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Pseudogaze in Afterimages Journal of Vision (2016) 16(5):6, 1–10 1 Where are you looking? Pseudogaze in afterimages Caltech Brain Imaging Center, Division of Humanities and Social Sciences, California Institute of Technology, # Daw-An Wu Pasadena, CA, USA $ Department of Psychological and Brain Sciences, Dartmouth College, Hanover, NH, USA Laboratoire Psychologie de la Perception, Universite´ # Patrick Cavanagh Paris Descartes, Paris, France $ How do we know where we are looking? A frequent gaze seems exceptionally well defined and we seldom assumption is that the subjective experience of our feel that we don’t know where we are looking within a direction of gaze is assigned to the location in the world visual scene. The conceptual equivalence of mental and that falls on our fovea. However, we find that observers physical gaze is ingrained to the point that the two can shift their subjective direction of gaze among concepts share almost all their terminology, even in different nonfoveal points in an afterimage. Observers scientific and technical usage. Terms such as ‘‘gaze,’’ were asked to look directly at different corners of a ‘‘look at,’’ ‘‘fixate,’’ etc. do not distinguish between the diamond-shaped afterimage. When the requested corner subjective sense of visual targeting and the physical was 3.58 in the periphery, the observer often reported pointing of the eyes at something. that the image moved away in the direction of the Whereas a large body of work exists regarding the attempted gaze shift. However, when the corner was at perceived direction of visual targets in egocentric space 1.758 eccentricity, most reported successfully fixating at the point. Eye-tracking data revealed systematic drift (i.e., where a target is located with respect to the head during the subjective fixations on peripheral locations. or body), the location of visual fixation in retinotopic For example, when observers reported looking directly space has received less attention. One study that at a point above the fovea, their eyes were often drifting directly measured where a fixated target fell on the steadily upwards. We then asked observers to make a retina used adaptive optics (Putnam et al., 2005). These saccade from a subjectively fixated, nonfoveal point to high resolution images specify the absolute position of another point in the afterimage, 78 directly below their the stimulus on the retina with an accuracy of about 5 fovea. The observers consistently reported making arcsec (Roorda, 2011). Using this method, Putnam et appropriately diagonal saccades, but the eye movement al. (2005) showed that, for three observers, the center of traces only occasionally followed the perceived oblique fixation was offset from the location of highest foveal direction. These results suggest that the perceived cone density by no more than 0.28 with an even smaller direction of gaze can be assigned flexibly to an attended dispersion of about 0.058. The offset was in different point near the fovea. This may be how the visual world directions for each observer. These measurements acquires its stability during fixation of an object, despite corroborated earlier studies, which did not have access the drifts and microsaccades that are normal to the absolute position of the fixation direction, but characteristics of visual fixation. could show that the retinal locations used to fixate targets had a dispersion of less than 4 arcmin (Stein- man, 1965; Kowler & Blaser 1995). These results show that the retinal location chosen for fixation is typically Introduction quite stable and close to the physical center of the fovea. As the eyes move around, the point in the visual However, this offset between the fixation target and world that lands on our fovea shifts. It is commonly the fovea is not constant but has a strong dynamic assumed that our perceived direction of gaze is component as our physical gaze is typically far less determined by the location in the world that falls steady than our mental experience would indicate. directly on our fovea. Subjectively, the center of our When we fixate on a visual target, our eyes constantly Citation: Wu, D.-A., & Cavanagh, P. (2016). Where are you looking? Pseudogaze in afterimages. Journal of Vision, 16(5):6, 1–10, doi:10.1167/16.5.6. doi: 10.1167/16.5.6 Received July 17, 2014; published March 10, 2016 ISSN 1534-7362 This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Downloaded From: http://jov.arvojournals.org/pdfaccess.ashx?url=/data/Journals/JOV/935062/ on 05/12/2016 Journal of Vision (2016) 16(5):6, 1–10 Wu & Cavanagh 2 drift and jump, by up to 0.58, around the point we feel eye (Gru¨sser, Krizicˇ, & Weiss, 1987), again suggesting we are fixating (Ortero-Millan, Troncosco, Macknik, that the subjective direction of gaze is not obligatorily Serrano-Pedraza, & Martinez-Conde, 2008; for a linked to the fovea. If an observer is asked to make an review, see Rolfs, 2009). Movements of this magnitude eye movement to the location of a small