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Chapter 4

PSYA01 Chapter 4 Notes.docx

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Steve Joordens

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Chapter 4: Sensation and Perception Sensory adaptation: sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditions. Our senses encode the information our brains perceive: Sensing light: • Sensation: simple stimulation of a sense organ; basic 1. The length of a light wave determines its hue, or what registration of light, sound, pressure, odor or taste as parts of the body interact with the physical world. humans perceive as color. • Perception: takes place at the level of your brain: it is the 2. The intensity or amplitude of a light wave-how high the organization, identification and interpretation of the peaks are-determines what we perceive as the brightness of light. sensation in order to form a mental representation. 3. Purity is the number of distinct wavelengths that make up • Our senses are the means by which we experience the world; everything we learn is detected by sense organs the light. Purity corresponds to what humans perceive as and transmitted to our brains by sensory nerves saturation, or the richness of colors. The human eye • Senses depend on the process of transduction: what takes place when many sensors in the body convert • Light reflected from a surface enters the eyes via the physical signals from the environment into encoded neural transparent cornea, which bends the light wave and sends signals sent to the CNS. it through the pupil, a hole in the colored part of the eye- Psychophysics: the iris (controls the size of the pupil and hence the amount of light that can enter the eye.) Fechner: developed this approach to measure sensation and • Behind the iris, the thickness and shape of the lens adjust perception: methods that measure the strength of a stimulus and to focus light on the retina (a light sensitive tissue lining the the observer’s sensitivity to that stimulus. back of the eyeball) where the image appears upside down and backward. Absolute threshold: the simplest quantitative measurement minimal intensity needed to just barely detect a stimulus; is a boundary. • Accommodation: process by which the eye maintains a clear image in the retina. JND (just noticeable difference)- the minimal change in a stimulus that can just barely be detected.  weber’s law: the JND • If eyeball is too long, images are focused in front of the of a stimulus is a constant proportion despite variations in intensity. retina, leading to nearsightedness (myopia) Signal detection theory: response to a stimulus depends both on • If eyeball is too short, images are focused behind the a person’s sensitivity to the stimulus in the presence of noise and retina, leading to farsightedness (hyperopia). on a person’s decision criterion. cells, which collect neural signals from the rods and cones and transmit them to the outermost layer of the retina, Phototransduction in the retina where neurons called retinal ganglion cells organize the signals and send them to the brain. • Two types of photoreceptor cells in the retina contain light- sensitive pigments that transduce light into neural • The bundled retinal ganglion cells axons form the optic impulses. nerve, which leaves the eye through a hole in the retina. The hole in the retina doesn’t contain rods or cones and • Human retina contains two general types of hence has no mechanism to sense light and this creates a photoreceptors: 125 million rods (function mainly in dim blind spot, which is a location in the visual field that light; are very sensitive to light but cannot detect change in produces no sensation on the retina. hue. Visual info they provide lacks sharpness) and 6 million Receptive fields cones (function when the level of illumination is bright enough to see things clearly. Responsible for acute daytime vision and for color perception) • Most retinal ganglion cells respond to input not from a single retinal cone or rod but from an entire patch of • Cones detect color, operate under normal daylight adjacent photoreceptors lying side by side, or laterally, in conditions, and allow us to focus on fine detail. the retina. • Rods become active under low-light conditions for night • A particular RGC will respond to light falling anywhere vision. within that small patch, which is called its receptive field, the region of the sensory surface that, when stimulated, • Rods contain the same photo pigment and provide no causes a change in the firing rate of that neuron. information about color and sense only shades of gray. Perceiving colour • Fovea- an area of the retina where vision is the clearest • S.I.Newton color is nothing but our perception of and there are no rods at all. wavelengths from the spectrum of visible light. • the absence of rods in the fovea decreases the sharpness • Shortest visible wavelength purple • Longest visible wavelength red of vision in reduced light. Trichromatic color representation in the cones • The cones are