1. Contribution of spatial and temporal integration in heading perception
Nadejda Bocheva, Miroslava Stefanova, Simeon Stefanov
Institute of Neurobiology
Bulgarian Academy of Sciences

2. Aim of the study
To evaluate the relative contribution of motion and form information in heading perception
Optical flow or optic flow is the pattern of apparent motion of objects, surfaces, and edges in a visual scene caused by the relative motion between an observer and a scene.
Given a constant gaze direction, the singularity within the optic flow field — the point from which the vectors representing the optic flow diverge — indicates the current heading direction.

3. Stimulus generation

4. Stimuli
Static

5. Stimuli
“Flicker”

6. Stimuli
Motion

7. Stimuli
Combined

8. Parameters Size 30 deg of arc Dot number 50 Lifetime 100 ms
Speed 4 deg/s
Dot separation 2 deg of arc
Coherence 70%
Center position: shifted to the left or right from the center by 0.67 to 4.67 deg of arc
20 repetitions
Subjects: 10 observers aged yrs (mean 37.9 ± 12.8), 4 male and 6 female

9. Results: sensitivity

10. Optimal cue combination
Predicted: 3.27 deg
Observed: 1.70 deg

11. Optimal cue combination: motion and “flicker”
Observed: Combined”1.70 deg;
“Flicker”: 2.29 deg; Motion: 4.22 deg
Predicted: 2.01 deg

12. Diffusion model for 2AFC tasks (Ratcliff & McKoon, 2008)
Parameters
Drift rate v – average slope of information accumulation process
Boundary separation a – amount of information needed for decision
Starting point z - indicator of an a priori bias in decision making.
Mean non-decision time t0 - average duration of all non-decisional processes
Inter-trial variability of the non-decision component st0- range of values for the distribution of the duration for the non-decisional processes

14. Results: RT

15. Results: RT Relative starting point
Duration of the non-decision processes

16. Results: RT Threshold separation Drift rate
For a given decision task, a single boundary value leads to the most correct answers per unit time, a metric that is often referred to as reward rate. To optimize reward rate, participants needed to use more narrow boundaries in the difficult condition

17. Conclusions
The presence of form information significantly improves the perceived direction of heading
The contribution of form information is based on temporal integration of the sequential frames
At slow motion speeds the temporal integration of form information has a higher weight
In dynamic conditions the amount of information necessary for a decision is not constant
Heading perception is not determined by an instant snapshop, but evolves over time

18. Thank you for your attention!