Image Processing : 5. Human Vision Aleix M. Martinez - - PDF document

1 Image Processing:

• 5. Human Vision

Aleix M. Martinez aleix@ece.osu.edu

The Human Eye

2

3 Diopters

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TT]E EYE Stimulus:

• n

Stimulus:

• n

The more of a given region, on or off , the stimulus filled, the greater was the response, so that maximal on responses were obtained to just the right size circular spot, and maximal off responses to a ring ofjust the right dimensions (inner and outer diameters). Typical recordings of responscs to such stimuli are shown on this page. The center and surround regions interacted in an anrago- nistic way: the effect of a spot in the center was diminished by shining a second spot in the surround-as if you were telling the cell to fire faster and slower ar the same time. The most impressive demonstration of this interaction between center and surround occurred when a large spot covered the entire receptive ficld of the eanglion cell. This evoked a response that u,as much weaker rhan the rcsponse to a spot just filling the center; indeed, for some cells the effects

• f

stimulating the two regions cancelled cach other cornpletely. An of-tenter cell had just the opposite behavior. Its receptive field consisted

• f a small ccnter frorn which off responses

were obtained, and a surround that gave on responscs. The two kinds of cells were intermixed and seemed to be equally common. An off-center cell discharges at its highest rate in response to a black spot on a white background, because we are now illuminating only the surround of its receptive field. In nature, dark objects are probably jusr as common as light ones, which may help explain why information from the retina is in the form of both oll-center cells and off-center cells. lf you make a spot progressively larger, the rcsponse inrproves until the receptive-field center is filled, then it starts to decline as more and more of the surround is included, as you can see from the graph on the ltcxt page. With a spot covering the etrtire field, the center either just barely wins oLlt over the surround, or the result is a draw. This effect explains why neurophysiologists Le-li: Four recordings fronr a typical on- center retinal ganglion cell. Each record is a single sweep

• f the oscilloscope,

whose duration is 2.5 seconds. For a sweep this slow, the rising and falling phases

• f the

impulse coalesce so that each spike appears as a vertical line. To the left the stimuli are

• shown. In the resting srate

at thc top, there is no stimulus: firing is slow and morc or less

• random. The lower three

records show responses to a srnall (optin-rum size) spot, a larple spot covering the receptive-field cen- ter and surround, and a ring covering the surround only. Rigftr: Responses

• f an off-

center retinal ganglion cell to the sante set

• f stinruli shown at the left.

Simple and complex cells

CHAI)T'Ell I Optic nerve Optic chiasm Optic tract Lateral geniculate nucleus Prirnary visual cortex Optic radiations

CHAPTER 7 138 The corpus callosum is a thick, bent plate

• f axons near the center of this brain sec-

tion, made by cutting apart the human ce- rebral hemispheres and looking at the cut surface.

condition called agenesis

• f the corpus

callosum. Occasionally it may be com- pletely or parrially cut by the neurosurgeon, either to treat epilepsy (thus pre- venting epileptic discharges that begin in one hemisphere from spreading to the other) or to make it possible to reach a very deep tumor, such as

• ne in the

pituitary gland, from above. In none of these cases had neurologists and psy- chiatrists found any deficiency; someone had even suggested (perhaps not seri-

• usly) that the sole

function of the corpus callosum was to hold the two cere- bral hemispheres together. Until the r95os we knew little about the detailed connections

• f the corpus callosum. It clearly connected

the two cerebral hemispheres, and

• n the basis
• f rather

crude neurophysiology it was thought to join precisely corresponding cortical areas

• n the

two sides. Even cells in the striate cortex were assumed to send axons into the corpus callosum to termi- nate in the exactly corresponding part

• f the

striate cortex

• n the
• pposite

side. In 1955 Ronald Myers, a graduate student studying under psychologist Roger Sperry at the University of Chicago, did the first experiment that re- vealed a function for this immense bundle

• f fibers.

Myers trained cats in a box

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THE CORPUS CALLOSUM AND STET{EOPSIS

containing two side-by-side screcns

• nto which hc could project

imagcs, for example a circle

• nto onc screcn

and a square

• nto thc other.

He taught a cat to press its nose against the screen with the circle, in prefcrcnce to the one with the square, by rewarding correct responses with food and punishing mistakes mildly by sounding an unpleasantly loud buzzer and pulling the cat back from the screen gently but firmly. By this method the cat could be brought to a fairly consistent performance in a few thousand trials. (Cats learn slowly; a

Here the brain is seen from above. On the right side an inch or so of the top has been lopped off. We can see the band of the cor- pus callosum fanning out after crossing, and joining every part of the two hemi- spheres. (The front of the brain is at the top of the picture.) THE CORPUS CALLOSUM AND STEREOPSIS

side world-except,

• f course,

for any input that area might receive from the left occipital lobe via the corpus callosum, as you can see from the illustration

• n this page. He then looked for responses

by shining light in the eyes and recording from the right hemisphere with wire electrodes placed

• n the corti-

cal surface. He did record responses, but the electrical waves he observed appeared

• nly at the inner border of area

17, a region that gets its visual input from a long, narrow, vertical strip bisecting the visual field: when he used smaller spots

• f light, they produced

responses

• nly when they were flashed

in

In his experiment, Whitteridge cut the right

• ptic tract. For information to get from

either eye to the right visual cortex, it now has to go to the left visual cortex and cross in the corpus callosum. Cooling either of these areas blocks the flow of nerve impulses. Corpus callosum Cortex

The Arnolfini Portrait Jan van Eyck, 1434

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74 Perception and Artistic Style Figure 4.3 The diagram shows the inconsistencies in perspective in the van Eyck porhait shown in figure 1..1.

• ne would expect

them to be readily noticeable but in fact they are not The t as vi Two both , world horizc € vok€ r horizc identir that or lines i 2.6). t angle paper-r constn be four Figure 4 illusion.

La Crau Vincent van Gogh

Perception and Artistic Style 1, "Le Crau". (Reproduced by Amsterdam).

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Cues Intrinsic to the Eye 1,39 Figure 7.38 Focal points implicit in van Gogh's Le Crau are shown in this diagram.

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