The Retina and Fovea Rods and Cones Light & Dark Adaptation Blind Spot, Blood Vessels The Fovea and Acuity.

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Presentation transcript:

The Retina and Fovea Rods and Cones Light & Dark Adaptation Blind Spot, Blood Vessels The Fovea and Acuity

Gregory, 1997, p.43. Optic Nerve

Central Retina

Blood Vessels

Stabilized Retinal Image An image whose retinal position remains constant regardless of eye movements. Quickly fades. Usually prevented by tremor, microsaccades. Shows that constant change is needed for vision.

The Blind Spot

Portion of the retina through which the optic nerve and blood vessels pass and an area without photoreceptors. Point in different directions in the two eyes. Not obvious due to the cortex “filling in”.

Williamson, 1983, p.185. Light Levels

Adaptation Ignoring constant (stabilized) stimuli Filling in holes in image Lighting Intensity Discounted –Sunlight : moonlight ~million:1 –Need only ~100:1 for reflectances –Two sensor systems (rods, cones) –Dark Adaptation

Neurons in the retina Photoreceptors –Rods –Cones Bipolar Cells Ganglion Cells (output to brain) Also Horizontal and Amacrine Cells

Retina

Goldstein, 1989, Plate 2.1

Rods Cones Goldstein, 1989, p.75. Photoreceptors

The Fovea

Photoreceptor Distribution

Section Through the Fovea

Retinal Circuitry

Photoreceptors Rods “Blue” Cones “Yellow” Cones –“Green” –“Red”

Brightness Ranges in Vision Photopic: (Cone) –High light levels (daylight) –Cones –High acuity –Fast Scotopic: (Rods) –Low light level (night vision) –Rods –Low acuity –Slow Mesopic: –Twilight –Rods and cones

Dark Adaptation

Adaptation

Adaptation Test

Adaptation and After Effects Adaptation is a decrease in sensitivity during the adapting stimulation. An after-effect is a change in viewing other stimuli after the adapting stimulation ends.

Adaptation

Adaptation Test

Moiré Patterns produced by interactions between two repetitive patterns. Movement in some art may be due to after- effects interacting with the current view.

Photoreceptors and Light Photopigments absorb photons. –Rhodopsin, R,G,B cones all similar Starts chemical reaction Results in an electrical response. Dark adaptation changes the gain between absorption and electrical response.

Solso, 1994, p.11. Photopic (Cone) Sensitivity Perception of Brightness “Yellow” cones –Red + Green –Not “blue” Peak in yellow- green. Red and Violet very dark when no hue seen