Chapter 4: Sensation 1 Introduction To survive we must know the world around us because most objects in the world are charged with meaning. So Where does knowledge come from? The simple answer is through our senses. 2 The Origins of Knowledge 3 1
The Origins of Knowledge Empiricisms (experience) says that we collect information from our senses and transform that experience into knowledge. John Locke says that human mind is like a blank tablet (tabula rasa) on which experience writes. Empiricists argued that all knowledge comes through stimuli that excite the senses. John Locke (1632-1704) www.artunframed.com 4 Stimuli To get information, energy from objects or events (distal stimuli) impinges on the sensory surface generating proximal stimuli. http://www.stlukeseye.com Distal Stimulus Proximal Stimulus 5 Empiricism However, there are problems with empiricism, e.g., how do we perceive qualities like, depth, size, and shape, not directly given in the proximal stimulus? The empiricists answer by asserting that much of perception is built up through learning by association. How is the hand closer than the car, for they cast an image on the retina that is similar in size. 6 2
Empiricism Empiricists argue that our perception of depth, size, and shape etc., are embedded in the proximal stimulus, as cues. And the use of these cues are based on learning (association). Henderson State University: Garrison Hall Size of the pillars gets smaller as it recedes in the picture and gives us the sense of depth. We learn to associate seeing the pillars get shorter as we move in the corridor and thus perceive depth by associating seeing with our previous learning. 7 Nativism Philosophers like Kant pointed out that humans do not perceive sensory information by simple association only, but use innate mental categories to perceive incoming stimuli. Immanuel Kant (1724-1804) http://www.markrietmeijer.nl 8 Categories 1. We perceive an object as a whole and not as bits of experience as the associationists claimed. The apple is a single object (category: unity) and not bits of texture, color, and brightness etc. 2. Likewise, perception of depth is also a unitary experience, not bits of objects perceived, that differ in size. 3. Kant suggested that perception was based on categories like these which were inborn (innate). 9 3
Psychophysics 10 Psychophysics The dispute between empiricists and nativists was a debate among philosophers The charting of relationship between physical stimuli and psychological experiences began with the field of psychophysics founded by Gustav Fechner. Psychophysics asks questions like, what changes in our perception of a sound as the frequency of the sound waves change? 11 Psychophysics and Dualism Fechner realized that sensations and the stimuli that produce them belong to two totally different realms, one to the mental realm (sensation) and the other to the physical realm (stimuli). However each realm could be measured separately. Gustav Fechner (1801-1887) upload.wikimedia.org 12 4
Two Realms Physical Stimulus Sound pressure Light flash Sucrose solution Hydrogen sulfide gas Sandpaper Registered Sensation Auditory sensation Visual sensation Sensation of sweetness Rotten egg smell sensation Course textural sensation 13 Measuring Sensory Intensity 1. How can we measure a strong smell from a weak one? How can we measure a loud sound from a faint one? Answers to such questions led to quantitative measurement of sensation. 2. How can we measure differences between sensations made by blue versus green light? How can we measure differences in sensations caused by sour versus sweet taste? Answers to such questions led to qualitative measurement of sensation. 14 Difference Threshold Before Fechner, Weber (1834) pointed out that just noticeable difference (JND) between two stimulus intensities was a constant ratio (k = δi/i S ). vlp.mpiwg-berlin.mpg.de Ernst Heinrich Weber (1795 1878) 15 5
Comparison - Standard Stimulus Intensity (δi) 2/2/2009 Weber Fraction δi/i S = K, where δi = I C I S I C = intensity comparison Stimulus I S = Intensity standard stimulus (I C I S )/I S = K If I S = 50g and I C = 51g, Weber fraction K, equals: (51-50)/50 = K 1/50 = K or K = 0.02 How much weight would be required to notice a difference if the weight of standard stimulus (I S ) was 150 gram? (I C I S )/I S = K (I C - 150)/150 = 0.02 (I C - 150)= 0.02 X 150 (I C - 150) = 3 or I C = 153 16 Weber Fraction Graphed 10 510 9 8 7 6 5 4 3 2 1 0 50 100 150 200 250 300 350 400 450 500 Standard Stimulus (g) Intensity (I S) 459 408 357 306 255 204 153 102 51 0 Comparison Stimulus (g) Intensity (I C) 17 Modality Sensitivities Based on his measurements, Weber concluded that there were different JNDs for different senses. Thus senses differed in their sensitivities. Sensory Modality Weber Fraction (δi/i) Vision (brightness, white light) 1/60 Kinesthesis (lifted weights) 1/50 Pain (thermally aroused on skin) 1/30 Audition (tone, moderate loudness) 1/10 Pressure (cutaneous pressure spot ) 1/7 Smell (odor of India rubber) 1/4 Taste (table salt) 1/3 18 6
