Like many of you, I am currently studying my Masters through the University of Sydney. The great thing about studying is not only am I increasing my own knowledge about the world of neurophysiology, but I get to share and discuss this with my fellow colleagues. One of the units I am studying at the moment is an ‘Introduction to Clinical Neurophysiology Techniques’. When I found out that my weekly learning material was on visual evoked potentials (VEPs), I was quietly happy and content, thinking that it was a week I could take it easy… but I was wrong.

I like performing VEPs. It gives me a break from the world of EEG, whilst being a simple test, that provides useful information for the patient. Aside from making sure my impedances were acceptable; measuring the distance from the screen; covering one of the patient’s eyes, ensuring they could see the little red dot in the middle of the screen; and stayed awake throughout the tests; I didn’t think there was much more to it. Sure, I could detect when there was an asymmetrical difference and when the recording was attenuated – but I could never full wrap my head around what it all meant. Nor did I realise that we could be doing more to help localise the patient’s abnormalities. I know a few of the senior scientists may be rolling their eyes at the moment, but for other junior scientists, I think we often get caught in the trap of thinking we know everything once we have mastered the basics.

So here is what I have learnt.

What we tend to perform in our departments are ‘full field’ VEPs. I can hear a few of you thinking ‘What? But we only test one eye at a time’ – which is what my initial thought was. Essentially, each eye has two visual fields – one left and one right. The visual field that is closest to our temporal regions remain ipsilateral as the optic tracts pass through the optic chiasm; whereas the visual fields closest to our nose switch to the contralateral side. Once the optic nerves travel through the optic chiasm, they continue onto the lateral geniculate nucleus, before synapsing in the occipital cortex. I could tell you about how the visual fields can then be broken down into quarters as they pass the inferior and superior retinal fibres, but I won’t confuse you too much.

So to recap – we all perform ‘full field’ VEPs on each eye as part of our routine testing. But here is where it gets a little more complicated, but where I think we could all do better in getting the best bang for buck.

Full visual field testing is good at tell us that there is a problem. If we notice that the left VEP latencies are slower than the right, then we can determine that there is something wrong with the left optic nerve, before the optic chiasm – but that is all we can say. This is where ‘half field’ VEPss come into play.

Half field VEPs use the same set up as full field VEPs. However, instead of the patient looking at a full screen of checkerboards, one half of the screen is black, with the red dot in the middle. By only stimulating half of the visual field, we are able to localise abnormalities post the optic chiasm. All four half fields should be tested. If both of the temporal visual fields are abnormal, we can determine that the patient has a lesion at the optic chiasm. Whereas, abnormalities in both of the left visual field would indicate a lesion post the optic chiasm.

Understanding what the half field waveforms mean can be a bit tricky due to the occipital cortex dipoles. Essentially, information that travels along the optic nerve, carrying the left visual field information lands in the left primary visual cortex. However, due to the way the neurons are arranged in the cortex, these waveforms will be detected in the right electrode. A way to think about it is that the visual field you are interested in will be detected in the ipsilateral electrode. Have I confused you yet?

Now that I know that this procedure exits, with minimal extra effort for the scientist performing the test, I think that it is worthwhile exploring more in our departments. To be able to pinpoint a patients abnormality give me more confidence interpreting the results I produce. I know that this extra component won’t necessarily be applicable for all of our patients, but how do we really know until we start doing them?

We're only seeing half the picture