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Jonathan Nassi; Specialized circuits relaying primate parallel visual pathways to area MT. Journal of Vision 2008;8(17):6. doi: 10.1167/8.17.6.
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© ARVO (1962-2015); The Authors (2016-present)
Parallel pathways in the primate visual system parse the retinal input into relatively independent magnocellular (M), parvocellular (P), and koniocellular (K) streams. Chromatic opponency carried along the P and K pathways remains anatomically separate and distinct from the non-opponent signals carried along the M pathway through the lateral geniculate nucleus (LGN) and into the input layers of primary visual cortex (V1). Within V1, however, these pathways appear to intermix and it is unclear how each pathway contributes to the computations performed in extrastriate visual cortex. If color supports motion perception, then it is likely that color and motion cues integrate somewhere along the cortical visual hierarchy. Dorsal stream area MT is known to be specialized for motion processing and is a likely location where color signals could meaningfully contribute to the perception of motion. Nevertheless, MT is thought to be dominated both functionally and anatomically by inputs from the M pathway, with little or no contributions from the P or K pathways. We used rabies virus as a transynaptic tracer of anatomical circuits to assess the contributions of M, P, and K pathways to area MT of macaque monkey. We were particularly interested to determine whether MT receives any inputs from the chromatically opponent P or K pathways. We found that the main ascending input through layer 4C of V1 to MT is indeed dominated by the M pathway. Nevertheless, the P pathway does reach MT along alternative routes, including a surprisingly robust, disynaptic input from the P layers of the LGN. Additional input from the P pathway may arrive into MT through a less direct connection involving layer 4C of V1 and extrastriate cortical area V2. While the functional implications of P and K inputs to MT remain unclear, each of the specialized circuits described above likely provides MT with a unique combination of M, P, and K pathway signals and informs highly specific visual computations and tasks.
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