6- The Retina Flashcards
Where is the retina?
At the back of the eye
How many cells are in the retina?
100 million+
How does light reach photoreceptors in the retina?
Light passes through overlying tissue
What is the advantage of the inside-out arrangement in the retina?
Pigmented epithelium below photoreceptors absorbs light that passes entirely through the retina that minimises light scattering
What do photoreceptors do?
Convert electromagnetic radiation to neural signals
What are the two types of photoreceptors?
Cones and rods
What are the 3 main regions of photoreceptors?
Outer segment, inner segment, synaptic terminal
Where is there a higher ratio of rods to cones?
In the peripheral retina
Why are rods more sensitive to light?
There is a higher ratio of photoreceptors to ganglion cells
Where is the only place cones are found and why?
At the fovea as they have a high acuity and low sensitivity
Why is there more convergence in the rod system?
Increasing sensitivity while decreasing acuity
What is phototransduction?
The conversion of light to neural signals
What do rods and cones contain?
Photosensitive pigment
What does the absorption of photons of light by pigment generate?
Electrical signal
What does the G protein-linked receptor respond to?
Light rather than neurotransmitters
What happens to Na+ channels in the dark?
They partially open
What happens to Na+ channels in the light?
They close and rods hyperpolarise
How do the 3 different pigments in cones differ?
Wavelengths and spectral sensitivity
How do we perceive colour?
Through visible light in the electromagnetic spectrum
What is ability to perceive light dependent on?
The information we are given
What are the 3 different types of cones in humans?
Red, blue, green
Vision in cones
Photopic (daytime) and colour vision
Main location of cones
Fovea
Why do cones have high acuity?
1 cone to 1 ganglion cell
Vision in rods
Scotopic (night time) and noncolour
Main location of rods
Periphery
Why do rods have low acuity?
Many rods to 1 ganglion cell
What cells in the retina fire action potentials?
Ganglion cells
How do other cells in the retina respond to stimulation?
With graded changes in membrane potential
What is the direct pathway of ganglion cells to fire action potentials?
Photoreceptor –> bipolar cell –> ganglion cell
What happens after light hyperpolarises photoreceptor cells?
They stop releasing glutamate to bipolar cells, which depolarise and release neurotransmitters to ganglion cells
How is the visual field mapped?
Point-to-point on the retina
What lateral connections in the retina modify responses?
Connections of horizontal and amacrine cells
100 million receptors are recoded to how many ganglion cells?
1.25 million
What does each receptive field for each ganglion cell do?
Combines input from a number of photoreceptors
When do cells in centre-surround receptive fields respond best?
When there’s contrast
2 types of retinal ganglion cells
M-type and P-type
What are the 3 differences between types of retinal ganglion cells?
Appearance
Connectivity
Electrophysiological response properties
Where is input to M-type cells primarily from?
Rods
Where is input to P-type cells primarily from?
Cones
What information does the retina extract?
About differences in brightness/contrast and colour
What is included in retinofugal projection? (7)
- Optic nerve
- Optic chiasm
- Lateral geniculate nuclei
- Optic radiation
- Primary visual cortex
- Hypothalamus (nonthalamic)
- Superior colliculus (nonthalamic)
Role of the hypothalamus in retinofugal projection
Biological rhythms control, including sleep and wakefulness
Role of the superior colliculus in retinofugal projection
Orients eye in response to new stimuli
What is contralateral wiring?
Left hemifield projects to right side of brain and vice versa
Which axons cross and which stay ipsilateral?
Ganglion cell axons from nasal retina cross
Temporal retinal axons stay ipsilateral
Where does information cross?
At the optic chiasm
What would a lesion in a visual hemifield mean?
We lose parts of the visual field
What would a lesion to the optic tract mean?
We would lose information on a contralateral hemifield
What type of structure is the lateral geniculate nucleus?
Thalamic
What is the first synaptic relay in the primary visual pathway?
Lateral geniculate nucleus
What is monocular input?
Input from two eyes is kept separate
What are receptive fields of the lateral geniculate nucleus similar to?
Ganglion cells
What are neighbouring regions of the visual field processed by?
By neighbouring regions of the visual cortex
What areas is more of the cortex devoted to?
Areas of high acuity
How is the retina stimulated?
By a point of light
Where is the peak of the broad distribution of activity in the striate cortex?
At the corresponding retinotopic location
How are cells with same eye preferences grouped?
Into ocular dominance columns
What do cells within the same column share?
The same ocular dominance and receptive fields in similar areas of the visual field
What is the receptive field of a cortical visual neuron?
The area of a visual field within which it is possible for a visual stimulus to influence firing of a given neuron
What is orientation selectivity?
Cells respond to particular orientation
What is direction selectivity?
Cells respond to direction of movement
What is an orientation column?
Orientation of nearby neurons is related
What is a cortical module?
Each module that is capable of analysing every aspect of portion of the visual field
What do ocular dominance columns respond to?
Respond more to information from 1 eye
3 things caused by damage to the primary visual cortex
Scotomas, completion, and blinsight
What is a scotoma?
An area of blindness in the contralateral visual field due to damage to the primary visual cortex
What is blindsight?
A response to visual stimuli outside conscious awareness of ‘seeing’
4 aspects of functional areas of the secondary and association visual cortex
- Neurons in each area respond to different visual cues
- Lesions result in specific deficits
- Anatomically distinct
- Retinotopically organised
What analysis is involved in the dorsal stream?
Analysis of visual motion and visual control of action
Where is area MT?
In the temporal lobe
How is the dorsal stream arranged?
Into direction-of-motion columns
What is akinetopsia?
Motion blindness
How is akinetopsia caused?
Damage to middle temporal area of cortex
What deficiency is implicated in akinetopsia?
Deficiency in ability to see movement progress in normal smooth fashion
What perception does the ventral stream control?
Perception of the visual world and recognition of objects
What does the area V4 control?
Colour vision and shape perception
What does damage to area V4 cause?
Achromatopsia- partial or complete loss of colour vision
What does damage to the fusiform gyrus (face area) cause?
Prosopagnosia- inability to distinguish faces
What is visual perception?
Identifying and assigning meaning to objects
What are retinal ganglion cells sensitive to?
Contrast and wavelength of light
What is involved in the striate cortex?
Orientation and direction selectivity, binocularity
What response does the extrastriate cortical areas control?
Selective responses to complex shapes
