370: Light & the Eye Pt3

1. Rods and Cones
1. Different spectral sensitivity scales
2. 3 cones taken together are most sensitive around 560 nm (yellowish)
3. The rod type is most sensitive around 500 nm (bluish green)
4. Purkinje Shift
1. At twilight bluish things seem brighter, while reddish/yellow things seem dimmer
2. This happens as you shift from reliance on cones to reliance on rods so since rods are most sensitive at around 500 nm (bluish green) blues start to get brighter
5. Rods
1. Take longer to adapt but they are more sensitive
2. With something like twilight where it gradually gets darker they adjust at about the same rate
2. Spectral Sensitivity Curves
1. Rods are most sensitive at about 500
2. Cones are most sensitive at...
1. Short
2. Medium
3. Long
4. Average = about 560
3. Basic Concepts
1. Convergence
1. why it's hard to be sensitive and detailed at the same time
2. Usually several receptor cells feed into one neuron
1. 126 million rods and cones converge to 1 million ganglion cells
3. Occurs in all sensory systems
4. Greater convergence leads to:
1. Higher sensitivity to faint signals (soft sounds, dim lights, light touch...)
2. Less specificity/acuity of signal detail (sound pitch, precise location of stimulus)
5. Higher convergence of rods than cones
1. greater convergence = greater sensitivity and less acuity
1. Rods individually take less light to respond
2. Trade off is that rod system cannot distinguish detail
2. 120 rods to one ganglion 
3. 6 cones to one ganglion
4. Cones in fovea have 1 to 1 relation to ganglion cells (no convergence)
5. Rods and cones send signals vertically through:
1. Bipolar cells
2. Ganglion cells
6. Signals sent horizontally (convergent):
1. Horizontal cells
2. Amacrine cells
6. Convergence Reduces Detail
1. All-cone foveal vision results in high visual acuity
2. One to one wiring leads to ability to discriminate details
3. Trade off is that cones need more light to respond than rods
2. Lateral Inhibition
1. Adjacent neurons compete with one another.  Why?
2. Adjacent sensory receptors send signals that inhibit each other in a competitive matrix
3. This occurs in several sensory systems, resulting in center-surround receptive fields
   4. Aids in detection of perceptual features
   5. In the retina, adjacent photoreceptors inhibit each other
4. Photoreceptor mutual inhibition helps with:
1. Edge enhancement, resulting in the mach band illusion
2. emphasis on relative brightness, rather than absolute brightness
5. Other illusions arise from lateral inhibition such as the Hermann Grid
1. Hermann Grid
1. People see an illusion of gray images in intersections of white areas
2. Signals from bipolar cells cause effect
3. Receptor stimulated by white areas inhibit the response of neighboring cells more than receptor stimulated by black areas
4. The lateral inhibition causes a reduced response which leads to the perception of gray
2. Mach Bands
1. People see an illusion of enhanced lightness and darkness at borders of light and dark areas
2. Low intensity (dark) areas have smallest input
3. High intensity (light) areas have largest input
3. Neural Circuits
1. Combining convergence and lateral inhibition helps solve perceptual problems
4. Neural pathways from retina to primary visual cortex