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
appreciably before parvocellular signals. The relative
speed of these two channels cannot be accurately predicted
based solely on axon conduction speeds, however. Other
factors, such as different degrees of convergence in the
magnocellular and parvocellular channels and the retinal
circuits that feed them, can affect the time it takes for
magnocellular and parvocellular signals to activate cortical
neurons. We have investigated the relative timing of visual
responses mediated by the magnocellular and parvocellular
channels. We recorded individually from 78 magnocellular
and 80 parvocellular neurons in the LGN of two anesthetized
monkeys. Visual response latencies were measured for small
spots of light of various intensities. Over a wide range
of stimulus intensities the fastest magnocellular response
latencies preceded the fastest parvocellular response latencies
by about 10 ms. Because parvocellular neurons are far more
numerous than magnocellular neurons, convergence in cortex
could reduce the magnocellular advantage by allowing parvocellular
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 advantage.
This observation calls into question inferences that have
been made about ordinal relationships of neurons based
on timing of responses.