How do you know what's where? Cognitive psychologists believe this requires "binding" - the ability to associate certain colors or shapes with each other and with certain locations in space, giving rise to a coherent perception of the world. It turns out that at least one unlucky person lacks this ability we often take for granted: the patient is known as RM, as reviewed by Lyn Robertson in this article on binding.
RM suffered two strokes, damaging both sides of his occipito-parietal cortex (see the image above). This region of the brain is known to be important for spatial computations; this pattern of damage will often result in Balint's Syndrome, characterized by three primary problems: the inability to perceive more than one object at a time (simultagnosia), the inability to reach towards objects that are being focused on (optic apraxia), and severe problems in changing which object the eyes are focused on (optic ataxia). Such patients are essentially blind outside the focus of their attention, and cannot locate, reach for, or track the spatial movements even of items that are within their focus of attention. In some ways, this represents the complete dissolution of spatial awareness; Robertson quotes a description of Balint's "as if there is no there, there."
And so it seems that RM has lost his ability to "bind" objects together into a coherent visual scene. Crucially, this also includes bindings that do not explicitly involve space. For example, when presented with a red circle and a blue square, RM is nearly as likely to say that he saw a red square or blue circle as he is to say he saw either of the correct color-shape bindings (known as an "illusory conjunction"). RM performs similarly when binding motion to shape, and color to size, suggesting this is in fact a bona fide impairment of all visual bindings, not just those involving location (e.g., color-location or shape-location bindings).
Similarly, RM has difficulty in visual search tasks in which he must find a red "X" surrounded by red "O's" and green "X"'s. Why might this be the case? Binding is required to differentiate the conjunction of "red" & "X" from "green" & "X" and from "red" & "O". Sadly, RM performs at chance on such tasks.
Yet there are strikingly similar tasks on which RM shows no impairments. For example, RM has no apparent difficulty in identifying the shape of particular objects (in which each contour of a shape might need to be "bound" to the larger whole shape of which it is a part). This profile of deficits suggests that the damage is limited to the dorsal visual processing stream, but that the ventral processing stream (the one usually responsible for object recognition) is intact.
A 2006 article by Robertson and Treisman provides more details on this aspect of RM's condition. For example, RM can indicate whether three dots on a page could be connected to form a straight or a bent line, but is very poor at determining the direction in which the line bent; similarly, he can identify scrambled from normal faces, but not whether they are upside-down or upright (a similar dissociation holds when he's asked to identify letters and their orientation). Finally, he can successfully read words like "NO" (which requires that the word be recognized as a spatial configuration of letters distinct from "ON") but cannot do so with novel words - for example, discrimination of "ZO" from "OZ" is no different from change.
Robertson suggests that RM's tragic condition can help to inform our understanding of visual binding. She claims that spatial attention will be crucial for any complete account of binding, given that all binding seems to involve spatial attention. Robertson suggests that one promising model involves the construction of "feature maps" by the ventral processing stream which are co-registered or aligned by spatial attention, acting throughout the dorsal stream.
Of course, Patient RM is not alone. Gillen & Dutton (2003) report a case of Balint's syndrome in a 10-year-old, who suffered a cerebral hemorrhage in the left and right parietal lobes. The child nonetheless appeared to recover well until 10 years of age, at which point he had difficulty in school with reading long words, and had shown lingering signs of simultagnosia (for example, bumping into people as if they were not there). His vision was normal, and yet he had difficulty copying from the blackboard and drawing simple shapes (though he could describe them perfectly well). Amazingly, the neurologists were able to suggest several common sense coping strategies which enabled the child to attend a mainstream school with only 2-3 hours of additional tutoring per week, although the condition itself did not appear to change at a one-year followup.
Balint's syndrome makes a very interesting contrast with synaesthesia (a topic that has captivated the blogosphere lately - see this fascinating article at Madam Fathom). While Balint's syndrome represents the dissolution of space and the capacity for feature-binding, synaesthesia represents "hyperactive" binding processes, such that color and shape or color and sound might be inextricably intermingled. So synaesthesia is a kind of inverse binding problem. Tomorrow's post will cover a recent study investigating whether the brain regions damaged in Balint's syndrome are the same as those which are hyperactive in synaesthetes.
Related Posts:
If I had to choose one neurological disorder to be affected by...
Dopamine to the rescue: the Binding Problem
Perceptual Binding Takes Time
Are we seeing the world or creating it? Perception & Sensory Integration
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Isn't Balint's amazing for what it says about attention (tragic too, of course). The patient can see one object in a given space, but not multiple objects that take up the same amount of space! I was familiar with Robertson's earlier articles on it, so I'll be really interested to read the recent you mention that she wrote w/ Treisman.
I've been reading your blog with interest for some time now, so I'm grateful for the link you provided to my new blog and the post I did on perception. Thanks!
neat - I wasn't aware of that aspect of Balint's! This seems to indicate that object-based attention can act without problems, but attention cannot be space-driven and thereby allocated to a region of space that may contain multiple objects. Would you agree?
sorry it took a couple of days...
Interestingly, people w/ dorsal simultaneous agnosia (manifest in Balint's) can read short words, but only when spelling them out backwards, one letter at a time. They are surprised to discover that each new letter exists but then do see it - separately. I first wondered whether it would be the same reading Hebrew, but then I read that even when two objects are superimposed on top of one another, they see only one. So does that mean that object attention is actually working? I'm not sure.
Coslett and Saffran 1991 is quoted in David Andrewes' Neuropsych book as defining the syndrome as "an inability to interpret complex visual arrays despite preserved recognition of single objects." When these patients navigate around the house, they have to use particular landmarks. When they watched TV characters arguing, they would think it strange, because they only saw one. Andrewes discusses it as a disconnection or destruction of the dorsal "where pathway" while the "what pathway remains." Which would confirm your interpretation. But that would mean that object based presentation has a default setting for single objects only, and that multiple objects must only be defined as multiple things within a given region of space. This is partly semantic but, it seems to me that disconnection within the object pathway is more likely. The notion of object attention is a little tricky anyway, since it always seems to me that the effects of object attention are generally being added to spatial. I.e. you can have spatial attention without object attention, but not object w/o spatial attention.
I'm actually going to do some research next year on how precisely objects are defined in regard to object attention. It's a very top down phenonmenon, I think. In a study with contour objects - there's nothing actually there - and yet we perceive an "object." I wonder what precisely is needed for an object to be declared by our object system to exist over and above the space it exits in - particularly in a two-dimensional environment. And in us "normal" folks perceiving complex arrays, something must hold the notion of more than one objects existing in a single space if all of us can only focus on one of them at a time.
The Andrewes' book also talks about Balints' patients having "sticky fixation." This is a saccade disorder resulting from superior colliculus lesions. So perhaps this is all about an inability to disengage object-based attention once one has fixed it somewhere.
this is the second half of my reply. Actually, if I had bothered to re-read your post which I haven't looked at since wednesday, I would have seen you had already discussed some of the things I'd mentioned. But I wonder about 3 things: disengagement, the fact that the object pathway may have stopped talking to the spatial pathway, or some frontal area that organizes objects into complexes, which would ultimately chunk the individual objects within a space as a single unit if we spent enough time processing them. Disengagement seems alluring because, when guided in reading backwards, patients do see the next letter.