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Perception of Intra-Saccadic Motion

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Perception of Intra-Saccadic Motion Chapter 10 Perception of Intra-saccadic Motion Eric Castet Abstract A typical saccadic eye movement lasts about 40 ms. During this short period of time, the image of the stationary world around us rapidly moves on the retina with a complex accelerating and decelerating profile. The reason why this 40 ms retinal motion flow does not elicit motion perception in everyday life is an issue that has received considerable interest. The present chapter first presents a brief history of the main ideas and experiments bearing on this issue since the seventies. Some key experimental paradigms and results in psychophysics are then described in detail. Finally, some suggestions for future investigations, both psychophysical and physiological, are made. A major goal of the chapter is to pinpoint some fundamental confusions that are often encountered in the literature. It is hoped that understanding these confusions will help identify more clearly the theoretical points – among which the role of temporal masking – on which scien- tists strongly disagree. 10.1 Introduction The stationary world around us does not appear to move during saccadic eye move- ments. Early authors already wondered why we are not aware of the activity elicited during the short saccadic period (about 40 ms) in which the image of the world does move on the retina (Dodge 1900, 1905; Holt 1903). This intra-saccadic issue should not be confused with another one usually referred to as the trans-saccadic fusion issue (Deubel et al. 2002). In the latter case, the problem is to understand why the E. Castet (*) Dynamics of Visual Perception and Action, Institut de Neurosciences Cognitives de la Méditerranée, CNRS and Université de la Méditerranée, 31 Chemin Joseph Aiguier, 13402, Marseille, France e-mail: [email protected] U.J. Ilg and G.S. Masson (eds.), Dynamics of Visual Motion Processing: 213 Neuronal, Behavioral, and Computational Approaches, DOI 10.1007/978-1-4419-0781-3_10, © Springer Science+Business Media, LLC 2010 214 E. Castet 2-frame shift in position occurring between the pre- and post-saccadic images does not usually elicit any displacement percept. To explain why the world does not appear to move during each saccade, two extreme theories are proposed which are actually nonexclusive. The first theory postulates an active suppression process originating from central nervous structures and operating during the saccade in order to inhibit visual areas. In such a frame- work, “extra-retinal” signals, conceptually similar to an efference copy, are trig- gered by the oculo-motor command and sent to visual structures. The other general theory does not postulate any extra-retinal signal and relies on visual and/or retinal spatio-temporal processes such as the well-known temporal masking. In the last two decades, a “preference” for the extra-retinal suppression theory seems to have emerged. More precisely, the idea that the motion-processing system, and thus motion perception, is actively and selectively suppressed during saccades can be often found in the literature. This is illustrated below by a few exemplary citations. “There is now good evidence that perception of motion is strongly sup- pressed during saccades (rapid shifts of gaze), presumably to blunt the disturbing sense of motion that saccades would otherwise elicit.” (Burr et al. 1999). “During fast saccadic eye movements, visual perception is suppressed. This saccadic sup- pression prevents erroneous and distracting motion percepts resulting from saccade induced retinal slip.” (Georg and Lappe 2007). “[…] this fits well with the idea that saccadic suppression reflects the visual system’s attempt to ignore the retinal image motion induced by saccades.” (Kleiser et al. 2004). “The purpose of the saccadic suppression of motion may be to block out unreliable motion signals that would be produced by a saccade” (Shioiri and Cavanagh 1989). This preference is also found in several recent reviews (Burr and Morrone 2004; Ross et al. 1996, 2001). The goal of the present chapter is to offer a more balanced view of the issue if only because it has been shown that intra-saccadic motion perception can be easily elicited in humans (Castet et al. 2002; Castet and Masson 2000). There is much confusion in the literature that might explain the tendency to systematically assert that motion processing and motion perception are suppressed during saccades. Notably, the expression “saccadic suppression” is extensively used as though it were a unique process relying on a homogenous set of experiments. In contrast, I will attempt to show that there are different classes of experimental effects that might actually reflect totally different visual processes. Another concern is related to the general problem of consciousness, which is strongly debated in visual neu- rosciences. When we do not consciously perceive a