BACKGROUND: Object-based attention can group image elements of spatially extended objects into coherent representations, but its mechanisms have remained unclear. The mechanisms for object-based attention may include shape-selective neurons in higher visual cortical areas that feed back to lower areas to simultaneously enhance the representation of all image elements of a relevant shape. Another, nonexclusive mechanism is the spread of attention in early visual cortex according to Gestalt rules, which could successively add new elements to a growing object representation.
RESULTS: We investigated the dynamics of object-based attention in the primary visual cortex of monkeys trained to perform a contour-grouping task. The animals mentally traced a target curve through the visual field and ignored a distracting curve. Neuronal responses elicited by the target curve were enhanced relative to those elicited by distracting curve. Remarkably, the response enhancement was delayed for neurons with receptive fields farther from the start of the tracing process. We could therefore measure propagation speed and found that it was low if curves were nearby and that it increased if curves were far apart. The results are well explained by an "attentional growth-cone" model, which holds that the response enhancement can spread in multiple visual cortical areas with different receptive field sizes at a speed of approximately 50 ms per receptive field.
CONCLUSIONS: Our findings support an active role for early visual areas in object-based attention because neurons in these areas gradually spread enhanced activity over the representation of relevant objects.