Fechner used Weber s fraction to arrive at a new formulation (S = K log R) to develop the psychophysical scale, that showed a relationship between the physical and the psychological realms. Psychophysical Scale 19 JNDs 1. JNDs became the basis to develop the psychological (sensation) scale. With a defined increment in the stimulus intensity the subject reported sensing a difference between two intensities. 2. However, new research shows that individual s sensory systems are affected by internal and external noise. Sometimes they say they don t detect a difference when there is a difference between stimulus intensities at other times they do when there is none. 20 Signal Detection Theory (SDT) SDT takes into account the sensitivity of the subject amidst noise and can tease apart the subject s bias or the ability to make a decision from his sensitivity. Stimulus Present Stimulus Absent Decision Yes Hit False Alarm Decision No Miss Correct Rejection 21 7
Uses of SDT SDT has been successfully used in a number of practical fields like communication, medicine, and law. 1. How does the radar operator detects and decides whether the approaching plane is that of the enemy. 2. How does the histologist decides whether she can detect and make a decision about cancer in a patient. 3. And finally how can a jury decide whether someone is guilty or not. 22 The Functioning of the Senses 23 The Functioning of the Senses 1. Seeing, hearing, tasting, smelling, and touching are 5 basic senses, including others, like vestibular, kinesthetic and skin sensations. 2. Each sensory cell transform physical energy of the stimulus into electrochemical process which the brain can understand. This is called transduction. 3. For each sense, stimulus energy can be coded for psychological intensity or quality. 24 8
The Functioning of the Senses 4. This coding is achieved through changes in neural firing for stimulus intensity. Rapid firing more intense stimulus. 5. Different types of neurons fire to register stimulus quality. Specific neurons work as labeled lines to process stimulus quality. This idea is based on law specific nerve energies (Müller, 1826). 6. Stimulus quality may also be registered by patterns of neuronal firing. 25 Amongst senses, vestibular sense registers bodily acceleration in the linear and the angular dimensions. It is processed through the semicircular canals in the inner ear. Vestibular Sense 26 Hearing 27 9
Hearing To hear, the ear is sensitive to sound pressure, which can vary in intensity, frequency and timbre. The intensity and frequency are represented below. 28 Sound Intensities Sound Intensity (db) Manned Spacecraft launching (150 ft) 180 Loudest rock band on record 160 Pain threshold (approximately) 140 Loud thunder 120 Shouting 100 Noisy automobile 80 Normal conversation 60 Quiet office 40 Whisper 20 Rustling of leaves 10 Threshold of hearing 0 29 Sound Frequencies Sound Frequency (Hz) Top note of grand piano 4244 Top note of piccolo 3951 Top range of soprano voice 1152 Top range of alto voice 640 Middle C 256 Top range of baritone voice 96 Top range of bass voice 80 Bottom range of contra bassoon 29 Bottom range of grand piano 27 Bottom note of organ 16 30 10
Timbre Timbre or quality of sound is based on complex waves. They are essentially a summation of simple sine waveforms. 31 Simple vs. Complex Sounds A simple sound frequency (or pure tone) is produced by a tuning fork. However most of the sounds are complex, e.g., piano sound. Complexity of sound represents its timbre. 32 The Ear Sound waves pass through the ear canal to vibrate the eardrum adhered to three bones called ossicles. Vibrations from ossicles are delivered on to the oval window of the cochlea to transduce sound signals to auditory impulses in neurons. 33 11
