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• W2187367908 abstract "Department of Psychology, University of Washington,Seattle, WA 98195, USASummaryThe standard view of neurons in early visual cortex is thatthey behave like localized feature detectors [1–7]. Here wedemonstrate that processing in early visual areas goesbeyond feature detection by showing that neural responsesare greater when a feature deviates from its contextcomparedtowhenitdoesnotdeviatefromitscontext.Usingpsychophysics, fMRI, and electroencephalography method-ologies, we measured neural responses to an orientedGabor (‘‘target’’) embedded in various visual patterns asdefinedbytherelativeorientationofflankingstimuli.Wefirstshow using psychophysical contrast adaptation and fMRIthat a target that differs from its context results in moreneural activity compared to a target that is contained withinan alternating sequence, suggesting that neurons in earlyvisual cortex are sensitive to large-scale orientationpatterns. Next, we use event-related potentials to show thatorientation deviations affect the earliest sensory compo-nents of the target response. Finally, we use forced-choiceclassification of ‘‘noise’’ stimuli to show that we are morelikely to ‘‘see’’ orientations that deviate from the context.Our results suggest that early visual cortex is sensitive toglobal patterns in images in a way that is markedly differentfrom the predictions of standard models of cortical visualprocessing.ResultsInseparateexperiments,wemeasuredneuralresponsesusingpsychophysical contrast adaptation, human fMRI, and event-related potentials (ERPs) to an oriented Gabor stimulus(‘‘target’’) embedded in various visual patterns as defined bythe relative orientation of flanking stimuli. Specifically, wevaried whether a central target deviated from its context bychanging the orientation of distant gratings while leaving theimmediately neighboring flankers unchanged. For example,we hypothesized that the neural response to a verticallyoriented target grating would be greater when it deviatedfromtheorientationofitsflankers(horizontalflankers,HHVHH)compared to when it was grouped into an alternatingsequence of orientations (VHVHV). Keeping the localorientation configuration around the target the same acrossconditions (e.g., HVH) eliminates effects of simple spatialsummation [8–10].Contrast Adaptation: Deviations from the Context Resultin More AdaptationOur first experiment used psychophysical contrast adaptationto infer the magnitude of the neural response in early visualcortex to the target stimulus [11–13]. To quantify adaptationstrength, we calculated the ratio of each observer’s contrastdetection threshold for a target before and after adaptation.Our assumption is that more adaptation—as indexed by anincrease in postadaptation detection thresholds—reflectsstronger neural activity in response to the adapting stimulus[11, 14–21] (see also Figure S1A available online). Observerswere initially adapted to a stimulus for 30 s before performingatwo-intervalforcedchoicedetectiontask.A5stop-upadap-tation period was inserted between trials to maintain stableadaptation (see Figure S1B and Supplemental ExperimentalProcedures for detail).Usingaverticallyorientedtarget(V),wefoundmoreadapta-tion with orthogonal, horizontally oriented local flankers (HVH,‘‘orthogonal’’ condition) compared to vertically oriented localflankers (VVV, three-element ‘‘same’’ condition) [paired two-tailed t test, t(6) = 6.15, p < 0.001; Figure 1A]. In an additionalexperiment, we found that the horizontal and vertical flankersalone (no stimulus in the target position) did not produce anyadaptation, indicating that differences in adaptation observedbetween the same and orthogonal condition were due todifferences in the orientation relationship between the targetand flankers and not due to differences in the flankers them-selves (Figure S1A). Larger adaptation to a target with orthog-onally oriented flankers is consistent with electrophysiologystudies showing larger neural responses in V1 to a stimuluswhen it is surrounded by orthogonal as compared to iso-oriented stimuli [9, 22, 23]. One possible interpretation of thisresult is that neurons respond more strongly to stimuli thatdeviate from the context. However, because this conditionalso manipulates the local orientation arrangement of thetarget and flankers, this result in isolation can also be ex-plained through local orientation-selective spatial summationor normalization [8–10].Wethenaddeddistantflankerstotheoriginalthree-elementstimulitocreatefive-elementstimuli.Thisallowedustomanip-ulate whether the target deviated from the context withoutchanging the local (the target and its immediate neighbors)feature arrangement. Adding distant vertical flankers to thethree-element ‘‘same’’ condition, generating a five-elementsame condition (VVVVV), did not change the amount of adap-tation [t(6) = 1.23, p = 0.26], demonstrating that additionalvertical flankers do not necessarily alter responses to thetarget. Importantly, we observed a significant difference inour critical condition: adaptation to the three-element orthog-onal condition (HVH) was significantly reduced when distantvertical flankers were added, producing a five-element ‘‘alter-nating’’ condition (VHVHV) [t(6) = 4.25, p = 0.005], presumablybecause the target does not deviate from—and is groupedinto—the alternating pattern induced by the flankers. Asa further test, we directly compared the amount of adaptationbetween the alternating VHVHV condition and the orthogonalHHVHHcondition.Consistentwithourhypothesis,theamountofadaptationwassignificantly lessinthealternatingcondition(see Figure S1C).We observed the same results for horizontally (Figure 1B)and obliquely (Figure 1C) oriented targets. In all cases," @default.
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• W2187367908 date "2012-01-01" @default.
• W2187367908 modified "2023-09-28" @default.
• W2187367908 title "Report Long-Range, Pattern-Dependent Contextual Effects in Early Human Visual Cortex" @default.
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