Poverty involves both low income levels and high income uncertainty. Do both these dimensions of being poor capture attention in ways that distort decision-making and trap people in poverty? We examine these issues using real-life shocks faced by farmers in Brazil: random payday variation affecting income levels, and rainfall shocks that affect income uncertainty. We find that it is income uncertainty that systematically has adverse cognitive effects; low income levels affect only the poorest households. The net adverse impacts on cognitive function prevail even though both dimensions of poverty reallocate attention to scarce-resource tasks. These results broaden our understanding of the impacts of uncertainty by exploring a psychological channel distinct from risk aversion, and help reconcile apparently contradictory evidence on the cognitive impact of poverty in previous studies.