Visual vs Auditory Encoding

This assignment is in lieu of the group presentations. It will be due at the end of the
course. I expect 3-4 pages, comparing the coding of the visual vs auditory systems.
You should describe a total of four things: how the visual and auditory systems encode
information in the spatial and temporal domains. The paper I’ve posted by Ahissar should
give you tremendous insight into this topic. We have yet to learn about the auditory
system, including the volley principle and frequency coding in general, so it may make
sense to wait until later to finish this part of the assignment.
Two of the four descriptions are straightforward:
Place coding for vision
Place coding for vision refers to the cell identity. An observer can tell where something is
by which cells are responding to it. Imagine looking at a vertical line. The cells that are
stimulated on the retina tell us where the line is in space and how big the line is. The
spatial layout of cells corresponds to the physical space in front of us. We code the spatial
information spatially, with the cell identity.
Time coding for audition
Here we are talking about coding for frequency. If you are listening to a 200hz tone, a
cell’s firing rate might be 200hz. We encode the temporal information in time, with a cell’s
firing rate, simply matching the frequency of the tone we are hearing.
Two of the four descriptions are not straightforward:
Temporal coding for vision
Consult the Ahissar paper to understand this. Instead of considering the spatial layout of
the cells on the retina, imagine just a single cell. Again looking at just a vertical line,
imagine an eye movement. As the cell moves over the line it goes from not responding
before it gets to the line, then it responds when looking at the line, then it stops as the
movement takes it beyond the line. How this cell responds in time can tell us where and
how big the line is, encoding spatial information in time.
Place coding for audition
Over a certain frequency, time coding cannot work. Cells have a practical limit to the
firing rate. Even using the volley principle, time coding for tones can’t work above
5,000hz. For larger frequencies, we use place coding, cell identity. Just like V1 cells are
tuned to orientation, some inner ear cells are tuned for frequency. If we hear a 10,000hz
tone, the cells that are tuned to that frequency are the most active, so we can tell by which
cells are responding what the tone is; they do not match the firing rate.