1.6.1. The retina and human scene exploration behaviour
The visual field is the total area visible to both eyes during any fixation (Gibson, 1950), and it covers approximately 200-220 degrees visual angle (Harrington, 1981). Visual acuity, the ability to observe fine details in a visual scene, depends on the position of the
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stimuli in our visual field, with objects placed in central vision being perceived in much greater detail (Anstis, 1974). The anatomical structure of the human retina is such that there is a disproportionate concentration of cones, the photoreceptive cells specialised in colour perception, in the fovea centralis, which is a small region in the centre of the retina. Moreover, despite the fovea spanning only 5 degrees of the visual field, it is subserved by approximately 30% of the primary visual cortex (Schira, Tyler,
Breakspear, & Spehar, 2009). It can then be understood why in central (or foveal) vision acuity is considerably higher than in peripheral vision.
Because of the limited extent of the fovea, human observers must move their eyes in order to view specific parts of the visual scene in detail. The typical sequence observed during scene exploration is an alternation of fixations of 150 - 500 ms, when the gaze remains relatively stable, and saccades of 20 – 100 ms, during which the gaze position is rapidly changed (Bahill, Clark, & Stark, 1975; Wilming et al., 2017). As a
consequence the projection of the visual scene will be displaced across the retina, but human observers do not consciously perceive these displacements. Yet, when a similar retinal motion is induced by a moving stimulus instead of an eye movement, the same displacement is easily detected (Bridgeman, Hendry, & Stark, 1975). It has been suggested that a copy of the motor signals which drive the eye movements is sent to visual areas, to compensate for the retinal motion signals induced by saccadic eye movements (Sperry, 1950; von Holst & Mittelstaedt, 1950; Gauthier, Nommay, & Vercher, 1990; Souman & Freeman, 2008). The terms efference copy, extra-retinal signals and re-afferent signals are commonly used to indicate such neuronal activity. The exact mechanisms that allow the saccade-induced displacement to be compensated are still the subject of scientific debate, however (Irwin, Yantis, & Jonides, 1983; Bridgeman, Heijden, & Velichkovsky, 1994; Melcher, 2005; Cavanagh, Hunt, Afraz, & Rolfs, 2010; Zirnsak & Moore, 2014).
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1.6.2. Smooth pursuit eye movements
Next to fixations and saccades, the third type of commonly observed eye movements is referred to as smooth pursuit (Rashbass, 1961; Robinson, 1965; Keller & Heinen, 1991; Krauzlis & Stone, 1999), and can only be initiated when a smoothly moving visual stimulus is available to follow with the gaze. Human observers are capable of following targets with speeds up to 30° per second (Lisberger, Morris, & Tychsen, 1987), and are more accurate at horizontal trajectories than vertical or diagonal trajectories (Rottach et al., 1996).
To initiate and maintain tracking, the motor control of smooth pursuit eye movements must be closely related to visual motion signals. Two phases can be identified. During the initial 100 ms of motion, the stimulus moves across the retina, and a retinal signal drives the eye movement (Lisberger & Westbrook, 1985). In the next phase, the
stimulus is accurately being tracked. The properties of tracking behaviour are not based on a low-level motion signal, but instead share their properties with those of high-level perceptual judgments of motion (Yasui & Young, 1975; Beutter & Stone, 1998; Stone & Krauzlis, 2003). Indeed, smooth pursuit eye movements continue when the stimulus traverses behind an occluder, although at a reduced speed when the occlusion period becomes longer (Becker & Fuchs, 1985; Pola & Wyatt, 1997). The locus of this link between perception and action is proposed to be area MST (Dürsteler & Wurtz, 1988; Komatsu & Wurtz, 1989; Pack, Conway, Born, & Livingstone, 2006).
Since retinal motion is neutralised during the tracking phase of smooth pursuit, motion areas must integrate an efference copy of the eye movements themselves to encode the motion of the target that is being followed. Whereas area MT is retinotopic in
organisation, area MST is largely spatiotopic and continues to respond when a stimulus is stabilised on the retina (Newsome, Wurtz, & Komatsu, 1988; Thier & Ilg, 2005; Ilg,
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2008). It is thus proposed that MST integrates retinal and extraretinal signals to enable continued pursuit and allow the conscious perception of motion, despite the absence of a retinal motion signal. Indeed, the properties of trajectory perception are independent of the presence of eye movements (Dzhafarov et al., 1993; Krukowski, Pirog, Beutter, Brooks, & Stone, 2003).
1.6.3. Motion perception in foveal and peripheral vision
The position of the gaze also determines in which part of the visual field a stimulus will be processed. Visual acuity declines rapidly with retinal eccentricity (Anstis, 1974). Although human observers are not typically conscious of this during normal behaviour, it can be easily experienced by keeping the gaze fixed on a single point in space while directing visual attention away from foveal vision. Other differences between foveal and peripheral vision have been found in the processing of visual information; for example, it has been shown that the estimated size of objects in peripheral vision is smaller when compared to foveal vision (Baldwin, Burleigh, Pepperell, & Ruta, 2016), and that the mechanisms of contour integration are different beyond 10 degrees of retinal
eccentricity (Hess & Dakin, 1997, Hess & Field, 1999).
Visual information from outside foveal vision is important, however, since it is this information that will be used to decide where to direct the gaze next. Given that saccadic eye movements are made multiple times per second, a continuous analysis of the visual periphery is required to plan the next eye movement. Motion in particular is a strong cue to attract the gaze towards a peripheral location, to bring it into the fovea for detailed analysis (Dorr, Martinez, Gegenfurtner, & Barth, 2010; Mital, Smith, Hill, & Henderson, 2011). Indeed, the peripheral retina is encoded by large field motion neurons (Cleland & Levick, 1974; Walsh & Polley, 1985), which makes it relatively
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more sensitive to moving stimuli than the fovea (Edwards & Nishida 2004). Fahle and Wehrhahn (1991) showed that motion sensitivity in the periphery is especially high for horizontal and centrifugal motion and less so for vertical and centripetal motion. The latter finding could potentially be related to the properties of the ubiquitous optic flow motion signals that are generated during self-movement, which is typically centrifugal. Finally, some motion phenomena are unique to peripheral vision. In the Peripheral Drift illusion (Faubert & Herbert, 1999), a static image is presented that in the fovea indeed appears to be static. However when eye movements occur, the layout of the figure will stimulate specifically the motion detectors in peripheral vision, and cause an illusory impression of motion. In reality, the only motion occurring is the movement of the eyes. These eye movements are then not fully compensated for by the efference copy of the eye movement motor signals, and the retinal motion signals are instead assigned to the static stimulus. In the Furrow illusion (Anstis, 2012; see Chapter 6), a dot is shown to traverse vertically across a diagonally striped pattern, alternatingly white and black in colour. In foveal vision the vertical trajectory is perceived accurately, whereas in peripheral vision the dot will appear to move diagonally instead. These examples indicate that motion and trajectory perception can operate differently at different locations of the visual field.
1.7. The slalom illusion