TERMS & CONCEPTS

1. The Electromagnetic Spectrum: a continuum of electromagnetic energy that is produced by electric charges and radiated as waves which range from 10^-12m to 10^4m)

2. Wavelength: the distance between the peaks of electromagnetic waves

3. Visible light: energy within the electromagnetic spectrum that is perceivable (400 - 700nm); 1nm = 10^-9m (ie) the longest visible wavelengths are slightly less than 1/1000 of an mm long

4. Cornea: the transparent covering of the front of the eye that accounts for 80% of the eye’s ability to focus

5. Lens: accounts for 20% of the eye’s focus ability to focus, changes its shape to adjust focus due to ciliary muscles

6. Ciliary muscles: located in the lens, allow for increased focus by increasing lens curvature

7. Outer segments: the parts of the receptors that contain the visual pigments to react to light and trigger electrical signals

8. Fovea: used when looking directly at something, comprised of mainly cones, contains about 1% (50 000/6 000 000) cones in the retina

9. Macular degeneration: destroys the fovea of the elderly, creating a blind region in the central vision so that one loses sight of what they are directly looking at

10. Retinitis Pigmentosa: the peripheral retina degenerates and causes peripheral loss (ie) tunnel vision; can cause complete blindness if cones are attacked

11. Blind spot: the place on the retina where the optic nerve leaves the eye and ganglion cell fibres flow into the optic nerve; there are no rods and cones

12. Accommodation: an unconscious process of change in the lens’ shape caused by the tightening of ciliary muscles at the eye’s front to increase curvature and pull the focus point back onto the retina for distal stimuli that are nearby

13. Presbyopia: “old eye”; an inability to accommodate due to hardened lens and weakened ciliary muscles

14. Myopia: “nearsightedness”; blurry distant objects; parallel rays are focused before the retina due to refractive myopia or axial myopia

15. Refractive myopia: the cornea/lens bends light too much

16. Axial myopia: long eyeball

17. Hyperopia: “farsightedness”; blurry nearby objects; focus point of parallel lines is beyond the retina; caused by a short eyeball

18. Isomerization: the straightening of retinal when it absorbs light, thereby inducing a chain reaction that activates thousands of molecules to create electrical signals in receptors

19. Dark adaptation: the process of being able to see in the dark after some time; measured with a dark adaptation curve; increases in two phases (cone adaptation: 3-4mins) and (rod adaptation: 7-10mins to 20-30mins)

20. Sensitivity: 1/threshold; high threshold = low sensitivity

21. Light-adapted sensitivity: the sensitivity measured in light

22. Dark-adapted sensitivity: 100 000x > than light-adapted sensitivity

23. Rod monochromats: individuals who lack cones; used to study rod adaptation (achieved at 25mins)

24. Rod-cone break: the place where the rods begin to determine the dark adaptation curve

25. Visual Pigment Bleaching: when the retinal straightens after absorbing light and separates from the opsin, thereby causing a lighter colour

26. Visual Pigment Regeneration: the process by which the retinal returns to the opsin and its original shape for the process of transduction, thereby affecting the adaptation duration of cones and rods

27. Detached retina: when the retina becomes detached from the pigment epithelium, thereby preventing regeneration of visual pigments. The separated retinal cannot recombine with the opsin after bleaching occurs, causing blindness in that area of the visual field. The condition is permanent without corrective laser surgery.

28. Spectral sensitivity: eye’s sensitivity to light when responding to wavelengths; measured and compared for rods and cones using a spectral sensitivity curve

29. Monochromatic light: the light of a single wavelength; created with a spectrometer

30. Purkinje shift: the enhanced perception of short wavelengths during dark adaptation (e.g. noticeable green foilage at dusk)

31. Absorption spectrum: the amount of absorbed light vs wavelength of light

32. Cell body: Keeps the cell alive

33. Dendrites: branch out and receive electrical signals

34. Axon: contains fluid that conducts electrical signals

35. Ions: charged molecules; high concentration of (+)ve sodium outside the axon and high concentration of (-)ve potassium inside the axon

36. Permeability: the ease with which a molecule passes through the membrane

37. The rising phase of the action potential: the intake of +Na into the axon when a signal travels down the axon, thereby increasing the charge from -70mV to +40mV

38. The falling phase of the action potential: the release of -K out of the axon, thereby dropping the charge from +40mV to -70mV

39. Synapse: space between neurons

40. Neurotransmitters: chemicals stored in the synaptic vessels of the sending neuron that flow into a synapse and a receptor site

41. Receptor sites: specific to neurotransmitters

42. Excitatory response: occurs when the inside of the neuron becomes more positive via depolarization; increases the chances of action potential and increases the rate of nerve firings

43. Depolarization: an excitatory response that causes the charge of a neuron to change in the direction that triggers the action potential

44. Inhibitory response: occurs when the inside of the neuron becomes more negative via hyperpolarization; decreases the rate of firing and the chances of action potential occurrences

45. Hyperpolarization: an inhibitory response that causes the internal axon charge to move away from the depolarization level that is required for an action potential to occur

46. Neural circuits: interconnected groups of neurons in the retina

47. Neural convergence: occurs when multiple neurons synapse onto a singular neuron

48. Visual acuity: detail-oriented vision

CONTENT

What colours are associated with wavelengths?

• short: blue

• medium: green

• long: yellow, orange, red

When did the first eyes appear?