afterimage at are easily seen when they occur in the external world more than 38 eccentricity (Kommerell & Ta¨umer, 1972), (Murakami, 2003), yet we do not generally sense the the observers typically make a series of saccades, never visual motion caused by our eye movements of fixation. succeeding in foveating the target, which flees further Instead, it requires special techniques (Verheijen, 1961; with each eye movement. However, for an afterimage Murakami & Cavanagh, 1998; Murakami, 2003), or at less than 28 in the periphery, observers almost always highly controlled laboratory conditions (Haddad & made smooth pursuit eye movements, a finding Steinman, 1973) for us to perceive the movements in previously reported with mechanically stabilized stimuli our fixations.Thus, while our eyes are often jittering by Robinson (1965) for targets offset by 0.18 and by around the fixation, our subjective experience is more Brune and Lu¨cking (1969) for eccentricities of up to 18 that our gaze is locked onto the fixation point. This to 28. In one condition of Kommerell and Ta¨umer’s difference between the subjective feeling of unwavering experiment (1972), some observers showed smooth fixation and the objective measurement of rapid, pursuit to afterimages at up to 108 eccentricity. random jumps and drifts (Rolfs, 2009) indicates that These earlier studies were primarily investigations of the subjective feeling of where we are looking is not eye movement mechanisms, and observers were asked determined simply by what falls directly on, or even neither where they felt they were looking, nor whether slightly offset from, the fovea. It has been suggested the target itself appeared to be moving. that the perception of the motion caused by the eye A study by Pelz and Hayhoe (1995) also used movements of fixation may be suppressed based on the afterimages but did ask observers whether the after- retinal motion signals themselves (Murakami & Cav- image appeared steady or seemed to move. They anagh, 1998; Poletti, Listorti & Rucci, 2010; Arathorn, induced whole scene afterimages and found that when Stevenson, Yang, Tiruveedhula, & Roorda, 2013). observers were instructed to maintain their direction of However, the suppression of motion signals is not gaze, their eyes showed slow drifts and occasional sufficient—we see our fixation not only as lacking in saccades but, despite these, observers did not report the motion energy, but also as having a clearly fixed afterimage as moving. When the instructions to fixate position (i.e., not jumping from point to point or being were relaxed and subjects were told to ‘‘inspect’’ or smeared). So some additional mechanism may be steadying the perceived position of the fixation along ‘‘watch the afterimage,’’ saccades of up to 58 were seen, with the mechanisms that are suppressing its motion. but again without reports of movement of the A much larger discrepancy in apparent direction of afterimage. The authors did not remark upon observ- gaze happens just before a saccade. Hunt and ers’ subjective direction of gaze within the afterimage Cavanagh (2009), following up on an earlier study by scene, but it could be easily inferred that the eye Deubel, Irwin, and Schneider (1999), asked observers movements corresponded to intentional and successful to shift their gaze to a clock with a fast-moving hand shifts in the subjective location of gaze. Indeed, it was and report the time on the clock when their eyes the case that the observers’ attempts to look around in landed. Interestingly, the time they reported seeing on the scene were felt to be successful so long as the the clock when they landed corresponded to a time afterimage didn’t disappear or move (Hayhoe, personal before their eyes had moved. This result suggested that communication, 2015). observers sensed that they were looking directly at the In this paper, we ask observers to move their clock before they had begun the eye movement: The subjective direction of gaze to specified locations in the perceived direction of gaze had shifted before their eyes afterimage and report whether they felt they success- did. fully looked directly at the intended target, and whether These effects, found during fixation or just before the afterimage appeared to move. Eye movements were saccades, are suggestive but the interpretations are analyzed for components that varied systematically complicated by the speed of the events. To avoid this with the target. We find that when observers are temporal uncertainty, we used long-lasting afterimages requested to move their gaze to a target at less than 38 to measure whether the perceived direction of gaze from the fovea in an afterimage, they often report that could move independently of the fovea. they have subjectively fixated the target and, despite the It is well known that voluntary saccades will make a smooth pursuit and small saccades that this often small afterimage appear to move, following the eye elicits, they report that the target is not moving.
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