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Chapter 4

Chapter 4 Sensation & Perception.pdf

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PSYC 2510
Agnieszka Kopinska

Chapter 4: Sensation & Perception Monday, October 17, 201112:00 PM Sensation vs. Perception - Sensation-- the stimulation of sense organs (e.g., ear, taste bud) - Perception-- the selection, organization, & interpretation of sensory info Sensation - Stimulus-- any detectable input from the environment - Threshold-- the point at which a stimulus can first be detected - As stimulus intensity increases, probability of responding to stimulus gradually increases - Absolute threshold-- the intensity level at which the probability of detecting stimulus is 50% (50% of the time) - Just noticeable difference (JND)-- the smallest difference that can be detected between 2 intensity levels of a stimulus ○ Varies by sense - Weber's Law-- the change in stimulus that will be just noticeable is a constant ratio of the original stimulus ○ Does not always hold for stimuli of extreme intensity ○ Weber fraction ○ Example:  Weber fraction for perception of weight is 1/30  JND is 10g for a 300g weight (300*1/30=30g)  JND for a 900g weight is 30g (900*1/30=30g) instead of 10g. - Fechner's Law-- the magnitude of a subjective sensation increases proportionally to the stimulus intensity, but only to a certain point ○ Sensory scaling - Many sensory stimuli do NOT fit Weber's and Fechner's laws ○ E.g., electricity - Signal-detection theory ○ The detection of stimuli depends on decision-making processes & sensory processes ○ These processes are affected by factors beyond stimulus intensity (e.g., fatigue) ○ The individual doing the detecting is not passive Event Signal Noise Detect Hit False alarm Response Does not Miss Correct detect rejection - Sensory adaptation-- a gradual decline in sensitivity due to prolonged stimulation ○ Demonstrated with all senses - Subliminal messages ○ Messages beyond conscious awareness can, to a limited extent, influence behavior - Awareness is required for a stimulus to significantly influence behavior ○ E.g., Judas Priest trial The Visual System - Vision has two functions Creating an internal representation of the external world Unit 1 Page 1 ○ Creating an internal representation of the external world ○ Guiding your actions based on this information - Light is essential for us to see - Light waves shape how we perceive color Main properties of light waves: amplitude, wavelength, & purity - - Perceived as: brightness, hue (color) and saturation (shades), respectively - The eye is instrumental to vision - Lens ○ Essential for accommodation - Pupil ○ Constricts or dilates to let in less or more light ○ Trade-off between amount of light allowed and sharpness of image projected onto retina - Eye saccades: ○ Constantly occurring eye movements ○ Prevents sensory adaptation (where images would disappear) - Retina ○ Neural tissue (part of CNS) that lines the back inner surface of the eye ○ Processes images ○ Relays visual info to the brain - Optic disk ○ Exit point of optic nerves ○ Blind spot because an image cannot be reflected on this part (it's a hole) - Rods and cones ○ Photoreceptors (light-sensitive receptors) ○ Only ~10% of light reaches these receptors ○ We have about 20x more rods than cones ○ Rods-- allow us to see at night (in low light conditions) and contribute to peripheral vision ○ Cones-- allow us to perceive colors - Fovea ○ Center of the retina ○ Contains only cones ○ Visual acuity is best in this region - Dark/light adaptation ○ The processes in which our eyes become more/less sensitive to light in low/high illumination ○ Full adaptation takes ~30 minutes - Information processing ○ Retina -> rods and cones -> neural impulses via optic nerve -> brain ○ Receptive field-- the specific rods and cones that affect the firing of a given sensory neuron when stimulated ○ Lateral antagonism-- when neural activity in a cell opposes activity in surrounding cells - Optic chiasm ○ Point where optic nerves cross ("X") ○ Left axons go to left hemisphere, and vice-versa ○ Information from inner part of retina is processed in opposite hemisphere (crisscross) ○ Information from outer part of retina is processed in same hemisphere - Pathway 1: ○ Optic chiasm -> lateral geniculate nucleus (LGN) -> primary visual cortex (occipital lobe) ○ LGN includes specialized channels used in parallel processing  E.g., magnocellular and parvocellular channels - Pathway 2: Unit 1 Page 2 - Pathway 2: ○ Optic chiasm -> superior colliculus (midbrain) -> thalamus -> occipital lobe ○ Travelling through midbrain allows for coordination of visual input with other sensory input - Visual information processing in primary visual cortex is highly specialized ○ Sensitive to specific features of visual stimuli ○ Simple cells ○ Complex cells - Primary visual cortex -> further processing in other regions of the cortex ○ Ventral stream-- perception of form and color (vision for perception)  Some cells respond to faces, face-like structures, or specific body parts ○ Dorsal stream-- perception of motion and depth (vision for action) - Summary: ○ 1) Light source/image -> cornea -> pupil -> lens -> retina (rods & cones) -> optic nerve -> optic chiasm -> LGN (thalamus) -> primary visual cortex -> ventral or dorsal stream ○ 2) … -> midbrain -> thalamus -> primary visual cortex -> ventral or dorsal stream Color Vision - Humans can detect ~1 million colors - Combinations of wavelengths result in perception of various colors ○ Subtractive color mixing (paint) ○ Additive color mixing (light) - Von Helmholz's Trichromatic theory of color vision ○ 3 types of cones
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