The eyes never cease to move: ballistic saccades quickly turn the gaze toward peripheral targets, whereas smooth pursuit maintains moving targets on the fovea where visual acuity is best. Despite the oculomotor system being endowed with exquisite motor abilities, any attempt to generate smooth eye movements against a static background results in saccadic eye movements [1, 2] . Although exceptions to this rule have been reported [3] [4] [5] , volitional control over smooth eye movements is at best rudimentary. Here, I introduce a novel, temporally modulated visual display, which, although static, sustains smooth eye movements in arbitrary directions. After brief training, participants gain volitional control over smooth pursuit eye movements and can generate digits, letters, words, or drawings at will. For persons deprived of limb movement, this offers a fast, creative, and personal means of linguistic and emotional expression. To achieve smooth pursuit, direction-and speed-selective neurons in dorsal visual areas are thought to provide trajectory information to pursuit neurons in premotor cortex [1, 2] . Interestingly, these motion-sensitive neurons invert their preferred direction of motion when the contrast polarity of a moving stimulus is rapidly modulated in time [6] [7] [8] . This surprising modification results in a vivid perception of motion in a direction opposite to veridical (reverse-phi motion [9] [10] [11] [12] ; Figure 1 ) and can elicit reversed short-latency ocular following [13] . Moreover, a perception of reverse-phi motion of an otherwise static random-dot field undergoing counterphase flicker can be induced by eye movements alone [10, 12] . Under these conditions, the whole flickering static field is seen as moving in the same direction as the eyes, in contrast to normal viewing conditions where smoothly tracking a target entails a shift of the stationary surrounding field in a direction opposite to that of the eyes. I relied on these perceptual phenomena to design a flickering visual display in which eye movements induce a compelling perception of reverse-phi motion in the very direction of eye movements. Crucially, this motor-induced motion percept can be tracked by the pursuit system, enabling a perceptionaction loop: reverse-phi motion elicited by eye movements provides a moving visual substrate, which in turn feeds the pursuit system. Under these conditions, individuals can gain voluntary control over the feedback loop between smooth pursuit and the visual reverse-phi motion it induces. With little training, typically three 30 min sessions performed on different days, smooth eye movements can voluntarily be directed in any direction, as if ''surfing'' on a self-induced wave of reverse-phi motion. It then becomes possible to generate smooth renderings of digits, letters, and words or even one's signature. To test this novel communication device, six participants and the author trained to generate and control smooth eye movements. During a first 30 min session, they experience examples of reverse-phi motion [9, 10] and familiarize with a display screen filled with randomly distributed static disks modulated in contrast about the background luminance at a moderate temporal rate (>12 Hz). When the eyes are at rest, this display appears as a field of faint static disks. However, any movement of the eyes creates a shift of the visual pattern on the retina. The motion energy of the resulting spatiotemporal luminance profile stimulates motion sensitive neurons, eliciting a perception of reverse-phi motion flowing in the direction of the eyes [10, 12] , thus providing a sustainable visual input to smooth pursuit (Figure 1 ; see Experimental Procedures; see Movie S1 available online). To first ease the perceptual experience of reverse-phi motion, participants are advised to move and track their finger or a stick in front of a high-contrast flickering display. To further help participants sense their eye movements, a ring of colored dots locked to the eye position, and thus moving with the eyes, is initially superimposed on the flickering pattern. Participants then generate eye movements with both the ring and the flickering pattern. After this initial period, the feedback ring is removed to encourage participants to focus on oculomotor activity, and the contrast of the display is lowered while keeping reverse-phi motion above perceptual threshold (Movie S1). In addition to avoiding fatigue and discomfort, lowering contrast has important consequences that ease the mastering of smooth pursuit eye movements: lowering contrast alters the relative activity of less sensitive parvocellular neurons within the ventral pathway and the more sensitive magnocellular neurons projecting to dorsal areas involved in eye movement control [14, 15] . As a consequence, the salience of position and form cues is reduced at low contrast, thus minimizing the conflict between perceiving a motion of an otherwise static display. Low stimulus contrast also entails decreased perceived speed [16] and slower pursuit [17] and further provides a natural eye-speed controller because the visibility of a fast stimulus is markedly reduced at low contrast, thanks to the transfer function of the visual system [18] . Under low contrast conditions, participants perceive a faint motion flow, as snowflakes moved by the wind, and can smoothly move their eyes for long periods of time (>10 s, Movie S2). They then experience the perceptual effects of pursuit so as to gain cognitive knowledge on their eye motility. Afterward, participants perform several w30 s runs solely with the flickering display, producing specific patterns or drawing figures of their own. With training, participants can draw digits, letters, or small words. It is worth noting that participants do not see the traces of their eye movement during writing, and therefore blindly project the visuomotor imagery of the intended figure
REF used sustained smooth-pursuit eye movement for cursive writing.
Cursive Writing with Smooth Pursuit Eye Movements
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