Trends in Cognitive Sciences
Volume 3, Issue 9, 1 September 1999, Pages 329-336
Journal home page for Trends in Cognitive Sciences

Review
Perception of self-motion from visual flow

https://doi.org/10.1016/S1364-6613(99)01364-9Get rights and content

Abstract

Accurate and efficient control of self-motion is an important requirement for our daily behavior. Visual feedback about self-motion is provided by optic flow. Optic flow can be used to estimate the direction of self-motion (‘heading’) rapidly and efficiently. Analysis of oculomotor behavior reveals that eye movements usually accompany self-motion. Such eye movements introduce additional retinal image motion so that the flow pattern on the retina usually consists of a combination of self-movement and eye movement components. The question of whether this ‘retinal flow’ alone allows the brain to estimate heading, or whether an additional ‘extraretinal’ eye movement signal is needed, has been controversial. This article reviews recent studies that suggest that heading can be estimated visually but extraretinal signals are used to disambiguate problematic situations. The dorsal stream of primate cortex contains motion processing areas that are selective for optic flow and self-motion. Models that link the properties of neurons in these areas to the properties of heading perception suggest possible underlying mechanisms of the visual perception of self-motion.

Section snippets

Heading detection during eye rotation

Experimental investigations of visual self-motion perception have benefited tremendously from the availability of specialized 3-D graphics workstations that can simulate movement through virtual environments in real time. The most basic experiments use linear movement in simple random-dot environments devoid of recognizable image features (Fig. 1B). The resulting visual motion is presented on a large screen in front of the subject that covers a substantial part of the visual field. Heading

Path perception

The above descriptions assume linear motion and a rotational component induced by eye movement. However, a combination of translational and rotational self-motion also arises during movement along a curved path. In this case the rotation axis is not in the eye but at the center of the motion curve. This creates a further problem for heading detection from retinal flow because the flow field cannot specify the location of the rotation axis and hence the origin of the rotational component. The

Combining retinal flow with information about the environment

Another factor that could influence heading judgements is information about 3-D scene layout. Knowledge of the depth structure of the scene could aid the separation of translation and rotation, because the motion of objects in the flow depends on their distance from the observer. The motion of distant points can be used to estimate rotation while the motion of near points is more useful to obtain translational information (see Fig. 1E). Independent knowledge about the depth structure of the

Dynamic properties and saccadic eye movements

Saccadic gaze shifts disrupt the retinal flow and change the retinal projection of the direction of heading on average twice per second (Box 1). Heading judgements are possible for presentation times as short as 228–400 ms, that is, within the time available between two saccades12, 23. Yet, visual search for the heading direction is only rarely accomplished in a single saccade, indicating that heading direction is usually processed across successive saccadic intervals35.

Short presentation times

Mechanisms of heading detection

Electrophysiology in macaque monkeys has shown several areas in the posterior parietal cortex involved in optic-flow processing37. Most research has focussed on the medial superior temporal (MST) area, because this is the first area in the cortical motion pathway with genuine optic flow selectivity38, 39, 40. Many MST cells also respond to real movement of the animal, even in darkness41, 42. Cells in MST are selective for the location of the focus of expansion43, 44. MST responses during

Conclusion

Goal-directed spatial behavior relies heavily on vision. Retinal flow provides visual input to monitor self-motion, navigate and guide future movements, and avoid obstacles. This article has reviewed the large body of knowledge about how humans analyse retinal flow that has accumulated in psychophysical studies. Humans can in principle use retinal flow for the determination of heading, in addition to several other visual cues (see Box 3). To solve the problem of eye rotations robustly, the

Outstanding questions

  • The future motion path is often more important for the control of self-motion than the current instantaneous heading. How is path information obtained from retinal flow and extraretinal signals and how is the path predicted?

  • Humans can use many other cues besides optic flow for visual navigation (Box 3). How is optic flow combined with other navigational strategies?

  • Most previous studies have used passive judgements of heading. Normally, however, heading judgements are required during active

Acknowledgements

We thank Antje Grigo and Bart Krekelberg for comments on the manuscript. This review and much of the cited work was made possible by financial support from the Human Frontier Science Program (RG-71/96B and RG 34/96B), the German Science Foundation (SFB 509), and the Dutch Research Council (SLW-NWO 805-33.171-P).

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