As with most specialty subjects, we need to aim at a bit of a generalisation and it is true enough (for our purposes) to consider the rods as a band around the foveal area. The rods are actually completely absent from the foveal area but do have their peak density about 15 degrees off the visual axis of the eye (where 0 degrees is the fovea) and remain the predominant cell type all the way to the edges of the retina. You should note that cone cells are still present at the periphery but just at much lower concentrations. The density of rods and cone cells decreases out towards the edges of the retina and so does the relative proportions of the two.

Here is a good diagram to help you visulise the distribution:

[center][attachment=434]rodscones.jpg[/attachment]
Rods are purple, cones are greenish in this diagram. Note absence of rods at fovea with peak between 15-20 degrees off the visual axis (or 20-25 degrees if you consider the nasal-side of the fovea).[/center]

Cheers,

Rich

Thanks. But still a little unsure if the question is asked in the CASA exam where the rods are most conentrated, out of - at the retinal periphery, and at the fovea periphery, which to choose.

From the graph you can see the greatest concentration occurs at the foveal periphery and drops away the further you get from the fovea.

Cheers,

Rich

Thanks, will give feedback if I get the question tomorrow.

Exam went well, but didn't get that question so can't give feedback.

Congrats on the pass mate. Glad to hear it went well for you. Awesome.

Cheers,

Rich

Thanks, I was conscious on saying because for all I know it could be false, but I was chatting to another candidate who was also sitting his AHUF as first ATPL, we were discussing the exact question and he was adimate that he had that question in his CHUF and answered Foeva Peripheral and got marked wrong. He assured me if he got the question again he would tick Retinal. Also chatted to a Tafe lecturer who swore on Retinal Peripheral. Not by any means saying you're wrong, it just clearly seems to be a grey area I think as to what CASA deem correct. Anyways I'm just glad Human Factors can virtually dissapear out of my life now. All I know is look after your ears and eyes, don't smoke, beware of vertigo and illusions, night vision, hypoxia and hyperventilation symptoms and treatments, identify threats, take actions to avoid errors, and if and when erros do occur, your main concern is safe control of the aircraft 😀

Ok, I have checked into this more and find the problem lies with the assumed meaning of the word "periphery". It infers the edge of the retina but it looks like the actual anatomical definition is a little different. The retina contains the fovea at the central point then there is the "central retina" which is a band of only about 6 mm around the fovea. Beyond this is what is called the "retinal periphery". The retinal periphery is a term which describes the majority of the retinal area and not just an edge as the term would suggest.

Therefore, based on this, I would now agree the answer would be retinal periphery and not foveal periphery. The rod cells are indeed concentrated around the edge of the fovea (which made me think of foveal periphery) but the anatomical "retinal periphery" is the one which contains the greatest concentrations of rod cells.

I hope that clears up any confusion 🙂

Cheers,

rich

Hi Rich, thanks for that, that all makes good sense. Appreciate the follow up, and I'm sure students with either CPL or ATPL Human Factors to come will be even more greatful, especially those like me who were finding contradictory information on the matter. It's pedantic, and perhaps even a little ambiguous, but that's why we love CASA exams :blink:

Hi Bob,

I have HPL on Friday and wanted to try clarify one of the few remaining questions (I'm aware) I have.

The discrepancy still exists between the AFT and BT texts with ATF's ATPL HPL revision test 3 saying that night vision is enhanced by looking slightly off centre due to "the concentration of rods in the retinal periphery" and in your set 4 Q26 the correct answer is given as "...in the foveal periphery"

From the above it looks like the answer to Q26 should be d. Would you be able to clarify, I'm definitely going to get this question!?

Many thanks

I found a medical review article which defines the "peripheral retina" as "the area of the retina outside the macula", and explains that it is "the part of the retina that provides peripheral vision". I guess that explains the terminology.