Cochlea The cochlea is a snail shaped organ in the inner ear contains the basilar membrane. Hair cells on this membrane deform by its movement caused by fluid that runs through cochlear duct. 34 Place Theory: Hearing Originally proposed by Helmholtz (1857) and later confirmed by von Békésy (1961), the basilar membrane deforms at different places to gives rise to sensations of different frequencies (pitch). www.germannotes.com Helmholtz (1821-1894) Von Békésy (18299-1972) www.ling.su.se Basilar membrane perturbed 35 Frequency Theory: Hearing However, lower frequencies (50 Hz) cause the basilar membrane to vibrate equally over its entire surface, and very high frequencies (20,000 Hz) cannot be registered by single neurons. So how can we perceive different sound frequencies? Frequency theory suggests that pitch is determined by the firing rates (patterns) of neurons (Wever, 1965). Ernest Wever (1902-1991) www.parmly.luc.edu 36 12
Sound: Further Processing Hair cells in the cochlea transduce mechanical motion into neural signals which are processed by nuclei in the midbrain, thalamus (medial geniculate nucleus) and the auditory area in the temporal cortex. 37 Sound: Further Processing 1. When a sound arrives in the brain, it must identify and recognize its quality. So a note on the piano needs to be differentiated from a note produced by a harpsichord (timbre). 2. Brain needs to identify a sound (my cell phone) from another sound (someone else's cell phone). 3. Brain must also localize sound in space, whether it is coming from the left or the right. 38 Sound: Further Processing Neurons in the auditory cortex have a frequency map (tonotopic map). Similar preferred pitches tend to be located close to each other. Auditory cortex: cat 39 13
Vision 40 Visual Sense Visual sense is the most important sense for human beings. We trust this sense more than other senses and it utilizes the largest area in the human brain compared to other senses. 41 Eye: The Visual Organ Light travels through the cornea and the lens to reach the retinal layer. Iris controls the amount of light that goes through. And the lens thickens and thins (accommodation) to focus on near and far objects. 42 14
Retina: Photosensitive Layer 1. Photoreceptor Layer: Cones perceive color; are concentrated in the fovea Rods perceive light/dark; are completely absent in the fovea 2. Bipolar Cell Layer 3. Ganglion Cell Layer: axons converge to form optic nerve. Optic nerve exits eyeball forming blind spot. 43 Photoreceptors Rods: Peripheral retina Detect black n white Work in twilight conditions (120 million) Cones: Central retina Fine detail and color Work in daylight or well-lit conditions (6 million) 44 Photoreceptor Pigments 1. Rods: Rhodopsin Sensitive to light and breaks down easily when light present. Therefore signals bright versus dim. 2. Cones: Three photopigments Light chemically changes the photopigments. Different wavelengths cause different photopigments to react, helping us to perceive color. 45 15
Visual System Processes If a stimulus remains unchanged sensitivity to this stimulus decreases. Neurons and other sensory cells fire more strongly to novel input. 46 Visual System Processes In the same fashion, contrast is another factor that leads us to being sensitive to differences in stimulation. Brightness, edges, and shapes are still other factors that require change in stimulation. The gray on the whiter background looks darker than on a black background. 47 Contrast Effects: Illusions If you look at the grid on the right and focus in the middle, you see gray spots at the intersections. This illusory phenomenon is due to lateral inhibition of brain cells in the visual system (see below). 48 16
Contrast Effects: Mach Bands Contrast effects do not simply separate bright from dark, but accentuates brightness differences, as seen in Mach bands. Each band is homogeneous gray strip however, at each edge left side looks darker and right side lighter than the band s gray tone. 49 Contrast Effects: Mach Bands Figure A shows how stimulus intensity uniformly changes over Mach bands, however perceived intensity (Figure B) is a different story. Brightness and darkness on each band s edge, are accentuated. 50 Contrast Effects: Physiology Cell C is inhibited by cells B and D less strongly thus C sends a strong signal. Cell D is inhibited by its neighbors more strongly thus sends a weaker signal. Thus the edge is emphasized. 51 17
Color White light is a narrow band of electromagnetic radiation, composed of spectrum of wavelengths ranging from 400nm (violet) to 700nm (red). Shorter wavelengths than violet are called ultraviolet (seen by bees) and longer wavelengths than red are called infrared, sensed as heat. 52 Hue or Color The human eye can detect 7 million colors. Short wave lengths tend to be bluish in color and longer wave lengths tend to be reddish in color. Hue 53 Brightness Brightness is a dimension of color that differentiates black (low brightness) from white (high brightness). Chromatic colors also have different brightness levels. Brightness 54 18
