Central visual pathways

Question 1

What is the pathway for visual information flow?

Answer 1

Retino-geniculo-striate pathway/primary visual pathway

Optic nerve head - optic nerve - optic chiasm - optic tract - lateral geniculate nucleus of the thalamus (synapse) - optic radiation through internal capsule (synapse) - primary visual cortex (V1)/striate cortex/BA17 in calcarine fissure in occipital lobe

Pathway responsible for seeing

Question 2

What is the significance of the optic chiasm?

Answer 2

Means that 60% of the fibres from each optic nerve decussate and the remainder stay ipsilateral - data from the right visual field is registered in the left portion of the retina from each eye and then travels in the left hemisphere after decussation = contralateral control

Question 3

What is the pupillary light reflex and what is the process behind it?

Answer 3

Reduction of diameter of pupil that occurs when sufficient light falls on the retina

Bilateral projections from retinal ganglion cells to pretectal nuclei in the midbrain which then stimulate the Edinger-Westphal nuclei (preganglionic, parasympathetic) bilaterally which in turn stimulate the ciliary ganglion (via the occulomotor nerve) which innervates the pupillary constrictor muscle - decreasing pupillary size when activated

Direct and consensual responses are expected in normal physiology

Similar responses for accommodation and vergence

Question 4

What is the significance of the suprachiasmic nucleus?

Answer 4

Part of the hypothalamus, responsible for the regulation of circadian rhythms - receives input from retinal ganglion cells and depending on light levels, will output to pineal gland to regulate melatonin production

Question 5

What is the role of the superior colliculus?

Answer 5

Retinotectal pathway projects to superior colliculi in the brainstem which are responsible for head orientation in response to movement - toward/away

Neurons are sensitive to luminance change - appear/disappear/move

Provide a retinotopic spatial topography - movements at different points in the visual field will activate different parts of the colliculi

Also further projections to reinforcement learning mechanisms

Question 6

How is retino-geniculo-striate pathway information presented in the cortex?

Answer 6

As with the colliculi - in a spatial retinotopic map - retinal ganglion cells have distinct spatial relations to their projecting cells in the cortex ie neighbouring cells in the retina will excite neighbouring cells in the cortex

Majority of visual cortex dedicated to processing of foveal vision - most posterior - as a greater density of cells here = cortical magnification

On the medial surfaces of the L and R hemispheres; upper visual fields represented below the calcarine sulcus, lower visual field above

Question 7

What cell types are there in the LGN and what are their functions?

Answer 7

Parvocellular layer - small cell bodies; colour/fine detail/stationary or slow moving objects ie scene analysis and object identification; cones input

Magnocellular layer - large cell bodies; luminance change ie on/off movement - similar to superior colliculus; rods input

Koniocellular pathway - smaller, possibly involved in short wavelength transmission of cone info

Question 8

What is a receptive field of a visual neuron?

Answer 8

Area of the visual field within which a visual stimulus can increase or decrease the firing rate of that particular neuron ie firing rate increases when an object is within a neuron’s receptive field

Discovered by Hubel and Wiesel

Question 9

What 4 features were discovered about receptive fields of neurons in the R-G-S pathway?

Answer 9

1. receptive fields of neurons of foveal vision were smaller than those in the periphery of the retina
2. these fields were circular
3. they were monocular
4. many had excitatory and inhibitory areas

Question 10

How did the features of RGS neurons differ from neurons in V1?

Answer 10

Cells in V1 not interested in single points of light/dark but in bars of light/dark (contrast) and specific orientations

These neurons were split into simple and complex cell types

Question 11

What are the functions of simple and complex cell types in V1?

Answer 11

Simple - discrete on/off regions in straight lines (rather than circles) = respond to bars of light in a specific orientation at a specific location in a visual field; rarer

Complex - not discrete on/off regions, respond to the orientation of a straight edge REGARDLESS of its position in the receptive field ie only to 45 degree bars of light; some respond optimally depending on the direction of movement of that bar of light

Question 12

How are simple and complex cells organised in V1?

Answer 12

In vertical orientation columns - all cells within this column have the same receptive field in the same area of the visual field and both simple and complex cells within that column are activated in response to the same specific orientation of the stimulus

Areas are also split into ocular dominance columns for each eye (in layer 4 only), and by passing horizontally through the cortex, different orientations of stimuli will stimulate either L or R eye; neurons at the centres of the columns will respond exclusively to L or R eyes but those at the periphery will respond equally to inputs from either eye

Question 13

What is the flow of flow of information processing up to and including V1 at a cellular level?

Answer 13

On/off centre cells - simple cells - complex cells

Higher level receptive field processing is constructed by combining processing of lower order cells

Question 14

What is visual information is each level of visual cortex responsible for constructing?

Answer 14

V1 - main input, obtaining the edge/contrast data
V2 - similar to V1 but more complex shape characteristics, reacts to illusory contours
V3 - form
V3a - motion, depth
V4 - colour (constancy - wavelength comparison between neurons) (cerebral achromatopsia), form, stimulus saliency, orientation
V5/MT - (Middle temporal area) - global motion w/o colour, (damage = cerebral akinotopsia)

Question 15

What is the perceptual binding problem?

Answer 15

Split into:

Segregation problem (BP1) - how does the brain segregate elements in complex patterns of sensory input so they are allocated to discrete objects ie how to tell that out of the two objects in front of you, that the circle is blue and the square is yellow

Combination problem (BP2) - how objects, background and abstract/emotional features are combined into a single experience

Basically - the fact that all visual information is encoded together then further processed separately in different areas of cortex before recombination into a perception

Question 16

What are the roles of the dorsal and ventral streams?

Answer 16

Flow of information through brains lobes

Ventral - temporal lobe = scene analysis and object identification ie ‘What’ (V1-2-4 and on)

Dorsal - parietal lobe = spatial locations ie ‘Where’ (V1-2-MT and on)

Question 17

What are the cell types in the visual cortex and what neurotransmitters do they use?

Answer 17

Spiny (pyramidal and stellate) - project to other areas of cortex - excitatory/Glu

Aspinous/smooth dendritic - local arbours - inhibitory/GABA

Arranged in 6 layers (as elsewhere in cortex)

Question 18

Which layer do LGN axons terminate?

Answer 18

4C - then project to other layers

Question 19

What is stereopsis?

Answer 19

The integration of binocular vision resulting in the ability to perceive depth

Some binocular neurons (far and near cells + tuned zero cells) have receptive field properties that make them good candidates for this - receptive fields from L and R eye input data are slightly offset either in visual space or cortical organisation