This is the first in a set of psychobiological posts on sensation and perception. While it’s designed with people studying psychology in mind, they’re written in a way that’ll make them accessible to anyone interested in the subject.
As an overview, the eye feeds information to the optic nerve, which in turn feeds information through to the LGN — the Lateral Geniculate Nucleus, if you want to be really fancy — leading to the visual cortex and is then separated into one of two streams. When we’re going through each individual area, try to keep in mind where in the process we are to give yourself a better idea of why particular things occur.
|The main thing to take from this is that light is picked up by rods and cones, which passes information to the ganglion cells.|
Now, this is where things become a little bit difficult to follow. Ganglion cells can either be on-centre, off surround or off-centre, on-surround. I’ll refer to these as on-centre and off-centre, respectively, just to make things a little bit simpler. The ganglion itself, too, can either be inhibitory or excitatory, which means that it will either hinder or aid the processing of the stimuli. On-centre cells respond, naturally, the best when light is focused on the centre (so the inhibitory cells are in the surround and excitatory cells in the centre) and off-centre cells focus optimally when light is focused off the centre of its receptive field (excitatory in the surround and inhibitory in the centre). Depending upon the amount of inhibitory and excitatory response, a ganglion either will or will not fire, or will fire weakly if equal amounts of stimulation are provided to the centre and surround. Receptive fields are basically the area in which the cell will respond to. If you’re still with me after that, the next part will be simples. At the centre of our retina, so the area that our vision is focused on, is the fovea. It is about 2% of our eyesight, but contains 33% of all our ganglion cells. As we move away from the retina, the receptive fields become larger, and the receptive fields for cones are usually smaller than those of rods. An important point to take from this is that smaller receptive fields have greater capacity for detail. It is because of receptive fields that illusions such as the Hermann Grid occur:
|A Hermann Grid|
On-centre receptive fields are detecting light in the inhibitory portion in such a large quantity that the effects of the excitatory ganglions are being counteracted, and so we see the crossroads perceived as grey. However, lateral inhibition alone cannot explain more complicated illusions (such as White’s Illusion and tricks with contrast). These will be explained later.
|Diagrams usually explain better than walls of text.|
Following the chain down, we reach the Lateral Geniculate Nucleus (LGN). This section is part of the thalamus, also known as the relay centre of the brain. It has six layers, two being magnocellular and four being parvocellular layers (though there exists a third type, K, the contribution of this is not clear and so will not be covered in this article). The M layers deal with rods, the P layers deal with cones, both layers amplifying stimuli: M cells respond to differing levels of light intensity (for example, an object flying towards one’s face) and P cells are affected by different colours. Finally in the journey of sight, we reach the occipital lobe, specifically the visual cortex (and even more specifically, V1; the primary receiving area). The visual cortex magnifies information from the cones and absorbs detail from the fovea: information is not just preserved in this region, but is also added to. We can see this when we look at phenomena such as the blind spot: the optic nerve has no rods or cones immediately behind it and so cannot perceive anything in that region, however our brain fills in gaps between objects, such as missing lines for us. Information from this point is passed around on the basis of how we need to act on it. The dorsal stream is the practical pathway, and is used, for example, to discriminate between landmarks and areas. The ventral stream is the more cognitive pathway and is used to separate objects from one another. The easiest way I find for remembering these is by looking at their names: In your body you have a dorsal muscle, and so the dorsal stream is practical; when you vent, you are using your mind to think of words, and so the ventral stream is cognitive. With that said, that just about covers the knowledge required for how we see things. There are other aspects to consider — I mentioned one briefly when I said about White’s Illusion — that will be covered later, but this is everything needed to be able to rote rehearse how we process visual stimuli. It’s not too difficult as long as you think of things in a start-to-finish kind of way, and don’t get mixed up with details or try to jump into specific portions of it.