快猫短视频

Can you tell a snake from a pretzel? Some can鈥檛 鈥� and their experiences are revealing how the brain builds up a coherent picture of the world

AFTER her minor stroke, BP started to feel as if her eyes were playing tricks on her. TV shows became confusing: in one film, she was surprised to see a character reel as if punched by an invisible man. Sometimes BP would miss seeing things that were right before her eyes, causing her to bump into furniture or people.

BP鈥檚 stroke had damaged a key part of her visual system, giving rise to a rare disorder called simultanagnosia. This meant that she often saw just one object at a time. When looking at her place setting on the dinner table, for example, BP might see just a spoon, with everything else a blur (Brain, vol 114, p 1523).

BP鈥檚 problems are just one example of a group of disorders known collectively as visual agnosias, usually caused by some kind of brain damage. Another form results in people having trouble recognising and naming objects, as experienced by the agnosic immortalised in the title of Oliver Sacks鈥檚 1985 best-seller The Man Who Mistook His Wife for a Hat.

Agnosias have become particularly interesting to neuroscientists in the past decade or so, as advances in brain scanning techniques have allowed them to close in on what鈥檚 going on in the brain. This gives researchers a unique opportunity to work out how the brain normally makes sense of the world. 鈥淗umans are naturally so good at this, it鈥檚 difficult to see our inner workings,鈥� says Marlene Behrmann, a psychologist who studies vision at Carnegie Mellon University in Pittsburgh, Pennsylvania. Cases like BP鈥檚 are even shedding light on how our unconscious informs our conscious mind. 鈥淎gnosias allow us to adopt a reverse-engineering approach and infer how [the brain] would normally work,鈥� says Behrmann.

Although we may not give it much thought, our ability to perceive our world visually is no mean feat; the most sophisticated robots in the world cannot yet match it. From a splash of photons falling on the retina 鈥� a 3-centimetre-wide patch of light-sensitive cells 鈥� we can discern complex scenes comprising multiple items, some near, some far, some well lit, some shaded, and with many objects partly obscured by others.

The information from the photons hitting a particular spot on the retina is restricted to their wavelength (which we perceive as colour), and their number (which determines brightness). Turning that data into meaningful mental images is a tough challenge, because so many variables are involved. For example, the number of photons bouncing off an object depends both on the brightness of the light source and on how pale or dark the object is. 鈥淭he information that the visual system receives is very impoverished,鈥� says Behrmann.

It is in the visual cortex, located at the back of the brain, where much of the processing goes on. When items obscure each other, the brain must work out where one thing ends and another begins, and take a stab at their underlying shapes. It must recognise things from different perspectives: consider the image of a chair viewed from the side compared with from above. Then there鈥檚 the challenge of recognising novel objects 鈥� a futuristic new chair, for example. 鈥淪omehow, almost magically, we derive a meaningful interpretation of complex scenes very rapidly,鈥� says Behrmann. 鈥淗ow we do this is the million-dollar question in vision research.鈥�

So how does the brain work its magic? In the early 20th century, European psychologists used simple experiments on people with normal vision to glean some basic rules that they called the 鈥済estalt principles鈥�. For example, the brain groups two elements in an image together if they look similar, having the same colour, shape or size, for example. And if not all of an object is visible, we mentally fill in the gaps 鈥� that鈥檚 the 鈥渃losure principle鈥� (see 鈥淐onstructing reality鈥�).

The gestalt principles can only go part of the way to describing visual perception, though. They cover how we separate the different objects in a scene, but they cannot explain how we know what those objects are. How, for example, do we know that a teacup is a teacup whether we see it from above or from the side, in light or in shadow?

It鈥檚 here that people with visual agnosias come in handy. Behrmann had previously studied people with integrative agnosia, who have difficulty recognising and naming complex objects as a whole, and instead seem to pay unusual attention to their individual features. One person, for example, mistook a picture of a harmonica for a computer keyboard, presumably thinking the row of air-holes in the mouthpiece were computer keys (Journal of Experimental Psychology: Human Perception and Performance, ). Others have mistaken a picture of an octopus for a spider, and a pretzel for a snake.

