Rapid feature-driven changes in the attentional window

Abstract

Spatial attention must adjust around an object of interest in a manner that reflects the object's size on the retina as well as the proximity of distracting objects, a process often guided by nonspatial features. This study used ERPs to investigate how quickly the size of this type of "attentional window" can adjust around a fixated target object defined by its color and whether this variety of attention influences the feedforward flow of subsequent information through the visual system. The task involved attending either to a circular region at fixation or to a surrounding annulus region, depending on which region contained an attended color. The region containing the attended color varied randomly from trial to trial, so the spatial distribution of attention had to be adjusted on each trial. We measured the initial sensory ERP response elicited by an irrelevant probe stimulus that appeared in one of the two regions at different times after task display onset. This allowed us to measure the amount of time required to adjust spatial attention on the basis of the location of the task-relevant feature. We found that the probe-elicited sensory response was larger when the probe occurred within the region of the attended dots, and this effect required a delay of approximately 175 msec between the onset of the task display and the onset of the probe. Thus, the window of attention is rapidly adjusted around the point of fixation in a manner that reflects the spatial extent of a task-relevant stimulus, leading to changes in the feedforward flow of subsequent information through the visual system.

Figure 1

Natural examples of the need to adjust the size of the window of attention. The retinal image of a single poppy flower may be many degrees when viewed from a few centimeters (A) or a fraction of a degree when viewed from many meters away (B). In addition, when an observer is fixated at the center of a geranium flower (C), the appropriate attentional window will differ depending on whether the observer is trying to discriminate the flower or the surrounding leaves. Photo credits to Brian Michelsen (panel A and B) and Carly Leonard (panel C).

Figure 2

A) Examples of the task array and probe stimuli used in this paradigm. Note that black and white in the figure represent red and blue in the actual task display. On probe-present trials, either an inner or outer probe appeared behind the task array for 100 ms, with a stimulus onset asynchrony that varied between 33 and 283 ms. B) Schematic representation of the timing of the task array and probes. Note that a probe was not present on one-third of trials (e.g., the second trial in the timeline shown here). A fixation cross remained on the screen at all times.

Figure 3

A) Grand average ERP waveforms to inner probe and outer probe stimuli averaged over posterior sites. Note that these waveforms were created by subtracting the no-probe waveforms from the probe-present waveforms. Each row represents the data averaged over a pair of consecutive stimulus onset asynchronies (SOAs) between the onset of the task array and the probe. Every second pair of SOAs is shown. B) Average across all SOA bins in which the P1 effect was significant. C) Scalp voltage maps of the attention effect (trials on which the attended color was in the region of the probe minus trials on which the unattended color was in this region), averaged across the SOA bins after the P1 attention effect became significant. Note that the baseline used for component quantification purposes was −200 to 0 ms.

Figure 4

Mean P1 amplitude from 60–120 ms at posterior sites as a function of SOA bin and task-relevant region for both the inner probes (A) and outer probes (B). Asterisks indicate the point at which the difference between attention conditions became significant.