How neurons in the primary visual cortex (V1) of
primates process parallel inputs from the magnocellular
(M) and parvocellular (P) layers of the lateral geniculate
nucleus (LGN) is not completely understood. To investigate
whether signals from the two pathways are integrated in
the cortex, we recorded contrast-response functions (CRFs)
from 20 bush baby V1 neurons before, during, and after
pharmacologically inactivating neural activity in either
the contralateral LGN M or P layers. Inactivating the M
layer reduced the responses of V1 neurons (n =
10) to all stimulus contrasts and significantly elevated
(t = 8.15, P < 0.01) their average
contrast threshold from 8.04 (± 4.1)% contrast to
22.46 (± 6.28)% contrast. M layer inactivation also
significantly reduced (t = 4.06, P <
0.01) the average peak response amplitude. Inactivating
the P layer did not elevate the average contrast threshold
of V1 neurons (n = 10), but significantly reduced
(t = 4.34, P < 0.01) their average
peak response amplitude. These data demonstrate that input
from the M pathway can account for the responses of V1
neurons to low stimulus contrasts and also contributes
to responses to high stimulus contrasts. The P pathway
appears to influence mainly the responses of V1 neurons
to high stimulus contrasts. None of the cells in our sample,
which included cells in all output layers of V1, appeared
to receive input from only one pathway. These findings
support the view that many V1 neurons integrate information
about stimulus contrast carried by the LGN M and P pathways.
Excitation of the Pre-frontal and Primary Visual Cortex in Response to Transcorneal Electrical Stimulation in Retinal Degeneration Mice