptive field?
A. We can manipulate the pattern of its stimulus and observe it’s responses.
Q. How are receptive fields distributed across neurons responsible for visual processing?
A. Neurons are specialized with different types of receptive fields, which work together in conjunction.
Q. Name a few types of receptive fields discovered.
A. e.g. center-surround cells, edge detectors, movement detectors, etc
Axons from the LGN first reach the occipital lobe in a site called {Area V1}, mostly located {between the two hemispheres}.
Q. What strategy does Area V1 use to process visual input?
A. Divide and conquer: many different cells firing for many different positions, angles, etc.
Q. Name two advantages of parallel processing.
A. e.g. speed, holistic interpretation
Q. What’s the key organizational disadvantage of parallel processing in the brain?
A. How to coordinate their function?