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Stomatopod Vision  

Thomas W. Cronin, N. Justin Marshall, and Roy L. Caldwell

The predatory stomatopod crustaceans, or mantis shrimp, are among the most attractive and dynamic creatures living in the sea. Their special features include their powerful raptorial appendages, used to kill, stun, or disable other animals (whether predators, prey, or competitors), and their highly specialized compound eyes. Mantis shrimp vision is unlike that of any other animal and has several unique features. Their compound eyes are optically triple, each having three separate regions that produce overlapping visual fields viewing certain regions of space. They have the most diverse set of spectral classes of receptors ever described in animals, with as many as 16 types in a single compound eye. These receptors are based on a highly duplicated set of opsin molecules paired with strongly absorbing photostable filters in some photoreceptor types. The receptor set includes six ultraviolet types, all spectrally distinct, many themselves tuned by photostable filters. There are as many as eight types of polarization receptors of up to three spectral classes (including an ultraviolet class). In some species, two sets of these receptors analyze circularly polarized light, another unique capability. Stomatopod eyes move independently, each capable of visual field stabilization, image foveation and tracking, or scanning of image features. Stomatopods are known to recognize colors and polarization features and evidently use these in predation and communication. Altogether, mantis shrimps have perhaps the most unusual vision of any animal.


Visual Guidance of Natural Behavior  

Mary M. Hayhoe and Rachel A. Lerch

The essentially active nature of vision is revealed in the complex interplay of head, body, and eye movements as humans gather information to guide their actions in the natural visual world. This dynamic perception–action cycle has long been appreciated but has been difficult to investigate due to limitations in the available instrumentation both to measure eye and body movements and to present realistic stimuli in the context of active behavior. Technological developments have opened up a wider range of natural contexts where some degree of experimental control is possible, and the last two decades have ushered in a variety of insights that would otherwise be difficult to achieve in more constrained environments. Within the context of natural vision, humans make continuous sequences of sensorimotor decisions to satisfy behavioral goals, and vision provides the relevant information for making good decisions in order to achieve those goals. The components of a good decision include the task demands, the rewards and costs associated with the task, uncertainty about the state of the world, and information stored in memory. Natural behavior offers a rich domain for investigation, because it is remarkably stable and leads to novel questions, and the behavioral context helps specify the momentary visual computations and their temporal progression.


Eye Movements and Perception  

Doris I. Braun and Alexander C. Schütz

Voluntary eye movements and visual perception are closely intertwined in humans and nonhuman primates because of the limitation of high-acuity vision to a very small, specialized area at the center of the retina, the fovea. Only when the image of an object is projected on the foveal region by eye and head movements it is possible, to perceive fine visual details such as letters during reading. In order to improve visual perception and to benefit from high-resolution foveal vision, rapid saccadic eye movements frequently change the direction of both eyes to selected peripheral locations. Continuous sequences of these voluntary saccades and fixations determine what humans see and in how much detail they perceive objects and their visual surroundings. Where, when, and how humans move their eyes depends not only on the visual properties of the target object but also on their intentions and prevailing tasks. Accordingly, target locations for saccades differ depending on the things people do—whether they just look around, actively search for something, read, or do sports. Instead of the classical dichotomy of bottom-up and top-down processes, recent research on gaze behavior has focused on the dynamic interplay of factors such as task demands, rewards, scene content, temporal sequences, and individual and historical differences. Besides saccadic eye movements, humans are also able to rotate their eyes continuously when they pursue moving objects of interest. Smooth pursuit eye movements stabilize the image of a moving object on the foveal region and prevent degradation of the retinal target image resulting from motion smear. The use of pursuit eye movements also improves the prediction of future target movement. Pursuit initiation is often combined with interceptive saccades that direct the fovea to the moving target, and catch-up saccades that correct for small mismatches concerning eye and target position, speed, and/or direction. Because each eye movement alters retinal input, compensations for retinal displacements are needed to maintain a stable representation of the environment. Overall, both saccadic and smooth pursuit eye movements provide optimal uptake of visual information for perception and guidance of actions.