Fall15Module2.1

ART 100Fall 2015

Class 3

agenda 9.1.15

• function of the eye• how eye and brain work together to create vision• the active, constructed nature of vision

In general in this course, we are interested not in the “nature” of vision, but in its culture; in other words, how humans have developed languages of visual communication given our status as sighted creatures.

So this session is a bit of a departure.Today we study the eye, the brain and the dynamic process of visual perception, to understand how our perception works.

schematic diagram of how vision works(please note: this diagram is WRONG)

Most people assume that vision works as pictured in the diagram below.

Put in words: our vision is just what our eye sees and reports to the brain.

why is the eye/camera idea wrong?

There is no “image,” no picture in the eye at all. In the eye, light admitted through the pupil and focused through the lens differentially stimulates the neuron-rich tissue at the back of the eye (the retina), sending patterns of electrical impulses to the brain (specifically to the visual cortex), where the signals must be processed and interpreted to create what we see.

“the eye is like a camera”

This analogy holds up to a point.

The point at which it no longer holdsis the retina.

Please note: this diagram is TRUE up to a point and then becomes FALSE.

what happens in the retina?Light is converted to an electrical signal in retinal photoreceptors via a light-sensitive protein called rhodopsin

Transduction is the process by which electrical impulses are converted to chemical form. This occurs differently in rods and cones.

The signalling mechanism is quite sophisticated. It optimizes the information transmitted from the retina by using “inhibition” to reduce the signal in certain areas (thus boosting the rest).

cones: large range of intensities, color vision, work quickly, very sensitive to small changes, concentrated in centerrods: evolutionarily more recent, but outnumber cones 20 to only work in very low light, evenly distributed across the retina

these signals are transmitted via the optic nerve to the primary visual cortex

what happens in the visual cortex?This is where matters get really complicated!There is evidence that there are THREE separate systems that process these signals.

animal and human evidence for 3 discrete processing systems

“Although the visual processing mechanisms are not yet completely understood, recent findings from anatomical and physiological studies in monkeys suggest that visual signals are fed into at least three separate processing systems. One system appears to process information mainly about shape; a second, mainly about color; and a third, movement, location, and spatial organization.”

Human psychological studies support the findings obtained through animal research. These studies show that the perception of movement, depth, perspective, the relative size of objects, the relative movement of objects, shading, and gradations in texture all depend primarily on contrasts in light intensity rather than on color.”SOURCE:

http://www.brainfacts.org/sensing-thinking-behaving/senses-and-perception/articles/2012/vision-processing-information/

Please note: this diagram is still a bit misleading, but it’s a whole lot better than the previous one.

This provides an explanation for why black and white drawings appear every bit as convincing in their illusion of space as color drawings.

in the third system (depth/location/movement)

“About 60 years ago, scientists discovered that each vision cell’s receptive field is activated when light hits a tiny region in the center of the field and inhibited when light hits the area surrounding the center. If light covers the entire receptive field, the cell responds weakly.”

Another way to put this is: “the visual process begins by comparing the amount of light striking any small region of the retina with the amount of surrounding light.”This process is enhanced by “lateral inhibition,” in which all but the strongest signals are filtered out by the retina before even reaching the brain. (Preference for edges.)

http://www.brainfacts.org/sensing-thinking-behaving/senses-and-perception/articles/2012/vision-processing-information/

John Singleton COPLEYMrs. Ezekiel Goldthwaite1771oil on canvas50 1/8 x 40 1/8 inches

http://www.mfa.org/collections/object/mrs-ezekiel-goldthwait-elizabeth-lewis-32756

This human perceptual preference for “edges”—areas of high contrast betweenlight and shadow—is also exploited by artists wanting to create convincing three-dimensional illusions in their two-dimensional art.

how is color/brightness processed? (this appears to be an independent pathway)how are form/shape processed to produce object recognition? how are motion, depth, and spatial relations processed?when and how are all of these coordinated?

object recognitionHumans are capable of instantly recognizing people and objects in visually cluttered scenes.Machines cannot do this, yet.What can machines do? Current research on software teaching scene recognition.

http://vision.stanford.edu/documents/LiFei-Fei_ICCV07.pdf

summaryThe brain constructs your field of vision from electrico-chemical impulses sent by your eyes.The eye collects data on:

shapecolorposition/location/movement

These elements seem to be processed via discrete mechanisms in the visual cortex and coordinated into a coherent visual field.

The raw data entering the third system has to do with differences in light intensity. These signals are enhanced by the retina through the process of lateral inhibition and are subsequently interpreted by the visual cortex to produce our field of vision, which we experience as continuous and compelling rather than as a series of approximations of distance, size and depth via contrasts between light and shadow.

Our ability to, judge distance, move through space, avoid obstacles,—these are all INFERENCES drawn from information about contrasts between light intensity rather than actual visual data—even though we perceive them as properties of our vision.This is ANOTHER REASON why the eye/camera idea is completely misleading.

The eye adjusts for relative brightness/darkness and motion stabilization.

try these at home!http://serendip.brynmawr.edu/bb/blindspot1.htmlhttp://serendip.brynmawr.edu/bb/blindspot/http://serendip.brynmawr.edu/bb/latinhib.htmlhttp://dragon.uml.edu/psych/illusion.html

M.C. Escher (Dutch, 1898 – 1972), Drawing Hands, 1948, ithograph, 11 1/8 x 13 1/8 in

visual puzzles and optical illusions exploit the ambiguities in these systems