Experiment 3: Differences in Object Speed

Having tested the effects of top-down and bottom-up attention to ventral stream features on dorsal stream motion processing, we wanted to determine the attentional effects when the object feature that differentiates the superimposed surfaces is also processed in the dorsal stream. We

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have previously shown that bottom-up attention to speed differences reduces both direction repulsion and processing time (Perry et al., 2014). However, what remains unknown is the effect that top-down task demands have on perceived direction, and if they impart the same advantage of speeded processing times, as seen with differences in surface color and contrast, when the superimposed objects are differentiated by speed. Will top-down attention to speed differences produce an additional advantage over bottom-up speed differences, suggesting separate

mechanisms that are additive in nature? Or will there be no difference between top-down and bottom-up speed conditions suggesting early stimulus-driven integration of speed that cannot be further enhanced by top-down mechanisms?

3.5.1 Methods

Twenty new participants completed the following experimental conditions (10 each): 1) Bottom- up Speed (ages 17-22, 8 females, 2 males, Figure 3.5B), and 2) Top-down Speed (ages 17-25, 8 females, 2 males, Figure 3.5C). Direction Repulsion and Processing Time in these two

conditions were compared to the data previously collected in the Unicolor Control condition (Figure 3.5A) from Experiment 1. Visual acuity was normal or corrected-to-normal in all participants and none tested positive for color blindness using Ishihara plates. Informed consent was obtained from all participants and the research was approved by York University’s Human Participation Research Committee.

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Figure 3.5: Experimental paradigm for experiment 3. Again, the procedure was similar to that used in Experiments 1 and 2. Instead of differences in surface color or contrast however, in this experiment one surface moved at 3⁰/sec and at 6⁰/sec in the other surface.

The stimuli were the same as used in Experiment 1 except that one of the surfaces moved at 3⁰/sec and the dots in the other surface at 6⁰/sec. In the Top-down Speed condition,

participants were asked to give a direction response for the “fast” surface and the “slow” surface randomly ordered trial-by-trial, which required actively linking the speed to the direction of the surface. These top-down task demands require the categorization of speed into fast and slow, separate from the direction discrimination, so that judgements of each feature, rather than the combined velocity, is encoded into the object representation of each surface. All other

procedures and data analyses are the same as those used in Experiments 1 and 2.

3.5.2 Results Direction Repulsion

There was a significant effect of attentional task on Direction Repulsion when the superimposed objects were different speeds (F(2, 27) = 6.96, p = 0.004, Figure 3.6A). This effect was driven by

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a significant reduction in Direction Repulsion in the Bottom-up Speed condition (M = 8.42⁰ ± 0.57SEM) compared to the Unicolor Control (M = 15.36⁰ ± 1.87SEM, p = 0.003). Surprisingly, the addition of top-down task demands that actively link the speed of the surface to the

corresponding direction reduced this advantage. Direction Repulsion in the Top-down Speed condition (M = 11.29⁰ ± 4.14SEM) was not significantly different than in the Unicolor Control (p = 0.094), though there was also no significant difference in Direction Repulsion between the Bottom-up and Top-down Speed conditions (p = 0.289).

We have previously suggested (Perry et al., 2014) that multidimensional feature

selectivity likely underlies this reduction in direction repulsion with stimulus-driven differences in surface speed. Speed is a feature that forms conjunctions with direction in the dorsal stream. In other words, neurons in MT will co-process speed and direction, and in essence respond

selectively to different object velocities. In doing so, each velocity vector can then be processed by a separate pool of neurons within MT, and reduce the interference caused when trying to process two superimposed objects. This advantage of multidimensional tuning appears to be diminished when participants are required to actively attend to the speed category of the surface and report it along with the direction. By focusing on categorizing the speeds, the stimulus- driven effects are diminished. This is consistent with other studies which show that stimulus- driven attentional effects can be diminished by top-down task demands (Folk, Remington, & Johnston, 1992; Hillstrom & Yantis, 1994; Jonides & Yantis, 1988; Yantis & Egeth, 1999).

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Figure 3.6: The effect of speed and attentional task on direction repulsion and processing Time. A. There was a significant effect of attentional task on Direction Repulsion (DR) when the surfaces were different speeds (F(2, 27) = 6.96, p = 0.004). DR in the Bottom-up Speed

condition (M = 8.42⁰ ± 0.57SEM) was significantly less than in the Unicolor Control condition (M = 15.36⁰ ± 1.87SEM, p = 0.003). This advantage was reduced in the Top-down Speed condition (M = 11.29⁰ ± 4.14SEM) when compared to the Unicolor Control condition (p = 0.289). B. There was also a significant effect of task demands on Processing Time when the surfaces were differentiated by speed (H(2) = 18.92, p < 0.001). In both the Bottom-up Speed (M = 350ms ± 43SEM) and Top-down Speed (M = 590ms ± 122SEM) conditions, Processing Time was significantly less than in the Unicolor Control condition (Ws < 0.01, z = -3.79, p < 0.001 and Ws = 6.00, z = -3.34, p < 0.001).

Processing Time

Across the three conditions, we found that there was a significant effect of task-demands on Processing Time (H(2) = 18.92, p < 0.001, Figure 3.6B). Of particular interest, Processing Time was significantly reduced through bottom-up (M = 350ms ± 43SEM) and top-down (M = 590ms ± 122SEM) task demands when compared to the Unicolor Control (1870ms ± 312SEM; Ws < 0.01, z = -3.79, p < 0.001 and Ws = 6.00, z = -3.34, p < 0.001 respectively). There was no

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significant difference in Processing Time between the Bottom-up and Top-down condition (Ws = 28.50, z = -1.66, p = 0.105).

3.5.3 Discussion

As with contrast, stimulus-driven differences in surface speed are automatically integrated into dorsal stream object representations and in turn improve visual processing speed. Again, similar to contrast, top-down task demands to attend to the speed in addition to the direction of the surfaces did not add to the stimulus-driven advantage. Combined with the previous effects on direction repulsion, these results support two different mechanisms for motion processing along the dorsal stream: direction selectivity in area MT, which works on velocities and improves direction perception, and later decision-making circuits that work on object representations to improve processing time.

Our hypothesis is that when there is only one object feature differentiating the

superimposed surfaces, interference between the processing of each surface slows processing time, as is seen in the Unicolor Control condition. If the dorsal stream simply combined speed and direction into a velocity vector and passed this information downstream, the two

superimposed surfaces would again only be differentiated by one object feature (velocity) and processing time would slow, similar to when both surfaces are only differentiated by direction (Unicolor Control condition). Instead, speed, independent of direction, is integrated into a dorsal stream object representation downstream of direction computation in area MT. Evidence for this comes from the speed categorization necessitated by the top-down task demands that requires independent speed and direction processing. Processing Time in this case is no different from that seen with bottom-up task demands, which suggests that decision-making circuits work on object representations that treat speed and direction as independent object features. Speed can

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then be used as a second distinguishing object feature, like color and contrast, that allows for object selection mechanisms resulting in faster processing speeds.