Brains at the Brink: Left Neglect (Part 1)
What is it like when you lose an entire dimension of space?
Imagine for a moment that you woke up one morning and did not know what feet were. Not only that, imagine that you lacked the ability to even understand what feet were, or to recognise them, or conceive of them. Feet were simply not a thing that your mind could grasp. You had no memory of ever knowing what feet were, and in fact, had no awareness that anything was amiss. To your feet-deprived mind, everything was perfectly normal, nothing had changed, you were the same today as you were yesterday; except you weren't.
When we lose some critical function of our brain that provides important cognitive input, sometimes we are incapable of noticing its absence. The mind maintains a powerful illusion of complete unity and singularity of self, and despite damage that can range from cuts and scrapes to missing limbs, this illusion persists doggedly, relentlessly. Amputee patients can sometimes be all but convinced of the continued presence of the amputated limb, lacking only a visual confirmation of it, for example.
Our many powerful cognitive functions are scattered across the brain, but some of them live in “modules” whose neurons are grouped together closely into units. When one of those units stops working, that entire function no longer exists for your mind to integrate, and to you, it would be like it had never existed at all. If there were a specific module for recognising and conceptualising feet, and it were to stop working one day, any feet that might be in our memories wouldn't get deleted, but the information encoding the feet would be unreadable, and simply ignored by the brain.
Lets take this further. What if you lost the ability to know how many fingers you had? Not only that, but when asked to move just one finger, your brain couldn’t even conceptualise that action. No matter if someone else were to demonstrate the activity of moving one finger on their own hand, you would not be able to do it, and in fact, you would probably have been unable to understand or fully comprehend the other persons example. Surely, you’d think, you’d notice this missing ability if it one day just stopped working?
No, not even in this instance. The mind is fantastic at coming up with justifications or explanations for why things are the way they are, but even so, your mind won't notice that anything has changed. It's still receiving the same information from all senses, but further down the processing pipeline, parts of that information are no longer readable, and thus discarded.
Losing Space
Lets take a look at something that really does happen (rarely, thank goodness): Left Neglect.
It’s part of an overall condition called "hemispatial neglect", which covers a few different varieties of a condition whereby an entire part of a visual stimulus can no longer be conceived of, despite being clearly within the visual field of the patient. Left Neglect in particular comes in 2 main variants: egocentric neglect, which basically means one loses all left-sided spatial field information from their direct perspective (relative to the persons own midline, e.g belly-button), and allocentric neglect, where all objects in both visual field and in the mind lose any "left-sidedness", from the midline perspective our mind assigns to that object (if that object was another person, it would be their belly-button)
In other words, egocentric neglect is like losing your left eye, and only being able to see things to the right side of your own bellybutton, whereas allocentric neglect is almost like losing an entire dimension of space, so that any object you look at or think about or remember is only a right half, from the relative visual or mental perspective.
This can be demonstrated in quite remarkable ways. From the wikipedia on Hemispatial Neglect:
A patient may also be asked to read a page out of a book. The patient will be unable to orient their eyes to the left margin and will begin reading the page from the center. Presenting a single word to a patient will result in the patient either reading only the ipsilesional part of the word or replacing the part they cannot see with a logical substitute. For example, if they are presented with the word "peanut", they may read "nut" or say "walnut".
So in egocentric neglect, if the word or page is positioned so that it is either side of a patients midline (or in some cases, their head or even retina), the left half relative to that line will be neglected. The brain will simply pass over it, and continue with its other processing as normal. Sometimes the predictive power of the brain steps in to fill in details that it expects to be there, sometimes not. In the case of "peanut" for example, if the patient says "walnut", that means the brain is seeing "nut", perhaps deciding it can only be partial word since the first letter is lowercase, and automatically infers a full word that fits the details it has so far observed. Since it can't observe the "Pea" part of the word, well, you get the idea. However, shift the word rightward relative to whatever midline the brain seems to be associating with the neglect, and the full word appears.
In allocentric neglect, however, the midline isn't relative to the body, or the person at all. In fact, the left side of everything, from the minds perspective, is ignored. If you take a picture of another person’s face, turn it upside down, and present it to a patient with allocentric left neglect, their mind will turn it right side up and then ignore the true left side of the image. The minds ability to project coordinate systems onto objects in focus, and return information about that object relative to that projected coordinate system, is what has broken down, but only for that one side.
Something that might be about as close as we can get to imagining this experience is noting how no matter how we move our head (or eyes), we cannot see what is behind it (lest we use a mirror, which is cheating). When we turn our head to look in the other direction, we are changing perspective, so now that direction is no longer behind. We still cannot see backwards. We have a "field of view" at all times that is limited relative to the direction we point our head. Although we can get a sense of what might be out of our field of view based on other senses and memories and general spatial awareness, in hemispatial neglect, you have not even that. An entire spatial dimension simply isn’t.
The effects after a stroke can be wildly varied. The truth is, the best we can do is try to group together similar kinds of symptoms under umbrella terms in terms of day to day diagnostics. Every stroke is different. Even those that affect the same parts of the brain can be in slightly different places, and the brains themselves are not identical either. Like fingerprints, no matter how similar they may seem, they are all unique. Despite this, the study of stroke patients provides glimpses into the way different modules of the brain integrate and work together, and sometimes it even helps reveal the area providing particular functions.
Sense of Direction
I wouldn't know about these, because I've never had one, but I'm told they're useful.
The human mind is equipped with incredibly sophisticated simulation machinery. We can run simulations in our mind of all manner of physical processes, we can visualise objects in full even when only having seen them in part, we can replay memories and past events with our own adjustments and observe the predictions. By and large, our predictions can be stunningly accurate. Most people can create a top-down map of their neighbourhood without ever having seen an aerial image of it, and can plot routes with varying efficiency modes, such as distance to walk, distance to drive, terrain, energy expenditure overall, visibility in different lighting conditions, threat assessment, and more, and even draw it. You can even simulate your own emotional response to other simulations!
No matter what the current position and orientation of your body, head, or eyes is, you can mentally move your perspective into completely different directions. You can imagine yourself walking through a house, turning down corridors, walking up spiralling staircases, peeking into rooms, returning the way you came, and exiting out the doors and walking around the building from the outside, all while in the physical world you are sitting or standing perfectly still.
You’re doing that right now, in fact.
This is not to suggest that we get everything right; god knows I don't. We can oversimplify, we can over-complicate, we can fudge, either deliberately or unknowingly. We confabulate, confuse, contrive and catastrophise. The machinery is not perfect, but if you remember that it was built by nothing but one 4-billion-year-long running chemical reaction (and counting), starting from primordial soup, wow.
Conclusion
Don’t try this at home. Seriously. I do not recommend it. However, stay tuned for a Part 2 of this post in the future sometime, where I will try to explain what is going on in the brain when we navigate space, develop coordinate systems and mental models of spatial things. I’m not sure how far away that post is, so it might take me a while. Anyway, I hope you’ve learned some fascinating things from this one. Brains are weird!