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Sensation and Perception

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Object Substitution Masking

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The visual world is full of objects that interact in space and time, and overlap within these dimensions can influence the conscious perception of them—a concept referred to as visual masking.

Similar to someone wearing a costume disguise, the phenomenon occurs when a target item—such as a face—cannot be perceived due to the presence of a second object—a mask.

When a target is substituted with a stimulus that overlaps in part of the same spatial location, this is a form of visual masking called object substitution.

Based on the methods pioneered by Enns and Di Lollo in 1997, this video demonstrates how to design and implement an object substitution masking experiment, as well as how to analyze the data and interpret the results dealing with the conscious perceptions of shapes.

In this experiment, object substitution masking is induced in participants as they observe the presentation of four elements on a computer screen: a fixation cross, target display, mask, and response choices.

At the start of each trial, a fixation cross is shown, which consists of 50-mm lines in the center of the screen, and ensures that participants are paying attention.

This is followed by the second element, the target display: eight shapes that are randomly selected from a set of four images—a circle, square, diamond, and triangle. They are displayed around an invisible circle with a radius of 150 mm for 30 ms.

Immediately after is the third element, the mask, which consists of four black dots, each with a radius of 25 mm, arranged to form the four corners of a square just large enough to enclose one shape. The mask surrounds the location of the randomly selected target shape and remains visible for 30 ms during a trial.

A critical independent variable here is the stimulus onset asynchrony—SOA for short—defined as the time difference between the appearance of the target display and mask.

A positive stimulus onset asynchrony means that the mask will appear after the target display. With, for instance, an SOA of 50 ms, the target display is shown for 30 ms, followed by a period of 20 ms where only the fixation cross is present before the mask comes on for 30 ms.

For SOAs less than 30 ms, say 10, the target display is shown for 10 ms before the mask becomes visible. After another 20 ms, the target display goes away and the mask remains for 10 more ms.

The time that the target display and mask are both onscreen is referred to as stimulus overlap. This is maximal when the stimulus onset asynchrony is zero.

When the stimulus onset asynchronies are negative, the order of the elements is reversed: the mask appears before the target display. With an SOA of -10 ms, the mask is presented for 10 ms before the target overlaps for 20 ms. It then disappears, leaving the target display visible for an additional 10 ms.

With larger negative values, like an SOA of -50 ms, the mask is shown for 30 ms. There is then a period of 20 ms where only the fixation cross appears before the target display comes on for 30 ms.

Regardless of the SOA, the fourth and final element is the response display: four shapes arranged horizontally in the center of the screen. They are shown until the participant presses the key corresponding to the shape of their choice.

The dependent variable is the percentage of correct responses recorded across the number of SOAs. Object substitution masking is expected to be induced in a discrete range of time, resulting in performance with reduced accuracy during positive SOAs, when the mask occurs shortly after and overlaps with the target display.

To begin the experiment, greet one of the recruited participants in the lab and guide them through the consent forms. Then, have them sit comfortably, 60 cm away from the monitor of the test computer.

Explain the task instructions: they’ll see a ring of eight shapes, along with four dots that will appear at a random location. Indicate that this mask may sometimes overlap a shape in time, but could also either precede or occur after it.

Instruct the participant to remember the shape that appears in the space between, and when in doubt, to simply guess.

Once the main rules are understood, describe a few more points: They should press the spacebar to start each trial; and when the fixation cross appears on the display, they should not move their eyes during the trials.

Now, have the participant start the program and watch them as they complete a couple of trials. At this point, leave the room.

Without supervision, allow the participant to complete all 300 trials—20 for each stimulus onset asynchrony value. Note that there are 15 values—ranging from negative, with the mask preceding the target—to positive, with the mask following, including an overlap zone.

When the participant has completed the task, return to the room and thank them for taking part in the experiment.

To analyze the data, compute the response accuracy—as percent correct—across all of the stimulus onset asynchronies, and graph the averages for the 15 time points.

As predicted, with very large SOAs of 150 or 300 ms, positive or negative, performance was highly accurate because the mask and the target display were perceived as separate events.

Similarly, for the negative SOAs between -90 and -10 ms, performance was fairly accurate since the participant’s attention was directed to the correct location by the appearance of the mask before the target display.

However, accuracy dropped to 50% when the SOAs were near zero, as the stimuli overlapped and appeared too briefly to be perceived.

The critical range of SOAs consisted of values between 10 and 90, where the target display was shown before the mask. Here, performance was poor, dropping near chance level. This suggests that object substitution masking took place and that the four-dot mask was enough to confuse the brain before a conscious perception of the shape formed.

Now that you are familiar with Object Substitution Masking, let’s look at how it’s used in studies of conscious awareness, as well as those investigating the neural circuitry involved in visual perception.

This masking paradigm can be combined with repeated Transcranial Magnetic Stimulation, rTMS, to isolate brain circuits involved in conscious perception. A magnetic coil can be used to repeatedly induce small electrical potentials in the brain, causing a small portion of cortex to briefly deactivate.

In one study, Hirose and colleagues found that if the V5/MT+ regions of visual cortex, known to play a role in the perception of motion, were deactivated during the task, they were able to negate the effects of masking. This suggests that the rTMS disruption caused the mask and target to no longer be perceived as part of the same event, allowing the subject to see both.

Other researchers are investigating whether a stimulus needs to be perceived to influence behavior, like verbal priming. For more details on this effect, check out our video in the Cognitive Psychology collection, Verbal Priming.

In a study conducted by Goodhew and colleagues, they used a variation of the masking task—the four dots in the mask were either pink or blue—and asked participants to remember the color. The masks were presented with target stimuli consisting of the words PINK, BLUE, MAIL, HOUR, JQCG, and AWHF.

With an SOA of 200 ms, participants named the color of the mask faster when the target word was the name of the color than when the target was not. This was true whether or not the participant could correctly identify the target as being either a word or non-word, suggesting stimuli do not need to be perceived or enter consciousness to be useful.

You’ve just watched JoVE’s video on Object Substitution Masking. Now you should have a good understanding of how to design the elements and run the experiment, as well as how to analyze and assess the results.

Thanks for watching!

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