The Visual System

Gross structure of the eye Cornea –does most of the focusing lens Aqueous humor Vitreous humor Central fovea – ~4,000 receptors – 10 5 times better resolution than TV Suspensory ligaments Ciliary muscle Optic nerve

Facts about refraction The refraction system (cornea and lens) focuses an inverted image on the retina – most of the refraction is corneal, but the lens is the part of the system whose refraction can be altered for distance accomodation. Accomodation for near objects: ciliary muscles contract – tension in suspensory ligaments decreases – lens assumes a more spherical shape – greater light bending

The retina Consists of: –Photoreceptors (two basic types) – modified epithelial cells –4 major types of interneurons: Bipolar, horizontal, amacrine (2 nd order cells) Ganglion cells (3 rd order cells whose axons pass into the optic nerve)

Microscopic anatomy of the retina Note that this picture is oriented so that the eyecup faces downwards and light would strike the retina from the bottom of the picture.

At the fovea, the overlying layers of the retina are swept aside to improve illumination of the photoreceptors. The receptive fields of ganglion cells in this are quite small, so perception of fine detail is high.

Photoreception: primates are tricromats Visual pigments confer wavelength specificity: –Rhodopsin (rod receptors) 498 nm –Blue cones 437 nm –Green cones 533 nm (but there are some genetic variants) –“Red” cones 564 nm (actually, peak is in yellow-orange part of spectrum)

Most mammals are functionally colorblind Most other mammals that have been tested lack hue discrimination, even though some of them appear to have the retinal equipment to do so. In particular, most mammals do not perceive red light

Scotopic and photopic systems In primates, rod coupled system (scotopic system) operates in dim light, cone- coupled system (photopic system) operates in bright light. In terms of photoreceptor distribution, the photopic system dominates in the central retina and fovea centralis; the scotopic system dominates toward the periphery of the retina.

What you need to know about photoreceptor function Photoreceptors are depolarized in the dark and hyperpolarize when illuminated. The photoreceptor’s membrane potential modulates synaptic vesicle release in the way that you would expect. The transmitter released by photoreceptors can have inhibitory effects on some 2 nd order cells and excitatory effects on others.

How do we map receptive fields of retinal ganglion cells? An anesthetized animal (which doesn’t track moving stimuli with eye movements) views a video screen on which a stimulus spot is moved. The image of the spot falls on corresponding parts of the retinal surface. An electrode is advanced into the optic nerve of the stimulated eye. It randomly contacts a single ganglion cell’s axon. In the absence of an appropriate stimulus, the ganglion cell is spontaneously active at a low rate. The investigator moves the stimulus spot around widely on the screen until there is a response from the ganglion cell. The investigator then moves the spot over small distances and records the changes in the ganglion cell’s activity.

Two kinds of ganglion cells can be detected in the retinas of most mammals Static cells with annular receptive fields, either on center/off surround or off-center/on surround. The optimum stimulus for such cells is either a light spot on a dark background or a dark spot on a light background. Direction-sensitive cells that respond most vigorously to spots that move across their receptive fields in a particular direction.

Retinal ganglion cells are spot detectors Static cellsDynamic cells

Lateral inhibition in the retina determines annular receptive fields of ganglion cells The orange ganglion cell is an off center/on surround cell – the tan one is an on center/off surround cell. Notice that lateral inhibition is mediated by horizontal cells.

Neurons in the visual pathway Retina10 8 photoreceptors Optic Nerve10 6 axons of ganglion cells Lateral Geniculate Nucleus (thalamus) 10 7 interneurons Primary visual cortex10 9 neurons

Anatomic pathway from the retina to the primary visual cortex

In this partly dissected brain the green line traces the visual pathway from the optic chiasm to the lateral geniculate nucleus of the thalamus (loop), and then to the primary visual cortex

The left cortex is a map of the right side of the visual field, and vice versa. In the human visual system, and other animals where there is substantial overlap of the visual fields of both eyes, axons of ganglion cells in the medial (nasal) side of each retina decussate at the optic chiasm, whereas the ones on the lateral (temporal) side of each retina do not decussate. Thus, those parts of the retinas of both eyes that view the left side of the visual field pass information to the right visual cortex, and vice versa for the right side of the visual field.

Decussating and nondecussating retinothalamic pathways deal with the overlap

Thought Question What differences would you expect in the retinothalamic connections for those animals that do not have much overlap in the visual fields of the two eyes, compared to those that have significant overlap?

Visual deficits can arise from disease processes The pituitary tumor has destroyed the decussating fibers at the optic chiasm, causing tunnel vision. The cerebral infarct has destroyed the primary visual cortex on one side, causing loss of all information from the contralateral side of the visual field.

This is a summary of the visual deficits that could be expected from lesions at various points in the visual pathway

Form analysis in the primary visual cortex

Ocular dominance within the cortex can be revealed by a method that exploits the brain’s energy metabolism A monkey which had had one eye masked was injected with 3 H-labeled 2-deoxy D glucose. This glucose analogue is taken up by cells as if it were glucose, but can’t be metabolized. After a few minutes the animal was sacrificed and the visual cortex sliced for autoradiographs. The stimulated cortical columns accumulated the radiolabel at a higher rate than the unstimulated ones.

The experimental evidence for cortical organization A. single-cell records from cortical cells while specific stimuli are being delivered to the retina B. isotopic measurements that give a big picture of neuronal excitation patterns across the whole visual cortex in response to particular visual stimuli

Single-cell records: cortical cells are orientation detectors Cortical cells are most sensitive to lines, borders and edges. In some cases, the length of the bar is also critical. This receptive field structure is established by wiring each cortical cell to receive projections from a row of ganglion cells on the retinal surface. Simple cortical cells can be activated by static bars or edges of particular orientations Complex cortical cells are activated by flashing or moving bars of the correct orientation.

Orientation columns are revealed by a stimulus pattern of repeating stripes In this experiment a pattern of vertical stripes was presented in one part of the visual field, corresponding to the left part of the autoradiogram, while randomly oriented stripes were presented in the other part (right part of the autoradiogram). In the left part of the picture, only those columns that are specific for vertical orientations respond; in the right part, all columns respond.

The basic functional unit of the visual cortex is a hypercolumn In any single cortical column, all cells prefer the same orientation and are dominated by input from one retina. Adjacent columns in an ocular dominance band prefer slightly different orientations. A hypercolumn consists of two ocular dominance bands (one for each eye) each containing enough columns to account for all possible stimulus orientations.

A hypercolumn contains all of the information needed to analyze for all orientations from a single point on the cortical map, using input from both eyes In this sketch we are looking at the cortical surface This hypercolumn contains a total of 10 columns: 5 for each orientation dominated by the left eye and 5 corresponding ones dominated by the right eye

Developmental assembly of the visual cortex

In the cat, correct assembly of the cortical map depends on early visual experience Elimination of all visual stimulus to both eyes during a 1-2 week critical period soon after birth results in failure to form orientation-specific columns- the animal has severe visual deficits and visual experience after the critical period Restricting exposure to one particular orientation causes all columns to prefer that orientation – the animal can see vertical orientations but is blind to other orientations.

Thought question What do you suppose happens to the cortical wiring of the left cortex if the right eye is masked during the critical period?

A link to an interesting website that lets you experience optical illusions (and sometimes tries to explain them)

A link to an on-line anatomical atlas – the neurosyllabus portion may be helpful for the present course segment