Sensory Five - Vision part one Flashcards
What is the black box analogy of vision?
Input = light (through cornea and lens) Processing = (photosensitive cells, parrallel processing and receptive fields) Output = Perception
Where does processing of light occur first?
In the retina. Contains the sensory cells and an array of ganglion to process information.
What captures the image and perceives it?
The eye captures the image and the brain perceives it.
What is vision?
The process of detecting components of light
What is the spectrum of visible light to humans?
390 to 750nm
Do we perceive everything in our vision?
No, only that of which we are paying attention to
What is perception?
The ability to gain knowledge through the senses
Describe the basic anatomy of the human eye:
Ant: - Cornea \+ Aquous humor - Iris (plus cillary body) - Lens \+ Vitreous humor - Retina - Optic fibres
How does light enter the eye?
The cornea and lens channel light onto the retina (contains photosensitive cells)
What is unique about the retina?
It is divided in two.
Each eye has a Temporal and nasal side.
Each LGN receives input from the temporal side of one lens and the nasal side of the other. (i.e the left side of both retinas)
Describe the fibres pathway of the retina
Some fibres cross over through the optic chiasm before rejoining other fibres and innervating the LGN and then more optic fibers onto the Visual cortex
What is unique about each visual cortex?
The right visual cortex processes only information from the right side of both retinas and vice versa.
What is the visual field?
Portion of the visual field that can be seen at one time.
What is the total field?
The sum of right and left hemifields
Consists of one binocular zone (where visual fields of each individual eye cross) and two monocular fields (peripheral vision)
What is the LGN?
Lateral Geniculate Nucleus
Lesions upon which part of the pathways result in loss of which parts of the visual field?
1) lesion in the fibers of one eye pre optic chiasm result in no vision in that eye
2) Lesion in half of on eyes fibers pre optic chiasm results in loss of half of the retina where those fibres were coming from
3) Lesion in the optic chasm result in loss of the left eyes left field and the right eyes right field. ( as the half of the retina detects the opposite side of vision)
4) lesions post optic chasm and LGN result in a loss of vision in each eyes retina side corresponding to whatever side of the brain the lesion is on.
What is special about light from a particular side of the visual field incidenting on a side of the lens?
The right half of either lens detects light from the left visual field.
Where does information from the retina go?
Either the LGN or superior colliculus
Where does the LGN send information on to?
To the V1 area of the 1 cortex. From here it can be passed onto v1-v5
What is the function of V5?
V5/MT understands movement in vision
How many subcortical regions of the brain does the retina send to?
Four
What are the subcortical regions of the brain the retina sends information to?
1) The LGN (Thalamus) The major subcortical center relaying information to the 1 cortex. (not all info passed on)
2) Superior colliculus in the midbrain, reflex orientation
3) The suprachiasmatic nucleus - hypothalmus which relays circadian rhythms
4) The pretectum - midbrain, controls papillary light reflex.
Describe the structure of the retina:
Post: - Photoreceptors (cones + Rods) - Inner nuclear cell layer (bipolar) - Ganglion cell layer Ant
Light strikes here and is filtered through the layers to hit the photoreceptive cells.
Where do the ganglion cell layers axons project to?
They project to the LGN (optic fibers)
How is a signal transmitter through the retina?
EM signal is transformed into chemical and electrical signals within the photoreceptive cellular layer.
Do photoreceptors depolarize?
No, they have a spikeless mode whereby membrane potential and synaptic transmission are continuous (basal rate) and graded.
Under ambient conditions the graded potentials of retinal neurons are?
Operating at or near the midpoint of their response range, capable of signalling both increments and decrements of light. (basal around -40mV)
What are the two types of photoreceptors and how are they different?
Rods and cones.
Rods detect the intensity of light only (contrast, black and white) - Night vision, lower threshold to light.
Cones (three kinds) detect colour. Higher threshold to light. Detect wavelength and intensity.
How do cones detect colour?
The three cones each respond maximally to a particular wavelength of light
L = red M = Green S = Blue
What is the ratio of rods to cones in the retina?
1:20
what do photoreceptive cells detect?
Intensity and frequency of light.
What are the two types of vision?
Phototopic vision
Scotopic Vision
What is phototopic vision?
