The “optic flow” generated as we move through the world can provide us with important information about our speed and direction of self-motion (Gibson,
1950; Gibson, Olum, & Rosenblatt,
1955). However, optic flow alone is not sufficient to determine whether we are in fact physically moving. This is because identical patterns of optic flow will be generated when we move toward objects embedded in the scene and when the same group of objects moves toward us when we are stationary (refer to
Movie 1). The visual system often interprets global patterns of optic flow as being due to self-motion, and as a result, it is possible to induce compelling visually induced illusions of self-motion in stationary observers, known as vection (Brandt, Dichgans, & Koenig,
1973). The effectiveness of vision in generating vection depends on how the visual information is acquired by our eye(s) and integrated with the available self-motion information arising from other senses. The present study examines the role that eye movements play in the capture of visual motion information in stationary observers and how they may influence visual distinctions between self- and object motions.
The brain can supplement visual information about linear self-motion in depth with inertial signals from the otolith receptors of the vestibular system located in the inner ear. These receptors respond to linear head acceleration and can help resolve the self- versus object motion ambiguity raised by optic flow. For example, otolith signals are generated when the head moves from rest and during changes in its linear velocity, such as the deceleration that occurs when the head achieves a new stationary position (red zones in
Movie 2). However, these otolith projections do not signal head movements occurring at a constant linear velocity (green zone in
Movie 2). This means that even with multi-sensory contributions from inertial and visual signals, a residual perceptual ambiguity will often persist, leading to the generation of vection.
Normally, when we move angularly or linearly through space, compensatory slow-phase eye movements are induced in an attempt to stabilize the retinal image. Both visual and vestibular inputs drive these eye movements, which are coordinated via convergent signals arising from the primary vestibular receptors and global retinal stimulation consistent with self-motion (Rucker,
2010). The present study examines the role that these compensatory eye movements play in the visual perception of self-motion.
Optokinetic nystagmus (OKN) is a compensatory eye movement induced by visual displays that simulate maintained angular head rotation. The earliest eye movement research on vection examined the role of OKN in the onset of circular vection. For example, Brandt, Dichgans, and Büchele (
1974) found that the onset of vection correlated with the buildup of OKN over time. Taken by itself, this finding suggests that the engagement of compensatory eye movements might facilitate the onset of vection. However, as noted by Howard (
1982), circular vection can still be induced when OKN is suppressed with active target fixation (p. 392), which suggests that OKN buildup over time is not necessary for the induction of circular vection.
Displays that simulate linear head translation also induce eye movements that facilitate the maintenance of a stable visual image. The ocular following response (OFR) achieves optimal activation at extremely short latencies (<100 ms) in response to motion parallax simulating linear head translations in 3D space (e.g., Kawano & Miles,
1986; Miles,
1998; Miles & Kawano,
1986; Miles, Kawano, & Optican,
1986; Schwarz & Miles,
1991). These visually mediated OFRs are similar to the early phase of OKN (i.e., OKNe) and scale inversely with perceived viewing distance (Busettini, Miles, & Schwarz,
1991; Busettini, Miles, Schwarz, & Carl,
1994), providing retinal image stability for a given depth of field. Recently, we found that pseudo-randomly directing the observer's gaze around a radial flow display improved the overall strength of linear vection in depth (Palmisano & Kim,
2009). This gaze shifting advantage for vection may have been due to the increased OFR/OKNe that follows each saccade eye movement (e.g., Kawano & Miles,
1986; Lisberger,
1998).
However, the OFR/OKNe is also known to adapt rapidly during maintained viewing of radial flow (Miles et al.,
1986). This will also lead to the repeated adaptation of the OFR over a series of gaze shifting events, which will increase the retinal slip of the visual scene. It is therefore possible that either the enhancement of the OFR during gaze shifting conditions or the retinal slip generated by its repeated adaptation may lead to increases in vection strength.
Previously, we found that stabilizing eccentric gaze with a stationary eccentric fixation point (increasing retinal slip) impaired vection, compared to free viewing conditions (Palmisano & Kim,
2009). It is possible that this finding may have been caused by differences in the retinal stimulation produced by the active versus passive engagement of eye movements. The present study examined the changes in eye movements that accompany changes in linear vection in depth as observers passively viewed radially expanding optic flow displays. We focused on eccentric, as opposed to central, viewing, since eccentric viewing produces the strongest OFR in passive observers due to the greater motion in the periphery of radial flow displays.