CH 3 Flashcards
conduction velocity
signalling fast = magnocellular system
signalling slow= parvocellular system
magnocellular system
- signal from retinal magnocellular ganglion cells
- large neurons
- detects luminance, provides info about location of objects
parvocellular system
- receiving info from retinal parvocellular ganglion cells in retina
- small neurons
- sensitive to colour, discriminates fine details
optic radiation
Axons from the LGN project to primary visual cortex (V1)
Organizational Features of V1
a. Topography
b. Cortical magnification
c. Cortical modularity
d. Receptive fields
Topography
○ Retinotopic organisation : objects that are close to each other in the visual field are represented close to each other in the primary sensory cortex
○ Losing function in the left visual cortex: can’t see on the right side
○ Damage in the occipital cortex corresponds to location in the visual field
cortical magnification
what you focus on: a lot of neurons are responsible for processing that piece of info. Inversement, what I’m not focusing on (parafovially) is being processed by fewer neurons in the occipital cortex
Size is unevenly distributed but our brains make sense of it.
Cortical modularity
neurons close by process the same info
Off bipolar cells
preserve the sign (+/-) of the cone, are hyperpolarised = produce less glutame by light
On bipolar cell
reverse the sign (+/-) of the cone, are depolarised = produce more glutame by light
receptive fields
If a stimulus from a certain area of the visual field elicits the strongest response from a specific neuron, then that area of the visual field is the receptive field for the neuron in question. Neurons can also be especially receptive to a certain orientation of a stimulus (tuning curves
they become more and more complex at the
cortex because multiple LGN cells integrate into simple cells in V1, which integrate into complex cells, which integrate into hypercomplex cells.
—> Neurons become more and more specialized higher up in the visual processing stream, for example the fusiform face area and grandmother cells.
Dorsal stream
The “where” stream. Primary visual cortexàparietal cortex. Analyses space and motion.
Ventral stream
The “what” stream. Primary visual cortexàtemporal cortex. Analyses color and form
Perception
product of active visual processing
Can be described by 5 properties:
a. lightness/brightness
b. colour
c. form
d. depth/distance
e. motion
Lightness and Brightness
The subjective visual experience with light and dark; not objectively measurable. Driven by knowledge of the world
a. Simultaneous lightness/brightness contrast phenomenon
b. Lateral inhibition
c. The inverse problem
Simultaneous lightness/brightness contrast phenomenon
Patches that reflect the same amount of light may appear lighter or darker depending on the lightness of the background because there is more local contrast against a darker background
Lateral inhibition
The most active ganglion cells try to inhibit the ones next to them
the center of a receptive field is LESS ACTIVE with lighter backgrounds:
—> for the image with the darker background, retinal ganglia are highly active and do not inhibit the center
—> for the light background, retinal ganglia are less active and inhibit the center.
The inverse problem
The visual image on our retina can be the result of many
different sources.
In other words, many different sources can produce the same image on the retina
Colour
colours depend on the wavelength of light that is not absorbed, no reflected
seeing colours is based on 3 different types of cones with different properties
colour: created and processed in V4
3 perceptual qualities:
a. hue = actual colour
b. saturation = how colourful something is
c. brightness
3 different types of cones
each with their own absorption properties: able to absorb short, medium or long wavelengths
humans are trichromatic: have 3 types of cones
damage to V4: cerebral achromatopsia: can’t see colour
colour contrast
the same colour (wavelengths) can be perceived as different depending on the surrounding colours
colour constancy
different colours can be perceived as the same depending on the surrounding colours
Form
involves perception of :
a. line lengths
b. their orientation
c. their intersections.
—> shape depends on what your brain does, rather than the actual physical properties.
Size: Different sizes of shapes cause different activations of areas in V1. The activation depends on perceived size, not actual size.
Distance and depth
we perceive a 3D world from 2D images on the retina
depth perception depends on:
a. monocular component (one eye)
b. binocular components (both eyes)
monocular component
depends on learned cues
a. occlusion: shapes in the back are blocked by shapes in the front
b. relationship between size and distance: objects that are far away appear smaller
c. motion parallax: closer objects appear to move faster
Binocular component
since our eyes are about 6 cm apart, they create slightly different images from slightly different angles = retinal disparity
occlusion
shapes in the back are blocked by shapes in the front
relationship between size and distance
objects that are far away appear smaller
motion parallax
closer objects seem to move faster
binocular rivalry
Each eye receives a completely different
image. The experienced percept is not one that fuses both
images
Cyclopean fusion
occurring in V1, brings the images together
Motion
The subjective experience when a sequence of different but related images is presented to the retina over a short time
processed in V5
a. aperture problem
b. apparent motion
c. motion aftereffect
aperture problem
many different combinations of speed and direction may elicit the same perception of a motion
Apparent motion
Similar static images are presented closely in time give the illusion of movement.
Motion aftereffect
result of neural adaptation wherein you perceive movement in the opposite direction when looking away from a movement after a relatively long time.
Neurons respond less and less when observing the same movement for a long period of time, so looking away causes non adapted neurons to become more active, creating an illusion of motion in the opposite direction