In 2006, Behrmann put one of her patients, known as SM, through a series of experiments alongside people with normal vision. All were shown a set of three-dimensional objects on a screen, each made from two simple geometric shapes. Afterwards, the volunteers were shown a stream of these images, with a few new objects thrown in. Their task was to report whether or not they had seen the objects before.

While those with normal vision performed with nearly 100 per cent accuracy, SM made some intriguing mistakes. He knew he hadn鈥檛 seen an object before if it contained a new part, but those that had the same parts in a different configuration confused him. About half the time he mistook these for the familiar objects (Journal of Experimental Psychology: Human Perception and Performance, ).

To Behrmann, the results suggest that our brains normally construct objects from a series of smaller building blocks, which she calls our 鈥渧isual vocabulary鈥�. To recall our concept of an object, she says, we form a mental map of the way these parts fit together. It was at this stage that SM failed. 鈥淗e had a good representation of the parts, but understood little of how they were combined,鈥� Behrmann says.

Behrmann鈥檚 work could help resolve a long-standing dispute among vision researchers. One theory has it that to interpret what we see, we flick through a mental catalogue of objects we have seen before 鈥� and preferably, a view of these objects from every vantage point 鈥� to try to find a best fit with the current image. Behrmann鈥檚 study lends weight to a different view, that we remember the typical form of an object as a construction of a few generic building blocks.

Essential elements

It would be quicker to try to match a new object to a building block construction than flick through a catalogue of every single example of the object we鈥檝e ever seen, Behrmann says. Importantly, the basic building blocks, and their configuration, would probably stay the same whether we view the objects from different angles or in different lighting, making the comparison and recognition easier and more robust.

After object recognition comes the next stage of visual perception: conscious awareness of what we see. Other kinds of agnosia have posed some serious questions about this process. 鈥淎gnosias are a breakdown in the unity of consciousness,鈥� says Rocco Gennaro, a philosopher studying consciousness at the University of Southern Indiana in Evansville. 鈥淚t leads to an inability to perceive the coherent whole.鈥�

鈥淭hese disorders are a breakdown in the unity of consciousness, an inability to see the coherent whole鈥�

Take the case of visual form agnosia. People with this disorder cannot consciously discern the shape, size or orientation of objects, yet they have no problem manipulating those objects. Studies have shown that, for example, while they could not describe the angle of a slot in a specially built mailbox, they had no difficulty posting an object through it.

Intriguingly, there are also people with the opposite problem: they can describe the shape and size of an object, but with manual tasks they are clumsy, often missing the thing they are reaching for. This condition has been termed optic ataxia.

Brain scans have revealed that people with visual form agnosia tend to have damage to the ventral (lower) part of the brain鈥檚 visual area. People with optic ataxia, on the other hand, have damage to the dorsal (upper) part. This led to the idea that we have two streams of visual processing. The ventral pathway is necessary for perceiving or recognising an object, while the dorsal pathway deals with an object鈥檚 physical location in our visual field and, if we need to perform an action on it, guides the movement of our bodies. For this reason, scientists often refer to the two processes as the perception-action, or the what-where, streams of visual processing.

This idea was necessarily based on studies of just a few people. To make matters worse, most of the cases of visual form agnosia arose from carbon monoxide poisoning, which can cause widespread brain damage, so it was hard to work out which part of the ventral pathway was most affected.

So for some time the search was on for an agnosic with just a small area of damage to their ventral pathway. Last year Hans-Otto Karnath, a cognitive neuroscientist at the University of T眉bingen in Germany, found a stroke patient with damage only to the central structures of the ventral pathway. Besides providing further evidence for the perception-action theory of vision, this person鈥檚 symptoms suggest that this area is vital for our normal understanding of shape and contour information (Journal of Neuroscience, ).

