Physiology and Visual Pathway of The Eye

How the eye works

• First, light rays are reflected off an object and enters the eyes through the cornea, a transparent outer coat of the eye.

• The cornea refracts light and together with the lens, focuses light onto the retina. Contrary to popular belief, the cornea instead of the lens, provides more refractive power (two thirds) due to its highly curved surface and high refractive index.

• As the light rays pass through the pupil, the iris changes the size of the pupil via sphincter (parasympathetic) and dilator muscles (sympathetic) to regulate the amount of light passing through. For example, in bright light, the sphincter muscles contract and dilator muscles relax to decrease the diameter of the pupil. The opposite action happens in dim light.

• The retina is a highly complex structure comprising photoreceptors (rods and cones) and neurones, some of which give rise to the optic nerve fibres. It converts light into electrical signals.

• Photoreceptors in the retina which are the rods and cones are responsible for detecting light with light-receptive pigments. Transformation of light energy into nerve impulses depend on alteration of visual pigments contained in these photoreceptors.

• Cones are responsible for daylight vision. Subgroups of cones respond to different wavelengths of light (blue, green and red). They are concentrated at the fovea (macula) which is responsible for detailed vision. (reading fine print). In bright conditions, cones provide clear, sharp, central vision.

• Rods are responsible for night vision. They do not signal wavelength info. They form the large majority of photoreceptors in the remaining retina. Rods provide peripheral vision, allowing eyes to detect motion.

• On the retina, the image formed is real, inverted and smaller. As the light passes through the lens, its trajectory changes. Thus the image is upside down on retina.

Accomodation, or how the eye sees images at different distances.

• In order to produce a focussed image, the zonular fibres supporting the lens transmit changes to the ciliary body. This allows the lens to change its shape and refractive power.

• In the normal resting state:

1. our ciliary muscle is relaxed
2. the elastic lens tends to become thick
3. aqueous & vitreous humour push outward on the sclerotic coat
4. ligaments become taut / tensed
5. lens pulled into a thin shape
6. short focal length

• When a near object is brought to our eye:

1. sphincter-like action of circular muscle fibres + contraction of longitudinal muscle fibres
2. contraction of ciliary muscle
3. distance between edges of ciliary body decreases
4. relaxation of suspensory ligament
5. lens becomes thicker
6. focal length shortens
7. light rays converge earlier; image formed on retina

Transduction

• Besides rods and cones, other cells found in the retina are bipolar, horizontal, amacrine and ganglion cells.
• The light activates the photoreceptor cells (turns them "on") and they in turn convert the light signal into a neural signal. This is done through the action of protein molecules on the outer segment of the rods and cones known as photopigments.
• The absorption of light by the photopigments then produces a chemical reaction that changes the rate of neurotransmitter release at the receptor’s synapse with the bipolar cells. The greater the change in transmitter release, the stronger the signal that is then passed on to the bipolar cells.
• The neural impulses from the bipolar cells then activate the ganglion cells. The axons of the ganglion cells make up the optic nerve, which then conveys the information to the brain for further processing.

Decussation of Optic Axon

• All information from the right half of the visual field projects to the left half of the brain, and vice-versa.

• The axons of ganglion cells in the nasal part of the retina (i.e. near the nose) cross in the optic chiasm. They convey information about the visual field periphery
(lightly shaded colours).

• The axons of ganglion cells in the temporal part of the retina do
not cross in the optic chiasm.



Location and structure of the lateral geniculate nucleus

• The LGN on each side of the brain receives projections from ganglion cells
located in the contralateral nasal retina and ipsilateral temporal retina.

• The LGN is formed by layers which receive projections from different classes of
ganglion cells.

• The Magnocellular (M) layers receive projections from the retinal Y cells, and the
Parvocellular (P) layers from the retinal X cells.

• The projection from the lateral geniculate nucleus to the cortex forms the
optic radiation, which is one of the most distinctive fibre tracts in the human
brain.

• The optic radiation is topographically organised. For example, information
from the upper half of the visual field (which projects to the lower retina) is
conveyed by the most ventral axons, and terminate in the lower bank of the
calcarine sulcus.



Receptive Field Mapping

• The visual receptive field of a cell is a small “window” of the visual field.
Presentation of a stimulus within the receptive field can modify the
neuron’s activity.

• By moving the spot (stimulus) on the screen, the experimenter defines
the receptive field.

• By changing the stimulus characteristics, the neuron’s selectivity for
stimulus features is determined.

• The receptive field is the “window” of the visual field that the neurone is
analysing.

• Only stimuli presented within the receptive field can change the
electrical activity of the neurone.

• The inteconnections between the retina, thalamus and cortex are TOPOGRAPHIC: adjacent cells of the retina connect to adjacent cells in the cortex.

• This creates a map-like representation of the visual scene in our visual cortices. There is one map in each visual area



Responses of retinal cells

• Retinal ganglion cells are maximally excited by a spot of light that fills the
receptive field.

• Because of the inhibitory surrounds, they respond poorly to uniform
illumination.

• Different ganglion cells will be maximally excited by light of different
colours.

• Similar to small photometers; each “reads” the amount and type of light
coming from a point of the scene.


Orientation

• In the visual cortex, cells are no longer “spot readers”. They code for the
presence of oriented boundaries.

• In this example, the neurone responds only to a boundary between light and
dark, of a specific orientation

• There are weak or no electrical responses to uniform light within the
receptive field, or to different orientations.