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REHOVOT, Israel - December 9, 1997 - Conjuring up images in the "mind's eye" enhances our ability to see what's really there, Weizmann Institute scientists have found.
However, imagination greases the wheels of perception only when the images are drawn from short-term memory, the researchers report in the current issue of the Journal of Cognitive Neuroscience.
In contrast, reaching into our long-term memory to recall an image may interfere with our ability to perceive visual data. "It seems that, under certain circumstances, activating short-term memory can change the 'hard wiring' of the visual cortex, allowing people to see what they couldn't see before," says Dr. Alumit Ishai, who conducted this research during her doctoral studies under the guidance of Prof. Dov Sagi of the Weizmann Institute's Neurobiology Department.
The research also supports the idea that short- and long-term visual memory may be governed by separate mechanisms in the brain.
Reality and Imagery
Anyone who has looked for a friend in a crowd intuitively knows how visual imagery works: conjuring up a mental image makes it easier to identify the friend when he actually appears.As Prof. Sagi explains, memory and perception constantly work together because whenever we see something, we identify it based on images stored in memory."Without memory, the world we perceive would be incomprehensible." he says. "However, if memory is given too much weight, the result is hallucination."
But does the brain work in the same way when we imagine an object and when we actually see it? And does reaching into our memory to recall an image make it easier or more difficult to complete visual tasks? Philosophers and scientists have long sought answers to these questions, but previous studies have produced conflicting results.
The new research by Ishai and Sagi explains why: previous studies made no distinction between images drawn from short- and long-term memory.
Short-Term Versus Long-Term Memory
The Institute researchers have, for the first time, quantified the effects of visual imagery on perception by creating experiments in which subjects viewed subtle points of light, known as Gabor signals, on a computer screen. By varying the points' intensity, they were able to establish the exact threshold of brightness at which the light became visible to the participant.
First, participants viewed a computer display in which three Gabor signals appeared along a straight line, with the flanking signals situated at various distances from the central signal. As the distance between the flanking signals was increased, participants found it more difficult to see the central signal, eventually losing sight of it altogether. Then, the scientists adjusted the brightness of the central signal in order to establish the new threshold at which participants could see the target.
At this point, memory was called into action. Sagi and Ishai showed the participants a screen with only one Gabor signal, asking them to imagine the flanking signals they had recently seen. The results were surprising: When participants imagined the flanking signals, they were able to see the target more easily than in the previous trial. By activating their short-term memory, the participants lowered the threshold of brightness at which visual perception could occur. They actually improved their own ability to see.
Further experimentation linked this phenomenon specifically to short-term memory. The threshold of perception was lowered only when the participants were asked to imagine the flanking signals soon after seeing them on the screen. As more time elapsed, the flanking signals presumably "slipped" into the participants' long-term memory bank; calling up the image raised the vision threshold, making the target signal more difficult to see.
These results indicate that short- and long-term memory are represented differently in the brain and are controlled by different neural mechanisms, although it is not yet clear what these mechanisms are.
Sagi and Ishai hope that with further study of this phenomenon, it may someday be possible to harness the natural power of memory to "open the eyes" of those who suffer from vision problems.
This research was supported by the Israel Science Foundation administered by the Israel Academy of Sciences and Humanities, and by the Charles H. Revson Foundation.
The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.
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