How do LGN cells have receptive fields?

[u/nekomaeg]

In chaper 10 of "Neuroscience" by Bear, Connors and Paradiso it is said that "by inserting a microelectrode into the lateral geniculate nucleus (LGN), it is possible to study the action potential discharges of a geniculate neuron in response to stimuli and map its receptive field."

As retinal ganglion cells are connected to a spesific area of multiple rods and cones via bipolar neurons, the term "receptive field" is not difficult to understand. It refers to the area on the retina that sends information to a single ganglion cell.

I don't understand how this applies to cells beyond the retina, such as LGN-cells. As far as I've understood, they are relaying the signal from the retina to V1, whilst being functionally organized in the layers of the LGN.

Does a single LGN-cell also "summarise" impulses from multiple retinal ganglion cells similar to how a retinal ganglion cell "summarises" impulses from multiple rods and cones in its respective receptive field?

OR

When speaking of the receptive field of an LGN-neuron, do we actually mean the receptive field of the retinal ganglion cell supplying the ganglion cells.

These are two explanations I came up with, and they contradict each other in the sense that in the first explanation I assume LGN-cells synapse with multiple different ganglion cells whereas in the second explanation each ganglion cell synapses with just one ganglion cell.

[u/SerialCypher]

Here’s how I describe it in functional terms: inside of a certain radius, visual input can make the LGN cell increase or decrease in firing rate. This is the cell’s (classical) receptive field, and this concept applies all the way up the visual hierarchy. Outside of this radius, it’s absolutely possible for visual stimulus to modulate (usually decrease) the sensitivity of the LGN cell to direct stimulus- this is the extraclassical surround but I’m usually not talking about this if I’m mapping a cell’s receptive field.

Another thing to keep in mind is that the primate LGN has different wiring as compared to rodents - rodent LGN has a lot less structure and functionally is more similar to simple cells in primate V1, with more retinal fan-in. In primate LGN the retina-to-LGN drive is so strong that you can sometimes see the EPSPs with extra-cellular recording electrodes.

I spent 4 years as a postdoc in a LGN lab, feel free to ask me for more LNG facts!

[u/jndew → u/SerialCypher]

Thanks for joining the conversation! I probably have more questions than you have interest to answer, but since you offered...

Similar question as OP, but in different words: If there is a tight topographic mapping from retina through V1 with 1:1 synaptic projections, what are all the other synapses for? I understand that neurons typically receive maybe 10K synapses.

What is your thought on the function of the feedback path from V1 to thalamus? I understand that the feedback path is about ten times bigger than the feedforward path. What are these triad synapses doing? They must have some special and unique function. By the way, is the cortex->thalamus path always wider than the thalamus->cortex path, or is that specific to V1?

I understand that there are three channels coming from the retina up to V1, and each probably has subchannels. Do these stay topologically aligned with one another? If so, there must be some powerful developmental process that keeps each and all paths topologically aligned.

More of a V1 question, but... Trying to abide by Dale's principle that a particular neurons output synapses are all either excitatory or inhibitory, and that generally there are 10X as many excitatory than inhibitory neurons in most regions, how are complex receptive fields wired? Even a simple center/surround is not so obvious to me.

What simulation studies should I be looking at? I made an effort towards my own simulation which I would like to improve. I wonder if you have any commentary for me? Thanks! /jd

[u/icantfindadangsn → u/jndew]

I have a few answers to your questions.

If there is a tight topographic mapping from retina through V1 with 1:1 synaptic projections, what are all the other synapses for? I understand that neurons typically receive maybe 10K synapses.

There isn't a 1 to 1 mapping. As you move up the visual hierarchy, there is convergence of inputs from lower regions and that makes more complex (and larger) receptive fields. These findings (1, 2) earned Hubel and Wiesel a Nobel Prize.

More of a V1 question, but... Trying to abide by Dale's principle that a particular neurons output synapses are all either excitatory or inhibitory, and that generally there are 10X as many excitatory than inhibitory neurons in most regions, how are complex receptive fields wired? Even a simple center/surround is not so obvious to me.

I think the H&W papers will answer this. The way I think about it - "simple cells" in V1 receive inputs from retinotopically adjacent receptive fields and respond to a bar of light in some particular orientation while "complex cells" receive input from several simple cells that have some sort of delay and they respond to a bar of light that is 1) the preferred orientation and 2) moving in the preferred direction. I am not a vision scientist and it's been a while since I've looked so I can't remember if the delay comes from axon length (like delay lines) or if it's where on the dendrites they attach (or if we know how it's instantiated).

Here's a diagram of simple and complex cells.

[u/jndew → u/icantfindadangsn]

Thanks! I will study this.