BIEB 166 Lecture 23 (WI13)

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University of California - San Diego
Biol/Ecology, Behavior, & Evol
BIEB 166
James Nieh

Lecture 23 The vertebrate retina Two types of photo-receptor cells - Named base on their morphology - In the back of the retina ○ Light can pass through the other layers since they are mostly transparent - Evolution doesn't necessary to come up with the BEST solution but the solution that WORKS 1. Rods - For vision in low light conditions (very light sensitive) ○ Can detect a single photon - With more rhodopsins - Always generating a constant current in complete darkness - Inhibited by light - Situated in between, mostly absorb green photons - Eyes at night are least sensitive to red light 2. Cones - Vision in bright light conditions - Perception of color - Less sensitive to light than rods - 3 different types of cone cells (R&G&B) ○ Blue: broad range ○ Green: most sensitive to green, least sensitive to red ○ Red Turtle cone oil droplets - Different colors of oil droplets - Dividing up the photons that are coming in the eye ○ Discriminate between red/green/blue ○ Only thing that get through is the color of the droplet ○ Color filtering out all the other wavelengths - 5 different types of color ranges ○ UV, Blue, Green, Red, Orange, Rod - The more different type of cone cells, the better color vision - Species-specific Bipolar and ganglion cells - Responsible for hue discrimination and brightness contrast ○ Dominant frequency - Different colors of cone cells are connected to different ganglion cells ○ Receive input from multiple cone cells - Ganglion cells are able to detect which photoreceptors are more activated ○ Compare different cone types - Parts of retina red cone cells are more activated, then that is a darker color ○ Compare output of same cone types Color vision Monochromats - All-rod or single-pigment cone eyes sees only shades of gray ○ Do not have cone cells - Adapted to night vision Bush baby, hamster, deep sea fish - Little light penetrate to their habitats Dichromats Dichromats - Solution for animals that need to have good color vision and night vision ○ Prey at night - Color analysis is based on a color-opponent system in the ganglion cells and at higher brain centers which receive dual input from two adjacent cones with different pigments - All colors over a reasonable range can be represented by combinations of the outputs of these ganglion cells at higher brain centers 1. Discrimination test - Responsiveness to different colors - Discrimination ability from behavioral test ○ High curve, able to discriminate small changes in wavelength ○ Low absorption in the mid-wavelengths, cannot discriminate small changes of that color Chipmunk, tigers, dogs - Dogs cannot distinguish between red and green Trichromats - Animals with good color vision have three different cone pigments - The addition of a third pigment peaking a t a longer wavelength allows for a second color-opponent system that receives opposite inputs from this cone cell and the middle wavelength cone cell ○ Results in broader and more precise color discrimination Human - Unable to discriminate the small color changes in blue-green zone and the red-yellow zone Color blindness - Tends to respond more to the orange wavelength - Similar to a dichromat Honeybees - Cone cell responses is more evenly distributed - Better color vision than human - Learn a wide variety of colors to distinguish the background color ○ Precisely see different types of color - Food and foraging Advantages and disadvantages of color vision 1. Advantage - Higher spatial and temporal resolution ○ Able to see very sharply different parts of the image ○ Blurriness of the image ○ Individual cone cells turn on and off very rapidly - Lightness constancy ○ Ability to order the brightness of objects with different reflectance under different spectral illuminations ○ White-balance ○ An object looks the same under different lighting conditions  Particular appearance of the prey in different environment  Human perception of surface brightness appears to based up on surface reflectance (the proportion of light being reflected) rather than on the absolute amount of light being reflect  We care about the ratio -> look like the same - Improved object detection ○ Find prey/avoid predator - Improved spatial discrimination ○ Hues change with increasing distance ○ Objects that are closer appear brighter and appear to be at the longer wavelength side
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