Sensation (Ch.5) OBJECTIVES

1.b. Explain the difference between bottom-up and top-down processing.

Bottom-up processing is sensory analysis that begins at the entry level, with information flowing form the sensory receptors to the brain. Top-down processing is analysis that begins with the brain and flows down, filtering information through our experience and expectations to produce perceptions.

10. Explain the importance of color constancy.

Color constancy is our ability to perceive consistent color in objects, even though the lighting and wavelengths shift. This phenomenon demonstrates that our brains construct our experience of the color of an object through comparisons with other surrounding objects.

2.a. Distinguish between absolute and difference thresholds.

Each species comes equipped with sensitivities that enable it to survive and thrive. Psychophysics is the study of the relationships between the physical characteristics of stimuli and our psychological experience of them. Our absolute threshold for any stimulus is the minimum stimulation necessary for us to be consciously aware of it 50 percent of the time. Signal detection theory demonstrates that individual absolute thresholds vary, depending on the strength of the signal and also on our experience, expectations, motivation, and alertness. Our difference threshold (also called just noticeable difference, or jnd) is the barely noticeable difference we discern between two stimuli 50 percent of the time. As Weber's law states, to be perceptibly different, two stimuli must differ by a constant proportion (such as a 2 percent difference in weight), not a constant amount, of the original stimulus.

5. Describe the major structures of the eye, and explain how they guide an incoming ray of light towards the eye's receptor cells.

Light enters the eye through the cornea, a protective covering that bends the light ray. The iris, a ring of muscle, controls the size of the pupil, through which light enters. The lens changes shape to focus light rays on the retina, the inner surface of the eye, where receptor cells convert the light energy into neural impulses. After coding in the retina, the impulses travel along the optic nerve to the brain. Although the retina receives an upside-down image, the brain constructs the impulses it receives into an upright-seeming image. Distortions in the eye's shape can affect the sharpness of vision.

8. Define parallel processing, and discuss its role in visual information processing.

Parallel processing is the brain's natural mode of information processing, in which it handles many aspects of a problem simultaneously. This multitasking ability lets the brain distribute subdimensions of vision (color, movement, depth, and form) to separate neural teams that work separately and simultaneously. Other neural teams collaborate in integrating the results, comparing them with stored information, and enabling perceptions.

7. Discuss the different levels of processing of visual information traveling from the eye's retina to the brain's cortex.

Perceptions arise from the interaction of many neuron systems, each performing a simple task. Processing begins in the retina's multiple neural layers, and then the retina's 6 million cones and 120 million rods relay their information via bipolar cells to ganglion cells. Impulses travel along the ganglion cells' axons, which form the optic nerve, to the thalamus, and on to the visual cortex. In the visual cortex, feature detectors respond to specific features of the visual stimulus. Higher-level supercells integrate this pool of data for processing in other cortical areas. As sensory input passes through multiple levels of processing, it is influenced by our assumptions, interests, and expectations.

1.a. Contrast sensation and perception.

Sensation is the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment. Perception is the process by which we organize and interpret this information. Although we view sensation and perception separately to analyze and discuss them, they are actually parts of one continuous process.

3. Describe sensory adaptation, and explain how we benefit from being unaware of unchanging stimuli.

Sensory adaptation is our diminished sensitivity to constant or routine odors, sounds, and touches. We benefit from this phenomenon because it focuses our attention on informative changes in stimulation, rather than on unchanging elements in our environment.

11. Describe the pressure waves we experience as sound.

Sound waves are bands of compressed and expanded air. Our ears detect these changes in air pressure and transform them into neural impulses, which the brain decodes as sound. Sound waves vary in frequency and amplitude, which we perceive as differences in pitch and loudness.

9. Explain how the Young-Helmholtz and opponent process theories help us understand color vision.

The Young-Helmholtz trichromatic (three-color) theory proposed that the retina contains three types of color recptors. Contemporary research has found three types of cones, each most sensitive to the wavelengths of one of the three primary colors of light (red, green, or blue). Hering's opponent0process theory proposed two additional color processes (red-versus-green and blue-versus-yellow) plus a third black-versus-white process. Contemporary research has confirmed that, en route to the brain, neurons in the retina and the thalamus code the color-related information from the cones into pairs of opponent colors, as demonstrated by afterimages. These two theories, and the research supporting them, show that color processing occurs in two stages.

2.b. Discuss whether we can sense stimuli below our absolute thresholds and be influenced by them.

The priming effect and other experiments reveal that we can process some information from stimuli below our absolute threshold for conscious awareness. But the restricted conditions under which it occurs would not enable unscrupulous opportunists to exploit us with subliminal messages.

6. Contrast the two types of receptor cells in the retina, and describe the retina's reaction to light.

The two types of receptors in the retina are the rods and the cones, and they differ in their shape, number, function, location, and links to the brain. When light enters the eye, it triggers a photochemical reaction in the rods and cones, which in turn activates bipolar cells. The bipolar cells activate ganglion cells, and their axons (combined to form the optic nerve) transmit information (via the thalamus) to the visual cortex in the brain's occipital region. The more numerous rods, located mainly around the periphery of the retina, are more sensitive to light. Multiple rods send combined messages to a bipolar cell, and this pool of information lets us see rough images in dim light. Cones, concentrated in the fovea (at the center of the retina), are sensitive to color and detail. A cone may link directly to a single bipolar cell, and this direct line to the brain preserves fine details in the cone's message.

4. Define transduction, and specify the form of energy our visual systems converts into the neural messages our brain can interpret.

Transduction is the process by which our sensory systems encode stimulus energy as neural messages the brain can interpret. In vision, we convert light energy into these neural impulse. The energies we experience as a visible light are a thin slice from the broad spectrum of electromagnetic radiation. The hue and brightness we perceive in a light depend on the wavelength and intensity.