2 The First Steps in Vision

Transcription

1 2 The First Steps in Vision 2 The First Steps in Vision A Little Light Physics Eyes That See light Retinal Information Processing Whistling in the Dark: Dark and Light Adaptation The Man Who Could Not See Stars 2 A Little Light Physics Light: A wave; a stream of photons, tiny particles that each consist of one quantum of energy.

2 2 A Little Light Physics Light can be absorbed, diffracted, reflected, transmitted, or refracted. Absorbed: Energy (e.g., light) that is taken up, and is not transmitted at all. Diffracted: Bent, or having waves that are spread out, (e.g., waves of sound or light, as they pass through a narrow aperture). Reflected: Energy that is redirected when it strikes a surface, usually back to its point of origin. 2 A Little Light Physics Transmitted: Energy that is passed on through a surface (when it is neither reflected nor absorbed by the surface). Refracted: Energy that is altered as it passes into another medium, (e.g., light entering water from the air). 2 Eyes That See Light The human eye is made up of various parts: Cornea: The transparent window into the eyeball Aqueous humor: The watery fluid in the anterior chamber Crystalline lens: The lens inside the eye, which enables changing focus

3 2 Eyes That See Light Pupil: The dark circular opening at the center of the iris in the eye, where light enters the eye Vitreous humor: The transparent fluid that fills the vitreous chamber in the posterior part of the eye Retina: A light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve 2 The Human Eye 2 Eyes That See Light Refraction is necessary to focus light rays and this is done by the lens The lens can change its shape, and thus alter the refractive power: Accommodation Emmetropoia: The happy condition of no refractive error

4 2 Eyes That See Light Problems of Refraction: Myopia: When the light entering the eye is focused in front of the retina and distant objects cannot be seen sharply; nearsightedness. Hyperopia: When light entering the eye is focused behind the retina; farsightedness. Astigmatism: A visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea. 2 Optics of the Human Eye 2 Astigmatism Chart

5 2 Eyes That See Light The retina: Optics are similar to those in most cameras, including mechanisms for regulating the amount of light and lens for adjusting focal length for viewing near and far objects. 2 Eyes That See Light Using the ophthalmoscope, doctors can view the back surface of patients eyes: fundus 2 The Blind Spot

6 2 The Retina 2 Eyes That See Light Photoreceptors: Cells in the retina that initially transduce light energy into neural energy. When light hits the eye, the process of photoactivation begins. Light is transduced by rod and cone photoreceptors. Rods: Photoreceptors that are specialized for night vision. Some animals have mostly rod retinas, (e.g., rats, owls) Cones: Photoreceptors that are specialized for daylight vision, fine visual acuity and color. Most mammals have cones. 2 Rods and Cones

7 2 Photoreceptor Density across the Retina 2 The Rule of Thumb Visual Angle: refers to the size of an image on the retina. 2 Retinal Information Processing Various regions of the retina interact via lateral inhibition: Horizontal cells and amacrine cells. The vertical pathway: Photoreceptors, bipolar cells, ganglion cells. A variety of bipolar cells, for instance. Diffuse bipolar cells; Receive input from multiple cones. Midget bipolar cells; Receive input from a single cone.

8 2 Different Types of Retinal Ganglion Cells 2 The Receptive Field (Part 1) Receptive field: The region in space in which stimuli will activate a neuron. 2 The Receptive Field (Part 2)

9 2 Retinal Information Processing Kuffler mapped out the receptive fields of individual retinal ganglion cells in the cat Found that the spatial layout of the ganglion cell s receptive field is concentric Cell fires at its fastest rate when the spot size matches the size of the excitatory center; reduces its firing rate when the spot begins to encroach on its inhibitory surround; ON-center cell OFF-center cell does the opposite 2 Whistling in the Dark: Dark and Light Adaptation Two mechanisms for dark and light adaptation: Pupil dilation Photoreceptors and their replacement 2 Whistling in the Dark: Dark and Light Adaptation Neural circuitry of the retina accounts for why we are not bothered by variations in overall light levels. The visual system regulates the amount of light entering the eye, and by ignoring whatever variation in overall light level is left over.

10 2 The Man Who Could Not See Stars Retinitis pigmentosa: A family of hereditary diseases that involve the progressive death of photoreceptors and degeneration of the pigment epithelium. The Fundus: Normal vs. Retinitis Pigmentosa 2 Visual Fields: Normal vs. Retinitis Pigmentosa