Case 12
As may have been glancingly alluded to before, the human
visual system is really pretty slick when it comes to reconstructing the
universe from a noisy optical input channel. We look. We see. We
build internal models of the external world from the data provided by
our senses; and we then interpret the world on the basis of our
models. Thus, at the most fundamental organic level, as human beings,
we are scientists.
The neurological and cognitive processes by which this happens are not
well understood. There is reasonable information on the ins and outs of
various implicated subsystems, but exactly how all that gets integrated
into a perceived scene, and how the scene in turn feeds back to the
lower levels, is the subject of much speculation. It's a matter of
attempting to model the modelling.
One thing that helps in this endeavour is to look at edge cases,
where the system either breaks down or else manages interestingly not
to. The brain is extremely good at papering over its own cracks, but
sometimes that interior sleight of hand becomes -- or can be made --
apparent.
A famous example is the blind spot: a significant patch of the
visual field of each eye, around the point from which the optic nerve
heads off to the brain, has no light receptors. The literal content of
those patches is simply not there. And yet, we are never directly aware
of any absence: we don't -- can't -- see these gaping holes in
our field of view. The blind spots don't overlap, so when both eyes are
open the brain can fill in the gap from each side by using the
information from the other. With only one eye open, the filling in is
more a kind of guesswork, based on the surroundings. It's pretty
clever -- it will fill in complex patterns, for example -- but
nevertheless sometimes entirely wrong. It relies on congruity: if
what's in the blind spot doesn't match its surroundings, it won't just
not be seen, it will be seen not to be there.
In fact, the brain is filling stuff in all the time. Looking around --
at least when sober -- you generally have the impression of a crystal
clear panorama in which everything is present in extraordinary detail.
You may not be able to make out the minutiae on the more peripheral
items, but you can rest assured they will be there should you turn your
attention to them. At some level, of course, we are aware that we don't
see things as clearly on the edges as at the centre, but that doesn't
make the experience any less seamless. All the stuff we're not actually
seeing clearly, we still imagine to be clear. Just as with
the blind spot, the brain covers up its covering up.
While blind spots are universal, other visual failures may occur on an
individual basis. Damage to a portion of the retina or the nerves
feeding from it can lead to scotoma, which is just another kind
of blind spot, but one that hasn't been built in from the start. These
too will commonly be completed by the visual perceptual system,
especially if they are monocular, and often pass unnoticed unless
specifically looked for. Yet again, you typically can't see what you
can't see.
With significant binocular loss of visual field, as occurs often
in the elderly via age-related macular degeneration (ARMD), things can
get trickier. One possible consequence of the resulting loss of visual
acuity is Charles Bonnet Syndrome, in which the patient
experiences hallucinations within the absent portion of their
visual field.1
Two general classes of hallucination are observed: simple, meaning
flashes or geometrical patterns in the visual field, and complex, where
the hallucinated entities are fully-formed objects such as people or
animals, usually located in the scene in a contextually plausible
way.2 The distinction may
sometimes be quite murky, given the subjective nature of hallucination:
all there is to go on is the patient's own description. Although models
for either kind of hallucination are far from settled, it seems likely
that the two classes arise through different mechanisms. In particular,
simple hallucinations probably result from bottom-up processes,
which analyse the input signals in various mechanistic ways to identify
things like edges, orientations, surfaces and colours; while complex
hallucinations probably involve top-down processes, by which the
constituents of a visual scene are organised into conceptual units and
interpreted on the basis of some set of beliefs about what is
being seen.
Some fairly concrete models exist for bottom-up visual processing, and
for the corresponding simple hallucinations. There are banks of neurons
in different sections of the visual cortex that respond to basic
elements in the observed image, and various circumstances -- say,
changes in the relative activity of different neurotransmitters (as may
be caused by psychedelic drugs) or hypersensitisation from loss of
normal input (in CBS) -- may cause these banks to trigger abnormally.
Patterns of neural firing in these areas will be perceived as
(different, but related) patterns within the visual field, and many of
the sorts of patterns that are commonly reported by patients can be
nicely reproduced by the mathematics of dynamic systems and the
geometries of our sensory apparatus.3
Modelling complex hallucinations is trickier, because our understanding
of the higher level cognitive processes likely to be involved tends to
be rather vague and hand-waving. It's difficult to characterise the
top-down effects of, say, consciousness on the visual system when
we don't have a convincing model of what consciousness is, nor of what
the the upper levels of visual processing -- with which it presumably
interacts -- are doing. So although it seems intuitively likely that the
organic causes may be quite similar -- abnormal firing patterns in
particular functional groupings of neurons -- we don't currently have
any way of quantitatively representing those causes and their perceptual
consequences.
As an aside, estimates of the prevalence of CBS vary widely. This is
partly down to an uncertainty of definition, but also because people are
often extremely reluctant to report hallucinations, for fear of being
considered insane and institutionalised. CBS hallucinations are,
apparently, usually not especially upsetting in themselves, but the idea
of losing one's mind can be very scary indeed.
1 CBS is imperfectly defined, with different sources applying different criteria, not always requiring associated eye damage. It usually means visual hallucinations where the patient is aware that the hallucinations aren't real, and where there is no cognitive impairment -- which is to say, the hallucinator isn't insane. Such distinctions can, of course, be somewhat subjective.
2 This is true for hallucinations in general, not just for Charles Bonnet. The distribution of the two classes varies, though, between, say, schizophrenia or dementia and CBS.
3 Which is to say, the particular way the eye bone's connected to the brain bone. Now hear the word of the lord!
1 CBS is imperfectly defined, with different sources applying different criteria, not always requiring associated eye damage. It usually means visual hallucinations where the patient is aware that the hallucinations aren't real, and where there is no cognitive impairment -- which is to say, the hallucinator isn't insane. Such distinctions can, of course, be somewhat subjective.
2 This is true for hallucinations in general, not just for Charles Bonnet. The distribution of the two classes varies, though, between, say, schizophrenia or dementia and CBS.
3 Which is to say, the particular way the eye bone's connected to the brain bone. Now hear the word of the lord!