Aqueous Humour Vitreous Humour Rods and cones in

Aqueous Humour Vitreous Humour

Rods and cones in more detail Pigment epithelium

Distinctions: Cones Rods ~ 6 million/eye ~120 million / eye -mostly in fovea -wavelength-sensitive -insensitive to light intensity -mostly in periphery -insensitive to wavelength -very sensitive to light intensity -high acuity -low acuity -few-to-one relationship to ganglions (6: 1) -many-to-one relationship to ganglions (120: 1) OK, what about transforming light into nerve impulses? (transduction)

Rods & cones are similar to typical neurons, but also different… discs

My Visual Pigment diagram Retinal opsin

My Visual Pigment diagram Retinal opsin

My Visual Pigment diagram opsin

At rest (no light): Na+ Na+ Na+

A substance called c. GMP is holding the Na+ channels open. The cascade of activity when pigment bleaching occurs results in the breakdown of c. GMP, thus closing the Na+ channels. The cell hyperpolarizes, turning off the “dark current”.

In the dark: -receptors are releasing inhibitory neurotransmitter

In the dark: -receptors are releasing inhibitory neurotransmitter In the light: -hyperpolarized receptors stop sending inhibitory signals -results in increases in AP activity bipolars, ganglions

Functional architecture or How to build a feature detector (e. g. : a length detector)

Firing rate of neuron A 7 6 5 4 3 2 1 1 2 3 4 5 6 Receptors stimulated A This way to the brain 7

-can see how differing convergence leads to differences in aspects of rods and cones: e. g. acuity differences

This way to the brain

This way to the brain

This way to the brain

This way to the brain

This way to the brain

This way to the brain

-can see how differing convergence leads to differences in aspects of rods and cones: e. g. : acuity differences e. g. 2: light sensitivity

Assume it takes 10 units of activity for a ganglion to fire This way to the brain

2 units 2 2 2 Result: No response in brain This way to the brain

102 units 10 10 10 Result: Response in brain!! This way to the brain

This way to the brain

2 units This way to the brain

2 units 2 2 2 Net activity in ganglion: 12 units This way to the brain

2 units 2 2 2 Net activity in ganglion: 12 units This way to the brain

Firing rate of neuron A 7 6 5 4 3 2 1 1 2 3 4 5 6 Receptors stimulated A This way to the brain 7

Firing rate of neuron A 7 6 5 4 3 2 1 1 2 3 4 5 6 Receptors stimulated A This way to the brain 7

Time

Firing rate Time

Firing rate Time

Firing rate Time

Firing rate of converged-on neuron

Firing rate of converged-on neuron

Problem: the firing rate is influenced by more than just orientation. Intensity of the stimulus also influences channel activity

Firing rate of converged-on neuron Notice, it fires maximally at 20 cps. So, if it is firing at 10 cps, this neuron “knows” it isn’t a vertical line. 30 25 20 15 10 5 0 But what if we used a less intense line? Is 10 cps because it isn’t vertical, or because it isn’t very intense?

Firing rate of converged-on neuron The solution: look at the pattern of activity across several differently-tuned fibres. 30 25 20 15 10 5 0 20 10 20

OK, that’s fine in theory, but what actually exists in the brain? Well, centre-surround receptive fields are everywhere Moving higher up (into cortex), can find ‘simple’ cells, complex cells, end-stopped hypercomplex cells, etc.

The processing of the signal begins at the level of the retina in the form of lateral inhibition

20% of signal

10 units -2 10 10 8 6 6 8 -2 Notice that the stronger the response, the bigger the edge enhancement will be

Cell location Response strength

Mach bands 6 4 2

Physiologically-based illusions Lateral inhibition

Also: two types of ganglions identified: Magno and Parvo cells Parvo Magno Small, numerous large, fewer Small receptive field large receptive field Slow (20 m/s) Fast (40 m/s) Sustained response Transient response Colour sensitive not colour sensitive Low contrast sensitivity high contrast sensitivity Processes form/colour Processes location, movement

From the retina, projections go to two different places: Superior Colliculus (SC) and Lateral Geniculate Nucleus (LGN) Two visual pathways: tectopulvinar, and geniculostriate

Tectopulvinar pathway LGN Pulvinar nucleus eye S. C. temporal cortex Occipital cortex

Geniculostriate pathway

Lateral Geniculate Nucleus (LGN)

See p. 85 and 86 for more diagrams