The secondary visual cortex consists of V2, V3, V4 and V5. These areas all have slightly different functions, but the main thing is that neurons in higher areas respond to more complex stimuli. For example, think of combinations of lines, or whole objects. V4 is an area that is very sensitive to attention. Damage to this part of the visual cortex can cause a disorder called simultanagnosia.
V2 : This area receives a lot of direct input from the primary visual cortex (V1). The neurons in this area have, for the most part, the same properties as the neurons in V1. That is, they respond to stimuli with a specific orientation or color. However, in V2 there are also neurons that respond to more complex properties of a stimulus, such as combinations of orientations. You can think of this as neurons responding to a triangle rather than three separate lines that could form a triangle together.
V3 : The neurons in this area respond to even more complex properties than the neurons in V2. Here you can think of seeing a clock, for example. The neurons in V1 respond only to single properties of the clock (for example, the hand). In V2, the clock is already seen reasonably as a whole, where the neurons then also respond to the round shape of the clock. In V3 it is merged and you ‘get’ a picture of the clock as a whole, with numbers and hands.
V4 : In this area, properties of objects are also signaled. But, as is the case every time one transitions to a higher visual area, the neurons in this area respond to more complex shapes and combinations of shapes than the neurons in the previous area, V3. In addition, V4 has an important role in processing color and recognizing objects.
Brodmann Areas 18 and 19 together form the secondary visual cortex in the occipital lobe. V2, V3, V4 and V5 are located in this area. V2 lies directly around the primary visual cortex. V4 lies against the temporal lobe, and V5 lies against the parietal lobe.
The areas in the secondary visual cortex are not all equally sensitive to attention. For example, areas V2 and V3 are not very sensitive to this. That is, the neurons continue to fire at about the same rate, even when an object has not caught your attention. In V4, however, the neurons become more active when you have your attention on a stimulus. Because V4 has such an important role in recognizing objects, you will therefore also recognize an object more easily when you have attention focused on it.
Damage to the occipital lobe can lead to simultanagnosia. The name of this condition already indicates the kind of problems someone with simultanagnosia can have: it is difficult for this person to perceive multiple objects (simultaneously). When these people stand in front of a forest, they can only perceive one tree at a time. This makes it difficult for these individuals to understand the meaning of a particular space.
Author: Myrthe Princen (translated by Melanie Smekal)