Vision Flashcards
What do sensory receptors do?
Convert stimuli into useable information
What do vision/audition/somatosensory sensory receptors do?
Vision (light waves to chemical energy)
Audition (sound waves to mechanical energy)
Somatosensory (touch and pressure to mechanical energy)
Sensation
Registration by the sensory organs of physical stimuli from the environment
Perception
Subjective interpretation of sensations by the brain
Retina
Light-sensitive surface at the back of the eye consisting of neurons and photoreceptor cells
Fovea
Central region of the retina specialized for high visual acuity
Rod-free area (cones are most dense here)
Center of retina with densely packed photoreceptors (contributes to vision being better in the centre of visual field than the periphery)
Blind spot
Retinal region where axons forming the optic nerve leave the eye and where blood vessels enter and leave - No Photoreceptors!
Rod
Photoreceptor specialized for functioning in low light
Scotopic (active in low light, inactive in bright light)
Sensitive to small changes in brightness
Low acuity
Insensitive to colour/detail
High convergence of info to ganglion cells
Cone
Photoreceptor specialized for colour & high visual acuity
Photopic (inactive in low light, active in bright light)
Insensitive to small changes in brightness
High acuity
Sensitive to colour/detail
Low convergence of info to ganglion cells
What is the wavelength range of electromagnetic radiation that is visible to the human eye?
About 400 to 700 nm
400 (deep purple)
700 (red)
Violet < Blue < Green < Yellow < Orange < Red
Retinal neurons
Bipolar cell
Horizontal cell
Amacrine cell
Retinal ganglion cell (RGC)
Bipolar cell
Receives input from photoreceptors
Horizontal cell
Links photoreceptors and bipolar cells
Amacrine cell
Links bipolar cells and ganglion cells
Retinal ganglion cell (RCG)
Gives rise to optic nerve
Two (primary) visual pathways
Geniculostriate system (all the P and some M ganglion axons)
Tectopulvinar system (remaining M ganglion axons)
Result of RGC axons separating upon entering the brain
Tectopulvinar system
Pathway projections from the retina to the superior colliculus (midbrain) to the pulvinar region of the thalamus, then to to the parietal and temporal visual areas
From the eye, through the midbrain tectum, to the pulvinar (then to parietal and temporal lobe)
Bypasses the occipital visual areas
Geniculostriate system
Pathway projections go from the retina to the lateral geniculate nucleus of thalamus (LGN of thalamus) to layer IV of the primary visual cortex (V1; BA17)
The geniculostriate system bridges the thalamus (geniculate) and the striate cortex
Parvocellular cell (P layers)
[Geniculostriate Pathway]
(Parvo- = small)
Receives input mostly from cones
Sensitive to colour
Magnocellular cell (M layers)
[Geniculostriate Pathway]
(Magno- = large)
Receives input mostly from rods
Sensitive to light and moving stimuli
Describe the geniculostriate pathway
Info travels from right side of each retina to right LGN
or the left side of each retina to the left LGN
- allows to combine info from the two eyes and to segregate info from P and M ganglion cells
(6 thalamic LGN layers)
Info from contralateral side goes to layers 1, 4, 6
Info from ipsilateral side goes to layers 2, 3, 5
Layers 1 and 2 receive inout from magnocellular pathway
Layers 3 - 6 receive input from parvocellular pathway
What does V1 refer to?
Striate cortex (heterogenous, having neurons processing different information)
Visual pathways beyond the occipital lobe
Ventral stream
Dorsal stream
Ventral stream
“What” pathway
Pathway to the temporal lobe
Recognizing objects; global features vs. one feature
Recognize things
Dorsal stream
“Where/how” pathway
Pathway to parietal lobe
Sensitive to movement
Spatial ability
Where in space
How do I interact with them
Describe the receptive-field hierarchy
(moving from eyes to brain)
Receptive fields of many retinal ganglion cells:
Combine to form a receptive field of a single LGN cell:
The receptive fields of many LGN cells combine to form the receptive field of a single V1 cell
Where are the central/peripheral/top areas of the visual field represented in the brain?
Central = back of the brain
Peripheral = more anteriorly
Top = lower part of occipital
Different types of neurons and areas throughout the visual system that contribute to shape/colour
Retinal ganglion cells
Primary visual cortex
Temporal cortex
Retinal ganglion cells (RGC)
Shape is contructed in cortex from info from RGCs, as RGCs only send info about edges
RGCs do not detect shape, only light
Receptive field of a ganglion cell is concentric circles
- on-centre, off-centre
Luminance contrast
Amount of light an object reflects relative to its surroundings
- A ganglion cell tells the brain about the amount of light hitting a certain part of the retina relative to the average amount of light hitting the rest of the retina
What do RGCs send info regarding?
Send info about edges
The info from RGCs sent to the visual areas of the brain are not treated equally from all visual field regions
- Regions that have differences in luminance (areas along edges) are emphasized (edges form shapes)
How are shapes processed in V1?
Each V1 cell receives input from several RGCs
Orientation detectors - maximally responsive to orientation of bars of light
Simple cells (V1)
On-off receptive field arrangement
Complex cells (V1)
Receptive fields maximally excited by bars of light moving in a particular direction
Hypercomplex cells
Receptive fields maximally excited by moving bars of light (like complex) but also have a strong inhibitory area at one end of the receptive field
When does simple cell/complex cell/hypercomplex cell fire?
Simple: if light is along its “on” receptive field
Complex: if stimuli is at a certain angle to its circular receptive field
Hypercomplex: horizontal, moving light in its “on” receptive field
Stimulus equivalence
Recognizing that an object is the same across different viewing orientations
In terms of processing shapes in temporal cortex, what are the cells maximally excited by?
Complex visual stimuli (like faces)
Temporal lobe = less hard wired, more flexible
Trichromatic theory
Explanation of colour vision based on the coding of the primary colours: red, green, and blue
- colour we see if determined by the relative responses of the different cone types
- if all active, we see white
- explains colour blindness/deficiencies
Opponent processing
Explanation of colour vision that emphasizes the importance of the opposition of colours
- The beginning of colour processing in the cones follows trichromatic theory, opponent processing occurs afterwards
Colour constancy
The ability to perceive the colour of an object as remaining the same, even when the lighting changes
(bowl of fruit and green glasses example)
Muller cells
Run from front of front of retina to the back
Optic chiasm
Optic nerves partly cross before entering the brain
Medial path of each retina (nasal retina) crosses to the opposite side
Lateral path (temporal retina) travels back on the same side
Meaning info from the left visual field goes to the right hemisphere of the brain
Info from the right visual field goes to the left hemisphere of the brain
Both the tectopulivinar and geniculostriate pathways contribute to what two streams?
Dorsal and lateral (ventral) streams
What are cells in V1 (which receive inputs from LGN, and there from RGC’s) called?
Orientation detectors - maximally excited by bars of light oriented in a particular direction rather than by spots of light
