Labeled Line & Population Coding

Transcript

0:00 – 0:30  [Basic Definition]  In labeled line coding, an individual sensory receptor is selective for specific stimulus characteristics, and communicates to neurons in the central nervous system that are selective for the same characteristics.  For example a spiral ganglion afferent is selective for specific frequencies of sound and communicates to neurons in the CNS that are similarly selective. In population coding, stimulus characteristics are coded by a distributed pattern of neural activity and not by the activity of any one neuron, for example the pattern of activity across all glomeruli in the olfactory bulb can be used to decode the identity of an odor.

0:30-2:30  

So how are odors encoded when we get up to the bulb? One way this has been addressed is using calcium imaging of neurons in the olfactory bulb. This data was collected using GCaMP, our genetically encoded calcium indicator protein. Here it was expressed in cells in the olfactory bulb. This allows us to look at the change in activity when different odorants are applied to the nose.

Here we are imaging over a big swath of the olfactory bulb. And in color here we can see the size of the response: the more red the color, the stronger the response and the more blue, the weaker. These blobs that you see appearing here, those are actually individual glomeruli, where you have the axons from the two types of neurons coming together.

What’s seen generally when you apply odorants and collect these images is that individual odorants activate a broad swath of the glomeruli. And that should be expected based on what I told you before, that an individual odorant can activate many different receptor types, and all those receptor types send axons up into the bulb to different glomeruli. So we get sort of a broad distribution of activated cells.

So this is an example of what we call population coding, where stimuli are coded by different levels or patterns of activity across a population. The idea here is that you have to look at the whole population to know what the stimulus was, because many cells are responding to many different stimuli. Their responses aren’t specific to one stimulus. What is specific to a stimulus are the levels of activity or the patterns of activity across the population.

For example, we have a strong responding cell here, a weaker responding goal here, some weak responses elsewhere. And together that makes up the representation. So it’s not just what cells are active, but the patterns of activity or the levels of activity across the whole population. That’s the essence of a population code.

An important corollary here is that you can’t just identify the stimulus by looking at which cells are active, again because cells are going to be active for many different stimuli. But we’ll see later in the lecture that this is actually very different from what’s going on in the taste system.

2:30-3:00  [Parallel Vocabulary] In introductory classes you learned about labeled line and population coding.  But Neuroscience is an interdisciplinary field, so there are many words for these terms – For example, labeled line “coding” may also be called the labeled line “principle” or labeled line “theory” or labeled line “hypothesis”.  Similarly, population coding may also be called combinatorial coding.  Both labeled line coding and population coding fall within the umbrella category of neural coding, which includes many theories about how the nervous system represents sensory information.

3:00-4:00  [Here’s a real world example]  Generating tools that can decipher population coding is an active area of research – while patterns of activity may vary from one individual to another, scientists are beginning to develop algorithms that can identify sensory stimuli based on patterns of neural activity in the sensory systems.  In contrast, labeled line coding in the early visual and somatosensory systems is predictably consistent across individuals and knowledge of these pathways plays an important role in clinical neuroscience. If a patient complained of an abnormal sense of vision or touch, the nature of the abnormality could be used to precisely pinpoint the likely location of an injury that is causing the sensory abnormality. For example, if a patient complains of a dark spot in the upper right corner of their vision, knowledge of the labeled line pathway for vision could be used to narrow down the potential location of injury to a small number of subregions in either the eye, the optic nerve, the optic tract, the thalamus, the cortical white matter, or the primary visual cortex.  The location could be further narrowed down by determining whether the visual abnormality disappears when closing one eye or the other.

4:00-6:00 [Follow along with this example]

6:00-6:30 [Here are a few readings to help you review]
1) Neuroscience Exploring the Brain (Bear)

• Chapters 8-12 cover the Sensory Systems

2) Neuroscience by (Purves)

• Unit 2: “Sensation and Sensory Processing”