Visual Perception Flashcards
Akinetopsia:
- Inability to perceive motion even though other aspects of vision seem normal
- Can see difference in position (object that was here is now over there), but doesn’t see the object moving
Retina
◦ Light sensitive tissue lining back of eyeball
◦ Fovea (center of retina)
Cornea and lens
◦ Focus incoming light
◦ Muscle around lens tightens to bulge lens to focus nearby objects
◦ Muscle around lens relaxes to flatten lens to focus far away objects
Photoreceptors:
Located on retina
Rods and Cones
Rons
◦ Sensitive to low levels of light
◦ Can distinguish different intensities of light
◦ Can’t distinguish colour
◦ More rods than cons farther away from fovea
Cones
◦ Less sensitive than rods
◦ Need more incoming light to operate
◦ Sensitive to colour differences
◦ 3 different types, each respond to different wavelengths
◦ Important for acuity
• Ability to see detail
◦ In fovea, cons far out number rods (no rods at all in center of fovea)
Bipolar cells
Intermediate cells that are stimulated by photoreceptors which then excite ganglion cells
Ganglion cells
Spread uniformly across retina, but axons converge to form optic nerve
Lateral Geniculate Nucleus (LGN)
◦ Info from optic nerve sent here first, then transmitted to occipital lobe
◦ Located in thalamus
Lateral Inhibition
Once stimulated, cells inhibit activity of neighbouring cells
Edge enhancement
Lateral inhibition highlights a surfaces edge because cells in the middle will be inhibited more than cells at the edge
Single Cell Recording:
- Animal is immobilized and has electrodes placed outside optic nerve or brain
- Various patterns on a computer flashed in front of animals eyes
- Used to define a cell’s receptive field (size and shape of area in visual that the cell responds to)
Receptive Fields:
Hubel and Wiesel discovered existence of specialized neurons, each with a different type of receptive field and different kind of visual trigger
Center-surround cells
◦ Center has one response, surrounding ring has opposite response
◦ If both center and surrounding are stimulated, they cancel each other out
Edge detectors
◦ Fire at maximum only when a stimulus has an edge in a specific orientation (i.e. horizontal, vertical)
◦ Will still fire when they detect a stimulus orientated in a different way, but not as strongly
Parallel Processing: Area V1
◦ Site on occipital lobe where axons from LGN first reach cortex
◦ Contains cells to detect every kind of stimulus (horizontal, vertical…etc)
Parallel Processing: Area MT
Neurons are acutely sensitive to direction and speed of movement
Parallel Processing: V4
Neurons most sensitive to certain colour and certain shapes
P Cells
- Provide main input for LGN’s parvocellular cells
* Specialized for spatial analysis and detailed analysis of form
M Cells
- Provide input for LGN’s magnocellular cells
* Specialized for detection of motion and perception of depth
Ventral Stream:
What Stream
• Occipital lobe-> temporal lobe
• Crucial for identification of visual objects
• Damage can cause visual agnosia
◦ Inability to recognize objects in visual field
Where System/ Dorsal Stream:
- Occipital lobe-> parietal lobe
- Crucial for locating object in space
- Damage can result in difficulty reaching out for objects
Binding Problem:
• Task of reuniting various elements of a scene that are initially addressed by different systems in different parts of the brain
Spatial Position:
- Various brain areas all keep track of where an object is
* i.e. where a circle is, where the blueness is, where motion was detected, where things were still
Neural Synchrony:
- If neurons detecting 2 different attributes are firing together, then they must be detecting the same object
- i.e. vertical line moving down stimulates neurons detecting orientation, and neurons detecting movement, therefore the are detecting the same object
Attention:
- Attention important aspect in binding different features of a stimulus together
- Synchronized neural firing observed when paying attention to a stimulus, not observed in neurons activated by an unattended stimulus
Conjunction Errors
◦ Correctly detecting the features present in a visual display, but making mistakes about how the features are bound together
◦ i.e. Someone shown blue H and red T but reports seeing a blue T and red H
Gestalt Psychologists:
• Suggested that organization of perception of visual world is contributed by perceiver
• Explains why perception of a stimulus can differ from the stimulus itself
• Ie. Necker cube
◦ Hand drawn cube that can be viewed in 2 different ways (reversible)
Figure/Ground Organization:
Determination of what is the figure (object) and what is the ground
Proximity/Similarity:
- Perception guided by principles such as proximity and similarity
- If you see 2 objects that are close together and resemble each other, you assume these elements are part of the same object
Perceptual Constancy:
- We perceive the constant properties of objects in the world even though the sensory information we receive about these attributes changes whenever our viewing circumstances change
- i.e. object getting bigger as you get closer, but still knowing that its is still the same size
Size Constancy
Correctly perceiving the sizes of objects in the world despite changes in retinal image size created by changes in viewing distance
Shape Constancy
Correctly perceive shapes of objects despite changes in the retinal image created by shifts in viewing angle
Brightness Constancy
Correctly perceive the brightness of objects whether they’re illuminated by dim or bright light
Unconscious Inference:
- Unconscious calculation done to perceive elements correctly
- Size of image on retina x distance between you and object = size
- Also occurs for shape and brightness constancy
- Is the reason you misinterpret optical illusions
Binocular Cues:
Distance Cues
Binocular Disparity
Distance cues
Features of stimulus that indicate an object’s position
Binocular disparity
◦ Difference between 2 eyes’ views
◦ Provides info about distance relationships in the world
Monocular Cues:
- Depth cues that depend only on what each eye sees by itself
- Depends on adjustment that the eye must make to see the world clearly
- Lots of adjustment for nearby objects, little adjustment for objects far away
Pictorial cues
Creating an impression of depth on a flat surface (i.e. in pictures)
Interposition
Linear Perspective
Interposition
Blocking of your view of one object by another object
Linear Perspective
Pattern in which parallel lines seem to converge as they get farther and farther away
Motion Parallax
Projected images of nearby objects move more than those of distant ones
Optic Flow
Pattern of stimulation across visual field also changes as you move toward object
