"Long before it's in the papers"
August 03, 2010

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Color plays “musical chairs” in brain

Oct. 4, 2009
Courtesy University of Chicago
and World Science staff

Sci­en­tists have man­aged to trick peo­ple’s eyes in­to los­ing sight of a shape while con­tin­u­ing to per­ceive its col­or.

The re­sult: the col­or—divorced from the shape to which it “be­longs”—seems to go in­to an­oth­er shape, ac­cord­ing to the re­search­ers.

This find­ing, they say, re­veals a new prop­er­ty of sight. 

Col­or from one shape is trans­ferred to an­oth­er. Above, two pat­terns of stripes are in­com­pat­i­ble to the eyes; when one pat­tern is shown to each eye, the brain on­ly sees one. In the mid­dle, the stripe pat­tern fades away at right, but its red­dish hue re­mains. Be­low, the red reap­pears in half of the stripes on the left side when the ver­ti­cal stripe pat­tern is per­ceived. (Im­ages cour­te­sy U. Chi­ca­go)

The brain pro­cesses an ob­jec­t’s shape and col­or in two sep­a­rate path­ways. And al­though the shape and col­or nor­mally are linked, the rep­re­senta­t­ion of col­or can sur­vive alone, said Ste­ven Shev­ell, a Un­ivers­ity of Chi­ca­go psy­chol­o­gist who spe­cial­izes on col­or and vi­sion. When that hap­pens, he added, the brain es­tab­lishes a new link that binds the col­or to an­oth­er vis­i­ble shape.

“Col­or is in the brain. It is con­structed, just as the mean­ings of words are con­structed. With­out the neu­ral pro­cesses of the brain, we would­n’t be able to un­der­stand col­ors of ob­jects any more than we could un­der­stand words of a lan­guage we hear but don’t know,” said Shev­ell.

The new ex­pe­ri­ments, by Shev­ell and Wook Hong of Van­der­bilt Un­ivers­ity in Ten­nes­see, are de­scribed in the cur­rent is­sue of the re­search jour­nal Psy­cho­log­i­cal Sci­ence.

Col­or is nor­mally thought of as a fun­da­men­tal at­trib­ute of an ob­ject: a red Cor­vette, a blue lake, a pink fla­min­go. Yet de­spite this pop­u­lar no­tion, new re­search sug­gests that our per­cep­tion of col­or is mallea­ble, and re­lies heavily on bi­o­log­i­cal pro­cesses, Shev­ell said.

“An as­pect of hu­man vi­sion that we nor­mally don’t ap­pre­ci­ate is that dif­fer­ent fea­tures of an ob­ject, in­clud­ing col­or and shape, can be rep­re­sented in dif­fer­ent parts of the brain,” said Shev­ell.

If a per­son sees a bas­ket­ball com­ing, it is per­ceived as hav­ing a par­tic­u­lar col­or, shape and ve­locity. “The knit­ting to­geth­er, or what can be called ‘neu­ral glu­ing,’ of all those dif­fer­ent fea­tures so we see a un­ified ob­ject is a com­plex func­tion done by the brain. Our re­search fo­cused on how the brain does that,” Shev­ell ex­plained.

To study how the brain rep­re­sents the col­or of ob­jects, the re­search­ers used a tech­nique called bin­oc­u­lar ri­val­ry. The tech­nique pre­s­ents a dif­fer­ent im­age to each eye and thus pits sig­nals from the right eye against sig­nals from the left. 

“The brain has dif­fi­cul­ty in­te­grat­ing the two eyes’ in­com­pat­ible sig­nals. When the sig­nals from the two eyes are dif­fer­ent enough, the brain re­solves the con­flict­ing in­forma­t­ion by sup­press­ing the in­forma­t­ion from one of the eyes,” Shev­ell said. “We ex­ploited this fea­ture of the brain with a meth­od that caused the shape from one eye to be sup­pressed but not its col­or.”

The re­search­ers first showed sub­jects ver­tic­ally ori­ented green stripes in the left eye and a hor­i­zon­tally ori­ented set of red stripes in the right eye. “The brain can­not fuse them in a way that makes sense. So the brain sees only hor­i­zon­tal or ver­tical,” Shev­ell said. 