Luminance level (0.03 cd / m2), when the cone mechanism mediates vision
What is scotopic vision?
Level below which the rod mechanism is active ( night vision)
What is the mesopic range?
Combination of cones and rods i.e at twilight where you can see low levels of colour but not in a lot of detail
How many factors affect the the recognition of objects in our visual field?
Five
What are the five factors that affect the recognition of objects in our visual field?
- Background illumination: (contrast of an object against the landscape)
- Spatial frequency : How rapid the stimulus changes across space
- Wavelength: Detection by photoreceptors
- Dark Adaptation : Rods and cones recovery function
- Processing of information by the retina : Pathways that deal with simple components of the light message - shapes, colours , boarders.
Does the retina only see one image?
No, there are at least a dozen different representations of the visual world, each embodied at a separate sublayer of the inner plexiform layer amd carried by a seperate class of ganglion
i.e see lots but is filtered
What sort of information integration occurs in the ganglion of the retina?
Pathways for shape, contrast, wavelength, intensity for a cone or rod interact to form a single signal to the one ganglion cell that interacts with the cell pathways for a single photoreceptor
So what happens to all the information sensed at the photoreceptor?
Each information pathway is summed up at the level of the ganglion for that photoreceptor
Studies in vertebrates have led to the distinction of ganglion cells how? and what properpties?
By size.
Large ganglion cells:
- Have open radiate branching patterns
- Process fast transient impulses such as those caused my movement
Small ganglion cells:
- Process small stationary fine details
- Tonically activated messages.
In humans what are ganglion classes based on?
Projections and functions
How many classes of ganglion cells are there in humans?
Five
What are the classes of ganglion cells in humans?
Midget cells (Parvocellular, P Pathway, P cells)
Parasol cells (Magnocellular, M pathway, M cells)
Bi-stratified cells (K cells, Konicellular)
Melanopsin ganglion cells ( Photosensitive cells)
Lastly:
Other ganglion cells projecting to the SC (superor colliculus)
What is associated with ganglion morphology?
Function
How is ganglion morphology associated with function?
The direction of their branching determines what direction the light must come from in order for them to fire…
Dendrite orientation determines response.
What are two important ganglion cells in humans?
P ganglion cells and M ganglion cells
What is special about P ganglion cells in the retina?
- Colour sensitive
- Outnumber M ganglioon cells by 100:1
- A small concentric receptive field
- Produces a sustained, slowly adapting response that lasts as long as the stimulus is constant
- Weak response to stimuli moving across its receptive field
i.e signals presence, colour and duration of stationary stimuli
Cannot sense movement
What is special about M ganglion cells in the retina?
- Motion detectors
- Much larger than P ganglions
- Colour insensitive
- Large concentric receptive field
- Sensitive to small center of brightness variation
- Fast Adapting response to a maintained stimulus
- Responds maximally to stimuli moving across its receptive field
- Signalling temporal variations in movement of a stimulus
Where does motion detecting occur?
Motion is NOT encoded in the photoreceptor layers, but is encoded just a few synapses after (ganglion cells and Amacrine cells)
What cells are involved in motion detecting?
Direction Sensitive Ganglion Cells (DSGCs)
Starburst Amacrine cells
Describe the concept of visual motion
A coordinated variation of light intensity and time + Space in the environment
What is the role of starburst amacrine cells?
Regulators of DSGCs using ACh and GABA
In response to centrifugal movement
i.e if you turn your head your body doesnt sense stationary things as moving…?
What is special about DSGCs?
Different cells sense movement in different direcitons
Four types: Ventral Dorsal Nasal Temporal
How large is the receptive fields of DSGCs?
Fairly large and sensitive to small changes movement and direction
Do these cells see motion?
No motion is perceived in the brain
How is motion perceived in the brain?
By integration of all information in the cortical areas (V5)
There are two streams of visual information in the brain, what are they and what information do they carry?
The Dorsal stream (spatial processing : Location, movement spatial transformation, spatial relations)
Ventral stream (Object processing : Colour, texture, pictorial detail, shape and size)
What causes the motion afterimage?
Adaptation that occurs in the cortex, hence Interocular transfer)
What is the motion pathway?
- Cones and rods
- DSGCs
- Parasol cells
- M ganglions
- LGN
- V1 and V5/ MT
