Abstract
To study the relationship between the ventral and dorsal pathways in the processing of dynamic objects, we used structure from motion (SFM) stimuli composed of clouds of dots defining 3D shapes visible only when in motion.
In a first series of experiments, minimal presentation duration thresholds for the identification of shape and motion direction were measured for 3 shapes and 3 motion directions, embedded in motion noise sequences.
Duration thresholds range from 60ms (+−7.5ms) for motion identification to 100ms (+−26ms) for form identification, suggesting a temporal precedence of motion processing over form.
In a second series of experiment, the same class of stimuli was used with Rapid Serial Visual Presentation (RSVP) paradigm, designed to induce Repetition Blindness (RB), in order to determine whether the same hierarchical processes would be at work. If so, one would expect motion repetition to interfere with shape perception.
In different blocks of trials, observers (N=12) reported the number of occurrences they perceived of either 3D shapes (“shape task”) or motion directions (“motion task”). RSVP sequences were identical in both tasks.
We computed a repeated measure ANOVA with 3 factors: task, repetition over the task-relevant attribute and repetition over the irrelevant attribute.
In both tasks, we found a significant effect (F (1,11)=136. 4, p[[lt]].001) of the repetition of the relevant attribute (RB effect) and no significant effect of the irrelevant one.
We conclude that the RB effect, well known for static objects, can be found to 3D dynamic objects and that RB for motion direction also exists.
Despite the necessary processing of motion to perceive 3D shapes, under our conditions we did not find any interaction between motion and shape. These findings suggest that RB is task dependent and that Object files are goal directed entities rather than resulting exclusively from bottom-up processes.
supported by DGA/D4S/MRIS grant n° 06C0035