Features integrate along a motion trajectory when object integrity is preserved

Abstract

Information about a moving object is usually poor at each retinotopic location because photoreceptor activation is short, noisy, and affected by shadows, reflections of other objects, and so on. Integration across the motion trajectory may yield a much better estimate about the objects' features. Using the sequential metacontrast paradigm, we have shown previously that features, indeed, integrate along a motion trajectory in a long-lasting window of unconscious processing. In the sequential metacontrast paradigm, a percept of two diverging streams is elicited by the presentation of a central line followed by a sequence of flanking pairs of lines. When several lines are spatially offset, the offsets integrate mandatorily for several hundreds of milliseconds along the motion trajectory of the streams. We propose that, within these long-lasting windows, stimuli are first grouped based on Gestalt principles of grouping. These processes establish reference frames that are used to attribute features. Features are then integrated following their respective reference frame. Here using occlusion and bouncing effects, we show that indeed such grouping operations are in place. We found that features integrate only when the spatiotemporal integrity of the object is preserved. Moreover, when several moving objects are present, only features belonging to the same object integrate. Overall, our results show that feature integration is a deliberate strategy of the brain and long-lasting windows of processing can be seen as periods of sense making.

Figure 1.

The “two-stage” model. The first stage consists of Gestalt grouping and segregation processes, which establish a reference frame for each group. These reference frames are then used to attribute features to stimuli. Features are then integrated following their respective reference frames.

Figure 2.

The sequential metacontrast (SQM). Each line was presented for 20 ms with an interstimulus interval (ISI) of 20 ms (30 ms for the first ISI to obtain strong masking of the central vernier). The percept is two streams of lines expanding from the center. The presentation of each pair of lines is a frame. Frame 0 corresponds to the presentation of the central line. V (vernier): only the central line is offset, that is, the lower segment of the line is spatially offset to the right or to the left compared with the upper segment. AV (anti-vernier): a flanking line is offset. V–AV (vernier – anti-vernier): the central line and a flanking line are offset in opposite directions. V-PV (vernier – pro-vernier): the central line and a flanking line are offset in the same direction. Observers are instructed to attend to one of the streams (here the right stream) and to report the direction (right or left) of the perceived offset. Colors are for illustration purpose. All stimulus elements were white or red on a black background. Figure adapted from (Drissi-Daoudi et al., 2019).

Figure 3.

Experiment 1a. (a) The anti-vernier was presented in frame 7. In the Occluded condition, the lines of the attended stream in frames 3, 4, and 5 were occluded by a grey rectangle. The same three lines were missing in the Gap condition. Colors are for illustration purpose only. (b) V and AV show the offset calibrations with either the vernier (V) or the anti-vernier only (AV; the symbol of the occluded AV configuration is invisible because of the overlap with the other symbols; likewise, errors bars are often too small to be visible). In the next conditions, both the vernier and the anti-vernier were presented together. We plot performance with respect to the subjective ratings. Observers rated the stream in the Classic condition (blue) as unified, and offsets integrated indicated by a dominance level of about 50%. Similarly, the stream was perceived as unified in the Occluded condition (pink), and offsets integrated. In the Gap condition (purple), the stream was perceived more disjointed than in the other conditions, and offsets integrated less. Observers reported mainly the offset of the anti-vernier, as they were instructed to report the perceived offset direction at the end of the motion trajectory. Thus, offsets integrate across the occluder. However, if the spatiotemporal integrity of the stream is not preserved, the offsets integrate less. Circles indicate individual data. Error bars represent standard errors of the mean.

Figure 4.

Experiment 1b. (a) Experiment 1b was identical to Experiment 1a except that the streams diverged until frame 4 and then converged back to the center. (b) Similar to Experiment 1a, observers perceived the steam as unified in the Classic (blue) and Occluded (pink) conditions. The offsets integrated in these conditions. In the Gap condition (purple), the stream appeared more disjointed than in the other conditions. The offsets integrated less. Circles indicate individual data. Error bars represent standard errors of the mean.

Figure 5.

Experiment 2. (a) The anti-vernier was presented in frame 7 (290 ms). In the Occluded and Occluded_red conditions, the lines of the attended stream in frames 3, 4, and 5 were occluded by a grey rectangle. In the Classic_red and Occluded_red conditions, the lines of the attended stream from frame 5 to the end of the stimulus were red. (b, c, d and e) Dominance level as a function of the subjective ratings regarding stream unity (1 [“The motion stream appears completely disjointed”] to 6 [“The motion stream appears completely unified”]) in the Classic (b), Occluded (c), Classic_red (d) and Occluded_red (e) conditions. (f and g) Dominance level as a function of the subjective ratings regarding the integration of the red elements (1 [“The red elements appear to be completely separated from the motion stream”] to 6 [“The red elements appear to be completely part of them motion stream”]) in the Classic_red (f) and Occluded_red (g) conditions. V and AV show offset calibration. V–AV: observers were naïve. V–AV R[AV]: observers were informed of the paradigm and instructed to report the anti-vernier. Offsets largely integrated mandatorily across occlusion, even when there was a color change after the occluder. When there was no occluder and the stream chanded color mid-trajectory, integrations seemed to follow the subjective ratings. Offset integrated less when the stream was perceived as less unified and the red elements as less part of the stream. Circles indicate individual data. Error bars represent standard errors of the mean.

Figure 6.

Experiment 3. (a) Crossing condition. The percept was a white and a red stream crossing. Bouncing condition. The red stream had longer lines than the white stream to reinforce the percept of two streams bouncing on each other. (b) Dominance level in the different conditions. White and red bars represent observers attending to the white and red stream, respectively. In the Crossing condition, the vernier (V) and anti-vernier (AV) offsets presented in the white stream integrated when observers attended to the white stream (crossing_white). When attending to the red stream, observers reported the direction of the offset presented in the red stream (crossing_red), which was in the same direction as V (PV). In the Bouncing condition, two observers perceived a mixture of crossing and bouncing streams. Data from these two observers is not included. For the other eight observers, V and AV integrate in the white stream (bouncing_white) and observers reported the direction of PV in the red stream (bouncing_red). Thus, only features that belong to the same object integrate. Circles indicate individual data. Error bars represent standard errors of the mean.

Figure 7.

Data from the two observers that perceived a mixture of crossing and bouncing streams in the Bouncing condition. Data from one observer are in orange and the other observer's data are in green. Black diamonds indicate the expected dominance level when integration follows the perceived trajectory of the streams. For example, when attending to the white stream, if a bouncing trajectory is perceived, V and AV should integrate, and dominance level should be around 50%. If a crossing trajectory is perceived, V and PV should integrate, and dominance level should be above 75%. The data do not match the expected results.