By Kamila Radjabova, Monami Waki, Tim Wang, and Yu Xin Zheng VISION By Kamila Radjabova, Monami Waki, Tim Wang, and Yu Xin Zheng

THE STIMULUS: LIGHT ENERGY VISIBLE LIGHT SPECTRUM The light that we, humans, are able to perceive. Visible light is only a thin slice of the whole spectrum of electromagnetic energy. It ranges from shorter waves of blue-violet light to longer waves of red light Other organisms perceive differently from us. Bees, for example, can see ultraviolet light Wavelength: the difference from one wave peak to the next. → Determines the hue of the light wave. → Also determines the pitch of sound. Amplitude: The height of the wavelengths that determines the amount of energy in light waves; also known as the intensity, or brightness of the light. Frequency: the number of wavelengths that can pass a point in a given time. The shorter the wavelength, the higher the frequency.

The Eye Light enters the eye through the cornea, which protects the eye and bends light to provide focus. Then, the light passes through the pupil, a small adjustable opening. -The iris then dilates or constricts in response to the brightness of the light. -Surrounds the pupil and controls its size. -A colored muscle that may even respond to inner emotions. Behind the pupil is a lens that focuses incoming light rays into an image on the retina, a multilayered tissue on the eyeball’s sensitive inner surface. The lens focuses the rays by changing its curvature in a process called accommodation.

The Eye

The Retina The retina doesn’t “see” a whole image. Rather, its millions of receptor cells convert particles of light energy into neural impulses and forward those to the brain. The impulses are reassembled there into a perceived, upright-seeming image.

The Retina When a light-energy particle enters the eye, it makes its way through the retina’s outer layer of cells to its buried receptor cells, the rods and cones Rods detect black, white, and gray. It is necessary for peripheral and twilight vision when cones don’t respond Cones are concentrated in the central focal point in the retina called the fovea. They function in well-lit conditions and detect fine detail and color. Cones have their own hotline to the brain because direct connections preserves precise information and detail. They each transmit to one bipolar cell while Rods share these cells with each other. Focusing sends things in the peripheral to the rods in the outer regions of the retina and cause them to look blurry

Sensitivity in Dim Light The Retina Cones Rods Number 6 Million 120 Million Location Center Periphery Sensitivity in Dim Light Low High Color Sensitivity Detail Sensitivity

The Retina Light triggers bipolar cells to activate neighboring ganglion cells to carry the neural impulses to be distributed by the Thalamus via the Optic Nerve (axons of the ganglion cells twined together) Although the Optic nerve can carry around 1 million messages at once, it has a blind spot. We cannot perceive light there because there are no receptor cells located at the point which the optic nerve leaves the eye. Retina cells are so sensitive that slight pressure could cause them to misfire. When you close your eyes and gently rub the right side of your eyelid, you will notice light in your left eye,

The Retina

Following a single particle of light in the retina

The sensory information and neural impulses travel by the optic nerve to the occipital lobe. (visual cortex)

Feature Detection Feature Detectors: nerve cells in the brain that respond to specific features of stimulus, such as shape, angle, or movement These cells pass information of edges, lines, angles, and movements to other cortical areas The information is used by supercell clusters when perceiving faces and objects Different parts of the brain are activated when perceiving different things One can anticipate certain actions based on this information (anticipating the direction of a kick in soccer based on the player’s movements)

Parallel Crossing Parallel Processing: the processing of many aspects of a problem simultaneously by supercells. The brain’s natural mode of info-processing for many functions Humans to do many things subconsciously at once when perceiving an image Brain divides the processing of a visual scene to subdimensions of form, motion, depth and color To recognize something, the brain compares the constructed image to stored information

Review 🎃 Visual Sensation Retinal processing: photoreceptors (cones and rods) activates bipolar cells which activates ganglion cells Feature detection: detector cells respond to edge, lines, angles, and movements Parallel processing: supercell clusters processes motion, form, depth, and color Recognition: compares the image with the brain’s stored images A Pumpkin!

Color Blindness

Can you see the numbers/animals?

Two Theories Young-Helmholtz trichromatic theory suggests that the retina has 3 types of color receptors for the 3 primary colors red, green and blue. Other colors can be made by combining primary colors. Yellow, for instance, can be created by mixing red and green lights. Colorblindness is the result of the lack of functioning cones of one or more of the three colors. Opponent-process theory suggests that there exists 3 sets of opponent retinal processes: Red-green, yellow-blue, and white-black. For example, some neurons in the retina and in the thalamus are turned on by “green” message but off by “red” message and vice versa. This explains why one cannot perceive a “reddish green” message because the two messages cannot both travel at once. It is possible, however, to perceive message like “reddish blue”, or magenta, because the two messages travel in different channels. This also explains why we can see yellow despite being colorblind to red and green.

Current Understanding Color processing occurs in 2 stages: Following the Young-Helmholtz trichromatic theory, varying degrees of color stimulus trigger a certain combination of the 3 primary color receptors to fire Following the Opponent-process theory, the stimulus is then processed by the opponent-process cells If any member of each pair is inhibited, the other member is also inhibited (as we see in color blindness) If any member of each pair is stressed, the other member is expressed (afterimages)

🎨 Light versus Pigment The primary colors and secondary colors of Light and Pigment are flipped When you combine primary colors of light, you get white When you combine primary colors of pigment, you get black To clear any misunderstandings, green light + red light is yellow while green pigment + red pigment is closer to brown