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

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.

Stimulus generation

Stimuli Static

Stimuli “Flicker”

Stimuli Motion

Stimuli Combined

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 21-59 yrs (mean 37.9 ± 12.8), 4 male and 6 female

Results: sensitivity

Optimal cue combination Predicted: 3.27 deg Observed: 1.70 deg

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

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

Results: RT

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

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

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

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