Visual Perception

I’ve read a couple of interesting articles in the New Scientist over the last couple of months

Robson (2010) brought together some studies on people with different forms of agnosia to look at the nature of visual perception…

“It is in the visual cortex, located at the back of the brain, where much of the processing goes on. […] [The brain] must recognise things from different perspectives: consider the image of a chair viewed from the side compared with from above. Then there’s the challenge of recognising novel objects – a futuristic new chair, for example.”

“Behrmann, […] suggests that our brains normally construct objects from a series of smaller building blocks, which she calls our “visual vocabulary”. To recall our concept of an object, she says, we form a mental map of the way these parts fit together.”

“[Other studies have] led to the idea that we have two streams for visual processing. The ventral pathway is necessary for perceiving or recognising an object, while the dorsal pathway deals with an objects 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.”

“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: 10.1093/cercor/bhp298]”

“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.”

“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 “master map of location”, 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, 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.”

An earlier summary paper on brainwaves (Thomson, 2010) helped to add some detail to this idea of the binding function:

“[T]he term “brainwave” has a specific meaning in neuroscience, referring to rhythmic changes in the electrical activity of a group of neurons. Each neuron has a voltage, which can change when ions flow in or out of the cell. This is normally triggered by stimulation from another cell, and once a neuron’s voltage has reached a certain point, it too will fire an electrical signal to other cells, repeating the process. When many neurons fire at the same time, we see these changes in the form of a wave, as a group of neurons are all excited, silent, then excited again, at the same time.”

“At any one time, a number of these brainwaves are sweeping through the brain, each oscillating at a different frequency. […] Brainwaves may provide clarity in the electrical storm by synchronising all the activity corresponding to a single stimulus […] to a particular frequency, while neurons attending to another stimulus fire at a different frequency. This would allow brains cells to tune in to the frequency corresponding to their particular task while ignoring irrelevant signals[.]”

“The importance of signal synchronisation becomes clear when you consider that the different aspects of a sensation – colour and shape in vision, for example, – are processed in different parts of the brain before being sent to another region that binds them back together.”

“The rhythmic activity of brainwaves ensures that all the relevant signals relating to sensation arrive at the binding region at exactly the same time. This allows the receiving neurons to process the signals together recombining them into a single sensation. “If neurons are oscillating at the same frequency signals from a stimulus would be treated together because the firing came in at the same time, and at the same point on the oscillation, so that the object is perceived as a whole rather than the separate details,” explains Laura Colgin at the Norwegian University of Science and Technology in Trondheim.”

So…. more pieces to the puzzle… when you give someone an object they will perceive the different aspects of that object using different parts of the visual processing system in the brain. Signals from these different parts can be co-ordinated by the binding region because the cells at the different regions are firing at the same frequency. Once the signals are bound we are capable of perceiving the object in a whole, coherent fashion. BUT that person still needs to understand the object, that is, they need to be able to fit it into their own frames of reference. Behrmann’s work suggests that the person confronted with a new object fits that object into their existing visual vocabulary using simple construction blocks. What about weird or unusual objects? They can perceive it without being able to categorise it. Or they might try to make it fit a category. One of the handling objects I worked with, which caused a stir, looked like a sock with 4 fingers at the end. Kids and adults were perplexed – Was it a glove? Was it a sock? I think that there may be an underlying intuition or inchoate experience of ‘rightness’, of feeling that the object has been satisfactorily assigned to a particular category (“It’s a sock!”) It is worth thinking about how children deal with this – again they probably don’t have the visual vocabulary for dealing with objects that adults do, so to a certain extent object handling is about helping them create new vocabularies, both visual and verbal.

References

Robson, D. (2010) ‘Seeing isn’t believing’, New Scientist, 28 August 2010, 30-33

Thomson, H. (2010) ‘Brain beat’, New Scientist, 10 July 2010, 28-31

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About Bruce Davenport

Museum educator and researcher.
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