For their stu­dy, the re­search­ers de­vel­oped a new form of the tech­nique that al­lowed the hor­i­zon­tal pat­tern to be sup­pressed with­out elim­i­nat­ing its red col­or, which con­tin­ued on to the brain. They did this by fad­ing out the hor­i­zon­tal stripes while leav­ing their red hue in­tact.

At this point, the brain has a mu­si­cal chairs prob­lem. Both the red and green col­ors reach con­scious­ness but with only the one ver­tical pat­tern—one ob­ject but two col­ors. The sur­pris­ing re­sult was that the “dis­em­bod­ied red, which orig­i­nat­ed from the un­seen hor­i­zon­tal pat­tern in one eye, glued it­self to parts of the con­sciously seen ver­tical pat­tern from the oth­er eye. That proves the idea of neu­ral bind­ing or neu­ral glu­ing, where the col­or is con­nect­ed to the ob­ject in an ac­tive neu­ral pro­cess,” Shev­ell said.

“To us it seems au­tomat­ic,” Shev­ell added. “Ev­ery bas­ket­ball has a col­or. Eve­ry shirt has a col­or, but the brain must link each ob­jec­t’s col­or to its shape.”

* * *

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Scientists have managed to trick people’s eyes into losing sight of a shape while continuing to perceive its color. The result: the color—divorced from the shape to which it “belongs”—seems to go into another shape, according to the researchers. This finding, they say, reveals a new property of sight. The brain processes an object’s shape and color in two separate pathways. And although the shape and color normally are linked, the representation of color can survive alone, said Steven Shevell, a University of Chicago psychologist who specializes on color and vision. When that happens, he added, the brain establishes a new link that binds the color to another visible shape. “Color is in the brain. It is constructed, just as the meanings of words are constructed. Without the neural processes of the brain, we wouldn’t be able to understand colors of objects any more than we could understand words of a language we hear but don’t know,” said Shevell. The new experiments, by Shevell and Wook Hong of Vanderbilt University in Tennessee, is described in the current issue of the research journal Psychological Science Color is normally thought of as a fundamental attribute of an object: a red Corvette, a blue lake, a pink flamingo. Yet despite this popular notion, new research suggests that our perception of color is malleable, and relies heavily on biological processes, Shevell said. “An aspect of human vision that we normally don’t appreciate is that different features of an object, including color and shape, can be represented in different parts of the brain,” said Shevell. If a person sees a basketball coming, it is perceived as having a particular color, shape and velocity. “The knitting together, or what can be called ‘neural gluing,’ of all those different features so we see a unified object is a complex function done by the brain. Our research focused on how the brain does that,” Shevell explained. To study how the brain represents the color of objects, the researchers used a technique called binocular rivalry. The technique presents a different image to each eye and thus pits signals from the right eye against signals from the left. “The brain has difficulty integrating the two eyes’ incompatible signals. When the signals from the two eyes are different enough, the brain resolves the conflicting information by suppressing the information from one of the eyes,” Shevell said. “We exploited this feature of the brain with a method that caused the shape from one eye to be suppressed but not its color.” The researchers first showed subjects vertically oriented green stripes in the left eye and a horizontally oriented set of red stripes in the right eye. “The brain cannot fuse them in a way that makes sense. So the brain sees only horizontal or vertical,” Shevell said. For their study, the researchers developed a new form of the technique that allowed the horizontal pattern to be suppressed without eliminating its red color, which continued on to the brain. They did this by fading out the horizontal stripes while leaving their red hue intact. At this point, the brain has a musical chairs problem. Both the red and green colors reach consciousness but with only the one vertical pattern—one object but two colors. The surprising result was that the “disembodied red, which originated from the unseen horizontal pattern in one eye, glued itself to parts of the consciously seen vertical pattern from the other eye. That proves the idea of neural binding or neural gluing, where the color is connected to the object in an active neural process,” Shevell said. “To us it seems automatic,” Shevell added. “Every basketball has a color. Every shirt has a color, but the brain must link each object’s color to its shape.”