Rods and cones have different...frame rates?

[alt_f13]

So rods are more sensitive to light, right?

 

Does your brain make up for that by ghosting the part of the image you receive from the center of the eye into subsequent...frames?

 

I ask because I can see black lines and waves in my CRT monitor and TV out of the corner of my eye, but not when I'm looking at them dead on.

[Glider]

The reason for this is that rods are more numerous than cones in the periphery of the retina. Cones are concentrated around the fovea. Rods are more sensitive to light and changes in intensity. In low light where you want to see something (e.g. a dim star or a person in the shadows), you will see better if you look slightly to one side of the object.

 

As to your CRT monitor, the same principle applies. The flicker that exists in all CRTs is better detected by the high concentration of rod cells in the periphery of the retina.

 

Retinal cells don't have a 'frame rate' per se, so there is no 'ghosting' of frames. In good light, the cones are as active as the rods, it's just that they're responding to different qualities of light (wavelength rather than intensity).

[alt_f13]

I understand all of that, but how is it that the centre of the eye produces a continuous image? Is this done in the eye or the brain? It sounds like a mechanism of the eye. How is it then that the cones retain the frequency information? Some sort of resonance within the cells?

 

Because the rods are meant to pick up the change in intensity so well, I take it that the cones aren't merely picking up color from the screen between scans, as the intensity would be either negligably low, or nonexistant.

[Glider]

The eye produces a continuous image because different cells are being stimulated all the time. The eye is never stationary, it is in constant movement, even in a steady gaze. This is called nystagmus. If you google for the term, you will find that it is more often used to describe a clinical condition of uncontrollable rapid and jerky eye movements. However, at sub-clinical levels, nystagmus is normal and necessary for vision.

 

Experiments have been done where the ocular motor muscles have been temporarily paralysed, eliminating nystagmus. The result is that the cells in the fovea bleach after a few seconds and vision fades. The only way to refresh the image was to move the object. This projected the image onto fresh cells and vision returns

 

Cones don't really retain frequency information. It is simply that in cones, the substance that is sensitive to light (retinoids) in cones is sensitive to different wavelengths and each cone will only respond to its own particular wavelength.

 

Because the rods are meant to pick up the change in intensity so well, I take it that the cones aren't merely picking up color from the screen between scans, as the intensity would be either negligably low, or nonexistant.

I'm not sure what you mean here, but as I said, cones will only respond within their particular band of wavelength sensitivity. The entire retina (at least the dorsal part) contains both rods and cones. However, there are fewer cone cells the further you go from the fovea.

 

During normal daylight vision (photopic) the activity of cone cells in the fovea supresses the activity of rod cells in that area. So photopic vision is primarily mediated by cone cells which are less sensitive to changes in intensity and the flicker of a crt monitor is below their detection threshold.

 

Low light (mesopic) vision is primarily mediated by rod cells which can detect the flicker of a crt monitor, but the light coming from a crt is bright enough for photopic vision (if you can see colour, you are using photopic vision), so rod cell activity in the fovea is suppressed. Thus, the only way to detect the flicker of a crt monitor is to use the rod cells in the periphery of the retina (i.e. out of the 'corner' of your eye) which are not being supressed as there are very few cone cells in the periphery of the retina.

[alt_f13]

The eye produces a continuous image because different cells are being stimulated all the time.

Even so, the cells immediately around a cell that *should* pick up a black scan line don't.

 

You can see the ghosting effect I was referring to when you close your eyes while looking at a bright point of light. The point remains for a while, burned into your vision. I was wondering if that was the same mechanism that causes you to blur scans together into a single, fluid image.

 

While on the subject of eyes, how is it that putting pressure on the eyeball produces "light," such as when getting hit in the eye with a ball.

[Glider]

Even so, the cells immediately around a cell that *should* pick up a black scan line don't.

That's because, as I said, rod cell activity is supressed by cone cells during photopic vision. So, unless you use your peripheral vision (where there are fewer cones to supress rod activity) you won't detect these scan lines.

 

You can see the ghosting effect I was referring to when you close your eyes while looking at a bright point of light. The point remains for a while, burned into your vision. I was wondering if that was the same mechanism that causes you to blur scans together into a single, fluid image.

It's not 'ghosting', it's bleaching. If you look at an extremely bright light, the cells that the image falls on quickly become exhausted and stop functioning until they recover. So, it's not really ghosting because the after image is more like a hole in your visual field rather than a genuine image.

 

Cone cells are slower to respond than rod cells (at least in humans) so we have a flicker fusion threshold (FFT) of about 16Hz in normal photopic vision. Any flicker above this rate will not be detected. Pigeons have a higher FFT so, for example, if you showed a pigeon a cine film (24Hz), the pigeon would see a series of still images, where humans see a continuous moving image. With human retinal cells, if you present a series of images above 16Hz (crt = ~50 - 60Hz) no flicker will be percieved.

 

You have to remember that the eye is not a video camera. There is no 'frame rate' or 'refresh rate' in the eye. Instead there are a series of mechanisms in the eye and the visual cortex that have evolded specifically to detect pattern, motion and detail.

 

While on the subject of eyes, how is it that putting pressure on the eyeball produces "light," such as when getting hit in the eye with a ball.

Because, whilst retinal cells are specialised to respond to light, mechanical deformation will also trigger action potentials in them.

 

If you press on your eye, the retina gets bent out of shape. This will cause retinal cells to fire. Action potentials are all the same, regardless of what caused them. Action potentials originating in the retina pass down the optic nerve and terminate in the primary visual cortex. Any volley of APs terminating in the visual cortex will always be interpreted as patterns of light because that's what the visual cortex does. It interprets volleys of APs as patterns of light.