An invisible fingerprint

Rather late in the day I have been reading Francis Crick’s popular book (reference 1) about the search for consciousness: Crick as well as being one of the team that sorted out the double-helix, was also largely responsible for making consciousness studies respectable, rather than a plaything for philosophers, cranks and amateurs, back in the late 1980s. This book turns out to be well-written and accessible book, a book which has stood the test of time well – and I shall come back to it in due course. In the meantime, I notice the striking business of what looks very like fingerprints at the back of the brain, which one of them snapped above, at around page 142 of my copy.

In what follows, it should be borne in mind that I am glossing a gloss, so the story here is at some remove from the facts on the ground. There is much simplification and much omission; hopefully not too much error.

Human eyes, along with those of other primates and many carnivores, are front mounted and both point forward, with considerable overlap between the visual fields of the two eyes. Overlap which gives binocular vision; which gives better perception of direction and distance; and so, giving better grabbing and handling of nearby objects, possibly prey, that is to say food.

The human head is so organised that the right-hand half of the brain deals with the left-hand portion of the visual field and the left-hand half deals with the right. Contralateral and ipsilateral being terms used in this context. In order to achieve this, there is a sort of nervous version of Clapham Junction – where the lines to Waterloo Station cross those to Victoria – called the optic chiasma. The part of the brain of present interest is the primary visual cortex at the very back, also called V1, also called Brodmann area 17, also called the striate cortex. For which see reference 3. With the primary visual cortex being wrapped up, as it were, by the secondary visual cortex, also called V2.

So each half of the striate cortex is taking input from both eyes, but it seems that any one neuron is only sensitive to input from one of them. The present question is how are those neurons organised? Are they all randomly and more or less evenly mixed up over the whole of the relevant part of V1? Are they organised into two neat sectors, one sector for one eye, the other sector for the other?

It seems that the answer has been around at least since the mid-1980s, and, diving down into the reference provided by Crick, I eventually got to reference 2, from where I have lifted the snap at the top of this post. With humans and some, but not all, of the other primates doing this.

Very roughly speaking a picture of a (two-dimensional) patch of cortex, viewed from above, showing a pattern of lines very much like those of a fingerprint. The white lines are columns of neurons dealing with input from one eye, the black lines are columns dealing with the other. With the columns being perpendicular to the plane of the image. With ocular dominance being the term used in this context, for more on which see reference 4.

So we are, as it were, a quarter of the way along the road towards integrating the four streams of input from the eyes – the two red and the two blue streams in the snap above – into the unified image that we usually see, that we are conscious of. Put another way, we are close to having integrated the two streams arriving at one half of V1 into one.

I don’t know how stable this pattern is, but it does vary both between species and between individuals. And I dare say left is not identical to right. Nor do I know at what age the pattern emerges, becomes visible. I dare say not at birth.

Speculating, on the one hand, we all see much the same stuff. So the V1 and downstream neurons must be programmed in such a way that these differences between individuals don’t matter. It all adds up to much the same thing in the end. On the other hand, it may well be that differences in these pattern do account for small differences in the way that we see things, for the fact that I don’t see things in quite the same way as you do – although designing an experiment to look into those differences, even just to identify them, might be challenging.

Maybe such differences could be expressed in topological terms, reasonably robust to different ways of projecting the much-folded surface of the striate cortex onto the plane – a projection which might involve some careful cutting – a matter which occupies much of reference 2. Better still, such topological terms might be evaluated in situ as it were, without needing to bother with projections. With a catch there being that projections onto the plane might be a convenient way-station on the route to adding up lots of brains to get some kind of average, some kind of average in which the feature of interest emerges from the noise.

Complications

The opening snap of a fingerprint like pattern is derived from work on a live then dead macaque monkey, the sort of work which may not be allowed on monkeys now and is not allowed on humans. I suppose that stripes have been observed in humans post-mortem and I assume for present purposes that those stripes can be explained in the same terms as those in the monkey.

One can also draw rather different, coloured maps on the same V1 showing orientation, the edge direction which the neurons respond best to. A property which might be regarded as giving a second layer of information. Maps which recall the pinwheels which Winfree talks of in connection with the heart, with the property there being firing phase in time, rather than direction in space. See, for example, reference 7.

In humans, the area (in square millimetres or whatever) of V1 varies a good deal from person to person, perhaps as much by a factor or two or three, and which interacts with both the cross-sectional area of the optic tracts leading to it and the density of the neurons on it. Notwithstanding, it appears to be of the order of 25 square centimetres per hemisphere – more than might appear from the outside because of all the folding. For all of which see reference 8.

While the maps from retina to V1 are retinotopic, which means that an image on the retina can be traced to something broadly similar on the surface of V1, given that the VI images are striped, these maps are not continuous at the level of the stripes.

Given that individual V2 neurons appear to respond to input from both eyes, one supposes that binocular differences, of importance in estimating distance, have either already been dealt with – or are dealt with elsewhere.

Downstream from V1 & V2, visual processing is organised in two streams, the ventral pathway and the dorsal pathway. These two pathways are supposed to deal with different aspects of visual processing.

The striate cortex is not named for the stripes of present interest. Rather from the single stripe, the stripe of Gennari, named for its 18th century discoverer. From the Latin ‘stria’ for furrow or channel.

Conclusions

Perhaps these fingerprints will make it to some future series of  the detective soap ‘Vera’. As the very latest thing in medical forensics.

References

Reference 1: The astonishing hypothesis: The scientific search for the soul – Francis Crick – 1990. 

Reference 2: The Complete Pattern of Ocular Dominance Stripes in the Striate Cortex and Visual Field of the Macaque Monkey – S. Levay,’ M. Connolly, J. Houde, D. C. Van Essen – 1985. The source for the fingerprint.

Reference 3: https://en.wikipedia.org/wiki/Visual_cortex. 

Reference 4: https://en.wikipedia.org/wiki/Ocular_dominance_column. 

Reference 5: https://www.edoctoronline.com/. The advertisement infested source for the diagram of the visual pathway. The original source is given bottom right.

Reference 6: https://en.wikipedia.org/wiki/Macaque.

Reference 7: When Time Breaks Down: The Three-Dimensional Dynamics of Electrochemical Waves and Cardiac Arrhythmias – Arthur T. Winfree – 1987.

Reference 8: Correlated size variations in human visual cortex, lateral geniculate nucleus, and optic tract – Timothy J. Andrews, Scott D. Halpern, Dale Purves – 1997.