Integration mechanisms for heading perception

Temporal stability of human heading perception

Humans are capable of accurately judging their heading from optic flow during straight forward self-motion. Despite the global coherence in the optic flow field, however, visual clutter and other naturalistic conditions create constant flux on the eye. This presents a problem that must be overcome to accurately perceive heading from optic flow-the visual system must maintain sensitivity to optic flow variations that correspond with actual changes in self-motion and disregard those that do not. One solution could involve integrating optic flow over time to stabilize heading signals while suppressing transient fluctuations. Stability, however, may come at the cost of sluggishness. Here, we investigate the stability of human heading perception when subjects judge their heading after the simulated direction of self-motion changes. We found that the initial heading exerted an attractive influence on judgments of the final heading. Consistent with an evolving heading representation, bias toward the initial heading increased with the size of the heading change and as the viewing duration of the optic flow consistent with the final heading decreased. Introducing periods of sensory dropout (blackouts) later in the trial increased bias whereas an earlier one did not. Simulations of a neural model, the Competitive Dynamics Model, demonstrates that a mechanism that produces an evolving heading signal through recurrent competitive interactions largely captures the human data. Our findings characterize how the visual system balances stability in heading perception with sensitivity to change and support the hypothesis that heading perception evolves over time.

Spared ability to perceive direction of locomotor heading and scene-relative object movement despite inability to perceive relative motion

BACKGROUND

All contemporary models of perception of locomotor heading from optic flow (the characteristic patterns of retinal motion that result from self-movement) begin with relative motion. Therefore it would be expected that an impairment on perception of relative motion should impact on the ability to judge heading and other 3D motion tasks.

MATERIAL AND METHODS

We report two patients with occipital lobe lesions whom we tested on a battery of motion tasks. Patients were impaired on all tests that involved relative motion in plane (motion discontinuity, form from differences in motion direction or speed). Despite this they retained the ability to judge their direction of heading relative to a target. A potential confound is that observers can derive information about heading from scale changes bypassing the need to use optic flow. Therefore we ran further experiments in which we isolated optic flow and scale change.

RESULTS

Patients' performance was in normal ranges on both tests. The finding that ability to perceive heading can be retained despite an impairment on ability to judge relative motion questions the assumption that heading perception proceeds from initial processing of relative motion. Furthermore, on a collision detection task, SS and SR's performance was significantly better for simulated forward movement of the observer in the 3D scene, than for the static observer. This suggests that in spite of severe deficits on relative motion in the frontoparlel (xy) plane, information from self-motion helped identification objects moving along an intercept 3D relative motion trajectory.

CONCLUSIONS

This result suggests a potential use of a flow parsing strategy to detect in a 3D world the trajectory of moving objects when the observer is moving forward. These results have implications for developing rehabilitation strategies for deficits in visually guided navigation.

Age-related changes in fine motion direction discriminations

The present study used an equivalent noise method to characterize the sources of reduced performance in fine discrimination of motion with age. We varied the density of the displays, the speed and speed variability and the temporal correlation of dots' motion in successive frames to assess their effect on the sensitivity to motion direction. The results showed that, in all experimental conditions, the older observers had higher levels of internal noise. Both age groups used the stimulus information less efficiently at slow speed and most efficiently when the moving elements were uncorrelated across frames. The older observers were less efficient than the younger observers in all conditions except at high speed where the efficiency of the two age groups was the same. We fitted two biologically plausible models to the experimental data: a modified version of the local-to-global direction encoding model (Dakin et al. in Vis Res 45:3027-3049, 2005) and a model for pooling of motion information in medial temporal area (MT) where the neuronal responses were correlated (Huang and Lisberger in J Neurophysiol 101:3012-3030, 2009). The modeling results indicate that the correlation in neuronal responses is essential to characterize the influence of speed and speed variability on the sensitivity to direction information. For the younger observers, a single set of parameters can account for the effect of noise and the spatio-temporal parameters of the stimuli, while, for the older observers, a change in the correlation of neuronal activity and the directional tuning bandwidth with the levels of external noise is needed. The findings are discussed with respect to optimal use of the dynamic information to overcome the negative effect of aging.

Global flow impacts time-to-passage judgments based on local motion cues

We assessed the effect of the coherence of optic flow on time-to-passage judgments in order to investigate the strategies that observers use when local expansion information is reduced or lacking. In the standard display, we presented a cloud of dots whose image expanded consistent with constant observer motion. The dots themselves, however, did not expand and were thus devoid of object expansion cues. Only the separations between the dots expanded. Subjects had to judge which of two colored target dots, presented at different simulated depths and lateral displacements would pass them first. Image velocities of the target dots were chosen so as to correlate with time-to-passage only some of the time. When optic flow was mainly incoherent, subjects' responses were biased and relied on image velocities rather than on global flow analysis. However, the bias induced by misleading image velocity cues diminished as a function of the coherence of the optic flow. We discuss the results in the context of a global tau mechanism and settle a debate whether local expansion cues or optic flow analysis are the basis for time-to-passage estimation.