Posts tagged visual representations

Many of us have steeled ourselves for those ‘needle in a haystack’ tasks of finding our vehicle in an airport car park, or scouring the supermarket shelves for a favourite brand.

A new scientific study has revealed that our understanding of how the human brain prepares to perform visual search tasks of varying difficulty may now need to be revised.

When people search for a specific object, they tend to hold in mind a visual representation of it, based on key attributes like shape, size or colour. Scientists call this ‘advanced specification’. For example, we might search for a friend at a busy railway station by scanning the platform for someone who is very tall or who is wearing a green coat, or a combination of these characteristics.

Researchers from the School of Psychology at the University of Lincoln, UK, set out to better explain how these abstract visual representations are formed. They used fMRI scanners to record neural activity when volunteers prepared to search for a target object: a coloured letter amid a screen of other coloured letters.

Their findings, published in the journal ‘Brain Research’, are the first to fully isolate the different areas of the human brain involved in this ‘prepare to search’ function. Surprisingly, they show that the advanced frontal areas of the brain, usually key to advanced cognitive tasks, appear to take a backseat. Instead it is the basic back areas of the brain and the sub-cortical areas that do the work.

Dr Patrick Bourke from the University of Lincoln’s School of Psychology, who led the study, said: “Up until now, when researchers have studied visual search tasks they have also found that frontal areas of the brain were active. This has been assumed to indicate a control system: an ‘executive’ that largely resides in the advanced front of the brain which sends signals to the simpler back of the brain, activating visual memories. Here, when we isolated the ‘prepare’ part of the task from the actual search and response phase we found that this activation in the front was no longer present.”

This finding has important implications for understanding the fundamental brain processes involved. It was previously thought that the Intra-parietal region of the brain, which is linked to visual attention, was the central component of the supposed ‘front-back’ control network, relaying useful information (such as a shape or colour bias) from frontal areas of the brain to the back, where simple visual representations of the object are held. If the frontal areas are not activated in the preparation phase, this cannot be the case.

The study also showed that the pattern of brain activation varied depending on the anticipated difficulty of the search task, even when the target object was the same. This indicates that rather than holding in mind a single representation of an object, a new target is constructed each time, depending on the nature of the task.

Dr Bourke added: “While consistent with previous brain imaging work on visual search, these results change the interpretations and assumptions that have been applied previously. Notably, they highlight a difference between studies of animals’ brains and those of humans. Studies with monkeys convincingly show the front-back control system and we thought we understood how this worked. At the same time our findings are consistent with a growing body of brain imaging work in humans that also shows no frontal brain activation when short term memories are held.”

(Source: lincoln.ac.uk)

It is natural to imagine that the sense of sight takes in the world as it is — simply passing on what the eyes collect from light reflected by the objects around us.

But the eyes do not work alone. What we see is a function not only of incoming visual information, but also how that information is interpreted in light of other visual experiences, and may even be influenced by language.

Words can play a powerful role in what we see, according to a study published this month by UW-Madison cognitive scientist and psychology professor Gary Lupyan, and Emily Ward, a Yale University graduate student, in the journal Proceedings of the National Academy of Sciences.

"Perceptual systems do the best they can with inherently ambiguous inputs by putting them in context of what we know, what we expect," Lupyan says. "Studies like this are helping us show that language is a powerful tool for shaping perceptual systems, acting as a top-down signal to perceptual processes. In the case of vision, what we consciously perceive seems to be deeply shaped by our knowledge and expectations."

And those expectations can be altered with a single word.

To show how deeply words can influence perception, Lupyan and Ward used a technique called continuous flash suppression to render a series of objects invisible for a group of volunteers.

Each person was shown a picture of a familiar object — such as a chair, a pumpkin or a kangaroo — in one eye. At the same time, their other eye saw a series of flashing, “squiggly” lines.

"Essentially, it’s visual noise," Lupyan says. "Because the noise patterns are high-contrast and constantly moving, they dominate, and the input from the other eye is suppressed."

Immediately before looking at the combination of the flashing lines and suppressed object, the study participants heard one of three things: the word for the suppressed object (“pumpkin,” when the object was a pumpkin), the word for a different object (“kangaroo,” when the object was actually a pumpkin), or just static.

Then researchers asked the participants to indicate whether they saw something or not. When the word they heard matched the object that was being wiped out by the visual noise, the subjects were more likely to report that they did indeed see something than in cases where the wrong word or no word at all was paired with the image.

"Hearing the word for the object that was being suppressed boosted that object into their vision," Lupyan says.

And hearing an unmatched word actually hurt study subjects’ chances of seeing an object.

"With the label, you’re expecting pumpkin-shaped things," Lupyan says. "When you get a visual input consistent with that expectation, it boosts it into perception. When you get an incorrect label, it further suppresses that."

Experiments have shown that continuous flash suppression interrupts sight so thoroughly that there are no signals in the brain to suggest the invisible objects are perceived, even implicitly.

"Unless they can tell us they saw it, there’s nothing to suggest the brain was taking it in at all," Lupyan says. "If language affects performance on a test like this, it indicates that language is influencing vision at a pretty early stage. It’s getting really deep into the visual system."

The study demonstrates a deeper connection between language and simple sensory perception than previously thought, and one that makes Lupyan wonder about the extent of language’s power. The influence of language may extend to other senses as well.

"A lot of previous work has focused on vision, and we have neglected to examine the role of knowledge and expectations on other modalities, especially smell and taste," Lupyan says. "What I want to see is whether we can really alter threshold abilities," he says. "Does expecting a particular taste for example, allow you to detect a substance at a lower concentration?"

If you’re drinking a glass of milk, but thinking about orange juice, he says, that may change the way you experience the milk.

"There’s no point in figuring out what some objective taste is," Lupyan says. "What’s important is whether the milk is spoiled or not. If you expect it to be orange juice, and it tastes like orange juice, it’s fine. But if you expected it to be milk, you’d think something was wrong."

(Source: news.wisc.edu)