Senses (Hearing, Vision, Olfaction, Gustation) Flashcards
Detail 4 problems affecting refraction potential in the lens. How are they mitigated?
Chromatic aberration: red light has lower refractive index than blue, so dispersion of λs on retina.
- Retina has yellow pigment over fovea and fewer blue cones to compensate for more blue light refracted into centre.
Spherical aberration: edges of lens bends light eccentrically
- Cornea is ellipsoid
- Refractive index of lens is lower at edges
Glare: light scatters in cornea
- Reduced by choroid
Diffraction: light scattered by restricted aperture
- Pupil can change size according to light intensity
What is the point spread function?
A way to quantify blur caused by the optical system.
Tested for using grating patterns - smallest grating that is still distinguishable at given contrast.
What consequence does convergence have in the retina?
No 1:1 mapping as huge convergence for receptive fields.
75,000 rods to 5000 bipolar cells converge to 1 ganglion cell.
Density is roughly 1 receptor every 2-3µm
How is pupil size controlled? What are the consequences of different sizes of pupil?
Dilation:
- Sympathetic nervous system control of radial dilator muscles
- Receives more light and minimises diffraction
- Decreases depth of field
Constriction:
- Parasympathetic control from Edinger-Westphal nucleus in brainstem to ciliary ganglion.
- Increases depth of field and minimises aberrations
- Increases diffraction and reduces light intake.
Give some ways that the eye is adapted to cope with a large range of photon intensities (range of 10^12)
Pupil diameter: copes with roughly 1 order of magnitude
Types of receptor:
- Photopic vision (cones) at high illumination
- Mesopic vision (both rods and cones)
- Scotopic vision (rods) at low illumination
Adaptation of each receptor type:
- Rods have slower adaptation (30mins)
What is a Purkinje shift and what does it show?
A shift in peak colour sensitivity at lower light intensities to shorter wavelengths
Gives evidence for different cell populations, since lower light intensities utilise more peripheral retina which has higher S cone density.
Describe the behaviour of a centre red; surround green receptive field.
ON centre for red; ON surround for green
- Red light on centre and green light on surround causes maximum firing (both areas depolarise bipolar cell as sign reversing)
- Red light over whole area causes weak firing
- Green light over whole area means bipolar cell does not fire
- Green centre; red surround means cell inhibited from firing (maximum hyperpolarisation of bipolar cell)
Describe the difference between and distribution of RGN for the magno and parvo pathways:
M pathway: Parasol cells with a larger receptive field
P pathway: midget cells with smaller receptive field.
In peripheral retina: roughly 50/50 distribution. In centre: 90% midget cells (higher spatial acuity)
Compare and contrast midget and parasol cells:
Parasol cells (5% of ganglion cells):
- Includes ON and OFF types
- Faster and more transient responses (faster adapting)
- More sensitive to low-contrast stimuli
Midget cells (90% of ganglion cells):
- Includes both ON and OFF
- Slower and more sustained responses (can deal with higher contrast)
- Can be colour opponent ganglion cells
How is light energy transformed into an electrical signal?
- Rhodopsin/iodopsin in outer disc segments absorbs photons
- Retinal molecule in the opsin changes conformation activating a Gtransducin coupled receptor
- PDE activated which degrades cGMP
- Results in closure of cGMP gated Na+ channels
- Hyperpolarises cell, and resuces internal Ca2+, reducing glutamate release
How is retinal recycled in retina?
Unstable metaopsins will split within minutes – trans retinal released to be converted back to cis
- Trans retinal transported to epithelial cells
- Reduced to all-trans retinol (= vitamin A)
- Converted back to cis retinal to be transported back
How is a photoreceptor returned to a dark state?
Goal is to increase Ca2+ levels
Transducin modulated mechanisms:
- Phosphorylation of metarhodopsin leads to arrestin activation = stops transducin activating PDE
- Transducin has intrinsic GTPase activity
Drop in intracellular Ca2+ has -ve feedback effect upregulating recovery speed:
- Increases guanylate cyclase activity to restore cGMP levels
- Upregulate arrestin levels as recoverin is activated
- Increases Na+ channel affinity for cGMP
How does a drop in intracellular Ca2+ (caused by light) oppose further hyperpolarisation?
- Recoverin activity increases, increasing rhodopsin phosphorylation and reducing transducin activity.
- Accelerates guanylate cyclase activity
- Increases channel affinity for cGMP using Ca-CAM
How do different bipolar cells show sustained vs. transient responses?
Sustained response:
- b3/b7 cells express lots of kainite receptors
- Results in tonic response
Transient response:
- b2 cells express high AMPA density
- Phasic stimulation (higher frequency)
Describe the route of neurons from the retina to V1:
Optic nerve
Optic chiasm
Optic tract
Lateral Geniculate Nucleus (Thalamus)
— Small number to hypothalamus and superior colliculus) —
Optic radiations
Brodmann’s area V1
Describe the organisation of visual information the LGN
Layered organisation:
- Parasol cells input into magnocellular neurons (M pathway) to layers 1&2
- Midget cells input into parvocellular neurons (P pathway) layers 3-6.
- In between layers input from koniocellular neurons giving colour information (mainly blue-yellow)
Layers 1,4,6 are from contralateral eye and layers 2,3,5 are from ipsilateral eye so same areas of visual field are kept together.
Describe how the retinotopic map in V1 is organised:
Layers:
- Six layers
- Koniocellular into II and III
- IVCα: mainly magnocellular neurons
- IVCβ: parvocellular neurons
Columns:
- Each area corresponds to area in visual field.
- Horizontal connections but mainly ocular dominance (each IVC neuron receives input from one eye)
- Determines orientation preference
Rows:
- Blobs (centered on ocular dominance stripe) and interblobs
- Blobs receive input from M pathway, P-B sub-pathway and koniocellular cells.
- Interblobs receive information from P-I sub-pathway (direction selectivity) and M pathway
What information do blobs and interblobs process?
Interblobs: shape processing
- Direction selectivity
- input from P-I pathway and M
Blobs: movement and colour
- Colour processing (yellow-blue from koniocellular and red-green from P-B)
Both interblobs and blobs combine information with columnar info on orientation (simple, complex and hypercomplex cells)
What is the difference between the P and M pathways?
P-pathway is important for high spatial acuity and colour vision
While the M-pathway is important for high visual sensitivity and motion vision
Give examples of higher level processing:
- Composite shape response (e.g. corners)
- Disparity (depth perception)
- Border ownership signals (is something foreground or background?)
- Contextual information (e.g. fill in scotoma (blind spot) using surrounding area
What is the ventral stream for vision?
The ‘What’ pathway:
- Receives information from P pathway: blob (colour) and interblob (shape)
- Sensitive to complex shapes (e.g. curvature)
- Show lighting; colour and size consistency
What does V4 connect to for higher level processing?
- Parahippocampus: memory recall of objects
- Pre-frontal cortex for decision making
- Temporal lobe for learning and memory
What is visual agnosia? What types are there?
Abolished ability to recognise objects while vision remains good = the ventral stream gone wrong.
Apperceptive agnosia: Can recognise objects but not copy them
Associative agnosia: can copy objects but not identify them.
Specific damage can cause other effects e.g. prosopagnosia (type of associative) with inability to recognise faces.
What is the dorsal stream? What is the evidence for this being separate from the ventral stream?
The ‘Where’ pathway.
Balint’s syndrome suggests different streams:
- Preserved recognition but impaired eye movements and reaching toward objects.
- Shown by posting experiments
