The Contaminated Present

A new study has found that visual memories of recent events may invade our visual perceptions of the current moment.

A Real Scientific Image of a woman picturing some dots.

The research, published in the journal Psychonomic Bulletin & Review, found that when subjects pictured motion in one direction while looking at motion in another direction, they tended to perceive more dramatic motion away from the direction they were picturing – as if the imagined movement actually affected their relative point of view. These results provide the most impressive evidence yet that our “mind’s eye” interacts with the same neurocircuitry that processes actual visual stimuli.

Even so, those visual memories have only a subtle effect on our perception of the present – which is why Vanderbilt psychologists Dr. Sang Wook Hong and Dr. Min-Suk Kang, and their team, had to design a clever experiment to bring that effect to light. The experiment was centered around a visual illusion known as motion repulsion:

This illusion is produced when two sets of moving dots are superimposed, with dots in one set moving in a different direction from those in the other set. Under these conditions, people tend to misperceive the actual directions of motion, and perceive a larger difference between the two sets of motions than actually exists.

The motion repulsion illusion usually produces its effect by showing the two sets of dots simultaneously – but for this experiment, the researchers decided to see if they could produce the same illusion by showing the subjects one set of moving dots, while asking them to hold the other set in visual working memory – in other words, while picturing the direction in which the other set was moving.

While the mental images of recent motion didn’t create any full-blown hallucinations, the subjects did report some intriguing alterations in their perceptions of the dots’ movement:

If they were first shown dots moving in one direction and later shown dots moving in a slightly counterclockwise direction relative to the first presented dots, they reported the counterclockwise movement to be more dramatic than it had actually been.

Though this might not seem all that astonishing, it provides support for a controversial idea: that sense-processing brain areas like the primary visual cortex may play an active part in working memory processes such as the visuospatial sketchpad.

It appears that visual perception of reality isn’t an “on/off” phenomenon at all – rather, objects become more or less visually real to us based on the amount of neural activity they stimulate. Light waves typically trigger retinal neurons to send high-priority messages up to the visual cortex, while mental images apparently activate similar brain regions more subtly, through a more circuitous route. And that may be equally true for our other senses as well.

For instance, when we’re hovering on the border between dreaming and wakefulness, the boundaries between working memory and reality can become blurred. Sights and sounds from the dream can seep into the waking world, or our minds may weave a sensation of cold, or a buzzing alarm clock, into the strange plot of the dream.

This all points back to one of the most exciting implications of modern neuroscience: reality isn’t something we passively take in – it’s something our brains actively construct throughout every moment.

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