test #2

your brain’s interpretation of the messages your receptors sent to you E.g. When you see a tree, your perception is not in the tree, but in your brain

Vision is just perception - E.g. When you see a tree, your perception is not in the tree, but in your brain - E.g. You feel something with your fingers, but you don’t feel it in your fingers. You feel it in your brain You see something only when light from it alters your brain activity - You do not perceive an object unless the rays bounce back from the object and return to your eyes

• Muller considered sensory nerves the seat of specific energies, and that a specific nerve energy exists for each sense organ • Activity by a particular nerve (i.e. set of neurons) always conveys the same type of information to the brain • Whatever excites a particular nerve establishes a special kind of energy unique to that nerve • Each of our senses has specialized receptors that are sensitive to a particular kind of energy • I.e. impulses in certain neurons indicate light; impulses in other neurons indicate sound, touch, or other sensations– the brain interprets the action potentials from the auditory nerve as sounds, those from the olfactory nerve as odors, and so forth

1) Sensory receptors receive stimuli which is taken to the brain for processing - E.g. stimuli like sound waves, light waves like a baseball player hitting a ball or an observer watching the baseball player run bases on the field 2) Then, the brain organizes the information and produces a reaction - E.g. the observer claps for the baseball player Each sense organ responds to different forms of stimuli - Our 5 senses take into the brain sound, light, taste, touch, and smell, but the brain responds to the sensory input differently depending on the situation - “Just because we see light or hear sounds does not meant that we will act in the same way each time we feel those sensations. The brain encodes these messages and responds appropriately to the situation”

ELECTROMAGNETIC SPECTRUM - ranges from 400 nm to 700 nm and travels in wavelengths and measures frequencies - GAMMA RADIATION: the shortest wavelength and highest frequency - VISIBLE LIGHT: portion of the spectrum that we can see (in the middle)

from short wavelengths to longer wavelengths the visible portion of the spectrum is much smaller relative to the rest of the spectrum - There is a lot of radiation around us which is “invisible” to our eyes (we only perceive that which our receptors are tuned to preceive)

• Red = longest visible wavelength • Violet/blue = shortest visible wavelength • Blue, green, and red = primary visible colours/wavelengths No single primary colour can be created from the other two, but all other colours can be formed by combining blue, green, and red in various proportions e.g. sunlight is made up of wavelengths in th eultraviolet, visible, and infrared portions of the spectrum

Humans have receptors that respond to a narrow spectrum of light, i.e. “visible light” (which we can see) Other animals may have a more restricted or greater spectrum, i.e., they have visual receptors more sensitive to ultraviolet radiation

HUE - the wavelength of light, which we commonly call ‘colour’ SATURATION - how pure a colour is (bright red > fire engine red)

theory that Colour perception occurs through the COMBINED stimulation/activity of 3 kinds of cones (receptors)– red, green, and blue - Each cone responds to a broad range of wavelengths of light, but some more than others … The ratio of activity across all 3 cones allows us to discriminate between wavelengths and determine the colour - what matters is the the activity of a cone relative to the other cones, not the response of any one cone - To see any single cone colour, you need all 3 cones to be active at the same time - Each cone type is giving a different colour code to tell you what you’re seeing - Every cone responds to a different light, but at different levels - E.g. light at 550 nm excites the medium-wavelength and long-wavelength receptors about equally and the short-wavelength receptor almost not at all; this ratio of responses among the three cones determines a perception of yellow-green - E.g. if you see the colour red, youre not seeing it just because the red cone is activated, but because all cones are activated at different levels The 3 kinds of wavelengths/cones - Blue = short-wavelength, high-frequency form of light (S-CONE) - Red = long-wavelength, low-frequency (L-CONE) - Green = medium-wavelength, somewhere in between (M-CONE) - … Together, these 3 cover the whole spectrum of light visible to humans

1) The 3 cones are unevenly distributed - Short-wavelength (blue) cones are about evenly distributed across the retina, the other two kinds are distributed haphazardly, explaining why moderate-sized patches of the retina have only medium- or longwavelength cones 2) The 3 kinds of cones are not equally abundant - There are many more M- and L- cones than there are S-cones It is easier to see tiny red, yellow, or green dots than blue dots - Overall, the cones in the retina’s periphery are so scarce that you have no useful color vision 3) M- and L- cones are more sensitive to light/brightness than S-cones - The S-cones are less important to how humans perceive brightness but still as important in perceiving colour as other cones - More intense light increases the activity of all three cones without much change in their ratio of responses (but S-cones the least), and increases the perceived brightness of the colour - E.g. monochromatic green light hits the retina; the green light has a medium wavelength, meaning that the reflected light would trigger a very weak response in the S-cone, a strong response in the M-cone, and a relatively weak response in the L-cone

We perceive colour in terms of (paired) opposites The brain has a mechanism that perceives colour on a continuum from red to green, another form yellow to blue, and another from white to black by 3 channels 1st– transmits green or red but cannot do both at the same time 2nd– transmits blue or yellow 3rd– transmits black or white After you stare at one color in one location long enough, you fatigue that response and tend to swing to the opposite

AFTERIMAGE Visual information persists and continues to be processed in our brain even after it is gone Support for the opponent-process theory Can last for several seconds NEGATIVE COLOUR AFTERIMAGE Specific type of afterimage where we overstimulate the rods and cones, and as such, those cells least “fatigued” process the opposite colour image

bipolar cells are excited by one set of cones and inhibited by another cones after prolonged exposure to the wavelength cones that results in increased activity, the fatigued cell decreases its response, which results in the response of the cell that inhibits that colour • E.g. bipolar cell increases its activity with medium-wavelength cones (green) and decreases its activity with long-wavelength cones (red), and after long exposure to green light, that cell is decreasing its activity/level of response (inhibition) and results in red

white or gray

2

Deficiency (abnormality of cone) or absence (due to gene failing to develop) of any of the cones results in an inability to discern that colour • E.g. In red-green color deficiency, people have trouble distinguishing red from green because their long- and medium-wavelength cones have the same photopigment instead of different ones

Specialized cells in the retina that receive input from rods and cones, and play a crucial role in colour processing Have their own photopigment– MELANOPSIN, which is sensitive to light (specifically blue/short wavelength light) Respond to the average itnensity of light– respond slowly when light comes on and decline slowly when the light ceases

retinal ganglion cells

TRICHROMATIC THEORY - Tries to explain how certain mixtures of wavelengths lead to colour - Proposes 3 cone photoreceptor types that respond maximally to short (blue), middle (green), and long (red) wavelengths OPPONENT-PROCESS THEORY - Tries to explain how we percieve colours in terms of opposites - Proposes 3 channels of G/R (green/red), B/Y (blue/yellow), and B/W (black/white)

SCLERA CORNEA IRIS PUPIL LENS RETINA OPTIC NERVE

The white portion of the eye that serves as an outer layer and protects the eye

The hole/black circle/opening in the center of the iris where light enters the eye Becomes larger in low light and smaller in bright light

Transparent layer forming the front of the eye Helps to focus/bend light as it enters the eye Not adjustable