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
