The syndrome of unilateral spatial neglect (USN) after right-hemisphere damage is characterized by failure of salient left-sided stimuli to activate an orienting response, attract attention, and gain access to conscious awareness. The explicit failure processing left-sided visual information is not uniform, however, and patients seem to be more successful performing certain visual tasks than others. The source of this difference is still not clear. We focus on processing of visual scene statistical properties, asking whether, in computing the average size of an array of objects, USN patients give appropriate weight to objects on the left; disregard left-side objects entirely; or assign them an intermediate, lower weight, in accord with their tendency to neglect these objects. The interest in testing this question stems from a series of studies in healthy individuals that led Chong and Treisman [Chong, S. C., & Treisman, A. Statistical processing: Computing the average size in perceptual groups. Vision Research, 45, 891–900, 2005a; Chong, S. C., & Treisman, A. Attentional spread in the statistical processing of visual displays. Perception & Psychophysics, 67, 1–13, 2005b] to propose that processing of statistical properties (like the average size of visual scene elements) is carried out in parallel, with no need for serial allocation of focal attention to the different scene elements. Our results corroborate this suggestion, showing that objects in the left (“neglected”) hemispace contribute to average size computation, despite a marked imbalance in spatial distribution of attention, which leads to a reduced weight of left-side elements in the averaging computation. This finding sheds light on the nature of the impairment in USN and on basic mechanisms underlying statistical processing in vision. We confirm that statistical processing depends mainly on spread-attention mechanisms, which are largely spared in USN.

Even when our attention is dedicated to an important task, background processes monitor the environment for significant events. The mismatch negativity (MMN) event-related potential is thought to reflect such a monitoring process. Nevertheless, there is continuing debate concerning the susceptibility of the MMN to attentional manipulation. We investigated the trial-by-trial relationship between brain activity related to change detection, reflected in the MMN, and visual psychophysical performance—while varying task difficulty. We find that auditory change detection is indeed “automatic” in that MMN remains robust despite increasing (visual) task load. However, the MMN amplitude and latency are susceptible to both visual load and to momentary attentional fluctuations as reflected in success or failure to identify a following visual target. We conclude that background central auditory processing is sensitive to the demands of a visual task, and fluctuates based on moment-to-moment allocation of attentional resources to the visual task.

What mechanism underlies serial order memory? Studying preverbal serial memory shows that macaque monkeys reproducing a sequence of items can acquire knowledge of item ordinal position. In our previous experiment, macaques were repeatedly presented with image lists (first shown sequentially and then simultaneously on a touch screen together with a distractor chosen randomly from other lists). The task was to touch list images in the correct order. The monkeys' natural tendency was to categorize images by their ordinal position or number because their most common error was touching the distractor when it had the same ordinal number (in its own list) as the correct image. Item-to-item associations were used to complete the categorization strategy. Proposing a dynamic image-salience hypothesis for serial recall (based on category-to-image influence and a salience computation for identifying touch targets), we now study the category label characteristics in the context of this hypothesis. We found that these category labels are absolute, ordinal-number-based categories (first, second, etc.), not relative memorized as relative distance from the beginning and the end of the list, and not based on fixed ranking of reward contingency/image familiarity. Even isolated from item–item associations, the categories demonstrate category tuning (as well as the corresponding overlap of adjacent ordinal number codes). Moreover, monkeys choose images by proximity of their category to the current touch number, irrespective of the accuracy of the preceding choice. Category tuning itself is symmetric relative to correct ordinal position, but is skewed by other factors (reward, etc.). Tuning width increases with list length, with a concurrent increased use of item-to-item associations for determining touch order.

We address two longstanding conflicts in the visual search and unilateral neglect literature by studying feature and conjunction search performance of neglect patients using laterally presented search arrays. The first issue relates to whether feature search is performed independently of attention, or rather requires “spread attention”. If feature search is “preattentive,” it should survive neglect. However, we find neglect effects for both feature and conjunction search, suggesting that feature search, too, has an attentional requirement. The second controversy refers to the space-or object-based nature of neglect following unilateral right-hemisphere parietal lobe damage. If neglect were a purely spatial phenomenon, then we would expect no detriment in performance in the right (nonneglect) field, and diminished performance for the whole left (neglect) field. On the other hand, if neglect were purely object-based, we would expect diminished performance on the left side of the search array, irrespective of its location in the visual field. We now demonstrate a combination of strong object-based and space-based neglect effects for conjunction search with laterally placed element arrays, suggesting that these two mechanisms work in tandem.