• The Cambrian period (570-500 million years ago)

• They appeared as eyespots on primitive animals (eg flatworms - distinguished light/dark but not specific environmental features)

What is the process of vision?

1. Transformation occurs as light enters the eye via the pupil

2. Then becomes focused by the cornea and lens to form sharp images of the proximal stimulus on the

3. Retina which transmits signals to the

4. Optic nerve which contains optic fibres that conduct signals to the lateral geniculate nucleus of the thalamus

5. Before transmission to the visual receiving area of the cortex (v1/striate cortex)

What are the two types of visual receptors?

1. Rods

2. Cones

Compare rods and cones

How does the brain perceptually compensate for the blind spot?

The brain creates a perception that matches the surroundings to fill in the blind spot and make a coherent perception

What is considered normal vision?

20/20 vision

How does distance affect the cornea-lens combination?

>20ft away: light enters the cornea and lens in a parallel form which is then focused on the retina

<20ft away: light enters the cornea and lens at an angle which pushes the focus point beyond the retina(out of focus)

What triggers electrical signals in visual receptors?

the absorption of light by the visual pigment molecules in the outer segments of the rods/cones

What are the proteins of the visual pigments?

1. Opsin

2. Retinal

What happens to the visual pigment proteins in light vs dark?

What causes the increased sensitivity during dark adaptation?

demonstrated by William Rushton (1961), sensitivity increases with an:

• increased chemical concentration (visual pigments)

• increased chemical reactions (regeneration)

How is cone/rod spectral sensitivity measured?

• Cones: look directly at a test light

• Rods: measured in the dark using test flashes on the periphery

What are the absorption spectra for the visual receptors?

• Rods: 500 nm (blue-green light)

• Cones: (S/short wavelength pigment) 419nm, (M/medium wavelength pigment) 531nm, (L/large wavelength pigment) 558nm

What are the key components of the neuron?

Cell body, Dendrites, Axon (nerve fibre)

How many neurons are in the eye?

100 million in each eye

How are electrical signals recorded?

• From the axons of neurons, using recording and reference electrodes for signal detection

• At rest: the difference between electrodes is -70mV (ie) inside the axon is more negative than outside; this phenomenon is the resting potential

• When stimulated: the axon is 40mV more positive than the outside; this phenomenon is the action potential

What are the properties of an action potential?

1. 1 millisecond long

2. Propogated: allows the electrical signal to travel down the axon without a size decrease, thereby enabling long-distance transmission

3. The size of an action potential is not affected by stimulus intensity

4. The size of an action potential is affected by the rate of firing

5. Refractory period: the time between the occurrence of one nerve impulse and the other that can be generated in the axon

6. The upper limit for the refractory period is 500 - 800 impulses per second

7. Spontaneous activity: action potentials occurring in the absence of stimuli that establish a baseline for the level of neuron firings

What is the perceptual process of neural transmission?

1. Rods & Cones generate signals

2. Horizontal cells transmit the signals across the retina’s neurons

3. Bipolar cells receive these signals

4. Amacrine cells relay these signals (between B and G cells)

5. Ganglion cells receive the signals

6. Ganglion axons transmit signals to the optic nerve

How does convergence occur in vision?

The Eye

• Each eye contains 126 million receptors and 1 million ganglion cells

• Convergence occurs when several neurons synapse onto one singular neuron. Hence, this disproportion of receptors to cells causes convergence

• 1 ganglion cell receives signals from 126 receptors simultaneously

The Retina

• There are also 120 million rods in the retina and 6 million cones

• Therefore, ganglion cells receive signals from 120 rods and 6 cones

• Since multiple neurons cause the firing of the same ganglion cell, separate spots of light fire the same cells and hence, no specific information about the two separate spots of light is discernable

The Fovea

• The cones here have “private lines” to ganglion cells

• There is a 1:1 distribution of signals from cones to ganglion cells, hence there is no convergence

• When two separate spots of light appear, two separate ganglion cells are fired which provides information that there are in fact two separate spots and produces visual acuity

Day vs Night sensitivity

• Day Time: use cones for vision, sensitivity is at 560nm

• Night Time: use rods for 420 - 580nm

How is visual acuity measured in infancy?

• Preferential Looking (PL) Technique: (due to babies’ spontaneous looking preferences) two stimuli are presented and the experimenter observes the infants’ eyes to determine where they are looking. To prevent bias, the experimenter does not know which stimulus is being presented on the left/right. If the infant looks at one stimulus more than the other, they conclude that the infant can differentiate

• Visual Evoked Potential (VEP) Technique: electrodes on the infants’ visual cortex record signals that measure acuity to provide an objective measure of the visual system’s detail detection capacity. It indicates a better acuity

• Both methods indicate acuity is poorly developed at birth (20ft/400ft - 20ft/600ft at 1 month).

What is the Visual Acuity of Infants?

• Acuity increases at 6-9mnts followed by a levelling-off period; full adult acuity is achieved after 1yr

• Low acuity results from poorly developed cone receptors in the fovea; widely spaced and small outer segment size that contains less visual pigment hence not absorbing light effectively since light gets lost in the spaces between the cones and cannot be used to see

• Acuity is decreased because of poorly developed visual areas of the brain that contain fewer neurons and synapses.

• As they age, neuron/synapse numbers increase and the cones become more densely packed

• The peripheral retina appears adult-like