Signals relayed through the magnocellular layers of the LGN travel on axons
with faster conduction speeds than those relayed through the parvocellular
layers. As a result, magnocellular signals might reach cerebral cortex app
reciably before parvocellular signals. The relative speed of these two chan
nels cannot be accurately predicted based solely on axon conduction speeds,
however. Other factors, such as different degrees of convergence in the ma
gnocellular and parvocellular channels and the retinal circuits that feed t
hem, can affect the time it takes for magnocellular and parvocellular signa
ls to activate cortical neurons. We have investigated the relative timing o
f visual responses mediated by the magnocellular and parvocellular channels
. We recorded individually from 78 magnocellular and 80 parvocellular neuro
ns in the LGN of two anesthetized monkeys. Visual response latencies were m
easured for small spots of light of various intensities. Over a wide range
of stimulus intensities the fastest magnocellular response latencies preced
ed the fastest parvocellular response latencies by about 10 ms. Because par
vocellular neurons are far more numerous than magnocellular neurons, conver
gence in cortex could reduce the magnocellular advantage by allowing parvoc
ellular signals to generate detectable responses sooner than expected based
on the responses of individual parvocellular neurons. An analysis based on
a simple model using neurophysiological data collected from the LGN shows
that convergence in cortex could eliminate or reverse the magnocellular adv
antage. This observation calls into question inferences that have been made
about ordinal relationships of neurons based on timing of responses.