Vision notes
***these notes should not be construed to be a comprehensive list of ‘everything we need
to know about vision for the final exam’. I’m just trying to give you a bit more help with
organizing your thoughts about vision in light of my perception (confirmed by strong
show of hands) that you’re finding this material difficult.
FIRST THING: remember the general principles that you learned—
1. all senses need transduction to convert energy fluctuations in the external
world into changes in electrical potentials in the nervous system.
2. all senses convey specific information about some aspect of the physical
world
3. each senses responds to an (adaptive) range of stimuli
4. each sense employs a variety of different codes for different aspects of a
stimulus
5. in the nervous system, there are specialized systems for processing
submodalities of a sense
6. the senses are mostly interested in changes and discontinuities
In our discussion of vision, most of our time has been spent considering points 4 and 5,
with some attention paid to 1 and 6 and at least a mention of 2.
Early in the module, following our discussion of transduction (which is pretty
straightforward as given in the text and requires you to memorize the biochemical steps
that start with absorption of a photon by photopigment and end with the closing of
sodium channels in the lamellae of the outer segment), we considered retinal processing
fairly extensively. The main reason for this is that the organization of the retina contains
marvelous examples of principles 5 and 6. The duplex retina, with one foveal system
specialized for colour and high spatial frequencies (detail) and one extrafoveal system
specialized for sensitivity is really the beginning of functional specialization in vision. It
will be important for you to understand the consequences of different ratios of
convergence (high convergence of rods to retinal ganglion cells and low convergence of
cones to retinal ganglion cells) to see how this is so and to see how the duplex retina
represents a brilliant solution to some basic constraints in neural integration – you can’t
have both high resolution and high sensitivity at the same time from the same set of cells,
so evolution has given you two sets of cells, each specialized for one or the other.
I spent some time talking about lateral inhibition in the horseshoe crab and its
relationship to centre-surround antagonism in receptive fields in the mammalian retina.
Both of these very highly related phenomena illustrate principle 6 because they
demonstrate that the visual system is tuned to change. SO tuned to change in fact that
when it finds it in the form of an edge, it actually exaggerates it.
In addition to these two very important points about retinal processing, we also spent
some time looking at the contrast between the way the visual world is actually taken in at
the retinal level (a series of very short fixations of very small parts of the world separated
by rapid eye movements (saccades) during which the world essentially goes dark) with
the phenomenology of vision (how the whole visual world, in full colour and detail, feels
as though it is present to us at all times). The lesson here is that basic anatomy and
physiology of the retina show that the perceptual world is constructed by the brain. Our
eyes are not just passive receivers, like cameras taking snapshots. Centre-surround
antagonism and the associated illusion of Mach bands demonstrates much the same kind
of thing – our visual systems don’t just receive the world – they make it for us,
emphasizing those things that make a difference (just like as in principle 3) and ignoring
those things that are of no use to us.
And all of this starts in the retina.
Following this extended examination of the retina, we spent a fair bit of time looking at
the anatomical and physiological organization of visual cortex. I told you the story of
Hubel and Wiesel’s discoveries of the functional organization of V1. There were several
reasons for the detail. One reason is that I think the story is such an interesting one. I
probably didn’t emphasize this in class enough (and it’s difficult to appreciate unless you
were on the scene at the time – which I wasn’t actually – I only look that old at certain
times of the term! – but I’ve thought about this a lot) but it was a huge surprise even to
Hubel and Wiesel themselves that the properties of cells in visual cortex were as openly
accessible and understandable as they were – given what I’ve said about the importance
of processing edges, one would predict that that is what cells would be interested in, or
‘tuned for’. When they first poked relatively big and clumsy electrodes into these
complex networks of millions of cells, though, Hubel and Wiesel were rightly not
optimistic about how things would turn out and were amazed at the simple elegance with
which their cortical recordings offered up profound truths about how brains worked.
Another reason for spending this time on V1 is that it helps to set up your understanding
of the next steps in cortical processing in V2 and beyond. As I said in class, I think that
the cytochrome oxidase story can be confusing until you realize that, in this context at
least, nobody cares so much about what cytochrome oxidase is or what it is doing, just
that it acts as an interesting marker in cortex for further functional subdivisions (blobs –
thin stripes – V4, etc.). All of this is important because, even apart from the interesting
consequences of certain kinds of lesions to higher levels of cortex (the agnosias), it
suggests strongly that the visual system is organized into a large number of submodules,
each of them specialized for some aspect of visual processing.
The last part of the lectures in vision had to do first with the agnosias and the evidence
from them for submodality coding and then to the efforts of some to try to organize the
colourful matrix of visual cortical areas into a larger scheme. One of the most successful
of these larger schemes has been the so called two cortical visual systems hypothesis of
Ungerleider and Mishkin, explained on the video segment and also a bit in class.
Basically, their idea was that there are two processing ‘streams’ through all of those
visual cortical areas – a dorsal stream for localizing stimuli and a ventral stream for
indentifying stimuli. This kind of division of labour was built on the back of much older
ideas coming from studies of animals such as frogs, toads, and hamsters and also from
both early (Gordon Holmes) and more recent (Larry Weiskrantz) case observations of the
effects of brain damage. The vision unit ends with a brief discussion of why Ungerleider
and Mishkin probably don’t quite have things right – the correct functional division of
labour in the the visual cortical systems is probably ‘what’ vs. ‘how’ and not ‘what’ vs.
‘where.’ An experiment which nicely demonstrates the difference will be/was discussed
in class.