Heading Optic flow patterns

When we want to reach some goal (e.g., a gate at the end of a field), we need to control our heading, or point towards which we are moving. Gibson (1950) emphasised the importance of optic flow patterns. When someone is moving forwards, the point towards which he or she is looking (the focus of expansion) appears motionless. In contrast, the visual field around that point seems to be expanding. Graziano, Andersen, and Snowden (1994) identified neurons in the medial superior temporal area that responded most to patterns of dots expanding outwards, and these neurons may provide the physiological basis for the perception of optic flow patterns.

As optic flow provides relatively precise information about the direction in which someone is heading, it follows from Gibson's (1950) theoretical position that heading judgements should be fairly accurate. In fact, heading errors of between 5° and 10° were reported in most early research (e.g., Warren, 1976). With that low level of accuracy, it is doubtful whether optic flow could provide adequate information for the control of locomotion.

Warren, Morris, and Kalish (1988) argued that there were some limitations with previous research. Heading judgements were generally obtained by requiring the participants to point, and this may be an insensitive measure. Accordingly, Warren et al. used a rather different task. They produced films consisting of moving dots, with each film simulating the optic flow pattern that would be produced if someone were moving in a given direction. The participants' task was to decide whether the person seemed to be heading to the left or to the right of a stationary target positioned at the horizon of the display. The mean error with this measure of heading accuracy averaged was about 1.2°. As Warren et al. (1988, p. 659) concluded, "optical flow can provide an adequate basis for the control of locomotion and other visually guided behaviour."

Theoretical accounts

Various aspects of optic flow might be of crucial importance to the perception of heading. Gibson (1950) proposed a global radial outflow hypothesis, according to which it is the overall or global outflow pattern that specifies the direction of heading. Alternatively, there is the local focus of outflow hypothesis (discussed by Warren et al., 1988), according to which the direction of heading is determined by locating the one element in the flow field that is stationary (the focus of expansion).

The focus of expansion is of most value when the individual in motion is looking directly at where he or she is going. However, car drivers often look at the line in the centre of the road or at the kerb instead. According to Lee (1980), drivers are more likely to use general optic flow information than the focus of expansion. When the driver is on course, the optic flow lines and the edges of the road will coincide. If the two do not match, then the driver is in danger of leaving the road.

Evidence against the local focus of outflow hypothesis was provided by Warren et al. (1988). They found that heading judgements were very accurate even when there was no stationary element in the visual environment.

Evaluation

Optic flow patterns generally and the focus of expansion specifically may contribute towards our ability to head in the right direction. However, Gibson's approach does not take account of the fact that movement on the retina is determined by eye and head movements as well as by the optic flow pattern. As a result, the focus of expansion on the retina does not correspond with the point towards which someone is heading when eye movements lead the individual to be looking in a slightly different direction (Loppe & Rauscheck, 1994).

Cutting, Springer, Braren, and Johnson (1992) adopted a different approach. They assumed that eye movements are useful in the control of heading, whereas Gibsonian approaches have regarded eye movements as an unwanted nuisance. Eye movements that track an object provide valuable information, because objects closer to the observer than the fixated object appear to move faster in the visual field and in the opposite direction to objects further away. This so-called differential motion parallax applies to all objects except those directly in line with the point at which the individual is heading. Cutting et al. (1992) hypothesised that eye movements are controlled by differential motion parallax, and this helps to ensure the accuracy of heading behaviour.

The evidence for this hypothesis is mixed. In support, Cutting et al. (1992) found that their participants exhibited worse judgements of direction of heading when the information provided by differential motion parallax was misleading. However, Warren and Hannon (1988) found that eye movements are not necessary in order to judge heading direction accurately. Cutting et al. (1992) admitted that differential motion parallax is not valuable for car drivers or pilots, and that instead the optic flow pattern may be used.

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