[Comparative scaling of afferent and efferent motion detection in man: linear functions with different slopes]

Illusory changes in the perceived speed of motion derived from proprioception and touch

In vision, the perceived velocity of a moving stimulus differs depending on whether we pursue it with the eyes or not: A stimulus moving across the retina with the eyes stationary is perceived as being faster compared with a stimulus of the same physical speed that the observer pursues with the eyes, while its retinal motion is zero. This effect is known as the Aubert-Fleischl phenomenon. Here, we describe an analog phenomenon in touch. We asked participants to estimate the speed of a moving stimulus either from tactile motion only (i.e., motion across the skin), while keeping the hand world stationary, or from kinesthesia only by tracking the stimulus with a guided arm movement, such that the tactile motion on the finger was zero (i.e., only finger motion but no movement across the skin). Participants overestimated the velocity of the stimulus determined from tactile motion compared with kinesthesia in analogy with the visual Aubert-Fleischl phenomenon. In two follow-up experiments, we manipulated the stimulus noise by changing the texture of the touched surface. Similarly to the visual phenomenon, this significantly affected the strength of the illusion. This study supports the hypothesis of shared computations for motion processing between vision and touch.NEW & NOTEWORTHY In vision, the perceived velocity of a moving stimulus is different depending on whether we pursue it with the eyes or not, an effect known as the Aubert-Fleischl phenomenon. We describe an analog phenomenon in touch. We asked participants to estimate the speed of a moving stimulus either from tactile motion or by pursuing it with the hand. Participants overestimated the stimulus velocity measured from tactile motion compared with kinesthesia, in analogy with the visual Aubert-Fleischl phenomenon.

Motion perception during selfmotion: The direct versus inferential controversy revisited

According to the traditional inferential theory of perception, percepts of object motion or stationarity stem from an evaluation of afferent retinal signals (which encode image motion) with the help of extraretinal signals (which encode eye movements). According to direct perception theory, on the other hand, the percepts derive from retinally conveyed information only. Neither view is compatible with a perceptual phenomenon that occurs during visually induced sensations of ego motion (vection). A modified version of inferential theory yields a model in which the concept of extraretinal signals is replaced by that of reference signals, which do not encode how the eyes move in their orbits but how they move in space. Hence reference signals are produced not only during eye movements but also during ego motion (i.e., in response to vestibular stimulation and to retinal image flow, which may induce vection). The present theory describes the interface between self-motion and object-motion percepts. An experimental paradigm that allows quantitative measurement of the magnitude and gain of reference signals and the size of the just noticeable difference (JND) between retinal and reference signals reveals that the distinction between direct and inferential theories largely depends on: (1) a mistaken belief that perceptual veridicality is evidence that extraretinal information is not involved, and (2) a failure to distinguish between (the perception of) absolute object motion in space and relative motion of objects with respect to each other. The model corrects these errors, and provides a new, unified framework for interpreting many phenomena in the field of motion perception.

Processing spatial information in the sensorimotor branch of the visual system

We distinguish two representations of visual space: a cognitive representation drives perception, and a sensorimotor representation controls visually guided behavior. Spatial values in the two representations are separated with the Roelofs effect: a target within an off-center frame appears biased in a location opposite the direction of the frame. The effect appears for a verbal measure (cognitive) but not for a jab at the target (sensorimotor). A 2-s response delay induces a Roelofs effect in the motor measure, showing the limit of motor memory. Motor error is not correlated with reaction time. Subjects could strike one of two identical targets, a process involving choice, without intrusion of a Roelofs effect, showing that the sensorimotor system can use its own coordinates even when a cognitive choice initiates the motor processing.

A neural model of smooth pursuit control and motion perception by cortical area MST

Smooth pursuit eye movements (SPEMs) are eye rotations that are used to maintain fixation on a moving target. Such rotations complicate the interpretation of the retinal image, because they nullify the retinal motion of the target, while generating retinal motion of stationary objects in the background. This poses a problem for the oculomotor system, which must track the stabilized target image while suppressing the optokinetic reflex, which would move the eye in the direction of the retinal background motion (opposite to the direction in which the target is moving). Similarly, the perceptual system must estimate the actual direction and speed of moving objects in spite of the confounding effects of the eye rotation. This paper proposes a neural model to account for the ability of primates to accomplish these tasks. The model simulates the neurophysiological properties of cell types found in the superior temporal sulcus of the macaque monkey, specifically the medial superior temporal (MST) region. These cells process signals related to target motion, background motion, and receive an efference copy of eye velocity during pursuit movements. The model focuses on the interactions between cells in the ventral and dorsal subdivisions of MST, which are hypothesized to process target velocity and background motion, respectively. The model explains how these signals can be combined to explain behavioral data about pursuit maintenance and perceptual data from human studies, including the Aubert--Fleischl phenomenon and the Filehne Illusion, thereby clarifying the functional significance of neurophysiological data about these MST cell properties. It is suggested that the connectivity used in the model may represent a general strategy used by the brain in analyzing the visual world.

