Figure 1.
Each trial started with a fixation point lasting between 1 and 4 seconds (randomized across trials). In the audiovisual conditions (AVc, AVi) two or three sounds appeared before the visual display in order to avoid a surprise effect at the onset of the first sound. This was followed by the visual display with or without a sound (AVc and AVi or V, respectively). Participants were asked to find a horizontal or a vertical bar in the visual display while maintaining their gaze on the fixation point at all times. They were asked to answer as fast and as accurately as possible by pressing the space bar on the keyboard. One trial lasted a maximum of 10 seconds during which the participant was expected to have detected the target. After detection, participants were asked to identify the orientation of the detected target (vertical or horizontal). If the participant had not detected the target, he was nevertheless asked to make a guess. Therefore, this design allowed quantifying two dependent variables: reaction times (RTs – with a 10 sec imparted limit for the participant’s detection) and identification rate. In subsequent analysis, trials in which the target was not detected within 10 s were discarded for RTs. The experiment was run in 3 pseudo-randomized blocks corresponding to the display condition (V, AVc and AVi).
Table 1.
Summary of linear mixed regression analyses.
Figure 2.
Effect of number of distracters on RT and identification rate collapsed over all temporal rates.
Mean response times (A) and detection rates (B) per condition and per subject as a function of set size. Bars denote two SEM. A significant interaction was found between display condition and set size for RTs. The slope of the curve RTs = f(set size) was significantly lower in condition AVc than in condition AVi. The number of distracters affected the visual search less when a sound was synchronized with a target color change than in the absence of sound (V) or in desynchronized condition (AVi).
Figure 3.
Main effect of temporal rates collapsed across all set sizes on reaction times and identification rate.
Mean response times (A) and detection rates (B) per condition (V: crosses, AVc: filled circles and AVi: open circles). Bars denote are two SEM. A sound synchronized with the visual target color change fastens RTs for all temporal rates (Fig. 3a) and improves target detection only below 1.4 Hz (Fig. 3b). The level of significance between AV (AVc and AVi) and V conditions are reported as follows: *p<0.05; **p<0.01; ***p<0.001, #p<0.055.
Figure 4.
Grouped RTs and identification rates as a function of set size.
Grouped RTs (upper panels) and identification rates (lower panels) as a function of set size for temporal rates under (A) and above (B) 1.4 Hz. Bars are two SEM. In the below 1.4 Hz group, slopes in AVc are shallower than in AVi and V conditions; this suggests that visual search is less impaired by distracters in AVc. In the above 1.4 Hz group, no significant differences in slopes were found.