What does the dot-probe task measure? A reverse correlation analysis of electrocortical activity

Abstract

The dot-probe task is considered a gold standard for assessing the intrinsic attentive selection of one of two lateralized visual cues, measured by the response time to a subsequent, lateralized response probe. However, this task has recently been associated with poor reliability and conflicting results. To resolve these discrepancies, we tested the underlying assumption of the dot-probe task-that fast probe responses index heightened cue selection-using an electrophysiological measure of selective attention. Specifically, we used a reverse correlation approach in combination with frequency-tagged steady-state visual potentials (ssVEPs). Twenty-one participants completed a modified dot-probe task in which each member of a pair of lateralized face cues, varying in emotional expression (angry-angry, neutral-angry, neutral-neutral), flickered at one of two frequencies (15 or 20 Hz), to evoke ssVEPs. One cue was then replaced by a response probe, and participants indicated the probe orientation (0° or 90°). We analyzed the ssVEP evoked by the cues as a function of response speed to the subsequent probe (i.e., a reverse correlation analysis). Electrophysiological measures of cue processing varied with probe hemifield location: Faster responses to left probes were associated with weak amplification of the preceding left cue, apparent only in a median split analysis. By contrast, faster responses to right probes were systematically and parametrically predicted by diminished visuocortical selection of the preceding right cue. Together, these findings highlight the poor validity of the dot-probe task, in terms of quantifying intrinsic, nondirected attentive selection irrespective of probe/cue location.

Keywords: EEG; anxiety; attention; emotion; face processing; hemispheric differences/laterality.

Figure 1

Task and Conditions. Each trial began with a 1–2 second fixation circle, followed by either a neutral or angry face on either side of the fixation circle. Four cue conditions were implemented: neutral – neutral (N-N), neutral – angry (N-A), angry – neutral (A-N), and angry- angry (A-A). In all conditions, both faces flickered for 2000 ms at different frequencies, to elicit separable SSVEP signals. Finally, a probe was presented on either the left or right side of the fixation circle, until participants responded to the orientation of the probe (either vertical or horizontal), in which case the probe disappeared from the screen. Note that these images are not drawn to scale.

Figure 2

The SSVEP Signal. Panel A depicts the grand mean (N=21) frequency spectrum across all participants and conditions, taken from a sensor just right of Oz, across the time window highlighted in grey in Panel B (.5 – 2 seconds). Panel B depicts the grand mean across time from the same sensor. Time 0 is the onset of the frequency-tagged face cues (15 and 20 Hz tags), and the 2-second mark indicates the onset of the probe.

Figure 3

Individual participants’ response speed. Each line represents the mean response speed of an individual participant across the four facial expression conditions: neutral-neutral, angry-neutral, neutral-angry, and angry-angry. No statistical differences were detected across the facial expression conditions.

Figure 4

SSVEP as Related to Response Time. Panel A depicts the time-varying SSVEP power at the tagging frequency of the right face (either 15 or 20 Hz) and the SSVEP power at the tagging frequency of the left face (either 15 or 20 Hz), during the cue presentation. The power of the tagging frequency is quantified as the mean across the contralateral sensor cluster, shown in red. The power is shown separately for trials where participants later responded either fast or slow to the subsequent probe. Panel B depicts the mean power across the scalp at the tagging frequency of the face cue, during the late cue window (1 – 2 seconds after onset of the cue). Topographies are shown separately by the tagging frequency of either the right or left face cue, for subsequent fast versus slow responses to the probe, and by whether the probe appeared on the left or the right.

Figure 5

SSVEP as Related to Response Time: Quartile analysis. The SSVEP power of the congruent cue, taken from the one second cue interval just preceding the dot-probe onset. EEG trials were split into quartiles by dot-probe response time, separately for each visual field of the probe (Left and Right), to examine the hypothesis that faster response times are preceded by larger SSVEP power, or more selective attention to the previous cue.