Module 18

Vision: Sensory and Perceptual Processing

❖ Light Energy and Eye Structures

■ Our eyes receive light energy and transform it into neural messages

➢ The Stimulus Input: Light Energy

■ The stimuli striking our eyes are pulses of electromagnetic energy that our visual system perceives as a color

■ On the spectrum‘s one end are short gamma waves, the other end are the mile-long waves of radio transmission

■ Light travels in waves, and the shape of those waves influence what we see

● Wavelength: the distance from the peak of one light or sound wave to the peak of the next. Electromagnetic wavelengths vary from the short blips of gamma rays to the long pulses of radio transmission.

◆ Wavelength determines:

➢ Hue: the dimension of color that is determined by the wavelength of light; what we know as the color names blue, green, and so forth.

➢ Intensity: the amount of energy in a light wave or sound wave, which influences what we perceive as brightness or loudness. Intensity is determined by the wave's amplitude (height).

➢ The Eye

■ Light enters the eye through the cornea, which bends light to help provide focus

● Cornea: the eyes clear, protective outer layer, covering the pupil and iris.

● The light then passes through the pupil

◆ Pupil: the adjustable opening in the center of the eye through which light enters.

➢ Iris: a ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening.

■ The iris responds to your cognitive and emotional states

◆ After passing through the pupil, light hits the transparent lens in the eye

➢ Lens: the transparent structure behind the pupil that changes shape to help focus images on the retina.

■ Retina: the light-sensitive inner surface of the eye, containing the receptor rods and cones plus layers of neurons that begin the processing of visual information.

■ Accommodation

● The process by which the eye’s lens changes shape to focus near or far objects on the retina.

❖ Information Processing in the Eye and Brain

➢ The Eye-to-Brain Pathway

■ In the back of the eye, the retina’s nearly 130 million buried receptor cells, the rods and cones

● Rods: retinal receptors that detect black, white, and gray, and are sensitive to movement; necessary for peripheral and twilight vision, when cones don’t respond.

● Cones: retinal receptors that are concentrated near the center of the retina and that function in daylight or in well-lit conditions. Cones detect fine detail and give rise to color sensations.

■ Ganglion cells form the optic nerve

● Optic nerve: the nerve that carries neural impulses from the eye to the brain.

■ The eye has a blind spot

● Blind spot: the point at which the optic nerve leaves the eye, creating a “blind” spot because no receptor cells are located there.

■ Cones

● Cones cluster in and around the fovea

◆ Fovea: the central focal point in the retina, around which the eye’s cones cluster.

● Many cones have their own hotline to the brain

◆ One cone transmits its message to a single bipolar cell, which relays the message to the visual cortex

➢ These direct connections preserve the cones’ precise information, making them better able to detect fine detail

◆ Cones can detect white, but also enable you to perceive color

◆ In dim light, they become unresponsive, meaning you see no colors

■ Rods

● Located around the retina’s outer regions

● Remain sensitive in dim light, and they enable black and white vision

● They have no hotline to the brain

● If cones are soloists, rods perform as a chorus

◆ Several rods pool their faint energy output and funnel it onto a single bipolar cell, which sends the combined message to your brain

➢ Color Processing

■ If no one sees the tomato, is it red?

● No.

● The tomato rejects the long wavelengths of red, so it’s everything but red

● The tomato’s color is our mental construction

◆ Like all aspects of vision, our perception of color resides not in the object itself but in the theater of our brain

■ Herman von Helmhotz and Thomas Young

● Knew that any color can be created by combining the light waves of three primary colored—red, green, blue

◆ Formed a hypothesis:

➢ The eye must therefore have three corresponding types of color receptors

■ Young-Helmholtz trichromatic (three-color) theory was confirmed by researchers

● Young-Helmholtz trichromatic (three-color) theory: the theory that the retina contains three different types of color receptors—one most sensitive to red, one to green, one to blue—which, when stimulated in combination, can produce the perception of any color.

■ About 1 person in 50 is colorblind

● Usually male because the defect is genetically sex linked

● Most people with color-deficient vision are not actually blind to all colors

■ Ewald Hering

● Found a clue in afterimages

● Hypothesized:

◆ Color vision must involve two additional color processes, one responsible for red-versus-green perception, and one for blue-versus-yellow perception

➢ Confirmed by researchers: the opponent-process theory

■ Opponent-process theory: the theory that opposing retinal processes (red-green, blue-yellow, white-black) enable color vision. For example, some cells are stimulated by green and inhibited by red; others are stimulated by red and inhibited by green.

■ Color processing occurs in two stages:

● The retina’s red, green, and blue cones respond in varying degrees to different color stimuli, as the Young-Helmholtz trichromatic theory suggested

● The cones’ responses are then processed by opponent-process cells, as Hering’s opponent-process theory proposed

■ Feature detectors

● Nerve cells in the brain’s visual cortex that respond to specific features of the stimulus, such as shape, angle, or movement.

● Feature detectors receive information from individual ganglion cells in the retina

● This specific information is passed to other cortical areas, where teams of cells respond to more complex patterns

➢ Parallel Processing

■ Processing many aspects of a problem simultaneously; the brain’s natural mode of information processing for many functions, including vision.

■ You brain integrates information projected by your retinas to several visual cortex areas and compares it with stored information, thus enabling your fusiform face area to recognize a face

■ Visual processing

● Letters (for example) reflect light rays onto the retina, which triggers a process that sends formless nerve impulses to several areas of the brain, which integrates the information and decodes its meaning