Chapter 7: Processing the Image Review structure of the eye Review structure of the retina Review receptive fields –Apply to an image on the retina –Usage in fine art Lateral Inhibition –Edge enhancement –Lightness contrast –Optical Illusions

The Human Visual System The photoreceptors turn light into electrical signals The nerve cells in the retina do some low level image processing Then transmit the signal to the optic nerve and the brain

Review Structure of Eye

Structure of the Retina Light

Concept Question The image that illuminates your retina is not the same thing as the image that your brain receives. Why is this? A.The lens and cornea distort the image B.The nerve cell system does low-level image processing C.The photoreceptors record the image inaccurately

Structure of Retina Cones (large dots) Rods (small dots)

Receptive Fields: Retina Excitatory regions Inhibitory regions

Nerve Cell Structure Ganglion Cell Light

Receptive Field on the Retina The size of the receptors and the receptive field are shown here much larger than actual size!

Image on the Retina

Image on the Receptive Fields

Lateral Inhibition: Example Here we have zoomed the image to get a more reasonable size for the receptive fields

Lateral Inhibition: Example Some inhibitory region is illuminated, response decreased

Lateral Inhibition: Example Receptive field uniformly illuminated, response is unchanged

Lateral Inhibition: Example Entire excitatory region illuminated, part of inhibitory region not illuminated, response increased For this receptive field, is the response A.Increased B.Decreased C.Unchanged

Lateral Inhibition: Example Entire excitatory region illuminated, most of inhibitory region is illuminated, response only slightly increased

Lateral Inhibition: Example Very little excitatory region illuminated, some inhibitory region illuminated, response slightly decreased For this receptive field, is the response A.Increased B.Decreased C.Unchanged

Lateral Inhibition: Results

Seurat: Original Painting

Seurat: With Edges Adjusted

El Greco: Christ in the Garden of Gethsemane Background gets darker right before edge

Simultaneous Lightness Contrast Occurs when the lightness of an area is influenced by neighboring regions Our perception of lightness is not objective, but depends on the surrounding area The center square on the right looks lighter because the surrounding area is a darker gray

Simultaneous Lightness Contrast The effect is more pronounced if you stare at the ‘x’ so the squares are in your peripheral vision This is because lateral inhibition acts over greater distances in the peripheral areas of the retina X

Lateral Inhibition & Edge Enhancement Which of the disks is lighter? X

Lateral Inhibition & Edge Enhancement Notice that the right circle disappears if you look at it in your peripheral vision. This is because the edge is not very sharp X

Hermann Grid Illusion You should see dark spots at the intersections between the white lines

Hermann Grid Illusion Which receptive field sends a larger signal to the brain? A) B) A B

Hermann Grid Illusion: Explanation When the image is on the first receptive field there is more light falling on the surround (inhibitory) than in the second position So there is more suppression and the illusion of a dark spot at the first location

Hermann Grid: Reversed You should now see light spots at the intersections between the black lines

More Illusions Each vertical band has equal light intensity across its width. However the left side of each bar appears darker than the right side due to lateral inhibition at the edges more inhibition (looks darker) less inhibition (looks lighter)

Image Processing: Edge Cues We rely on edges for information about uniform regions in- between This processing cues the brain to make generalizations which may not be objectively correct We process (distort reality) by generalizing the edge contrast conclusion to the non-edge regions. We jump to a conclusion or fill in based on edge information

Successive Lightness Contrast In simultaneous lightness contrast, a signal received at a different place in your receptive field inhibits response. In successive lightness contrast, a signal received at one time inhibits response in the receptive field a later time.

Successive Lightness Contrast We know from dark adaptation that the sensitivity of the retina changes depending on the intensity of light hitting it. Prolonged or intense stimulation by an image on the retina desensitizes those parts of retina. Those parts of the retina have a weaker response to subsequent stimulation.

Negative Afterimages Stare at the black dot in the center of the left image for about 30 seconds, then look at the dot on the right.

Negative Afterimages