Strong support has grown for the idea that the information supporting safe visually-guided navigation lies in the global pattern of velocity vectors in retinal flow. This pattern contains information about the instantaneous speed and direction of motion of points of light across the retina. However, this is just one way of conceptualizing the potentially useful information in retinal flow.

An alternative conceptualization, which takes into account second-order information (i.e., not just velocity, but change in velocity; acceleration) was presented by Wang & Cutting (1999). These authors demonstrated the potential usefulness of particular types of motion parallax in retinal flow; defined in terms of change in the direction and velocity of local object-image relative motion. Specifically, they hypothesized that observers make use of three categories of local relative motion when moving through cluttered scenes: Accelerating divergence (AD; indicating unsure heading, but most likely outside a far reference object), decelerating divergence (DD; indicating heading inside a nearby reference object), and convergence (CV; also indicating heading inside a nearby reference object).

In a series of experiments using two-object displays we have examined the predictions of this tripartite model. While some support was found, results indicated that participants may be attuned to information in displays that is not captured by the three categories of relative motion. In particular, our data suggested that continuous differences underlying these categories may be used by participants to perceive the direction of their path. Because of the heuristic (i.e., uncertain) information provided by AD, it was possible to simulate different paths of movement which gave rise to this same relative motion category. Contrary to Wang and Cuttingís (1999) predictions, observers differentiated between these paths, despite the presence of the same relative motion category in the display (i.e., AD). Heading was perceived to lie on the inside of the near object, but the distribution of responses around the far object was variable across AD conditions. In regard to the other motion categories, DD and CV, it was found that observers performed poorly at identifying the simulated path direction in DD conditions, but performed well in CV conditions.

This pattern of responses is consistent with the idea that participants were sensitive not only to the categorical differences identified by Wang and Cutting, but perhaps also to continuous changes in the variable underlying the three categories; acceleration. AD represents an instance of positive acceleration with varying rates of change which appear to be linked to changes in heading judgments. DD and CV represent relatively slow velocity change and rapid velocity change instances of negative acceleration (not to be confused with deceleration) respectively. While this information for guiding locomotion is quite unlike Gibson's (e.g., 1954) original hypothesis about optic flow (and also different from many of the ideas that have grown out of Gibson's work), it nevertheless represents ecological information of the kind that Gibson sought (Reed, 1996). That is to say, the ability to regulate retinal image acceleration, for example, keeping such acceleration positive or negative during locomotion is the ability to pass to the inside or outside of the objects that furnish the environment respectively.

References:

Cutting JE, Wang RF, Fluckiger M, Baumberger B (1999). Human heading judgments and object-based motion information. Vision Res. 39: 1079-1105.