Mod 14 Basic Concepts and VisionPresentation Transcript
Introduction to Sensation and Perception: Vision (Module 14)
How do we construct our representations of the world?Our sensory receptors detect physical energy from the environment and translate it into neural signals. This is called sensation.At this level of detection, sensory analysis uses bottom-up processing (…is sensory analysis that begins at the entry level, with information flowing from the sensory receptors to the brain).However, we also select, organize, and interpret our sensations—a process called perception.During perception, our mind processes what our senses detect. At this level, sensory analysis uses top-down processing (… is information processing guided by higher-level mental processes, as when someone constructs perceptions drawing on experience and expectations).
Bottom-up Processing Analysis of the stimulus begins with the sensereceptors and works up to the level of the brain and mind. Letter “A” is really a black blotch broken down into features by the brain that we perceive as an “A.”
Top-Down ProcessingInformation processing guided by higher-level mental processes as we construct perceptions, drawing on our experience and expectations. THE CHT
Sensory and perceptual processes form a continuum. Failures occurring anywhere between sensory detection and perceptual interpretation can distort our view of the world.EX: Patient E.H. suffered from propopagnosia, which is the inability to recognize and connect with the outside the world. Complete sensation was present but perception was incomplete. If shown an unfamiliar face, there was no response. If shown a familiar face, the autonomic system reacted by perspiring. If shown her own face, she still could not recognize it. This is what is known as a lack of top-down processing. She is unable to relate her stored knowledge to the sensory input.
Making Sense of ComplexityOur sensory and perceptual processes worktogether to help us sort out complex images. “The Forest Has Eyes,” Bev Doolittle
Sensing the World Senses are nature’s gifts that suit an organism’s needs. Because of particular receptor cells in a frog’s eyes, a frogcan detect the motion of insects and can therefore thrive. Afrog could be surrounded by motionless insects and starve to death!A male silkworm moth is sensitive to female sex-attractant odor and can be affected up to a mile a way!We as human beings are sensitive to sound frequencies that represent the range of human voice.
Psychophysics A study of the relationship between physicalcharacteristics of stimuli (such as intensity) and our psychological experience with them. Psychological Physical World World Light Brightness Sound Volume Pressure Weight Sugar Sweet
SensationSensing the World:Some Basic Principles Thresholds Sensory Adaptation
Exploring the SensesWhat stimuli cross our threshold for conscious awareness? (Absolute Threshold)
Absolute ThresholdTo some kinds of stimuli we are especially sensitive. If we were to stand on top of a mountain (on a dark, clear night), most of us would be able to see a candle flame on top of another mountain 30 miles away.Most of us would be able to feel the wing of a bee on our cheek.Most of us would be able to smell a drop of perfume in a 3 room apartment.Our awareness of these faint stimuli illustrates our absolute thresholds— which is the minimum stimulation necessary to detect a particular light, sound, pressure, taste, or odor 50% of the time.EX: A hearing specialist exposes an individual to varying sounds in order to figure out their absolute threshold for hearing certain pitches.
Absolute ThresholdAbsolute Threshold: Minimum stimulation needed to detect a particular stimulus 50% of the time. Proportion of “Yes” Responses 0.50 1.00 ------Subliminal Threshold below the threshold of 0.00 conscious perception 0 5 10 15 20 25 Stimulus Intensity
Subliminal ThresholdHave you ever tried a cessation program that delivered you messages subliminally?For example, listening to audiotapes that are supposed to help you lose weight, stop smoking, or improve your memory?These audiotapes are usually masked by soothing ocean sounds and unheard messages such as “I am thin”, “Smoke tastes bad”, or “I do well on tests because I have total recall of information”.These subliminal messages are all in an attempt to influence our behavior.We can unconsciously sense subliminal stimuli and without our awareness can to some extent be affected by it.
