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Binocular Vision and Functional Stereopsis 321 Journal of Vertebrate Paleontology 26(2):321330, June 2006 2006 by the Society of Vertebrate Paleontology The more anteriorly directed the eyes, the greater the width of the BFoVan increase achieved generally at the expense of the total field of view (TFoV), the panorama visible without head or eye movements. The tradeoff between TFoV and binocular overlap is often regarded as a feature that can be used to distin-

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  • BINOCULAR VISION IN THEROPOD DINOSAURS

    KENT A. STEVENS11Department of Computer and Information Science, University of Oregon, Eugene, Oregon 97403 U.S.A., kent@cs.uoregon.edu

    ABSTRACTThe binocular fields of view of seven theropod dinosaurs are mapped using sculpted life reconstructionsof their heads and techniques adopted from ophthalmic field perimetry. The tall, narrow snout and laterally facing eyesof the allosauroids Allosaurus and Carcharodontosaurus restricted binocular vision to a region only approximately 20wide, comparable to that of modern crocodiles. In contrast, the coelurosaurs Daspletosaurus, Tyrannosaurus, Nanotyr-annus, Velociraptor, and Troodon had cranial designs that afforded binocular fields between 4560 in width, similar tothose of modern raptorial birds. Binocular field width and predatory style (ambush versus pursuit) is examined for extanttaxa, along with a discussion of cranial adaptations that enhance binocular vision. The progressive increase in frontalvision in the tyrannosaurids culminates in broader binocular overlap than that of a modern hawk. The visual acuity andthe limiting far point for stereopsis is estimated for Tyrannosaurus based on reptilian and avian models.

    INTRODUCTION

    In most vertebrates, the optical design of the eye results in aretinal image or monocular field of view (MFoV) between 160and 170 in diameter (Walls, 1942). The orbits are oriented an-terolaterally such that in virtually all species the optic axes aredirected at least slightly anteriorly, i.e., optic axis divergence(AD) is less than 180. Consequently, the fields of view coveredby the left and right eyes overlap partially, providing a region ofspace directly ahead of the animal visible to both eyes simulta-neously, i.e. binocularly.

    The region of overlap, the binocular field of view (BFoV),affords the animal several visual advantages. Binocularly-drivenneurons, cells in the central nervous system that receive inputfrom two neural pathways, can use probability summation ofthe two statistically independent signals to suppress uncorrelatedneural noise and thereby gain a slight enhancement in the de-tection of low contrast luminance featuresan advantage fornocturnal vision (Thorn and Boynton, 1974). A wide anteriorfield of binocular vision can also assist in the perception of thedirection of travel during locomotion (Jones and Lee, 1981), andbinocularly driven neurons sensitive to the direction of motioncan discriminate objects moving in depth towards the observer(Regan et al., 1979), also useful in locomotion. The most dra-matic advantage of binocular vision, however, is the perceptionof depth and the range of objects seen within the BFoV: nearerobjects appear sharply delineated in depth against the surround-ing background, subtle variations in curvature across the surfaceare revealed, and the distance to target points can be judgedallwithout requiring the observer to reveal itself by moving its head.

    Lateral separation between the two eyes results in a pair ofimages from slightly different perspectives: consequently, anearby target point seen binocularly will project to a slightlydifferent relative location on the left and right retinae. This bin-ocular parallax, analogous to the motion parallax induced byhead movement and the parallax used by surveyors in triangu-lation, can support the perception of the range or absolute dis-tance of a given target, such as the determination of whetherprey is within striking range. The more sophisticated perceptualprocess of stereopsis ( seeing solid) derives from detectingdifferences in binocular parallax, known as binocular disparities(Mayhew and Longuet-Higgins, 1982). Stereopsis permits theperception of surface curvature and relief across regions wherebinocular disparity varies continuously and the detection of sur-face boundaries where disparity varies discontinuously. Objects

    appear sharply demarcated in depth against their background,thus breaking camouflage. While the spatial information pro-vided by binocular depth perception is similar to that induced byshifting the head laterally or bobbing vertically, stereopsis hasthe advantage for predation of not revealing the presence of thepredator by self-movement.

