Big Fish - Little Fish -

We've been building the experimental equipment...

Easy test equipment for the rest of the school to measure their blind spot angle - they look through the hole and slide the test card back and forth until they find the point at which their blind spot hides the dot - then we read off the distance and make a look table to find which angle that corresponds to! Quick and easy! We'll bribe them with a thinky so they do the experiment for us!

(Here's thinky the monocular-illusion-dragon! (look at him with one eye closed... ) 

We made lots of him and everyone except those with an over-intelligent visual cortex thought he was very cool/wierd)

Field-of-vision test box - we look through the hole in the middle and we will position LEDs poking into the box that we can light up and see if we see them whilst we look straight ahead. We will be able to measure field of view and sensitivity to how bright they are and sensitivity for different colours :)

Fair enough!

We need to be sure that our random flash LED light really is random or else sneaky brained visual cortexes will keep checking over where the light flashes most often, and we need to know if we are going to use it as an electronic dice for gambling of course! (Not encouraged by the 'Out of Sight' Blog or the Royal Society of course).
Lots of us did a tally count test of lots of blinks...

I've put the results in a spreadsheet to make it easier to see :)

Is that what we expect? It is difficult to say exactly - you could just be very lucky and keep rolling sixes even if it is uniformly random, but there is a test you can do which tells you the probability that you would get results this different from the ideal equal proportions of each number - the Chi-squared test. You can see that there is a bigger than 10% chance that our results would be at least this uneven for most individuals and for the combined total - student 6 looks to be a bit odd though... 99.5% probability that data like theirs isn't generated by a fair circuit! Maybe the capacitor was big so their LEDs flashed slowly, and then if they were testing quickly the LEDs selections would not be independent (or maybe they didn't like 5s and cheated?)

Production line

The MASH toys were fun but for our visual field test equipment we need to place the LEDs where we want them so they need long leads...

We went into production line mode exploiting the readily available child labour!

Result!

For our Visual Field Test equipment, we also want the lights to be out most of the time and only come on when we press the button - this is kind of the opposite of the MASH circuit which shows the light most of the time and cycles round when we press the switch. It needs a bit of logic changing! 

The MASH circuit holds the ENABLE pin 13 of the 4017 counter high most of the time by connecting it to +5V by a 100k resistor which means the counter is stopped and only one LED is lit and stays lit. When we poke the yellow wire to earth, that pulls the ENABLE pin to earth (0V) which lets the counter go again and the lights start to flash round. Instead, we want the ENABLE pin to normally be connected to earth (by a little resistor) and pull it high (ie stop) when we press the switch to connect it directly to 5V. That's an easy rewiring.

Finally, we also want the LEDs to be off whilst the 4017 counter cycles round and round, and only the randomly selected LED to be lit when we press the switch. This is trickier, but if we just think about the logic levels, we want the LED anodes to NOT be earthed most of the time and only connect to earth when we press the switch. We could easily connect them all to the switch instead of earth, but the switch for the INHIBIT pin is normally earthed and goes high when we press it which is the exact opposite to what we want! The answer is to put an invertor in between which changes 5V to 0V and 0V to 5V. We could do this with a chip but there isn't much space and the easier way is to use a Transistor Resistor Logic (TRL) inverter circuit - shown below. It works for us with a 2N3904 transistor and Rbas 100k and Rcol 10k.

Finally (really finally!) we can use all the outputs of the 4017 instead of just 6 of them by changing the reset pin to connect to pin 11 instead of pin 5.

Making a difference

Science can only make a difference if we tell people what we are doing - in university we have to publish our work so other scientists can find out what we are doing and come up with even better ideas! We did a poster about our club to show next year's pupils what we do and what they might get involved with next year - nice!

(by the way - it's not strictly true that we all have a blind spot... those of us that are octopuses don't)

Angular analysis...

Here are our data from our blind spot measurements

We can look at these in a Histogram - we sort the angles into size ranges and plot how many are in each range. If we make a sensible choice for the size ranges we can see where most of the measurements lie and how spread out they are (too many small size ranges and we just see the individual measurements like in the list: too few and they all end up in the same range and it doesn't illuminate our data...)

This looks like there is a typical value in the 15-20 degree group and some spread about that value. This is quite normal for many properties measured in nature where values are 'randomly' spread around a central value. The spread can be due to natural variation (like for example peoples height) and also due to measurement error.

We can fit a Normal distribution - a mathematical characterisation of the variation - which gives a mean and a standard deviation parameter. The mean is the average value and the value we would Expect to find if we measure someone new. The standard deviation is a measure of how spread out the measurements are. Most (about 95%) values are within two times the standard deviation of the mean value. In the next figure you can see the red curve which is the fitted distribution showing the relative Likelihood of a given value being measured.

Of course, the more measurements we make, the more Confident we can be about these values. In fact we have quite a small Sample (only 9 of you recorded measurements) and so the mean value is uncertain. We can estimate how uncertain the estimate of the mean is (!), and the green curve shows the likelihood of the true mean angle taking different values.

The mean angle of our blind spot is therefore found (with 95% confidence) to lie between 16.2 and 21.5 degrees horizontally from the line of central fixation.

This is bigger than Wikipedia tells us (if you look up blind spot on Wikipedia it gives 12-15 degrees. Why are our numbers different?)

If you look closely, the wikipedia entry comes from a US military spec document for the design of optical displays and equipment that needs to be viewed so is a rather specific source for the angle. Another reference (http://www.ncbi.nlm.nih.gov/books/NBK220/ - also US) gives 12-17 degrees.

Possible reasons why our number is different include...

1) Errors (did we measure it badly?)

2) Bias due to not considering how big the blind spot is (ie did we always measure the outer limit of the blind spot which is about 5 degrees wide?)

3) Random discrepancy due to our small sample size (we just happened to measure eyes with big angles to the blind spot...)

4) Wikipedia's source reference is wrong or untypical?

5) We are both right but UK children's eyes have a bigger angle that US soldiers eyes (they are changing as we grow? or US soldiers exclude people who can't see perfectly so form a biased sample? or US soldiers are mostly men whereas we are fairly equally boys and girls? or ...)

We could test some of these ideas by measuring a bigger sample of eyes, perhaps improving our techniques, sampling boys and girls separately, and also recording how old the eye is that we measure...

Chemical codes

To see our photos you need to go to this link

 

and log in with a google account. In case you haven't got one I made one for you!
It is corbetscience@gmail.com 

Oh and you need a password! It is 8 92 t - 8 9 - 14 g 1 t except of course it isn't (I've coded it to be safe and secure - it is all lower case and there are no spaces but the hyphens are real. You know when you've cracked the code!)

You can also use this account to edit and add stuff to the blog (be sensible!)

M.A.S.H.

To measure the field of view we need to make our own visual field test equipment - in particular, we need to make one to test peripheral vision that we can also use to investigate rod and cone cell sensitivity in our peripheral vision...

I call it M.A.S.H. - "Multi-light Automated System to measure Human field of view"

I think we can adapt a system from card, sticky-back plastic and an electronic dice project...
Here is a electronic dice design we could use to control LEDs

...and here is a simplified version we can make on our proto-board kit using the 555 Timer in astable mode to send a stream of counts to a 4017 Counter chip with a reset after 6 counts. If the counts from the 555 Timer are fast, it will be random and allow us to light a random LED whilst our subject looks at a central point.

(or you could just play MASH?)

Anamorphic illusions

One of us found these photographic optical illusions - very cool! :)

click to watch!

And this is a nice optical illusion project...