In this video, you'll learn how neurons in the visual pathway become sensitive to simple features in the visual scene. We'll talk about the important concepts of receptive fields, we'll review how synapses work, and we'll talk about how the centers around organization that is present in many areas of the visual pathway serves to enhance contrast detection. Last time, I posed the problem of how we were able to perceive the visual scene as composed of objects with boundaries. Recall that the optics of the eye ensure that light from a particular position in the visual scene lands at a particular position in the retina. So, individual photoreceptors are sensitive to what's happening only at a single place in the visual scene. So, photoreceptors here will be sensitive to light that comes from here. Photoreceptors located here will be sensitive to light that comes from here. And, photoreceptors that are here will be sensitive to light that comes from there. The location that a photoreceptor is sensitive to is known as the receptive field. We're going to talk a lot about receptive fields, and I think it's one of the most important concepts in all of neuroscience. So, photoreceptors have receptive fields due to the optics of the eye, the pupil and the lens, which keep light sorted based on where it came from. The photoreceptors are connected with other neurons in the retina, and these later stage neurons inherit the receptive fields of the neurons that they receive input from. To understand how this works, let's review the basic structure of neurons and the synapses that connect them. Most neurons have three general parts: the cell body or soma, a set of dendrites, and one or more axons. The neuron is an information processing unit, and the direction of information flow is from dendrite to soma to axon. So, from here, through here, and along the axon. Neurons are connected to other neurons via synapses, and synapses can be formed from dendrite to dendrite, from axon to axon, or from axon to dendrite. Synapses between dendrites or between axons are a bit beyond the scope of this class, so we're going to focus on the synapses between axons and dendrites. When one neuron fires an action potential, that action potential travels down the axon to reach the presynaptic side of the synapses that that neuron forms with other neurons. The action potential sets in motion a biochemical chain reaction that culminates in the release of small molecules known as neurotransmitters. Those neurotransmitters are released from the axon side of the synapse and travel across the small space between the axon of one neuron and the dendrite of the next. These neurotransmitter molecules then bind to receptors on the postsynaptic side. The re, the dendrite side of the synapse. The receptor molecules cause ion channels in the receiving neuron to either open or close. Depending on the kind of ion channel, whether it is opened or closed, and whether or not it permits the passage of sodium, potassium, or something else, governs what kind of electrical effect is produced. The effect on the postsynaptic neuron may be to make it more positive or more negative. Generally speaking, if the change in potential is positive, it is said to be an excitatory synapse, and if it is negative, it is referred to as an inhibitory synapse. Inhibitory synapses are a means of flipping the sign of a signal and can be used to make neurons respond in the opposite way from their inputs. There are a lot of different types of neurotransmitters in the brain, and you've probably heard of a few of them, such as serotonin, dopamine, endorphins, adrenaline, glutamate, or GABA. These are all types of neurotransmitter. Medications that affect the brain often exert their effects by altering the impact of one or more of these neurotransmitters on their receptors. We won't talk about this further in this class, but it's an important area of brain science. Okay, back to our neurons. If activity happens simultaneously in excitatory synapses in enough of the dendrites, then the soma of the neuron is depolarized enough to trigger an action potential. Generally, the action potential doesn't occur in the dendrites. Rather, a decision of sorts about whether or not to fire the action potential is made in the soma, and the action potential then travels down all the axons of that neuron, and the process is repeated at all the synapses that that neuron forms with other neurons. So, to summarize. Each neuron collects inputs from a bunch of other neurons and sends output to yet another bunch of neurons. Every time this happens, information is pulled together and interesting new response properties can emerge. For example, let's talk about a kind of neuron called a retinal ganglion cell. Retinal ganglion cells are the last stage of processing in the retina. From there, signals proceed on into the brain. There are actually several types of neurons between the photoreceptors and the retinal ganglion cells, but I'm going to ignore that and talk about the net input that retinal ganglion cells receive that originates from those photoreceptors. Retinal ganglion cells and many later neurons in the visual pathway have a property called center surround organization. This means that the input that they receive originates from a population of photoreceptors and is transmitted to these neurons with a combination of excitatory and inhibitory synapses. One type of synapse occurs in a central region, and the other type occurs in an annulus, surrounding that region. Some neurons receive net excitatory in the center and net inhibitory from the surround, and other receive net inhibitory in the center and net excitatory synapses in the surround. The end result is that retinal ganglion cells are more sensitive to patterns of light that are not uniform, but are changing in a way that, that matches their center surround organization. That can make them most responsive to light spots on a dark background, which is referred to as having an on center and off surround, or dark spots on a light background, which is referred to as having an off center and on surround. Here's a video recorded by scientists David Hubel and Torsten Wiesel, who won the Nobel Prize for their pioneering work on the response properties of neurons in the visual system. In this video, you'll see the visual scene that was placed in front of a cat while neurons were recorded from its visual pathway. specifically, these, this neuron was recorded in a structure called the lateral geniculate nucleus of the thalamus, which is the next stop after the retina in the visual pathway. You'll hear spikes fired by these neurons as little clicks or popping sounds. In real life, neurons don't actually make an audible sound at all, but you can listen in as if through a Geiger counter if you send the electrical signals they make to an ordinary stereo system. So, I'd like you to notice two things in this video. First of all, be alert for the receptive field. The neuron is sensitive to light only in a particular region of the visual scene. And secondly, watch out for the center surround organization of this neuron. See if you can tell by listening whether the neuron is an on-center, off-surround, or an off-center, on-surround type of neuron. [SOUND]. Hopefully, you all said it was an on-center, off-surround neuron. The point is that these response properties, by emphasising local areas of contrast through regular patterns of excitatory and inhibitory synapses, seem to be a baby step towards identifying the boundaries of objects. And in the next video, we'll start connecting these polka dots, looking at visual cortex, where some fancier response properties emerge.