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Chapter 5

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Psychology 1000

CHAPTER 5: SENSATION AND PERCEPTION - synesthesia: a condition where stimuli are experienced by more than one sense. Literally means “mixing of the senses”  can experience sounds as colours or tastes as touch sensations that have different shapes - Sensory receptors must translate information into nerve impulses. Then, specialized neurons break down and analyze the specific features of the stimuli. At the next stage, these numerous stimulus pieces are reconstructed into a neural representation that is then compared with our knowledge of what particular objects look, smell, or feel like. This allows us to recognize the stimulus. - Sensation: the stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain - Perception: making “sense” of what our senses tell us – the active process of organizing this stimulus input and giving it meaning > SENSORY PROCESSES - more than just the 5 classical senses vision, audition, touch, gustation (taste), and olfaction (smell) - also senses that provide info about balance and body position, pressure, pain, temperature, foreign invaders in the immune system, etc - psychophysics: studies relations between the physical characteristics of stimuli and sensory capabilities  concerned with two types of sensitivity: absolute limits of sensitivity, and the differences between stimuli The Absolute Threshold: - asks “how intense must a stimulus be before we can detect it’s presence?” - researchers present stimuli of varying intensities and ask ppl if they can detect it - the absolute threshold: the lowest intensity at which a stimulus can be detected correctly 50% of the time (lower the absolute threshold, greater the sensitivity) Signal Detection Theory: - level of sensitivity for each sense can vary from person to person - can also vary based on factors such as fatigue, expectation, or potential significance of the stimulus - signal detection theory: concerned with the factors that influence sensory judgments - ask participants if they hear a tone  4 outcomes: hit, miss, false alarm, or correct rejection (figure 5.3 pg 162) - at low stimulus intensities, the decision criterion is influenced by both the participant’s and the situation’s characteristics  bold participants say “yes” more than conservative participants, and manipulating the rewards or costs for correct or incorrect responses can put more pressure to answer correctly The Difference Threshold: - the smallest difference between two stimuli that ppl can perceive 50% of the time - also called the just noticeable difference (jnd) 1 - Weber’s law: the jnd is directly proportional to the magnitude of the stimulus with which the comparison is being made, and can be expressed as a Weber fraction • Eg, the jnd value for weights is a Weber fraction of approx. 1/50  this means that if you lift a weight of 50 grams, a comparison weight must weigh at least 51 grams for you to be able to say it was heavier • Does not work at extremely high and low intensities • Table 5.2 pg 165 Sensory Adaptation - Sensory adaptation: sensory neurons are engineered to respond to a constant stimulus by decreasing their activity, and diminishing sensitivity to that stimulus (also known as habituation) - R.M. Pritchard  demonstrated that if tiny involuntary eye movements did not keep images moving around the retina, stationary objects would fade from sight - Sensory adaptation allows us to pick up informative changes in the environment by freeing our senses from the constant and mundane stimuli around us > THE SENSORY SYSTEMS Vision - stimulus for vision is electromagnetic energy, or light waves, measured in nanometers (nm)  humans can detect from about 700nm (red) to about 400nm (blue-violet) The Human Eye - light waves enter the eye through the cornea, a transparent protective structure at the front of the eye - Behind the cornea is the pupil, an adjustable opening that can dilate or constrict to control the amount of light that enters the eye. The pupil’s size is controlled by muscles in the coloured iris - Behind the pupil is the lens, an elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects - The lens focuses the visual image on the light-sensitive retina, a multi-layered tissue at the rear of the fluid-filled eyeball - Myopia (nearsightedness): the lens focuses the visual image in front of the retina, resulting in a blurred image for faraway objects. Usually occurs because the eyeball is longer than normal - Hyperopia (farsightedness): occurs when the lens does not thicken enough and the image is focused on a point behind the retina. Aging typically causes the eyeball to become shorter, which is why many older ppl need reading glasses Photoreceptors: Rods and Cones - the retina contains two types of light-sensitive receptor cells: rods and cones  there are about 120 million rods and 6 million cones in the human eye 2 - rods function best in dim light, and are primarily black-and-white brightness receptors. They are about 500 times more sensitive to light than cones, but cannot sense colour. - Cones are colour receptors, and function best in bright light - in humans, rods and cones are found throughout the retina except in the fovea, a small area in the center of the retina that contains only cones. Cones decrease in concentration as one moves away from the center of the retina - rods and cones send their msgs to the brain via bipolar cells and ganglion cells - bipolar cells have synaptic connections with the rods and cones, then they synapse with a layer of about one million ganglion cells, whose axons are collected into a bundle to form the optic nerve - Typically, many rods are connected to the same bipolar cell. They therefore can combine their individual electrical msgs to the bipolar cell, where the effect of the many signals may be enough to fire it. That is why we can detect a faint stimulus more easily if we look at it off to the side so that it does not fall on the fovea, but on the peripheral of the retina, where the rods are most dense - Cones in the fovea each have their own private line to a single bipolar cell, so