Aqueous Humour Vitreous Humour
Rods and cones in more detail Pigment epithelium
OK, what about transforming light into nerve impulses? (transduction) Distinctions: Cones Rods ~ 6 million/eye ~120 million / eye -mostly in fovea -mostly in periphery -wavelength-sensitive -insensitive to wavelength -insensitive to light intensity -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+ Na+ Na+
A substance called cGMP is holding the Na+ channels open. The cascade of activity when pigment bleaching occurs results in the breakdown of cGMP, thus closing the Na+ channels. The cell hyperpolarizes, turning off the “dark current”.
-receptors are releasing inhibitory neurotransmitter In the dark: -receptors are releasing inhibitory neurotransmitter
In the dark: In the light: -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)
A Firing rate of neuron A 2 3 4 5 6 7 Receptors stimulated 1 2 3 4 5 6 7 Receptors stimulated A This way to the brain
-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
e.g.: acuity differences -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
Result: No response in brain 2 units 2 2 2 2 2 2 Result: No response in brain This way to the brain
Result: Response in brain!! 10 units 2 units 10 10 10 10 10 10 Result: Response in brain!! This way to the brain
This way to the brain
2 units This way to the brain
Net activity in ganglion: 12 units 2 2 2 2 2 2 Net activity in ganglion: 12 units This way to the brain
Net activity in ganglion: 12 units 2 2 2 2 2 2 Net activity in ganglion: 12 units This way to the brain
A Firing rate of neuron A 2 3 4 5 6 7 Receptors stimulated 1 2 3 4 5 6 7 Receptors stimulated A This way to the brain
A Firing rate of neuron A 2 3 4 5 6 7 Receptors stimulated 1 2 3 4 5 6 7 Receptors stimulated A This way to the brain
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 Problem: the firing rate is influenced by more than just orientation. Intensity of the stimulus also influences channel activity
But what if we used a less intense line? Notice, it fires maximally at 20 cps. So, if it is firing at 10cps, this neuron “knows” it isn’t a vertical line. Firing rate of converged-on neuron But what if we used a less intense line? 30 25 20 15 10 5 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. Firing rate of converged-on neuron 30 25 20 15 10 5 20 10 0 10 20 10 0 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 10 10 10 10 -2 8 6 6 8 -2 Notice that the stronger the response, the bigger the edge enhancement will be
Response strength Cell location
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 (20m/s) Fast (40m/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. Occipital cortex temporal cortex
Geniculostriate pathway
Lateral Geniculate Nucleus (LGN)
See p.85 and 86 for more diagrams