densely packed in the foveathe distribution of cones directly affects visual acuity and explains why • Pattern of responding across the three types of cones objects off to the side, in your peripheral vision, aren’t so provides a unique code for each colour. clear. The light reflecting from those peripheral objects has • A genetic disorder in which one of the cone types is missing and in some very rare cases, two or all three, a difficult time landing in the fovea making the resulting causes a color deficiency a.k.a color blindness. image less clear. Color opponent system • The retina is thick with cells. The photoreceptor cells • Pairs of visual neurons work in opposition. form the innermost layer. The middle layer contains bipolar The visual brain • One interconnected visual system forms a pathway that • Streams of action potentials containing information courses from the occipital visual regions into the lower encoded by the retina travel to the brain along the optic temporal lobe. This ventral stream enables us to identify nerve, what we see. Includes brain areas that represent an • Half of the axons in the optic nerve that leave each eye object’s shape and identity. come from the retinal ganglion cells that code information • Another interconnected pathway travels from the occipital in the right visual field, whereas the other half code lobe through the upper regions of the temporal lobe into the information in the left visual field. parietal regions. This dorsal stream allows us to locate objects, to track their movements, and to move in relation • The two nerve bundles link to the left and right hemispheres respectively. to them. • The optic nerve travels from each eye to the lateral • Visual-form agnosia: brain damage to the ventral geniculate nucleus located in the thalamus. (Thalamus stream; the inability to recognize objects by sight. receives inputs from all of the senses except smell). • Optic ataxia: brain damage to the parietal section of the • From there, the visual signal travels back to the brain, to a dorsal stream; difficulty using vision to guide their location called area V1, the part of the occipital lobe that reaching and grasping movements. contains the primary visual cortex. • There, the information is systematically mapped into a Recognizing what we perceive: representation of the visual scene. • binding problem: how features are linked together so that we see unified objects in our visual world rather than free- Neural systems for perceiving shape • Perceiving shapes depends on the location and orientation floating or miscombined features. of an object’s edges. • Area V1 is specialized for encoding edge orientation. • Illusory conjunction: a perceptual mistake where features • Neurons in the visual cortex selectively respond to bars from multiple objects are incorrectly combined. and edges in specific orientations in space. • Some neurons fire when an object in a vertical orientation • Feature integration theory: the idea that focused is perceived attention is not required to detect the individual features • Other neurons fire when an object in a horizontal that comprise a stimulus BUT is required to bind those individual features together. orientation is perceived • Other neurons fire when objects in a diagonal orientation of • Perceptual constancy: even as aspects of sensory 45 degrees are perceived, and so on. signals change, perception remains constant. Pathways • One brain system identifies people and things and another • Perception is sensitive to changes in stimuli, but perceptual constancies allow us to notice the tracks their movements, or guides our movements in differences in the first place. relation to them. • Two visual streams project from the occipital cortex to visual areas in other parts of the brain. Principles of perceptual organization: before object recognition, the visual system must group the image regions that belong together into a representation of an object. We tend to perceive a unified, whole object rather than a collection of separate parts is • Apparent motion: the perception of movement as a result the foundation of gestalt psychology. of alternating signals appearing in rapid succession in different locations. 1. Simplicity • Change blindness: when people fail to detect changes to 2. Closure: we tend to fill in missing elements of a visual the visual details of a scene. scene, allowing us to perceive edges that are separated by gaps as belonging to complete objects. • Inattentional blindess: a failure to perceive objects that 3. Continuity: edges or contours that have the same are not the focus of attention. orientation have what the Gestaltists called “good continuation” and we tend to group them perceptually. Audition • Vision is based on the spatial pattern of light waves on the 4. Similarity: regions that are similar in color, lightness, retina. shape or texture are perceived as belonging to the same • The sense of hearing is about sound waves- changes in air object. pressure unfolding over
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