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? ☉