Notes

• One of the main functions of the retina is light adaptation.
• "Cells get used to the light stimuli".
• The photoreceptors adapt very quickly to the average level of stimulation.
• It adjusts its sensitivity to the overall level of illumination.

Human Light and Dark Adaptation

The dark adaptation curve on the right shows the threshold (how much light) do you need to perceive something, i.e., how strong the light stimulus has to be in order for us to perceive it as a function of time when you walk in a dark environment. With the time passing we become more and more sensitive, which lowers the (log) threshold of the amount of light needed for us to see "something". The first dotted-line is the curve describing the adaptation of cones, followed by the adaptation of rods, which induce a significant decrease in threshold levels. The result is that the retina as a whole changes its sensitivity.

On the left we have the curve describing the minimal amount of light that is needed to see something. However, in this graph the horizontal axis defines the light intensity of the background instead of time. Thus, we can see that with a low-intensity of background light the threshold for perceiving a light is much lower than when the background has an "overwhelmingly" high-intensity level. Again, the dotted line describes the adaptation of cones, while the full-line describes the adaptation of rods.

The Jungfrau Viewed from Wengen We care for surface reflectance, not light intensity. Contrast is proportional to reflectance.

The intensity of light in different parts of the image is dependent on: reflectance*illuminance. So, assuming that the illuminance is always the same in an image, the intensity is then determined by the reflectance of the elements. The relevant parameter is the contrast, which tells us how much each part of the image differ in terms of intensity. In particular, the contrast remains constant to changes in illumination.

Cones Responses Adapt to Background Illumination This graph shows how individual cones respond to flashes of different light intensity presented in different background light conditions. It shows that cones become less and less sensitive the higher the light level in the background, which shows that the cone has adapted. Different parts of the retina can be adapted to different light intensities. Light adaptation is somewhat local in space.

The above effect can be explained by the local contrast, which changes across space.

Ganglion Cells Adapt to the Mean Light Intensity

In this graph, the response of an individual ganglion cell measured in terms of how many AP/s the cell fires under different background light conditions. When the background is dark the cell is quite responsive for low intensity stimuli and then adapts and reduces the firing rate for low-intensity stimuli.