retinal event which lasts about 50 ms, does it mean that this event has to be erased in early visual areas, or does it mean that this brief period is “filled-in” by anterior and posterior retinal events? The answer to this question is crucial to make correct predictions regarding the neural processes leading to the intra-saccadic “blindness” in normal viewing. The first section of the chapter outlines the evolution of the ideas since the seventies without describing in detail the experimental effects. This is to help understand why some ideas seem to have become predominant while overlooking some key results available in the early literature. Then, a few key experimental effects, and their possible interpretations, will be described without pretending to be exhaustive. I will 10 Perception of Intra-saccadic Motion 215 rely mainly on psychophysical studies, as Chap. 10 by Mike Ibbotson will be devoted to physiological work on the intra-saccadic perception issue. 10.2 A Brief History of the Concepts 10.2.1 Up to 1982 In the seventies, it was believed that saccadic speeds were too fast for the visual sys- tem to resolve and caused therefore a blurring or smearing of the visual scene. In this context, two important studies showed that a form of temporal masking was the main factor preventing us from perceiving the smearing or “grey-out” induced by each saccade (Campbell and Wurtz 1978; Matin et al. 1972). The principle of Campbell and Wurtz’s experiments, which extended those of Matin et al. was simple (Fig. 10.1). When the experimental room was illuminated only during the time of saccades, observers perceived the scene as being smeared or greyed out. By grey-out, a decrease in the apparent contrast of the image was meant. However, as the duration of the light was extended beyond the end of the eye movement, the amount of smear- ing became progressively less. Only 40 ms of post-saccadic illumination of the room was sufficient to restore a sharp percept of the scene. In this case, the authors insisted that subjects did not perceive a smeared image followed by a sharp image but instead reported a single sharp percept. It was thus the presence of a post-saccadic image of the scene which made it possible to avoid the perception of the brief intra-saccadic grey-out (the effect of a pre-saccadic image was shown to be as efficient as a post- saccadic image). The authors referred to this temporal masking mechanism as a “sac- cadic omission” process (instead of saccadic suppression) in order to emphasize their main theoretical point: the basic process needed by the visual system to prevent per- cepts induced by the intra-saccadic stimulations cannot rely on a suppression (or on a dampening) process. If it were the case, the temporal flow of our perception would be constantly interrupted by a dark (or a dimmer) brief percept whenever we make a saccade. What is needed is a mechanism that preserves the perceptual continuity between the pre- and post-saccadic images, so that the brief period corresponding to the intra-saccadic stimulation does not entail any conscious percept at all. I will call this conceptual requirement the saccadic “temporal filling-in” issue. A few years later, the seminal study of Burr and Ross (1982) was published and turned out to have far-reaching and lasting consequences. This paper first started by noting that previous work had always assumed that the human visual system cannot resolve objects moving at high speeds. The authors decided to test this commonly held assumption by measuring the contrast threshold at which direction discrimination of very fast movements was possible – observers’ eyes were static. Their striking result was that the use of low spatial frequency gratings (or wide bars) as stimuli allowed observers to perceive motion at incredibly high speeds (even higher than usual saccadic speeds). Moreover, peak contrast sensitivity was identical at all speeds up to 800°/s and corresponded to a temporal frequency of about 10 Hz. 216 E. Castet Fig. 10.1 Schematic representation of Campbell and Wurtz’s (1978) results. (a) Intra-saccadic blur perception (or grey-out) is temporally masked by the pre- and post-saccadic images, thus preserving temporal continuity. (b) In the absence of temporal masking, intra-saccadic blur is clearly perceived 10 Perception of Intra-saccadic Motion 217 These amazing results led the authors to wonder why observers were not “startled during a saccade by the intrusion of low frequency components onto the scene?” To answer this question, they proposed for the first time “that during saccades motion sensitivity is dampened, precisely to avoid the disturbing consequences of saccadic image motion which would follow if it were left intact”. This motion sensitivity damping hypothesis was made more explicit in a paper which was published the same year (Burr et al. 1982). The paradigm and results of this study will be discussed later in order to focus on concepts in the present sec- tion.
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