Seeing and Perceiving

Modeling RGCs and the LGN To describe what cells in the retinal ganglion layer or the LGN respond to in more detail, we’re going to use results from single-unit recordings of those cells to guide us. This refers to a technique in which we place a small electrode into a cell that we’re interested in so that we can measure the changes in electric potential that correspond to the action potentials that most neurons produce when they send signals to other cells in the nervous system. If you don’t know what an action potential is, don’t panic: All I really need you to know about them is the following: 1)    When a cell is just kind of hanging out and doing nothing in particular, it produces action potentials (or “spikes”) every so often at a rate that we’ll call the base rate . 2)    Sometimes, a change in stimulation can make a cell fire more than the base rate. 3)    Sometimes, a change in stimulation can make a cell fire less than the base rate...

Visual processing in the retinal ganglion cells and the LGN To continue discussing how your vision works, we’re going to have to abandon structures that are relatively easy to see (like your retina – which is tricky, but not impossible, to see directly) and start talking about parts of your visual system that aren’t so accessible. Our next stop will be cells in two different locations: We’ll consider cells called retinal ganglion cells (or RGCs) and cells within a structure called the lateral geniculate nucleus (or LGN). We’ll be talking about these cells together because it turns out that they have very similar computational properties even though they’re located in different parts of your visual system. The retinal ganglion cells are located in a layer just “above” your photoreceptors if you’re looking at a cross-section of your eye, and receive inputs directly from the rods and cones. The lateral geniculate nucleus is a good bit further along – the retinal ganglion cells send p...

Mathematical Preliminaries To get the most out of these lessons, I need you to be familiar with a few different mathematical concepts. If you’ve had a course in algebra at some point in your life, you may dimly remember some of what’s below, but in case you don’t, I thought I’d go over the things you’ll need. This is a light treatment of these concepts, but it’s really all that we’ll make use of. If you want to know more, there are a ton of resources online to get into the mathematics more rigorously.              Besides these specific concepts, I also need you to be comfortable with basic arithmetic (adding, subtracting, multiplying, and dividing) and to have some fluency thinking about the graph of a function (y=f(x) and such). We’ve already used some material that relies on arranging numbers into lists (which we’ll call vectors) and soon we'll be working with arrays (which we’ll call matrices), but we won’t work with any ...