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Temasek Life Sciences Laboratory,1 Research Link, 117604 Singapore.*E-mail: snejana@tll.org.sg

DOI: 10.1016/j.cub.2008.01.029

Current Biology Vol 18 No 6R250Colour Vision: Cortical Circuitry forAppearance

Directly stimulating certain cortical neurons can produce a color sensation;a case is reported in which the color perceived by stimulation is the same asthe color that most effectively excites the cortical circuitry.Brian Wandell

One of the great achievements ofneuroscience is the completedescription of the early stages of colorvision. In the human retina there arethree types of cone containing differentlight absorbing pigments, each withits own unique sensitivity to thewavelengths of light. Because weacquire only three cone-type samples,and thus critically under-sample theavailable wavelength information,we are quite poor at resolving thewavelength information in a scene. Thedesign of nearly every modern imagingtechnology from displays to printersto cameras takes advantage of thefact that humans encode light usingonly three types of cone. Technologystandards show us how to captureand display enough information topersuade the cones that they arelooking at the original scene [1].

Cones are clearly central to colorvision, but the relationship betweencone responses and our colorperception is not straightforward.Retinal and cortical circuits processthe cone responses to create ourexperience of color. These processescan be revealed by visualdemonstrations in which the samecone photon absorptions producedifferent lightness and colorappearance (Figure 1). Some principlesof the neural coding mostimportantly, the fact that the conesignals are recombined in the retinainto three channels, known asopponent-colors, which are made upof sums and differences of localcone responses are also used inengineering standards, includingtelevision transmission and imagecompression.But, we do not have theories thataccurately predict the patterns of colorwe perceive. How cortical circuitryinterprets the encoded informationremains a grand challenge for colorscience. For many years, the locationof the essential cortical circuitry ofcolor was a very contentious point,with many investigators doubting thevery existence of any corticalspecializations for color. Neuroimagingand neurological case studies over thelast century demonstrate that signals inventral occipital cortex (Figure 2) areessential for the perception of color [2].For example, responses in a portion ofventral occipital cortex rise and fall assubjects alternately view colored andluminance-matched achromaticobjects [3]. Damage to these sameregions of cortex produces a syndromeknown as cerebral achromatopsia a color disturbance of cortical origin.Rather remarkably, in this syndromecolor perception is severely alteredwithout any obvious interference withother abilities, such as form, motion ordepth perception [4,5].

In a paper published recently inCurrent Biology, Murphey et al. [6]provide a glimpse into the relationshipbetween brain activity, brainstimulation and color perception. Theirwork bypasses the intricate colormachinery of the retina and cortex.Instead, they study a patient who hadan electrode array implanted in orderFigure 1. Two images which are identical apart from the shadow penumbra.

In the second image, the penumbra is replaced by a sharp edge coinciding with the checkers.Most people see a greater difference in the lightness of the spots in the shadow (top) than inthe paint (below). The appearance difference is not caused by differences in cone signals, butrather by the neural circuitrys analysis of the absorptions. (Reprinted from [16].)

DispatchR251to localize regions of healthy anddiseased cortex. The electrodes turnedout to be in a healthy location of cortex,near one of the regions that has beenidentified as particularly responsive tocolor. The location is a few centimetersanterior to the location identified inneurological cases of cerebralachromatopsia. The authors show thatthey can measure a significantresponse to colored stimuli by one ofthe electrodes; that some of the colorstimuli are more effective than others;and that stimulating cortex with thiselectrode evokes a visual sensationcorresponding to the most effectivecolor stimulus. Making suchmeasurements in the human brain isparticularly valuable because thesubject can offer a verbal description ofthe experience caused by thestimulation. The stimulation data addsupport to the wealth of neuroimagingdata suggesting a critical role for theseregions in color perception.

In the face of a neuroscienceliterature documenting the intricaciesof the molecular and neural circuitry ofcircuit function, results like this areamazing and puzzling. Somehow,current pulses from a 2.2 mm diameterelectrode near the cortical surfacegenerate just the right circuit responseto evoke a recognizable color percept.This study is not alone in reporting the

Figure 2. Specific regions in the human ven-tral cortex appear to contain circuitry essen-tial for various behaviors.

The location of a region essential for coloris indicated in green. Regions containingcircuitry essential for faces, motion and read-ing are indicated by red, blue and yellow.(Reprinted from [17].)effectiveness of such stimulation atevoking a recognizable percept. Aclassic investigation of stimulation inprimary visual cortex (V1) showeda correspondence between thereceptive field location and theperceived position of the evokedperceptual activity [7]. These have beenfollowed by a few other studies,including some that showed that lowstimulation levels in V1 can producephosphenes [8]; these are generallysmall spots or oriented lines but theycan appear in a variety of colors. Amore recent study [9] summarizingstimulation from many regions withinvisual cortex showed that stimulationproduces visual percepts ofvarying complexity. In that study,measurements were made in nearly1200 electrodes from 23 patients.About one-fifth of the surfaceelectrodes generated a visualperception, and it is likely that if onecounted the ability to modify a perceptthe percentage would have been evenhigher. The visual percepts caused bythe stimulation were well correlatedwith the conclusions of neurologicaland neuroimaging studies. Dependingon the electrode placement, subjectsperceived a range of forms from dots,to geometric shapes (triangles,diamonds), to hallucinations ofanimals, people or landscapes.

These results teach us that even thesimplest stimulation is capable ofstirring up a perceptually meaningfulresponse from the cortical circuitry.One possibility is that the complexmolecular and neural circuitry thatserves this portion of the brain istolerant of a wide range of potentialinputs, and that nearly any stimulationof this circuitry evokes a characteristic(resonant) response. The resonantresponse of these specific circuits isthe experience of color.

Historically, there have been fewelectrical stimulation measurements inthe human brain. This is likely to changeduring the next decade. The success ofdeep brain stimulators in alleviating thesymptoms of Parkinsons Disease [10],the hope that such methods will beuseful in other disorders [1113], andsignificant advances in the means tocontrol neural signaling will increasethe number and type of stimulationexperiments [14,15]. For theseapplications to succeed, we mustobtain a deeper understanding of theconsequences of stimulation forvarious types of perception, rangingfrom color and sight to emotions. Thework of Murphey et al. [6], combiningperceptual measurements, electricalmeasurements and electricalstimulation, is a useful contributiontowards that understanding.

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14. Zhang, F., Aravanis, A.M., Adamantidis, A.,de Lecea, L., and Deisseroth, K. (2007).Circuit-breakers: optical technologies forprobing neural signals and systems. Nat. Rev.Neurosci. 8, 577581.

15. Zhang, F., Wang, L.P., Boyden, E.S., andDeisseroth, K. (2006). Channelrhodopsin-2 andoptical control of excitable cells. Nat. Methods3, 785792.

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17. Wandell, B.A., Dumoulin, S.O., and Brewer, A.A.(2008). Visual cortex in humans. In NewEncyclopedia of Neuroscience, L. Squire,T. Albright, F. Bloom, F. Gage, and N. Spitzer,et al., eds. (Elsevier), in press.

Department of Psychology, Building 420Room 484, Stanford University, California94305-2130, USA.E-mail: wandell@stanford.edu

DOI: 10.1016/j.cub.2008.01.045

mailto:wandell@stanford.edu

Colour Vision: Cortical Circuitry for AppearanceReferences