The Eye

A Model for Sensing  Three components: Stimulus detection – a specialized sensory neuron Reception – where neurons receive information from the sensory neurons Integration – where information from receivers is processed  All the human sensory systems have these components.

The Visual System  Retina  Optic nerve (axons of ganglia in eye)  Layers in the thalamus (LGN) Secondary paths to SCN (circadian rhythms) and superior colliculi to control eye movement  Primary visual cortex

Adaptation  Adaptation -- diminishing receptiveness of a sensory receptor neuron. Habituation – also diminishing receptiveness but at a different level (within a neuronal circuit not a cell).  Adaptation is essential for the perception of change.  Dark and light adaptation permit vision under different environmental conditions.

Parts of the Eye  Cornea – protects eye and initiates focusing  Lens – focuses light on the appropriate spot on the retina  Iris – adjusts focus by opening and closing pupil to admit light  Pupil – hole that admits light

More Parts of the Eye  Aqueous and vitreous humor – fluid inside eye  Sclera – whites of eye  Retina – layer of photoreceptors at the back of the eye, responsive to light  Blind spot – place where optic nerve exits the eye  Fovea – spot of best focus and densest cones

Types of Photoreceptors  Rods – used for brightness perception and motion  Cones – used for color and form (shape) perception  Cones are wavelength-specific: Blue = 430 nm Green = 530 nm Red = 560 nm  Mixing all three equally produces white

Transduction  Photoreceptors release the neurotransmitter glutamate (glu) when depolarized. Depolarized in the dark. Hyperpolarized by light.  Only ganglion cells have action potentials. Photoreceptors produce graded response that provides input aggregated by bipolar cells.  Magno ganglion cells receive input from rods, parvo ganglion cells from cones

Bipolar Cell Receptive Fields  The receptive field is the area of the retina capable of changing the bipolar cell’s membrane potential  Two kinds of receptive fields: OFF cell – excitatory ON cell – inhibitory OFF and ON refers to light, not the cell  Center and surround are opposites

Edge Detection  The center-surround organization of the receptive fields of ganglion cells exaggerates the contrast at borders.  Visual processes “fill in” what occurs between borders (edges).  Contrast effects occur because we notice variations, not absolute magnitudes of light.

Color Contrast  Cones respond to specific wavelengths of light that determine hue.  Color cells have complementary surrounds that heighten contrast and strengthen their signal.  Opponents are: red/green, blue/yellow.