Visual response latencies of magnocellular and parvocellular LGN neurons in macaque monkeys

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.