Perception and Visual Illusions

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Definitions about perception, the eye, visual illusions, and constancy
Steph M
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Steph M
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Sensation The detection of external stimuli and the transmission of this info to the brain
Perception The processing, organisation, and interpretation of sensory signals
Bottom-Up Processing Perception based on the physical features of the stimulus
Top-Down Processing How knowledge, expectations, or past experiences shape the interpretation of sensory info
Transduction The process by which sensory stimuli are converted to signals the brain can interpret
Sensory Coding The process of our sensory systems translating physical stimuli into neural impulses
Sensory Receptors These receive physical or chemical stimulation and pass the resulting impulses to the brain in the form of neural impulses
Qualitative Info that consists of the most basic qualities of a stimulus
Quantitative Info that consists of the degree, or magnitude, of those qualities
Absolute Threshold The minimum intensity of stimulation that must occur before you experience a sensation
Difference Threshold The minimum amount of change required for a person to detect a difference between two stimuli
Psychophysics A subfield developed during the 19th century that examines our psychological experiences of physical stimuli
Weber's Law States that the just noticeable difference between two stimuli is based on the proportion of the original stimulus rather than on a fixed amount of difference - The more intense the stimulus the bigger change needed for you to notice
"Hit" If the signal is present and the participant detects it
"Miss" If the participant fails to detect the signal
"False Alarm" If the participant detects a signal that was not presented
"Correct Rejection" If the signal is not presented and the participant does not detect it
Response Bias A participant's tendency to report detecting the signal in an ambiguous trial
Signal Detection Theory (SDT) A theory of perception based on the idea that the detection of a stimulus requires a judgement, it is not an all-or-nothing process
Sensory Adaption A decrease in sensitivity to a constant level of stimulation
Cornea Light first passes through the eyes thick, transparent outer layer and focuses the incoming light
Lens The focused light from the cornea enters the lens and is bent further inward and focused onto the retina
Retina The thin inner surface of the back of the eyeball, it contains the sensory receptors that transduce light into neural signals
Pupil The dark circle at the centre of the eye, a small opening in the front of the lens, by contracting or dilating the pupil determines how much light enters the eye
Iris A circular muscle, that determines the eye's colour and controls the pupil's size
Accommodation When muscles change the shape of the lens to focus on objects that are close or far away
Rods Retinal cells that respond to low levels of lights and result in black-and-white perception (night vision)
Cones Retinal cells that respond to higher levels of light and result in colour perception (day vision)
Fovea The centre of the retina, where cones are densely packed
Photopigments Protein molecules that become unstable and split apart when exposed to light, this decomposition alters the membrane potential of the photoreceptors and triggers action potentials in downstream neurons
Ganglion Cells The first neurons in the visual pathway with axons, they are the first neurons to generate action potentials
Optic Nerve A bundle of axons which exits the eye at the back of the retina, there are no rods or cones where the optic nerve exits the eye, producing a blind spot in each eye
Optic Chiasm Where half the axons in the optic nerve cross, causing all info from the left side of visual space to be projected to the right hemisphere of the brain and vice versa
Ventral Stream Appears to be specialised for the perception and recognition of objects
Dorsal Stream Appears to be specialised for spatial perception
Object Agnosia The inability to recognise objects
Trichromatic Theory Colour vision results from activity in three different types of cones, each receptor is sensitive to different wave lengths
Opponent-Process Theory According to this theory, red and green are opponent colours so when we stare at a red image for some time, we see a green after image, creating the perception the red and green are opponents
Hue Consists of the distinctive characteristics that place a particular colour in the spectrum, dependent primarily on the light's dominant wavelength when it reaches the eye
Saturation Is the purity of the colour, this varies according to the mixture of wavelengths in a stimulus (more wavelengths = less pure)
Brightness Is the colour's perceived intensity, mostly determined by the total amount of light reaching the eye
Lightness Is determined by the brightness of the stimulus relative to its surroundings
Gestalt Principles These explain why we perceive sensory info as organised whole objects
Proximity The closer two figures are to each other, the more likely we are to group them together and seem them as part of the same object
Similarity We tend to group figures according to how closely they resemble each other, whether in shape, colour, or orentation
Continuity We tend to group together edges or contours that have the same orientation, it appears to play a role in completing an object behind anything that hides a portion of the object
Closure We tend to complete figures that have gaps
Illusory Contours We sometimes perceive contours and cues to depth even when they do not exist
Figure and Ground In identifying either figure, the brain assigns the rest of the scene to the background, assignment of figure and ground is ambiguous and the figures periodically reverse
Prosopagnosia Deficits in the ability to recognise faces but not in the ability to recognise objects
Fusiform Gyrus A region of the brain in the right hemisphere which may be specialised for perceiving faces
Binocular Depth Cues Cues of depth perception that arise from the fact that people have two eyes
Monocular Depth Cues Cues of depth perception that are available to each eye alone
Binocular Disparity A depth cue: because of the distance between the two eyes, each eye receives a slightly different retinal image
Steroscopic Vision The ability to determine an object's depth based on that object's projections to each eye
Convergence A cue of binocular depth perception; when a person views a nearby object, the eye muscles turn the eyes inward
Occlusion A near object occludes (blocks) an object that is farther away
Relative Size Far-off objects project a smaller retinal image than close objects do, if the far-off and close objects are the same physical size
Familiar Size Because we know how large familiar objects are, we can tell how far away they are by the size of their retinal images
Linear Perspective Seemingly parallel lines appear to converge in the distance
Texture Gradient As a uniformly textured surface recedes, its texture continuously becomes denser
Position Relative to Horizon - Objects below the horizon that appear higher in the visual field are perceived as being farther away - Objects above the horizon that appear lower in the visual field are perceived as being farther away
The Ames Box Ames boxes play with linear perspective and other distance cues, one room makes a far corner appear the same distance as a near corner
The Ponzo Illusion Two horizontal lines appear to be different sizes but are actually the same length, the two lines drawn together trick your brain into thinking they are parallel and receding into the distance
Motion Aftereffects May occur when you gaze at a moving image for a long time and then look at a stationary scene, you experience a momentary impression that the new scene is moving in the opposite direction from the moving image
Stroboscopic Motion Perception A perceptual illusion that occurs when two or more slightly different images are presented in rapid succession and are perceived as being in motion
Object Constancy Correctly perceiving objects as constant in their shape, size, colour, and lightness, despite raw sensory data that could mislead perception
Size Constancy - Correctly perceiving objects as constant in their size, despite raw sensory data that could mislead perception - Size constancy relies on knowing how far away the object is from us
Shape Constancy - Correctly perceiving objects as constant in their shape, despite raw sensory data that could mislead perception - Shape constancy relies on knowing what angle or angles we are seeing the object from
Colour Constancy - Correctly perceiving objects as constant in their colour, despite raw sensory data that could mislead perception - Colour constancy relies on comparing the wavelengths of light reflected from the object with those reflected from its background
Lightness Constancy - Correctly perceiving objects as constant in their lightness, despite raw sensory data that could mislead perception - Lightness constancy relies on knowing how much light is being reflected from the object and its background
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