Saturation Saturation is simply purity of a color. Saturation determines to what extent a color is chromatic (saturated) or achromatic (unsaturated). Saturation 55 Physiological Basis of Color What is the physiological basis of color? The answer lies in two mechanisms: 1. retina and, 2. the nervous system. 56 Color Receptors Normal human vision depends on three different kinds of cones, thus our color vision is trichromatic. We have short, medium and long-wavelength cones, each responding to different spectral colors, however there is overlap in responsiveness (see below). 57 19
Young-Helmholtz Theory Young (1801) and then Helmholtz (1866) suggested that human retina contains three receptors and that when long wavelength cones were activated we perceived redder hues and when short wavelength cones were activated we perceived bluish tones. And all other colors were perceived as combination of activation of cones. 58 Problems with Young-Helmholtz Theory 1. Young-Helmholtz theory of color vision explains why red, green and blue have special status. They all are primary colors, not based on mixtures of other colors. 2. However, the theory does not explain why yellow looks like a primary color, when it is a mixture of red and green. 59 Problems with Young-Helmholtz Theory 3. Another problem with Young-Helmholtz theory is that it cannot explain why colors come in pairs, e.g., simultaneous color contrast. The gray patch on the yellow square looks darker and bluer, and gray patch on blue square looks lighter and yellower. 60 20
Problems with Young-Helmholtz Theory 4. A clearer example of pair of colors can be appreciated when we study negative after images. Each color generates a negative after image. Red results in green after image and vice versa, and blue in yellow etc. Stare in the middle of the flower for a minute, blink twice and then look at the black dot on the right. 61 Opponent-Process Theory Herring (1872) and later Hurvich and Jameson (1955) proposed opponent-process theory of color vision, which suggests that red-green, blue-yellow and black-white colors are paired at the next level in the nervous system beyond retinal receptors. 62 Color Blindness Color blindness can be due to two defects in the visual system: 1. Defective opponent process, 2. Missing photopigment or both. Eight % men and.03% women suffer from color blindness with red-green form of color blindness as most common. 63 21
Shape Perception We perceive shapes through cells called feature detectors. Cells respond differentially to various stimuli, like Straight: horizontal, vertical, lines, Corners, angles, and even movement, velocities. With animal preparations (shown below) such feature detector could be outlined. 64 Shape Perception Different kind of responses are elicited in the retina and the nervous system which are combined to form feature detectors. 65 Shape Perception Feature detectors is not the only kinds of cells in the visual nervous system. There are cells that respond to complex shapes like faces, hands, fingers etc., 66 22
Some Final Thoughts 67 Active Perceiver All scientific evidence suggests that we actively shape our sensations. And these include perception of shape, motion, and edges. The visual brain s activity increases as we process information even more deeply. 68 Questions 1. The perspective that humans categorize and interpret incoming sensory information, offered as an alternative to the idea that human senses passively perceive the world, is known as: a. transduction. b. nativism. c. signal detection theory. d. empiricism. 69 23
Questions 2. The idea that different sensory qualities within a single sense (i.e., red versus green; sweet versus sour) are signaled by which neurons are firing more or less in response to the stimulus is known as theory. a. specificity. b. signal detection. c. pattern. d. frequency. 70 Questions 3. Contrast-effect illusions, such as the Mach bands, are produced by what mechanism of visual processing? a. lateral inhibition. b. negative afterimage. c. simultaneous color. d. feature detection. 71 Questions 4. The auditory receptors that transduce sound waves into neural firing are located in the: a. cochlea. b. outer ear. c. auditory canal. d. oval window. 72 24