Fangtu T. Qiu

Figure-ground mechanisms provide structure for selective attention

Attention depends on figure-ground organization: figures draw attention, whereas shapes of the ground tend to be ignored. Recent research has revealed mechanisms for figure-ground organization in the visual cortex, but how these mechanisms relate to the attention process remains unclear. Here we show that the influences of figure-ground organization and volitional (top-down) attention converge in single neurons of area V2 in Macaca mulatta. Although we found assignment of border ownership for attended and for ignored figures, attentional modulation was stronger when the attended figure was located on the neurons preferred side of border ownership. When the border between two overlapping figures was placed in the receptive field, responses depended on the side of attention, and enhancement was generally found on the neurons preferred side of border ownership. This correlation suggests that the neural network that creates figure-ground organization also provides the interface for the top-down selection process.

Neural representation of transparent overlay.

Perceptual transparency is a surprising phenomenon in which a number of regions of different shades organize into overlaying transparent objects. We recorded single neuron responses from Macaca mulatta area V2 to a display of two bright and two dark squares that appeared as two overlaying bars. We found that neurons assign border ownership according to the transparent interpretation, representing the shapes of the bars rather than the squares.

The speed of context integration in the visual cortex

The observation of figure-ground selectivity in neurons of the visual cortex shows that these neurons can be influenced by the image context far beyond the classical receptive field. To clarify the nature of the context integration mechanism, we studied the latencies of neural edge signals, comparing the emergence of context-dependent definition of border ownership with the onset of local edge definition (contrast polarity; stereoscopic depth order). Single-neuron activity was recorded in areas V1 and V2 of Macaca mulatta under behaviorally induced fixation. Whereas local edge definition emerged immediately (<13 ms) after the edge onset response, the context-dependent signal was delayed by about 30 ms. To see if the context influence was mediated by horizontal fibers within cortex, we measured the latencies of border ownership signals for two conditions in which the relevant context information was located at different distances from the receptive field and compared the latency difference with the difference predicted from horizontal signal propagation. The prediction was based on the increase in cortical distance, computed from the mapping of the test stimuli in the cortex, and the known conduction velocities of horizontal fibers. The measured latencies increased with cortical distance, but much less than predicted by the horizontal propagation hypothesis. Probability calculations showed that an explanation of the context influence by horizontal signal propagation alone is highly unlikely, whereas mechanisms involving back projections from other extrastriate areas are plausible.