    Binocular depth perception is not exclusively associated withpredators, nor is binocular depth perception all-or-none, as it isoften oversimplified. It is also frequently thought that a wideBFoV is associated exclusively with predators. A rabbit (141AD, 2732 BFoV), for instance, has a wider binocular field thana crocodilian (144 AD, 2426 BFoV); a cow (114 AD, 52BFoV) has more than a tawny owl (55 AD, 48 BFoV) (Walls,1942; Martin, 1993).

    The neural processes of stereopsis require the detection ofbinocular disparities of only minutes or seconds of arc. Resolvingfiner visual detail permits detection of finer binocular disparities,which translates to distinguishing smaller differences in distances(or, informally, to see in depth out to greater distances). Spatialacuity can be enhanced by increasing the retinal image scale(either optically or by increasing eye diameter), or by increasingretina receptor density (usually within a fovea or area centra-lis). For a given visual acuity, increasing the interpupillary sepa-ration increases the disparity proportionally; i.e., other factorsbeing equal, doubling the separation between eyes doubles bin-ocular depth acuity, a capability discussed later for Tyrannosau-rus.

    Binocular Vision and Functional Stereopsis

    In human vision, for which binocular depth perception is bestunderstood, stereopsis was first shown to be achievable purelyon the basis of binocular disparities using random dot stereo-grams that appear featureless monocularly but reveal depthwhen fused (Julesz, 1971). Binocular stereopsis provides a func-tionally independent means of detecting the presence and layoutof objects in visual space in the absence of any other visual cues.While surface curvature in depth and object boundaries wheredepth is discontinuous can be achieved by stereopsis alone, bin-ocular depth perception is most accurate when the visual systemintegrates disparity information with other depth cues that co-occur in natural scenes (e.g., accommodation, vergence, retinalmotion induced by head movements, and monocular three-dimensional cues such as shading and texture gradients; Foley,1978, 1980; Stevens, 1996).

    Journal of Vertebrate Paleontology 26(2):321330, June 2006 2006 by the Society of Vertebrate Paleontology

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  • Binocular stereopsis has been incontrovertibly demonstratedfor only a few non-mammalian species. While neurophysiologi-cal studies on a variety of mammalian and avian species havedemonstrated the existence of neurons that are driven binocu-larly, the most convincing demonstration of stereopsis is behav-ioral, e.g., using stereo goggles and a behavioral task requiringthe detection of depth in random dot stereograms in the falcon(Fox et al., 1977), the pigeon (McFadden and Wild, 1986), andthe barn owl (van der Willigen et al., 1998). Vertebrates withbinocular fields wider than roughly 20 almost invariably derivesome form of distance perception (at least the perception ofrange if not full stereopsis) from binocular disparity information(Pettigrew, 1991). The pigeon achieves stereopsis with only 22overlap (Martin, 1993). While some concern has been expressedthat binocular overlap does not necessarily imply stereopsis (e.g.,Molnar, 1991), the few known exceptions are highly specializedavians that might be regarded as secondarily stereo-blind, suchas the nocturnal oilbird, which lives in totally dark caves, and theswift (Pettigrew, 1986).

    In many birds, coordinated motor behavior involving eye con-vergence, accommodative changes with fixation changes, andhead movements suggest that stereopsis integrates with otherperceptual processes, in activities such as pecking in pigeons(McFadden, 1990, 1994) and prying in starlings (Martin, 1986).Compared to mammalian stereopsis, however, avian stereopsis ismore restricted in the range over which it is effective, and withinthat range, depth perception is relatively coarser than the mam-malian counterpart. This suggests that neural processing occursat a greater expense of disparity resolution than in mammals(McFadden, 1993). The spatial directions of greatest binocularsensitivity and monocular spatial acuity are often distinct inbirds. Falconiforms, for example, have highly divergent eyes andare bifoveate, with a central fovea aligned with the optic axis thatsupports the finest spatial acuity and a second more temporallylocated fovea specialized for binocular vision (Frost et al., 1990).Bifoveate birds alternate between aligning their head with thedirection of a target in order to fixate it binocularly, and rotatingtheir head by AD/2 (e.g., by 45 for the falcon) to permit scru-tiny of spatial details (Tucker, 2000). In contrast, mammals, withtheir single fovea but fine control of convergent eye movements,simultaneously achieve th