A stimulus is Subliminal if it is below your absolute threshold, meaning that you detect it less than 50% of the time. For instance, a microscopic cell is subliminal to you because you cannot see it with your naked eye.Subliminal advertisements (Drink Coke, eat popcorn etc.), do have an affect on you, (they prime you—predisposing you to making a particular decision) but do not persuade you.The final statement of subliminal messages is that much of our information processing occurs automatically, out of sight, and off the radar screen of our conscious mind.
Subliminal Threshold Subliminal Threshold: When stimuli are belowone’s absolute threshold for conscious awareness. Kurt Scholz/ Superstock
Difference ThresholdsHow does the magnitude of a stimulus influence our threshold for detecting differences?The Difference Threshold (a.k.a the just noticeable difference or jnd) is the lowest difference you can detect between any two stimuli 50% of the time. For example, you are just able to notice the difference between 1kg and 1.02kg half the time.
Weber’s LawTwo stimuli must differ Constant Stimulus by a constant (k)proportion (rather than Light 8% a constant amount), to be perceived as Weight 2% different. Weber Tone 3% fraction: k = ∆I/I.
SensationSensing the World:Some Basic Principles Thresholds √√ Sensory Adaptation
Sensory AdaptationDiminished sensitivity as a consequence of constant stimulation. Put a band aid on your arm and after awhile you don’t sense it.
Sensory Adaptation: What purpose does it serve?Although sensory adaptation reduces our sensitivity, it offers an important benefit: Freedom to focus on informative changes in our environment without being distracted by the constant chatter of uninformative background stimulation.Our sensory receptors are alert to novelty; bore them with repetition and they free our attention for more important things.This reinforces a fundamental lesson: We perceive the world not exactly as it is, but as it is useful for us to perceive it. (ex: smokers, body scents, etc.)
Now you see, now you don’t(a) A projector mounted on a contact lens makes the projected image move with theeye. (b) Initially the person sees the stabilized image, but as the sensory systembecomes fatigued, she begins to see fragments fading and reappearing.
SensationVision The Stimulus Input: Light Energy The Eye Visual Information Processing Color Vision
TransductionThe transformation of stimulus energy (sights, sounds, smells) into neural impulses.
The Stimulus Input: Light Energy Both Photos: Thomas Eisner Visible SpectrumLight is composed of electromagnetic waves with Wavelengths (distance fromone peak to another peak on a wave) and Amplitudes (height of the wave).
Physical Characteristics of Light 1. Wavelength (hue/color) 3. Intensity (brightness)
Wavelength (Hue)Hue (color) is the dimension ofcolor determined by thewavelength of the light.Wavelength is thedistance from thepeak of one waveto the peak of the next.
Wavelength (Hue) Violet Indigo Blue Green Yellow Orange Red 400 nm 700 nmShort wavelengths Long wavelengths Different wavelengths of light result in different colors.
Intensity (Brightness) Intensity: Amount of energy in a wavedetermined bythe amplitude. It is related to perceived brightness.
Intensity (Brightness) Blue color with varying levels of intensity.As intensity increases or decreases, blue color looks more “washed out” or “darkened.”
Parts of the eye• Cornea: Transparent tissue where light enters the eye.• Iris: Muscle that expands and contracts to change the size of the opening (pupil) for light.• Lens: Focuses the light rays on the retina.• Retina: Contains sensory receptors that process visual information and sends it to the brain.
The Lens Lens: Transparent structure behind thepupil that changes shape to focus images on the retina. Accommodation: The process by which theeye’s lens changes shapeto help focus near or far objects on the retina.
Retina Retina: The light- sensitive inner surface of the eye,containing receptor rods and cones inaddition to layers of other neurons (bipolar, ganglion cells) that processvisual information.