A review of the role of efference copy in sensory and oculomotor control systems

Efference copy is an internal copy of a motor innervation. In the oculomotor system it provides the only extraretinal signal about eye position that is available without delay, and it is shown to be the most important extraretinal source of information for perceptual localization and motor activity. Efference copy accompanies all voluntary eye movements and some involuntary ones, including pursuits, saccades, and the fast phases of vestibular and optokinetic nystagmus. Not all eye movements are accompanied by an efference copy; its presence is determined by a movement's function, not it dynamics. Because the gain of the efference copy mechanism is less than 1, and it does not take account of oculomotor delays and kinematics, it is supplemented by other mechanisms in achieving space constancy. It functions differently for perception and for visually guided behavior. There is only one efference copy for both eyes, reflecting Hering's law, and it is subject to adaptation.

Localization of objects in the peripheral visual field

We investigated visual localization by asking humans to point at visual objects without vision of their hand. The objects were luminous discs, presented stereoscopically at different distances, eccentricities and meridians with respect to the subjects' straight-ahead. Final pointing position was recorded by an electromagnetic search-coil technique. We found that the eccentricity of pointing responses towards peripheral targets was larger when subjects fixated straight-ahead rather than looked at the targets. This outcome confirmed our previous finding that target eccentricity in the peripheral visual field is overestimated. We further found that overestimation increased less than proportionally with target eccentricity, which suggests that the local magnification factor gradually declines in the visual periphery. A quantitative analysis indicated that the magnification factor is about 1.5 at the fovea, and approaches 1.0 at 10 degrees visual angle. Thus, our data support the hypothesis of a peri-foveal magnification effect which gradually subsides with increasing eccentricity. The observed magnification was similar for the horizontal and the vertical meridian. We found that the egocentric distance of pointing responses depends not only on the distance of the object pointed at, but also on the distance of a second object in the visual field. This outcome was in quantitative agreement with the predictions of Foley's model of interactive distance evaluation. Response depth, i.e. the difference in the response distances towards the two objects, was larger if both objects appeared near the center of the visual field rather than if one object appeared in the visual periphery.(ABSTRACT TRUNCATED AT 250 WORDS)

An acceleration illusion caused by underestimation of stimulus velocity during pursuit eye movements: Aubert-Fleischl revisited

When the eyes pursue a fixation point that sweeps across a moving background pattern, and the fixation point is suddenly made to stop, the ongoing motion of the background pattern seems to accelerate to a higher velocity. Experiment I showed that this acceleration illusion is not caused by the sudden change in (i) the relative velocity between background and fixation point, (ii) the velocity of the retinal image of the background pattern, or (iii) the motion of the retinal image of the rims of the CRT screen on which the experiment was carried out. In experiment II the magnitude of the illusion was quantified. It is strongest when background and eyes move in the same direction. When they move in opposite directions it becomes less pronounced (and may disappear) with higher background velocities. The findings are explained in terms of a model proposed by the first author, in which the perception of object motion and velocity derives from the interaction between retinal slip velocity information and the brain's 'estimate' of eye velocity in space. They illustrate that the classic Aubert-Fleischl phenomenon (a stimulus seems to be moving slower when pursued with the eyes than when moving in front of stationary eyes) is a special case of a more general phenomenon: whenever we make a pursuit eye movement we underestimate the velocity of all stimuli in our visual field which happen to move in the same direction as our eyes, or which move slowly in the direction opposite to our eyes.

Effect of context and efference copy on visual straight ahead

Bias in efferent commands to the eye changes the apparent straight ahead direction in an unstructured visual field, but has little effect in a normal visual environment. Naive subjects set a visible marker to appear straight ahead under monocular viewing conditions and while pressing on the viewing eye. Three background conditions were used: a naturalistic landscape photograph, a blank field, and a repeating checkerboard texture that provides strong contours but no information about visual direction. Effect of eyepress on straight-ahead judgments was small but significant with the landscape background, and larger with the blank field; the checkerboard texture yielded a bias halfway between the magnitudes of bias in the other two conditions. A visual capture theory predicts that the textured field should work like a blank one, while an oculomotor theory predicts that it should work like a natural one. Interpreted in this context, the results show the two theories to be about equally important in judging straight ahead. A second experiment with experienced observers and moving backgrounds gave the same result.

