Peter Bex
Professor Peter Bex

by Thea Singer

You’re hus­tling across Hunt­ington Avenue, eyes on the Marino Center, as the “walk” sign ticks down sec­onds: Seven, six, five…. Sud­denly bike brakes screech to your right. Yikes! Why didn’t you see that cyclist coming with your periph­eral vision?

Researchers in the lab of North­eastern psy­chology pro­fessor Peter J. Bex may now have the answer—one that may lead to relief for those with age-​​related mac­ular degen­er­a­tion, or AMD, an eye dis­order that destroys cen­tral vision: the sharp, straight-​​ahead vision that enables us to read, drive, and deci­pher faces. AMD could affect close to 3 mil­lion people by 2020, according to the Cen­ters for Dis­ease Control.

The problem with periph­eral vision—which people with AMD par­tic­u­larly rely on as their cen­tral vision fails—is that it’s noto­ri­ously poor because it’s sub­ject to “crowding,” or inter­fer­ence from sur­rounding “visual clutter,” says William J. Har­rison, co-​​author of the study and a former North­eastern post­doc­toral fellow. “You know something’s there, but you can’t iden­tify what it is.”

Bex and Harrison’s break­through paper, pub­lished last month in the journal Cur­rent Biology, uses com­pu­ta­tional mod­eling of sub­jects’ per­cep­tions of images to reveal why that visual befud­dle­ment occurs, including the brain mech­a­nism dri­ving it. That knowl­edge could pave the way for treat­ments to cir­cum­vent crowding.

An evo­lu­tionary compromise

Our vision oper­ates on a gra­dient: It has high res­o­lu­tion in the center and pro­gres­sively coarser res­o­lu­tion in the periphery. The dimin­ish­ment is an evo­lu­tionary neces­sity. “If we were to have the same res­o­lu­tion that we have in the center of our vision across our whole visual field, we’d need a brain and an optic nerve that were at least 10 times larger than they cur­rently are,” says Bex, who spe­cial­izes in basic and clin­ical visual science.

It’s that com­pro­mise that makes us vul­ner­able to crowding. “It’s not the phys­ical prop­er­ties of the eye—its shape, the number of photo receptors—that deter­mine what we see,” says Har­rison, who’s cur­rently a post­doc­toral fellow in psy­chology at the Uni­ver­sity of Cam­bridge. “It’s the wiring in the brain.”

Crowding, simply put, exem­pli­fies the lim­ited band­width of our visual pro­cessing system.

A new paradigm

Pre­vious attempts to under­stand crowding spring from that insight. One hypoth­esis holds that those lim­ited resources lead our brains to “average” the sym­bols in our periph­eral vision, in effect adding the dis­trac­tions to the target image and dividing the whole by the parts to pro­duce an uniden­ti­fi­able object. Another hypoth­esis sug­gests that each part is rep­re­sented accu­rately in the brain’s vision center, in the occip­ital lobe. But other regions of the brain—such as the frontal lobe, which is largely respon­sible for visual attention—don’t have the power to select the target image from among the others.

Fur­ther blur­ring the pic­ture, the pre­vailing hypotheses that explain crowding con­flict with one another, requiring qual­i­fiers for spe­cial cases.

But now Bex and Har­rison, using an inno­v­a­tive exper­i­mental design, have rec­on­ciled those dif­fer­ences in a single com­pu­ta­tional model based on how cells in the brain’s visual center rep­re­sent what we see. “Our model inte­grates a com­bi­na­tion of hypotheses and enables us to find evi­dence for any account of crowding,” says Harrison.

Decoding the images

Most vision tests for crowding ask people to deci­pher a single letter of the alphabet, say an “A,” sur­rounded by lines at var­ious angles and dis­tances, while focusing on a dot with their cen­tral vision. Hence they elicit either a right or a wrong answer (“Er, maybe it’s an “N”?).

crowding demonstration with several sticks on either side of a green dot
A demon­stra­tion of visual crowding: Fixate on the green spot. Without moving your eyes, see if you can iden­tify the letter on the left and on the right side of the dis­play. Most people will easily be able to iden­tify the letter on the left, whereas the same letter is almost impos­sible to iden­tify on the right because of crowding—the brain’s propen­sity to com­bine nearby visual infor­ma­tion into a single object.
Image by William Harrison

In Bex and Harrison’s study, how­ever, par­tic­i­pants viewed the image of a broken ring, sim­ilar to the letter “C,” and noted where in the image the opening appeared. The mea­sure pro­duced “an index of con­tin­uous per­cep­tions,” says Bex, thus per­mit­ting the researchers not only to know if crowding was occur­ring but also to create a com­puter sim­u­la­tion of how the cells in the brain actu­ally decoded the image.

Knowing how crowding works, when it hap­pens, and when it doesn’t, means we can start to modify the way infor­ma­tion is pre­sented to reduce the crowding,” says Bex. “And reducing it is what we need to do to help people with AMD.”