Postattentive vision.

Much research has examined preattentive vision: visual representation prior to the arrival of attention. Most vision research concerns attended visual stimuli; very little research has considered postattentive vision. What is the visual representation of a previously attended object once attention is deployed elsewhere? The authors argue that perceptual effects of attention vanish once attention is redeployed. Experiments 1-6 were visual search studies. In standard search, participants looked for a target item among distractor items. On each trial, a new search display was presented. These tasks were compared to repeated search tasks in which the search display was not changed. On successive trials, participants searched the same display for new targets. Results showed that if search was inefficient when participants searched a display the first time, it was inefficient when the same, unchanging display was searched the second, fifth, or 350th time. Experiments 7 and 8 made a similar point with a curve tracing paradigm. The results have implications for an understanding of scene perception, change detection, and the relationship of vision to memory.

Fractionating the binding process: neuropsychological evidence distinguishing binding of form from binding of surface features.

We present neuropsychological evidence demonstrating that the binding of form elements into shapes dissociates from the binding of surface detail to shape. Data are reported from a patient with bilateral parietal lesions, GK, who manifests left-side visual extinction along with many illusory conjunctions when asked to discriminate both surface and form information about stimuli. We show that there are effects of grouping on both extinction and illusory conjunctions when the tasks require report of object shape. In contrast, illusory conjunctions involving surface and form information were unaffected by grouping based on shape. In addition, grouping was stronger when forms were presented within the same hemifield than when they appeared in different hemifields, whilst illusory conjunctions of form and colour occurred equally often within and across hemifields. These results support a two-stage account of visual binding: form elements are first bound together locally into shapes, and this is followed by a second stage of binding in which shapes are integrated with surface details. The second but not the first stage of binding is impaired in this patient.

Which end is up? Two representations of orientation in visual search.

What is the orientation of an object? A simple line has an axis of orientation. That line, turned upside-down, is indistinguishable from the original line. Thus, the possible orientations of a line range from 0 to 180 degrees. Most objects, however, have an axis and a polarity. A polar object, turned upside-down, looks upside-down. Accordingly, the orientations of a polar object range from 0 to 360 degrees. A series of visual search experiments were run to determine if preattentive processes represent orientation in a 180 or a 360 degrees framework. Results suggest that preattentive orientation is represented in 180 degrees. Experiments 1 and 4 show that search for a target rotated 90 degrees from the distractors is more efficient than search for a target rotated 180 degrees from the distractors. Experiments 2, 3, and 5 use a variety of different stimuli to demonstrate that search for targets rotated 180 degrees from distractors is inefficient.

Visual search asymmetries in motion and optic flow fields.

In visual search, items defined by a unique feature are found easily and efficiently. Search for a moving target among stationary distractors is one such efficient search. Search for a stationary target among moving distractors is markedly more difficult. In the experiments reported here, we confirm this finding and further show that searches for a stationary target within a structured flow field are more efficient than searches for stationary targets among distractors moving in random directions. The structured motion fields tested included uniform direction of motion, a radial flow field simulating observer forward motion, and a deformation flow field inconsistent with observer motion. The results using optic flow stimuli were not significantly different from the results obtained with other structured fields of distractors. The results suggest that the local properties of the flow fields rather than global optic flow properties are important for determining the efficiency of search for a stationary target.