Vision Flashcards
How many rods vs cones?
100m v 7m.
Describe the mechanism of phototranduction.
Photopigment absorbs photon, undergoes conformational change that sets in motion a biochemical cascade lowering cGMP concentration, closing cGMP-gated channels permeable to Na+, hyperpolarising.
What isomer does phototransduction cause in rhodopsin?
All-trans.
In what sense is there a simple neural representation of the scene at the photoreceptor layer?
Neurons in bright regions hyperpolarised, dark depolarised.
What receptors do OFF vs ON bipolar cells express.
AMPA (depolarise to glu released in darkness) vs mGluR6 (hyperpolarise to glu released in darkness).
How are two parallel pathways for retinal output established at the level of ganglion cells?
ON bipolar connect to ON ganglion etc.
There are about X% as many ganglion as receptor cells. This means that…
1, retinal circuits must extract low-level features most useful to central visual system. This enhances regions of spatial contrast.
What are antagonistic centre-surround RFs? Why do bipolar cells have them?
RFs where opposite responses elicited when stimulus in surround as centre. Greater firing rate when adjacent light and dark aligned with RF regions. Lateral inhibition by horizontal cells.
Different populations of ganglion cells…
Transmit multiple neural representations of the retinal image along parallel pathways.
Parvocellular pathway (90%) function and projection:
Colour and fine detail (object detection). Projects to parvocellular LGN and then layer 4Cb of V1.
Magnocellular pathway (5%) function and projection:
Motion detection. Projects to MLGN, superior colliculus, and later 4Ca of V1.
Damage to PLGN does what opposed to MLGN.
PLGN, reduction in acuity and colour, MLGN, reduction in ability to perceive rapidly changing stimuli.
Retina recovers colour by…
Comparing responses of several types of cone photoreceptor maximally sensitive to different wavelengths.
Who did microspectrophotometry of human retinal tissue and found 3 receptors that absorbed different wavelengths maximally? What wavelengths?
Dartnall et al (1983). 419, 532, 558nm.
Why may metamers emerge? How is colour discriminated then?
Three cones have optimal stimuli but also overlapping wavelengths. Incoming light likely to activate two simultaneously. Discriminated by comparing firing of paired cones.
How does colour opponency work?
Pairs represented in opponent channels in which the responses elicited by each colour are mutually inhibitory. Isolates the differences in response to wavelengths between cones, reducing ambiguity caused by overlapping cone responses.
Light adaptation. What happens to responses of retinal ganglion when diff intensity flashes presented with constant background illumination? How about when increased? Why?
Sigmoidal curve. When increased, same curve but shifted to higher flash intensities. This is because threshold flash intensity is proportional to background intensity, in line with Weber’s Law of adaptation.
Light adaptation. What does pupillary reflex do? Though what is the problem?
Contracts when bright. Range over which it can occur much smaller than range of visible light, however. In bright, 2mm, dark 7-8. Just over one log unit, less than the 12 log units of visible light range (400-700nm).
Dark adaptation. How long pigment regeneration in cones vs rods.
Cones 9-10 minutes, rods 20-30.
How is photopigment regenerated?
Retinal pigment epithelium regenerates by recycling retinal and generates rhodopsin.
Who used microelectrode recordings in cat V1 while presenting bars of light against a dark background to show that cells in V1 layer WHAT show orientation selectivity, responding most strongly to bars in a particular direction.
Hubel and Wiesel (1959). Layer IV.
When do complex cells fire?
When line or edge (of p. orientation) traverses RF.
When do end-stopped cells fire?
Lines of certain length, corners.
What would double-opponent cells respond to?
Red in centre green surround or green centre red surround.
What might double-opponent cells contribute to? Why?
Colour constancy. Because emphasise local contrast over global illumination changes.
Who used radioactive glucose to show that the retinotopic map was inverted, flipped and distorted?
Tootel et al (1988).
Orientation-specific responses functionally grouped…
Into columns perpendicular to corticalsurface.
Occular dominance columns parallel to cortical surface, forming a chequerboard, …
With both eyes and all orientations represented in a small patch called a hypercolumn.
How many hypercolumns in V1?
13,000.
Who showed ‘blobs’ in hypercolumns? In layers II/III. How?
Wong-Reily (1979). Cytochrome oxidase staining.
Technical term for lazy eye:
Amblyopia.
What did Goebel et al (2001) show about people with V1 lesions?
Blind in corresponding region, but grasp objects in ‘blind’ VF at levels better than chance. Loss of visual awareness but not complete loss of vision. Alternate pathway for vision bypassing V1, perhaps via optic tract -> superior colliculus -> secondary thalamus -> extrastriate cortex.
Ventral stream originates in the X and projects to the X.
Occipital lobe, projects to the temporal lobe.
Dorsal stream originates in the X and projects to the X.
Occipital lobe, parietal lobe.
Role of dorsal stream:
Perception of motion and spatial relationships to guide action.
Goodale and Milner (X) showed that visual form agnosiac patient DF with XXXX damage from carbon monoxide poisoning could not describe the orientation of a slot, though could orient his hand correctly to insert it through.
- Bilateral ventral occipital lobe damage.
Those with optic ataxia have damage where?
Dorsal parietal regions.
What evidence is there for the idea that colour is processed in V4? ?Colour constancy?
Meadows (1974), achromatopsia (coloured stamps), ventral occipital lobe lesion in V4.
What evidence suggest the function of V4 is not only colour processing?
Roe et al (2012). Macaque singel cell recordings, neurons in V4 also respond to visual features such as shape and texture.
What evidence is there for the idea that V5 plays a role in visual movement perception?
Zihl et al (1983). Patient LM bilateral damage to V5, akinetopsia. Her world still frames.
Also Zeki et al (1991), PET, images of moving dots relative to static. V5 neurons become active.
What shows that RFs in V5 are more complex than V1?
Salzman et al (1990), V5 activation in response to movement depends on dot coherence.
