Vision Key notes

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Describe the different photoreceptor types and their connections

Light sensitive cells in the retina consist of two types of photo receptors: rods and cones.

Rods and cones contain photopigments that break down when exposed to light, triggering a series of stages that leads to neural impulses that are conveyed to the brain by the optic nerve.

The human retina has about 120 million rods and about 6 million cones.

Despite being fewer of them, cones are most important for seeing fine detail, and are most active in the daylight. Different types of cones are sensitive to different wavelengths of light, and are responsible for our ability to see colour.

Rods do not discriminate between different wavelengths, and they cannot discriminate fine visual detail. But rods are much more sensitive to light than cones, and so rods are used in dimly illuminated environments.

What are the types of cones?

Short-wavelength (S) cones - peak sensitivity at 440 nm (blue light)

Medium-wavelength (M) cones - peak sensitivity at 530 nm (green light)

Long - wavelength (L) cones - peak sensitivity at 560 nm (red light)

What are the visual pathways to the brain?

1.After leaving the eye the axons of retinal ganglion cells are bundled together to form the optic nerves (one for each eye).

2. These project posteriorly and medially toward the optic chiasm.

3. Here, roughly half the axons from the retina of each eye cross over to the opposite side of the brain. Axons from the temporal half of the retina of the right eye remain on the same side, but axons from the nasal half cross over to the left hemisphere. Similarly, axons from the temporal half of the retina of the left eye remain on the same side and axons from the nasal half cross over.

NOTE: This arrangement means that visual information from the right visual field is conveyed exclusively to visual areas in the left hemisphere, whereas visual information from the left visual field is conveyed exclusively to the right hemisphere.

4. Beyond the optic chiasm, axons in the optic tracts continue posteriorly until they form synapses with neurons in a part of the thalamus called the lateral geniculate nucleus (LGN; one in each hemisphere).

5. Neurons in the LGN send their axons posteriorly where they form synapses with neurons in the primary visual cortex. 90% of LGN axons terminate in the primary visual cortex / remaining 10% project to other areas, including the superior colliculus and pulvinar nucleus of the thalamus.

What are the interactions in the retina that allow visual perception?

There is a convergence of information from the photoreceptors, through the bipolar cells to the ganglion cells. These are combined to influence the response of a single ganglion cell.

Each ganglion cell responds to signals from one small cluster of photoreceptors, which in turn are excited by light from one small region of the visual field (this is known as the receptive field).

The photoreceptors exert an excitatory influence on the bipolar cells with which they form connections. Adjacent bipolar cells can then have either an excitatory or inhibitory influence on a single ganglion cell.

Receptive fields of ganglion cells tend to have a centre-surround organisation (like a doughnut).

ON cells are excited by light in the centre of their receptive field, and inhibited by light in their surrounding field.

In contrast, OFF cells are inhibited by light in the centre of their receptive field, and excited by light in their surrounding field.

These two simple types of receptive field provide a powerful mechanism for signalling edges and borders in the visual world

How does the primary visual cortex function and what is the role of simple cells?

LGN neurons provide input into the simple cells of the primary visual cortex (back of brain). Simple cells signal the presence of a line or edge; orientation of an object. The simple cells are either excited or inhibited by balance of on and off regions of the receptive field touched by incoming light.

Simple cells have an orientation preference - each cell fires preferentially to bars and edges of one particular orientation (e.g., vertical, horizontal, 45° clockwise from vertical, etc.). Strong response for vertically orientated objects.

Explain the anatomical structure of the primary visual cortex

The primary visual cortex is divided into several functional modules:

V1 = Primary visual cortex

V4 = colour

MT = Movement

Inferior temporal cortex = complex objects & faces

Name clinical conditions associated with impairments to regions: V1 and V4

Area V1 in each hemisphere contains a retinotopic map of the contralateral half of the visual field (right goes left & vice versa).

Damage to V1 or a diagnosis of 'Hemianopia' will result in blindness to the visual field contralateral to the impairment in the visual cortex.

Similarly for v4 damage to the left side in the cortex would cause colour blindness in the right (disorder called achromatopsia or Hemiachromatopsia).

Rarely occurs in isolation and usually found with other visual functional impairments.

Explain motion perception

Perception of motion allows us to discriminate form & structure.

We are biologically tuned into perceive motion as shown by examples in lecture:

E.g. when light sources were placed at joints of limbs, it was sufficient to perceive a person walking despite only being a collection of dots.

E.g Motion afterefect- rainfall video: caused by visual nuerons adapting when exposed to a continuously moving stimulus as in the waterfall illusion. The motion system is easily tricked into interpreting displace of a single visual image in space as continuous motion.

E.g. Motion blindess- disorder called akinetopsia, occurs when area MT is damaged in both hemispheres, but in which regions responsible for colour and form vision remain intact.Results in moving visual stimuli appearing like a succession of 'snapshots' e.g. leads to errors when pouring tea, crossing roads etc.

What is visual object agnosia?

Visual object agnosia is the loss of the ability to recognise familiar objects through the modality of vision.

Patients with this condition are still able to recognise objects through other sensory modalities, such as hearing and touch, indicating that they have not lost information about what the objects are called, what they mean and what they are used for.

Visual object agnosia typically occurs after unilateral or bilateral damage of the inferior part of the temporal cortex.

What is prosopagnosia?

A region of the inferior part of the temporal lobe called the fusiform gyrus is important for face recognition (uttner et al., 2002)

A problem in recognising familiar faces is called prosopagnosia (ability to recognise other objects may still be relatively good)