Optic Nerve, Blind Spot & FoveaOptic nerve: Carries neural impulses from the eye to thebrain. Blind Spot: Point where the optic nerve leaves theeye because there are no receptor cells located there.Fovea: Central point in the retina around which the eye’scones cluster. http://www.bergen.org
Test your Blind Spot Try is on your own time with the photo in your book. Close your left eye, and fixate your righteye on the black dot. Move the page towards your eye and away from your eye. At some point thecar on the right will disappear due to a blind spot.
Bipolar & Ganglion Cells Bipolar cells receive messages fromphotoreceptors and transmit them to ganglioncells, which converge to form the optic nerve. Cone -- Rod Neural impulse
Visual Information Processing Optic nerves connect to the thalamus in themiddle of the brain, and the thalamus connects to the visual cortex.
How is visual information processed in the brain?The retina processes information before routing it via the thalamus to the brain’s cortex.After the rods and cones process the visual input, the information travels to the ganglion cells (whose axons make up the optic nerve) and then to the visual cortex in the brain (located in the occipital lobe).
Feature Detection Feature Detectors: Nerve cells in the visual cortex that respond to specific features, such as edges, angles, and movement.Ross Kinnaird/ Allsport/ Getty Images Electrodes record how individual cells in this monkey’s visual cortex respond to different visual stimuli. Hubel and Wiesel won a Nobel prize for their discovery that most cells in the visual cortex respond only to particular features—for example, to the edge of a surface or to a bar at a 30-degree angle in the upper right part of the field of vision. Other cells integrate information from these simpler ones.
Shape Detection Specific combinations of temporal lobe activity occur as people look at shoes, faces, chairs and houses. Ishai, Ungerleider, Martin and Haxby/ NIMHDuring an fMRI, different areas of the brain “light up” when looking at certainobjects.
Visual Information Processing Processing of several aspects of the stimulus simultaneously is called parallel processing. Thebrain divides a visual scene into subdivisions such as color, depth, form, movement, etc.
Sometimes parallel processing is hindered in some way.For example: Mrs. M was a woman who suffered stroke damage to both sides of her brain. She became unable to perceive movement. She was unable to see people move yet they would miraculously appear in different places although she could not see how they had gotten there.There is also a phenomenon called Blindsight. This is when people who cannot consciously perceive can still remarkably locate objects but are consciously unaware of how they knew where the objects were or what their orientation was.
From Sensation to Recognition: A Simplified Summary of Visual Information ProcessingTim Bieber/ The Image Bank
Color VisionThe Young-Helmholtz Trichromatic (three color) theory and the Opponent-Process theory help contribute to our understanding of color vision.We know (or we think we know) that a tomato is red.Ask yourself this: If a tree falls in the forest and no one is around to hear it, does it make a sound?Now ask yourself this: If no one sees the tomato, is it red?
The answer is NO.The tomato is everything but red, because it rejects (or better yet, reflects) the long wavelengths of red.Also, the tomato’s color is our mental construction.Light rays are not colored. Color does not reside in an object; it resides within our brain; hence our ability to dream in color.
Color Vision Trichromatic theory: Young and von Helmholtz suggested that the eye must contain three receptorsthat are sensitive to red, blue and green colors, which when stimulated can produce the perception of any color. Standard stimulus Comparison stimulus Max Medium Low Blue Green Red
Color BlindnessGenetic disorder in which people are blind to green or redcolors. This supports the Trichromatic theory. They will not be able to perceive the number within the design.Show color blindness video: http://www.youtube.com/watch?v=CWyrp3hu4KE Ishihara Test
Color VisionOpponent Process Theory:Hering, a physiologist, found that when you stare at a green square for a while and then look at a white sheet of paper, you see red, which is green’s opponent color.If you stare at a yellow square and do the same, you will see blue, which is yellow’s opponent color.The Opponent Process theory is the theory that opposing retinal processes (red/green, yellow/blue, white/black) enable color vision.
Opponent Colors Gaze at the middle of the flag for about 30seconds. When it disappears, stare at the dot and report what you see. You should see the opponent colors.