Spatial summation and degree of center-surround antagonism
were examined in the receptive field of nonlagged cells
in the dorsal lateral geniculate nucleus (dLGN). We recorded
responses to stationary light or dark circular spots that
were stepwise varied in width. The spots were centered
on the receptive field. For a sample of nonlagged X-cells,
we made simultaneous recordings of action potentials and
S-potentials, and could thereby compare spatial summation
in the dLGN cell and in the retinal input to the cell.
Plots of response versus spot diameter showed
that the response for a dLGN cell was consistently below
the response in the retinal input at all spot sizes. There
was a marked increase of antagonism at the retinogeniculate
relay. The difference between the retinal input and dLGN
cell response suggested that the direct retinal input to
a relay cell is counteracted in dLGN by an inhibitory field
that has an antagonistic center-surround organization.
The inhibitory field seems to have the same center sign
(ON- or OFF-center), but a wider receptive-field center
than the direct retinal input to the relay cell. The broader
center of the inhibitory field can explain the increased
center-surround antagonism at the retinogeniculate relay.
The ratio between the response of a dLGN cell and its retinal
input (transfer ratio) varied with spot width. This variation
did not necessarily reflect a nonlinearity at the retinogeniculate
relay. Plots of dLGN cell response against retinal input
were piecewise linear, suggesting that both excitatory
and inhibitory transmission in dLGN are close to linear.
The variation in transfer ratio could be explained by sustained
suppression evoked by the background stimulation, because
such suppression has relatively stronger effect on the
response to a spot evoking weak response than to a spot
evoking a strong response. A simple model for the spatial
receptive-field organization of nonlagged X-cells, that
is consistent with our findings, is presented.