One example of parallel integrated systems of the human brain is obvious as we have two eyes. Images of the world from our left eye and our right eye are integrated to give us a unitary image (in 3-D) of the world.
Things we see form images on the inner back surface (the retina) of both of our eyes. Somehow these two images have to be put together in our brain.
These images are split into two halves. The images on the back of the left half of the left eye are sent to the left cortical hemisphere for processing (see the Figure 1 below). And images falling on the left half of the right eye are also sent to the left cortical hemisphere for processing.
Figure 1 Things you see in the right half of the world form images on the left back surface of the inside of both of your eyes. These images are then sent to your left cortical hemisphere for processing (shown in red on this figure). The opposite happens for images in the left half of your world (shown in blue on this figure). (redrawn from Gray, 1918)
The processing of images in the cortex initially takes place in an area called visual area 1 (or V1). Here an individual neuron only analyses a small piece of the image. In this area the images are analysed by neurons organised in the same general layout as those on the retina (so that bits of the image that were side-by-side in the eye are now analysed by neurons that are, essentially, side-by-side in the cortex). But in V1, neurons processing the image from the left eye don’t communicate with neurons processing the same bit of the image from the right eye.
Information from V1 is then passed on to another cortical area V2. Here neurons processing information from the left eye do ‘talk to’ neurons processing the same bit of the image (or something close by) from the right eye. In this way the two slightly different views of the world are put together, are integrated, to form what we experience as a single view of the world. This process also allows us to form a 3-D image of the world (and allows us to watch 3-D movies).
Well, that’s the plan anyway.
But it doesn’t work that well for everyone. We know that to avoid forming multiple personalities, you need the right childhood. And similarly, to avoid forming multiple visual systems, you also need the right childhood.
Your capacity to do all this develops in childhood. Neurons in V2 processing part of an image from the left eye are not automatically connected to the corresponding neurons processing the same part of the image from the right eye. The axons and dendrites of these corresponding neurons need to find each other and make the connections. Over many years, they find each other as they process the same bits of images.
If for some reason, however, you damage the muscles controlling where you point one of your eyes (or you have a congenital problem that has the same result), you won’t form images on the back of your eyes in corresponding positions. The neurons in V2 that need to learn to ‘talk’ to each other don’t. This can result in one system shutting down and as an adult relying only on images from one eye.
If, as has been often done in the past to avoid this, a child is made to wear eye patches (so that they learn to use both eyes), what can happen is that the child develops two unintegrated visual systems – one processing images from the left eye, and the other processing images from the right. And so what happens is that sometimes the adult will rely on one visual system, and at other times the other – but they are never able to form an integrated image of the world.
So here we have another system in the brain which comprises parallel systems which integrate in childhood to form an integrated system in adulthood. It is also a system for which inappropriate experience in childhood results in the formation of unintegrated systems in adulthood.
Julesz, B. (1971). Foundations of cyclopean perception. Chicago, IL: University of Chicago Presss.
Gray, H. (1918). Anatomy of the human body. Philadelphia: Lea & Febinger.
Pearlman, A.L. (1981). Anatomy and physiology of central visual pathways. In R. A. Moses (Ed.) Adler’s Physiology of the Eye (pp. 427-465). St Louis: The C. V. Mosby Co.