Johan Hulleman

Color-based grouping and inhibition in visual search : evidence from a probe-detection analysis of preview search

In four experiments, we examined selection processes in visual search using a probe detection task to measure the allocation of attention. Under preview search conditions, probes were harder to detect on old relative to new distractors (Experiment 1). This cannot be attributed solely to low-level sensory factors (Experiment 2). In addition, probe detection was sensitive to color-based grouping of old distractors and to color similarity between old distractors (Experiments 3 and 4). These effects were dissociated when the color of the old distractors changed but probe detection effects remained. Collectively, the data indicate both group-based suppression of distractors and the separate inhibition of distractor features in search.

The mathematics of multiple object tracking: from proportions correct to number of objects tracked.

This paper examines the relation between proportions correct responses and the number of items tracked in multiple object tracking (MOT). It analyses two of the principle methods used in MOT. The mark all method, where the participants have to mark all the items, is shown to be equivalent to sampling without replacement. For the probe one method, where participants have to indicate whether a particular item belongs to the target set, formulas are derived as well. The paper shows that it is not possible to determine the tracked number of target items (m) and distractor items (v) from the proportions correct answers when employing only one of these two methods. A combination of the mark all and probe one methods does not yield a unique relation between the proportions correct and m and v either, because of the interchangeability between tracking targets and tracking distractors.

What is "marked" in visual marking? Evidence for effects of configuration in preview search

Visual search for a conjunction target is facilitated when distractor sets are segmented over time: the preview benefit. Watson and Humphreys (1997) suggested that this benefit involved inhibition of old items (visual marking, VM). We investigated whether the preview benefit is sensitive to the configuration of the old distractors. Old distractors changed their location prior to the occurrence of the new items, while also either changing or maintaining their configuration. Configuration changes disrupted search. The results are consistent with object-based VM, which is sensitive to the configuration of old stimuli.

Is it impossible to inhibit isoluminant items, or does it simply take longer? Evidence from preview search

Visual search can be facilitated when participants receive a preview of half the distractors (the preview benefit in search; Watson & Humphreys, 1997). Donk and Theeuwes (2001) have argued that preview-based benefits are abolished if the display items are isoluminant to a background. This is consistent with the preview benefit being due to onset capture by the new stimuli. In contrast, the present experiments challenge this suggestion and show that preview benefits can occur under isoluminant conditions, providing that they are given enough time to occur. In Experiment 1, we showed that a preview benefit can occur even with isoluminant stimuli, provided that the old items are previewed for a sufficient time. In Experiment 2, we tested and rejected the idea that this advantage is due to low-level sensory fatigue for the preview stimuli. These findings indicate that the preview effect is not caused solely by onset capture. nt]mis|This rese was supported by a British Academy postdoctoral fellowship awarded to the first author and an MRC grant to the fourth author.

A new cue to figure–ground coding: top–bottom polarity

We present evidence for a new figure-ground cue: top-bottom polarity. In an explicit reporting task, participants were more likely to interpret stimuli with a wide base and a narrow top as a figure. A similar advantage for wide-based stimuli also occurred in a visual short-term memory task, where the stimuli had ambiguous figure-ground relations. Further support comes from a figural search task. Figural search is a discrimination task in which participants are set to search for a symmetric target in a display with ambiguous figure-ground organization. We show that figural search was easier when stimuli with a top-bottom polarity were placed in an orientation where they had a wide base and a narrow top, relative to when this orientation was inverted. This polarity effect was present when participants were set to use color to parse figure from ground, and it was magnified when the participants did not have any foreknowledge of the color of the symmetric target. Taken together the results suggest that top-bottom polarity influences figure-ground assignment, with wide base stimuli being preferred as a figure. In addition, the figural search task can serve as a useful procedure to examine figure-ground assignment.

Fast Color Grouping and Slow Color Inhibition: Evidence for Distinct Temporal Windows for Separate Processes in Preview Search.

The authors report 4 experiments that examined color grouping and negative carryover effects in preview search via a probe detection task (J. J. Braithwaite, G. W. Humphreys, & J. Hodsoll, 2003). In Experiment 1, there was evidence of a negative color carryover from the preview to new items, using both search and probe detection measures. There was also a negative bias against probes on old items that carried the majority color in the preview. With a short preview duration (150 ms) carryover effects to new items were greatly reduced, but probe detection remained biased against the majority color in the old items. Experiments 2 and 4 showed that the color bias effects on old items could be reduced when these items had to be prioritized relative to being ignored. Experiment 3 tested and rejected the idea that variations in the probability of whether minority or majority colors were probed were crucial. These results show that the time course of color carryover effects can be separated from effects of early color grouping in the preview display: Color grouping is fast, and inhibitory color carryover effects are slow.

Transcranial magnetic stimulation to right parietal cortex modifies the attentional blink.

The ‘attentional blink’ (AB) reflects a limitation in the ability to identify multiple items in a stream of rapidly presented information. Repetitive transcranial magnetic stimulation (rTMS), applied to a site over the right posterior parietal cortex, reduced the magnitude of the AB to visual stimuli, whilst no effect of rTMS was found when stimulation took place at a control site. The data confirm that the posterior parietal cortex may play a critical role in temporal as well as spatial aspects of visual attention.

Revisiting preview search at isoluminance: new onsets are not necessary for the preview advantage.

It has been argued that search performance underpreview conditions relies on automatic capture by luminance onsets (Donk & Theeuwes, 2001). We present three experiments in which preview search was examined with both isoluminant and nonisoluminant items (e.g., as defined by luminance onsets). Experiment 1 provided evidence against the automatic capture of attention by onsets. Search benefited when onset previews were followed by new onset stimuli, as compared with a full-set baseline matched for the number of new onsets but in which half the distractors appeared simultaneously at isoluminance. Furthermore, both Experiments 1 and 2 established a preview advantage when isoluminant targets followed onset previews, when compared with appropriate full-set baselines. Experiment 3 replicated this result, while showing that the preview benefit was disrupted by dual-task interference. The data indicate that new onsets are not necessary to generate a preview advantage in search. We discuss the data in terms of search’s benefiting from active inhibition of old onset-defined stimuli.

Differences between searching among objects and searching among holes

We report results from six experiments in which participants had to search for a “C” among “O” distractors. The search items were either holes or objects, defined by motion, contrast, or both. Our main findings were (1) it was easier to search among objects than to search among holes, and (2) the difference between search among objects and search among holes was primarily caused by grouping with the background. The data support the hypothesis that the shape of a hole is only available indirectly. We further note that, in our experiments, search performance for both holes and objects depended on the surface medium used to define the search items.

Is there an assignment of top and bottom during symmetry perception

We report data from a discrimination task in which participants had to decide whether a single-item display was either vertically symmetric or asymmetric. This decision was found to be easier when items with a top–bottom polarity were placed in an orientation where the wide end was the base of the stimulus and the narrow end was the top. The difference in reaction times between this orientation and its inversion was about 10 ms. We suggest that top and bottom labels are assigned to stimuli during the detection of bilateral symmetry.