Contribution of retinal versus extraretinal signals towards visual localization in goal-directed movements

In human subjects, we investigated the accuracy of goal-directed arm movements performed without sight of the arm; errors of target localization and of motor control thus remained uncorrected by visual feedback, and became manifest as pointing errors. Target position was provided either as retinal eccentricity or as eye position. By comparing the results to those obtained previously with combined retinal plus extraretinal position cues, the relative contribution of the two signals towards visual localization could be studied. When target position was provided by retinal signals, pointing responses revealed an over-estimation of retinal eccentricity which was of similar size for all eccentricities tested, and was independent of gaze direction. These findings were interpreted as a magnification effect of perifoveal retinal areas. When target position was provided as eye position, pointing was characterized by a substantial inter-, and intra-subject variability, suggesting that the accuracy of localization by extraretinal signals is rather limited. In light of these two qualitatively different deficits, possible mechanisms are discussed how the two signals may interact towards a more veridical visual localization.

Visual-vestibular interaction studied with stroboscopically illuminated visual patterns

Horizontal DC-electrooculograms were recorded in subjects rotating on a horizontal turntable sinusoidally at 0.1 Hz and 35 to 40 degrees amplitude. The subjects either fixated a stroboscopically illuminated vertically striped pattern (1.15 to 3.45 degrees period) rotating with the turntable or initiated Sigma-OKN before the rotation began and tried to maintain Sigma-OKN during rotation. In a third paradigm, interaction of vestibulo-ocular reflex (VOR) and Phi-OKN was studied. VOR-suppression by fixation was complete within the limits of EOG-recording precision (+/- 1 degree X s-1) for flash frequencies fs greater than 10 flashes X s-1. VOR-suppression decreased monotonically with fs between 10 and 1 flashes X s-1. A similar dependency on fs was found for VOR-suppression during Sigma- or Phi-OKN. Above 10 flashes X s-1 VOR-suppression remained incomplete; below 5 flashes X s-1 VOR-suppression was stronger with the Sigma-OKN paradigm than during fixation and depended on spatial frequency of the pattern. During sinewave rotation of the subject the perceived speed Vp of Sigma-movement correlated to the movement of gaze in space and not to the movement of the eye in head. In a control experiment with normal optokinetic stimulation, OKN-suppression by fixating a small flashing target was found to depend on fs in a similar way as VOR-suppression in the experiments described above.

On the relativity of perceived motion

Perceived stability of the visual world during eye movements is traditionally explained as due to the presence of extraretinal signals, equal in magnitude to retinal signals. Motion is perceived when the two signals differ. An experiment is reported in which motion thresholds were measured during smooth pursuit eye movements. The results show that the traditional view is incomplete. Motion is only perceived when the two signals differ by at least a just noticeable difference (JND), the magnitude of which depends on ocular velocity and is independent of the direction of stimulus motion relative to the eyes. The data lead to the rejection of theories according to which ocular velocity is under-represented in extraretinal signals. In addition they show that retinal image motion carries no information about stimulus motion. Perceived motion, direction and velocity are relative concepts. They depend on the JND and therefore they are relative to extraretinal signals. This principle explains the Filehne illusion and even predicts the Aubert-Fleischl phenomenon. A similar analysis can be applied to understand vestibular effects on motion perception.

Smooth pursuit eye movements and optokinetic nystagmus elicited by intermittently illuminated stationary patterns

Stationary periodic visual patterns (row of equally spaced dots or black-white stripes) of the period Ps illuminated stroboscopically with a flash frequency fs induce an apparent movement perception (sigma-movement) when slow eye movements are performed across the periodic pattern. The movement appears in the direction of the eye movements when the angular speed VE of the eyes corresponds to the following condition: Ve = k . Ps . fs [deg . s-1] (1) k is a constant and equals 1 (or exceptionally 2 or 3). The sigma-movement induces a sigma-OKN with an average angular speed of its slow phases corresponding to Eq.(1). sigma-OKN can be elicited when identical foveal or identical extrafoveal stimulus patterns are applied from flash to flash. A considerable random variability of the flash sequence does not interrupt the sigma-movement and the sigma-OKN. Both phenomena can also be elicited by a stimulus pattern with its periodicity hidden in spatial noise and this periodic pattern only becomes visible during the eye movements. It is argued that the sigma-phenomena are caused by efference copy signals of the gaze control system, which interact with the afferent signals (displacement of visual stimuli on the retina) at different levels of the afferent visual system. One interaction is supposed at a cortical level where the extrapersonal visual space is represented.