Abstract
Motion area V5/MT+ shows a variety of characteristic visual responses, often linked to perception, which are heavily influenced by its rich connectivity with the primary visual cortex (V1). This human motion area also receives a number of inputs from other visual regions, including direct subcortical connections and callosal connections with the contralateral hemisphere. Little is currently known about such alternative inputs to V5/MT+ and how they may drive and influence its activity. Using functional magnetic resonance imaging, the response of human V5/MT+ to increasing the proportion of coherent motion was measured in seven patients with unilateral V1 damage acquired during adulthood, and a group of healthy age-matched controls. When V1 was damaged, the typical V5/MT+ response to increasing coherence was lost. Rather, V5/MT+ in patients showed a negative trend with coherence that was similar to coherence-related activity in V1 of healthy control subjects. This shift to a response-pattern more typical of early visual cortex suggests that in the absence of V1, V5/MT+ activity may be shaped by similar direct subcortical input. This is likely to reflect intact residual pathways rather than a change in connectivity, and has important implications for blindsight function. It also confirms predictions that V1 is critically involved in normal V5/MT+ global motion processing, consistent with a convergent model of V1 input to V5/MT+. Historically, most attempts to model cortical visual responses do not consider the contribution of direct subcortical inputs that may bypass striate cortex, such as input to V5/MT+. We have shown that the signal change driven by these non-striate pathways can be measured, and suggest that models of the intact visual system may benefit from considering their contribution.
Highlights
A major unsolved problem in our understanding of the human cortex is uncovering the influence of weaker pathways, which may be overshadowed by activity derived from more dominant inputs
We have revealed that when V1 is absent due to damage in adulthood, V5/middle temporal area (MT)+ responds in a pattern typical of early visual cortex
The similarity to response patterns in V1 measured in control participants suggests that it may be direct subcortical input that shapes the motion coherence response in patients under these conditions
Summary
A major unsolved problem in our understanding of the human cortex is uncovering the influence of weaker pathways, which may be overshadowed by activity derived from more dominant inputs. By completely removing the influence of these driving cortical areas, the remaining innervations and responses can be studied. The primary visual cortex (V1) has neurons with small receptive fields that are modulated by multiple.
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