In fact, the closer neuroscientists look, the more modular our visual systems appear. MRI scans of people with and without agnosias have suggested that within the ventral stream, separate aspects of appearance are processed independently. This year, psychologist Cristiana Cavina-Pratesi at Durham University in the UK found that shape, texture and colour are all processed in individual regions (Cerebral Cortex, DOI: ).

Yet our experience feels markedly different. When we consciously see something, all these disparate elements are stitched seamlessly together, so we know instantly that an apple is smooth, green and round. The question of how we accomplish this is central to the study of conscious perception.

Some neuroscientists, like H. Branch Coslett at the University of Pennsylvania in Philadelphia, think that the brain binds all the different features from the ventral stream to a 鈥渕aster map of location鈥�, which is held in the dorsal stream. This binding process is so fundamental, he claims, that it is only once this link has been formed that an image can pop into our consciousness; without it, images lurk somewhere in the subconscious.

Coslett says that evidence for the binding theory comes from people with simultanagnosia, such as BP, who can only see one object at a time. These people tend to have damage to the posterior parietal lobe (PPL), which may be crucial for linking information processed by the dorsal and ventral pathways. The damage appears to reduce the amount of information that can be bound together, meaning affected individuals can only bind enough data for one object at a time, while everything else remains beyond conscious reach.

Further support for the idea came in 2008, when Coslett discovered a simultanagnosic man who had damage to his PPL, leading to some curious symptoms. Like other simultanagnosics, the man, known as KE, could only see one object at a time but, uniquely, he struggled to see more than one aspect of an object at a time. In one experiment, for example, he could report an object鈥檚 shape or patterning, but not both at once. When he read words written in coloured ink, he could read the word but not report the colour (Journal of Cognitive Neuroscience, ).

Yet KE was not completely blind to the features he couldn鈥檛 describe. In another experiment, he was asked to say what was depicted in line drawings of objects like lemons and tomatoes. Sometimes, these images were coloured appropriately 鈥� the lemon would be yellow, for example 鈥� while others would be mismatched. Although KE reported seeing no colour when naming the figures, his accuracy was better when the objects had the right colour, showing that some colour processing was occurring in his subconscious.

All of this fits with the binding theory. 鈥淜E could only link one channel 鈥� the colour, shape, or name of an object 鈥� to the 鈥榳here鈥� information at any one time,鈥� says Coslett. Without the necessary binding to the dorsal map, however, all of the other features were unavailable to his conscious mind.

What lies beneath

These findings seem to support the view emerging from several disparate fields of neuroscience 鈥� that the subconscious mind has a bigger role than previously supposed. When it comes to the mental faculties we prize as uniquely human, including creativity, language and aspects of memory and learning, subconscious thought processes are far from playing second fiddle to the conscious mind (快猫短视频, 1 December, 2007, p 42).

鈥淭he subconscious has a bigger role than we thought in the mental faculties we prize as uniquely human鈥�

To Coslett and other neuroscientists studying consciousness, our attention is like a spotlight that points to a specific location in our visual representation of the world around us. As the spotlight hits a particular region, it selects the relevant information bubbling away in our subconscious, binding the different features to their location, before they pop into our consciousness as a single, unified experience.

So important is the role vision plays in most people鈥檚 everyday lives that most research has concentrated on visual agnosias. Now the hunt is on for similar disorders that affect the other senses. Recently, for example, neurologists found a person who could understand speech but not other sounds. Coslett, meanwhile, is investigating whether simultanagnosics also have trouble binding other sensory sensations together, such as sights and sounds.

Understanding such deficits could reveal how the brain processes different types of sensory information. Indeed, Behrmann hopes that working with a wider range of senses might reveal general principles of sensory perception.

Press even further down this road and we could unlock one of the most fascinating mysteries of modern neuroscience: how the brain binds together all of our disparate sensory experiences into the single, flowing conscious experience that we call 鈥渢he present moment鈥�.