Acceleration is detected by comparing initial and final velocities Brian Timney, Samantha Kearney, and Benjamin Asa. Western University, London, Ontario, Canada

Visual Acceleration Necessary complement to motion perception Virtually neglected in the literature, most of what is available is quite old These older studies were technologically limited We decided to do some parametric studies and attempt to gain an understanding of the mechanisms underlying acceleration perception

How is acceleration detected? Acceleration may be detected in one of two ways Direct Thresholds determined by acceleration rate. This implies the existence of “acceleration detectors” that process rate of change of motion directly Indirect Thresholds determined by ability to detect that speed has changed This implies that the critical variable will be the difference between initial and final velocity, not the absolute acceleration rate

Experiment 1:Threshold acceleration rate Speed Time 200 ms constant velocity at beginning and end of presentation Acceleration rate varied from trial to trial

Experimental 1:Threshold acceleration rate We measured ability to detect acceleration for a range of starting velocities General Methodology 100 black dots in 5° x 5° white square aperture 2-interval, 2 AFC; Constant velocity vs Acceleration Constant stimuli: 6 acceleration rates, 25 trials/rate, 4 runs Report which interval contained dots that were accelerating Starting velocities: 1 to 32° s-2 2 presentation durations: 500ms and 1000ms

Acceleration Detection Performance Performance improved with acceleration rate Greater increases in acceleration were needed for faster starting velocities (Weber’s Law) Acceleration thresholds varied with presentation duration

Performance vs Final Velocity Original data replotted to show performance as a function of final velocity Each pair of curves represents one starting velocity Effect of presentation duration disappears Threshold is dependent on the speed difference between initial and final velocity

Experiment 2a: Varying the acceleration rate Experiment 1 provides good evidence for the indirect hypothesis, but a stronger test would be to decouple absolute acceleration rate from average acceleration over the acceleration period; To do this we introduced a speed plateau during the acceleration phase; If thresholds rely on detecting speed differences, then average acceleration over the presentation period should be constant at threshold

Experimental Paradigm Representation of two different stimulus presentations with different absolute acceleration rates but the same average acceleration rate In different sessions the duration of the plateau was varied

Absolute acceleration thresholds Acceleration thresholds, plotted as a function of the absolute acceleration rates, for starting velocities of: A) 2° s-1 and B) 10° s-1 Absolute acceleration threshold increases as a function of plateau duration

Average acceleration thresholds Acceleration thresholds, plotted as a function of the avaerage acceleration rates, for starting velocities of: A) 2° s-1 and B) 10° s-1 Average acceleration threshold is independent of plateau duration

Experiment 2b: Multiple acceleration rates Data from Experiment 2a suggest that recognizing that a target has accelerated is dependent on the average acceleration within the presentation period As a final test, we varied the acceleration rates during presentation even further

Experimental paradigm The acceleration period was broken down into three components: 250 ms initial acceleration 500 ms middle acceleration 250 ms final acceleration (identical to initial) The middle acceleration period could be positive (red), negative (blue), or zero (green) Velocity Time Acceleration Period

Absolute vs. average acceleration Thresholds vary markedly when plotted as a function of the first last acceleration phases When data are expressed in terms of average acceleration over all three phases, thresholds are invariant

Conclusions Thresholds for the detection of acceleration vary substantially depending on the temporal parameters of the stimulus presentation When data are expressed in terms of the average velocity over the course of the presentation, thresholds remain relatively constant The data suggest that the critical variable in the detection of acceleration is the difference between the initial and final velocities, regardless of the duration of the acceleration period All of the results provide strong support for the “indirect” model of acceleration perception.

Thank You