The Attentional Doughnut

Many think of visual attention as something like a spotlight moving across the visual scene: whatever it illuminates enters conscious awareness, and everything else is confined to preattentive limbo. This seemingly reasonable metaphor takes for granted that the areas to which we attend are roughly circular, contiguous, and, well, spotlight-light like, just like the receptive fields of neurons in visual cortex. Hence the intense surprise at the recent discovery that attention's "spotlight" shape may actually deform into a "donut" shape under the right conditions.

Muller and Hubner presented a steady stream of small flickering uppercase letters embedded in the center of a stream of large, uppercase letters, and measured an ERP component known as the steady-state visual evoked potential (SSVEP) which is known to be sensitive to changes in flicker frequency. Subjects were first told to monitor the large letters, and after a given amount of time, to monitor the smaller letters in the center of the larger ones. SSVEP amplitude is known to be larger for attended items than for unattended ones, and the waveform itself is nearly sinusoidal in response to different stimulus flicker rates.

If attention is shaped like a spotlight, viewers will actually be unwillingly attending to the smaller letters when they're supposed to be attending to the large letters; therefore, one would not expect SSVEP magnitude to change significantly when subjects attend to the small letters. However, the researchers found SSVEP amplitude increasing by almost 100% when subjects changed the location of their attention, suggesting that they were actually able to ignore the distractor letters in the middle, while attending to regions surrounding it!

Several control conditions ensure that this effect is not due simply to different responses in flicker rate (flicker rate was counterbalanced for large and small items), less attention being paid to the large items (subjects were asked to detect the target letter H in the to-be-attended stream), crosstalk between flicker frequencies (a complex demodulation process was used on the EEG waveforms, along with a low-pass filter at 2 Hz), attentional selection by spatial frequency (a global spatial frequency filter would not have differentially selected each stream, since their spatial frequencies are not mutually exclusive to one another), or gradient allocation of attentional resources within a beamlike area (there was no measurable target P300 for the ignored small letter stream). The researchers also claim that an object-based system of attention cannot account for their results because of the origin of the SSVEP signal (early visual areas) which would not be predicted if subjects were selecting on the basis of object identity, and that this pattern of results is consistent with several other imaging studies.

Other researchers, however, have criticized some of the methodology in this study. For example, Catena, Castillo, Fuentes & Milliken point out that they did not control whether subjects focused on the display or at a point before the display in space, which could have important consequences for how the image was displayed on the retina. Replications of this study seem to suggest that people are indeed intentionally blurring the image by fixating at a different place in depth, which means that the simplest explanation for the data is that attention is not shaped like a doughnut, but rather that subjects are using flexible strategies which make it appear so. Still, other studies have reported similar findings in cases where intentionally blurring one's vision would not seem to help.

What do these and similar findings mean for models of attention? The results would seem to suggest that attention does not necessarily occupy contiguous regions; but there is no definitive answer to whether the early visual effects (such as SSVEP) are actually caused by annular or ringlike attention, or are merely the result of activation by top-down object identity/feature processing.