Vision Flashcards
sensation
registration of physical stimuli from the environment by the sensory organs
perception
the interpretations of sensations by the brain
law of specific nerve energies
activity by a particular nerve always conveys the same type of info to the brain.
what is light?
Range of electromagnetic energy that is visible to humans:
-about 400 nanometers (violet) to 700 nanometers (red)
-nanometer (nm): one-billionth of a meter
blind spot:
region of retina without photoreceptors (optic nerve, blood vessels)
fovea
specialized centre of retina specialized for high acuity
myopia
Inability to bring distant objects into clear focus
Focal point of light falls short of the retina
hyperopia
Inability to focus on near objects
Focal point of light falls beyond the retina
presbyopia
Common form of hyperopia seen in older adults.
rods
Sensitve to low levels of light (dim light)
Used mainly for night vision
One type of pigment only
cones
Highly responsive to bright light
Specialized for color and high visual acuity
Located in the fovea only
Three types of pigment (RED, GREEN, BLUE)
phototopic
“Bright lights vision”
High acuity
Low sensitivity with few receptors
Low convergence
Fovea
Cones (RGB)
scotopic
dark/dim vision
Low acuity
High sensitivity with many receptors
High convergence
Periphery
Rods (only one pigment)
colourblindness
Genetic: sex-linked (X-chromosome)
5-8% of males are colourblind
0.5% of females are colourblind
Red-green colour blindness is more common than blue-yellow colourblindness
Result of some kind of anomaly in cones (e.g., low number or a complete lack of a particular cone)
trichromacy
Explanation of colour vision based on the coding of three primary colours: red, green, blue
Color we see is determined by the relative responses of the different cone types.
Pro: can explain different types of colorblind
Con: cannot explain afterimages
opponent-process theory
Emphasizes the importance of opposite colours
Red-green continuum
Blue-yellow continuum
Opponent processing in retinal ganglion cells
on-off/center-surround receptive fields
Approx. 60% of retinal ganglion cells
colour/brightness constancy
the ability to recognize colour/brightness despite changes in lighting is not easily explained by these theories
retinex theory
suggests the cortex compares info from various parts of the retina to determine the brightness and color for each area
magnocellular cell (M cell)
magno= large body
Receives input primarily from rods (evenly distributed throughout the retina)
Sensitive to light and moving stimuli
parvocellular cell (P-cell)
parvo= small body
Receives input primarily from cones (mostly foveal)
Sensitive to colour
geniculostriate system
Projections from the retina to the lateral geniculate nucleus to the visual cortex (striate cortex, primary visual cortex, V1)
tectopulvinar system
Projections from the retina to the superior colliculus to the pulvinar (thalamus) to the parietal and temporal visual areas.
dorsal visual stream “how”
Originates in the occipital cortex and projects to the parietal cortex
Action pathway- grasping, reaching, etc
ventral visual stream “what”
Originates in the occipital cortex and projects to the temporal cortex
Object recognition
optic ataxia
Difficulties making reaching movements, but otherwise normal vision
visual agnosia
Difficulties with recognizing objects, but otherwise normal vision.
medial pulvinar
projects to the parietal lobe
lateral pulvinar
projects to the temporal lobe
striate cortex
Cellular oganization that represents a functional unit
6 cortical layers deep and aprox. 0.5mm2
Information from each LGN is sent to adjacent cortical columns, thus maintaining the separation of into from each retina.
occipital (visual) cortex
At least 6 different visual reasons
primary visual cortex (V1)
Striate (striped) cortex
Receives input from the LGN (thalamus)
extrastriate cortex
Visual cortex areas outside the striate cortex
receptive fields
Region in the visual world that stimulates a receptor cell or neuron
Ganglion cells have RF on the retina
Coding location: light shone in one place on the retina will activate one ganglion cell, and light shone in another place will activate a different ganglion cell.
topographic map
Neural representation of the external world
Cells in the LGN and in the cortex also have RFs
More cortical tissue is devoted to cells in the fovea than in the periphery.
retinal ganglion cells
respond simply to the presence or absence of light in their receptive field; there is no coding of shape
Center-surround mechanism
On-center cells
Off-center cells
Retinal cells code edges, what about V1?
Primary Visual Cortex (V1)
Cells in V1 detect orientations
Excited by bars of light oriented in particular directions
simple cells
RF has a rectangular on-off arrangement
- The more light that shines in the excitatry zone, the more the cell responds.
The more light that shines in the inhibitory zone, the less the cell responds.
complex cells
Maximally excited by bars of light moving in a particular direction through the RF
hypercomplex cells
Maximally responsive to moving bars but theu also have a strong inhibitory area at one end of its RF.
temporal cortex
Maximally excited by complex visual stimuli (e,g, faces, scenes, houses)
Neurons in the temporal lobe alter their preference with experience (expertise)
Temporal cortex: respnods to objects (e.g. faces) when they are consciously recognized.