our visual acuity, or ability to see fine detail, is best when an image is focused right on the fovea  eagles and hawks have 2 fovea to allow them to see small prey - The optic nerve formed by the axons of the ganglion cells exits through the back of the eye producing a blind spot, where there are no photoreceptors  we are unaware of the blind spot because our perpetual system fills in the missing parts Visual Transduction - transduction: the process whereby the characteristics of a stimulus are converted into nerve impulses - rods and cones translate light waves into nerve impulses through the action of protein molecules called photopigments  absorb light to produce a chemical reaction that changes the rate of neurotransmitter release at the receptor’s synapse with the bipolar cells  greater the change, the stronger the signal passed on to the bipolar cell and then the ganglion cells  triggering of all three levels (rod/cone, bipolar cell, ganglion cells) the msg is sent to the brain Brightness Vision and Dark Adaptation - sensitivity of rods and cones depends on the wavelength of the light - rods are least sensitive to red, and cones are most sensitive to yellow-green - switch to yellow-green fire trucks for cones, and blue runway lights for rods - dark adaptation: the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination - after absorbing light, a photoreceptor is depleted of its pigment molecules for a period of time – during dark adaptation, the photopigment molecules are regenerated and sensitivity increases - cones reach maximum sensitivity after 5-10 minutes, and rods after about ½ hour - red light keeps rods in state of dark adaptation because they are insensitive to red wavelengths, thus retain high levels of photopigment and remain sensitive 3 Colour Vision - can distinguish an estimated 7.5 million different hues - two theories of colour vision The Trichromatic Theory - Young and Hemholtz - additive colour mixture: any colour can be produced by mixing green, blue, or red - said there are 3 types of colour receptors in the retina, and individual cones are most sensitive to wavelengths that correspond to either blue, green or red - signal from each receptor sent to brain and mixed to make original hue – pure white if there is an equal activation - problems  people with red-green colour blindness can still perceive yellow, and this theory cannot account for afterimages Opponent-process Theory - Hering (1870) - Proposed 3 types of cones  one which responded to red or green, another to blue or yellow, and a third to black or white - Explains afterimages by saying that if you stare at a colour long enough, the neural processes that register that colour become fatigued. When you look at a white surface, which reflects all wavelengths, a “rebound” opponent reaction occurs as each receptor responds with its opposing chemical reaction Dual processes theory - combines the two theories to account for colour transduction processes - cones do contain one of three different protein photopigments roughly corresponding to red, blue, and green – when mixed, can make other hues - However, opponent processes do not occur at level of the cones. Certain ganglion cells in the retina, as well as some neurons in the visual relay stations and the visual cortex, respond in an opponent process fashion by altering their rate of firing. - Red-green opponent processes are triggered by input from the red or green sensitive cones, but the blue-yellow opponent process is more complex  blue is triggered by blue sensitive cones, but yellow is triggered by simultaneous input from the red and green sensitive cones Colour-deficient Vision - ppl with normal colour vision are called trichromats - 7% of males and 1% of females have a deficiency in the red-green system, the yellow-blue system (dichromat), or both (monochromat), caused by an absence of hue-sensitive photopigment in certain cone types - monochromats see only in black and white Analysis and Reconstruction of Visual Scenes Feature Detectors - From the retina, the optic nerve sends nerve impulses to a visual relay station in the thalamus. From there, the input is routed to various parts of the cortex, particularly the primary visual cortex in the occipital lobe. There are at least 10 mappings from the retina to the brain  insurance against damage - Groups of neurons within the primary visual cortex are organized to receive and integrate sensory nerve impulses originating in specific regions of the retina  4 some of these cells are known as feature detectors  fire selectively in response to stimuli that have specific characteristics  certain neurons fire most frequently based on the orientation of a line (vertical, horizontal, diagonal) - Other classes of feature detectors respond to colour, depth, or movement, thus processing visual scenes using each simultaneously  parallel processing - visual association cortex: here successively more complex features of the visual scene are combined and interpreted in light of our memories and knowledge Audition - the stimuli for hearing are sound waves  sound is pressure waves in air - sound waves have two characteristics: frequency and amplitude - Frequency: the number of sound waves, or cycles, per second. - The hertz (Hz) is the technical measure of cycles per second (1 hertz equals 1 cycle). The higher the hertz the higher the perceived pitch - Amplitude: refers to the vertical size of the sound waves – that is, to the amount of compression and expansion of the molecules in the conducting medium - The sound wave’s amplitude is determined by the sound’s perceived loudness - Expressed in decibels (db), a measure of the physical pressures that occur at the eardrum Auditory Transduction: - The transduction system of the ear is made up of tiny bones, membranes, and liquid-filled tubes designed to translate pressure waves into nerve impulses - Sound waves travel into an auditory canal leading to the eardrum, a movable membrane that vibrates in response to the sound waves - Beyond the eardrum is the middle ear, a cavity housing three tiny bones  the hammer (malleus), anvil (incus) and sti
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