Evidence of a direct influence between the thalamus and hMT + independent of V1 in the human brain as measured by fMRI

In the present study we employed Conditional Granger Causality (CGC) and Coherence analysis to investigate whether visual motion-related information reaches the human middle temporal complex (hMT+) directly from the Lateral Geniculate Nucleus (LGN) of the thalamus, by-passing the primary visual cortex (V1). Ten healthy human volunteers underwent brain scan examinations by functional magnetic resonance imaging (fMRI) during two optic flow experiments. In addition to the classical LGN-V1-hMT+ pathway, our results showed a significant direct influence of the blood oxygenation level dependent (BOLD) signal recorded in LGN over that in hMT+, not mediated by V1 activity, which strongly supports the existence of a bilateral pathway that connects LGN directly to hMT+ and serves visual motion processing. Furthermore, we evaluated the relative latencies among areas functionally connected in the processing of visual motion. Using LGN as a reference region, hMT+ exhibited a statistically significant earlier peak of activation as compared to V1. In conclusion, our findings suggest the co-existence of an alternative route that directly links LGN to hMT+, bypassing V1. This direct pathway may play a significant functional role for the faster detection of motion and may contribute to explain persistence of unconscious motion detection in individuals with severe destruction of primary visual cortex (blindsight).