Visual response properties of neurons in the rat LGN
Balaji Sriram*, Philip Meier*, and Pamela Reinagel
*co-first-authors
We characterize the response properties of
visual neurons in the lateral geniculate nucleus (LGN)
of adult Long-Evans rats. Most cells were recorded
under isoflurane anesthesia; some cells were
recorded in awake, eye-tracked, passively viewing
conditions. We characterize the spatial frequency
tuning, direction selectivity, and contrast sensitivity of
neurons in the rat LGN.
We recorded extracellular spikes in response to
drifting sinusoidal gratings with varying direction of
motion and spatial frequency. In the anesthetized
state, some cells were band pass with a peak
response around 36° to 72° per cycle. However, more
than half the cells responded strongest to the lowest
spatial frequency that was reasonable to display on
the monitor (144°/cyc), suggesting that they were low
pass. Preliminary data indicate that most cells are
band pass in the awake state, with a peak response
about 18° to 36° per cycle. Some LGN cells show
significant direction selectivity in the anesthetized
state. This remains untested in the awake state.
For other cells, we presented a static square-
wave grating for 200 msec per trial, varying the grating
contrast from trial to trial. This stimulus was designed
to match stimuli viewed by other rats in an awake
behaving paradigm. Most LGN neurons reduced firing
as contrast decreased, and some completely ceased
to fire at low contrast suggesting that behavioral
performance at these contrasts was probably limited
by sensory encoding rather than cognitive confusion.
To fit the stimulus-response relationships in
these data, we used a generalized linear model (GLM)
that separately accounts for the effects of the stimulus
parameter of interest, refractoriness (spiking neural
history), and the state of the network (LFP).
We acknowledge support from: JSMcDonnel Foundation, NIH/National Eye Institute, San Diego Foundation Blasker Fellowship (BS), NSF Graduate Research Fellowship (PM).