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Perception and Sensation

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PSYC 2510
Agnieszka Kopinska

SENSATION & PERCEPTION  Sensation – the stimulation of sense organs (e.g. ear, taste bud)  Stimulus – any detectable input from the environment  Threshold - the point at which a stimulus can first be detected  as stimulus intensity goes up, the probability of responding to stimulus gradually increases  differences in detection of stimulus in point to differences in threshold – e.g. dog whistles  Absolute threshold – the intensity level at which the probability of detecting stimulus is 50% under ideal conditions  Just noticeable difference (JND) – the smallest difference that can be detected between two intensity levels of a stimulus  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  called the Weber fraction  not entirely accurate  Fechner’s law – the magnitude of a subjective sensation increases proportionally to the stimulus intensity, only to a certain point  Also referred to as “sensory scaling”  Many sensory stimuli do not fit Weber’s and Fechner’s laws!  Signal-detection theory  The detection of stimuli depends on decision-making processes and 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 Does not detect Miss Correct rejection  Sensory adaptation – a gradual decline in sensitivity due to prolonged stimulation  We can forget about current stimulus but forget about it to focus on more important things  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 THE VISUAL SYSTEM  Vision has two functions  Creating an internal representation of the external world  Guiding your actions based on this info  Light waves shape how we perceive color  Main properties of light waves: amplitude, wavelength and purity  Perceived as: brightness, hue and saturation respectively  The eye is instrumental to vision!  Lens – focuses image on the retina  Essential for accommodation  Put things in/out of focus  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  Retina – neural tissue (part of CNS) that lines the back inner surface of the eye; it processes images and relays information to the brain to the optic nerve  Optic disk – exit point of optic nerves where an image cannot be reflected – a blind spot  Rods – photoreceptors that allow us to see at night (in low light conditions) and contribute to peripheral vision  Cones – photoreceptors that allow us to perceive colors  Only about 10% of light reaches these receptors  We have about 20x more rods than cones  Fovea – the center of the retina that contains only cones  Visual acuity (sharpness of what you see) is best in this region  Responsible for central vision – any activity that require attention to detail (e.g. reading)  Eye saccades - constantly occurring eye movements  Prevents sensory adaptation (images would disappear)  Dark/light adaptation – the processes in which our eyes become more/less sensitive to light in low/high illumination  full adaptation takes about 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  left axons go to left hemisphere, and vise-versa  Pathway 1  Optic chiasm -> lateral geniculate nucleus (LGN) -> primary visual cortex (occipital lobe)  LGN includes specialized channels used in parallel processing  Pathway 2  Optic chiasm -> superior colliculus (midbrain) -> thalamus -> occipital lobe  Visual info processing in primary visual cortex is highly specialized  Sensitive to specific features of visual stimuli  Simple cells – need to be exposed to exact specific thing they’re made to respond to  Complex cells  Primary visual cortex -> further processing in other regions of the cortex  Ventral stream – perception of form and color (vision for perception)  Dorsal stream – perception of motion and depth (vision for action)  Summary  Light source/image  cornea  pupil  lens  retina (rods & cones)  optic nerve  optic chiasm  LGN (thalamus)  primary visual cortex  ventral or dorsal stream  OR … optic chiasm  midbrain  thalamus  primary visual cortex  ventral or dorsal stream  Color vision  Humans can detect about 1 million colors  Combinations of wavelengths result in perception of various colours  Subtractive colour mixings – colors get absorbed (mix them all up to get black)  Additive color mixing – colors are added (mix them all up to get white)  Von Helmholz’s Trichromatic theory of color vision  Three types of cones on the human retina (blue, red and green) allow us to see all colors  “color-blindness” linked to having only 2 of the 3 types of colors  the after-image phenomenon doesn’t support this theory  Hering’s Opponent process theory of color vision  Color perception depends on receptors that involve 3 pairs of colors opposing each other:  Red-green  Blue-yellow  Black-white  Supports after-image phenomenon  Both theories are accurate in some ways  The eye had 3 types of cones (red, blue, green)  Cells in the retina, the LGN, and the visual cortex responds to some colors and are inhibited by another (e.g. red vs. green)  Perception – the selection, organization and interpretation of sensory info  Highly subjective  Influenced by perceptual sets – a predisposition to perceive a stimulus in a certain way  Shapes inattentional blindness – disregarding completely another aspect of a stimulus  How we organize visual information also shapes our perception  Feature analysis – perceiving th
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