The Eye- Lecture 1 Flashcards
Pupil
Controls amount of light let into eye
Retina
Where transduction occurs by photoreceptors- Rods and Cones
Rods
Respond to intensity and brightness
Cones
Respond to colour and works best in bright conditions
3 Types of cone cells
Short cones- Responsive to blue light wavelength
Medium - Responsive to green light wavelength
Long - Responsive to red light wavelenth
What is colour blindness?
Due to change in the number of cones that an individual has of each type
Fovea
Where visual processing is most in detail (cones are primarily in the fovea right at centre of retina)
Optic Nerve
Axons of magnocellular and parvocellular cells form the optic nerve and leave through the blindspot- transfer of info from eye to brain
Different retinal cells
Photoreceptors- rods and cones
Bipolar cells
Ganglion cells
Bipolar cells
act directly or indirectly to transmit signals from the photoreceptors to the ganglion cells.
Ganglion cells
Receives visual information from photoreceptors via intermediate neuron- bipolar cells
Magnocellular pathway
Rod cells join M cells which forms the Magnocellular pathway
Parvocellular Pathway
Cone cells join P cells which forms the Parvocellular pathway
Parallel processing
Bishop (1933)
At each step of visual processing there are lots of parallel neurons in each layer (with lateral connections between them).
Found three different classes of axons found in the optic nerve process different sensory qualities related to vision
Hierarchical Processing
there is a transfer of information from the eye to the brain.
Each step involves some image processing and computation that builds on the last
Receptive fields of V1 receptors
Photoreceptors have receptive fields. They respond to light in point A but not point B.
Example of retinotopic organisation and receptive fields
Anatomically adjacent photoreceptors responded to physically adjacent points of light.
Retinotopic organisation
axons from specific regions of the retina project to a specific area of the lateral geniculate nucleus (LGN) and also the occipital cortex.
Lateral Geniculate Nuclues
multilayered structure that receives input from both eyes to build a representation of the contralateral visual hemifield.
Hubel and Wiesel 1981
Micro-electrode recording in cats- found a retinotopic map in V1
Also that the cells in V1 are sensitive to orientation as well
Different cells respond to specific orientations of stimulus by firing a burst of action potentials
Laminar architecture of V1 cortex
6 layers of cells
Input goes to layer IV (4)
Outputs from layers II/III (2/3)
After V1…
V2 > V3 > V3A
Where there is an increase of receptive field size as we move up the visual hierarchy
And- increasing specialisation of function
V2
Relay station which sends parvocellular inputs along ventral stream and magnocellular inputs along the dorsal stream
Ventral visual stream
gets parvocellular inputs and generates conscious representations of object shape and identity
Dorsal visual stream
gets magnocellular inputs and generates unconscious representations of object location and motion- rapid action control like returning a tennis serve
Leslie Ungerleider
Pioneer in Ventral and Dorsal stream research and exploration
Milner and Goodale (1995)
Carried out famous neuropsychological work on patients with dorsal and ventral stream lesions
V5
Early part of the dorsal stream
Neurons in this region are sensitive to motion in their receptive field
Damage causes akinetopsia- motion blindness
V4
Early part of ventral pathway
Sensitive to colour and shape
Damage causes Achromoatopsia- colourless world
Beckers and Hömberg (1992) AK
Akinetopsia induced by TMS of V5 area in humans
