Post by Shireen Parimoo
What's the science?
Selective attention involves attending to the appropriate, target information and ignoring distracting, irrelevant information. The dorsal attention network, which consists of the frontal eye fields (FEF) and the intraparietal sulcus (IPS), is active during tasks that require selective attention. Specifically, these regions are thought to be involved in the control of visual attention, as they bias posterior visual areas to enhance target processing and/or inhibit distractor processing. However, it is not clear whether the FEF provides unique regulatory input or whether both regions are independently involved in attentional control of distraction. This week in the Journal of Neuroscience, Lega and colleagues used transcranial magnetic stimulation (TMS) to identify the dorsal attention network regions causally involved in distractor suppression during a visual search task.
How did they do it?
Thirty right-handed young adults completed a visual search task in two separate sessions. In the task, participants saw four pairs of triangles in each quadrant of the screen and were instructed to identify the orientation of the target pair. The target was the pair of triangles with the same orientation (i.e. both pointing up or both down). On half of the trials, a pair of differently colored triangles served as the distractor, whereas on the other half of the trials, the distractor was absent and the non-target triangles were the same color as the target. In each session, participants completed six blocks of the task while TMS was applied to the IPS, FEF, and a sham stimulation region in each hemisphere. During TMS, a coil is used to non-invasively stimulate cortical brain regions by applying pulses of magnetic stimulation. The authors stimulated both hemispheres independently to determine whether the contribution of the dorsal attention network regions comes from one side of the brain more than the other. In the sham (control) condition, the TMS coil was placed between the FEF and the IPS to prevent cortical stimulation. In the task, the search array was presented for 50ms and the authors applied three 10 Hz TMS pulses 100ms after the onset of the search array, with a 100ms gap in between each pulse. After the search array disappeared, participants had two seconds to make a response for the target orientation.
The authors analyzed participants’ reaction times on correct trials using linear mixed effects models, a statistical technique that accounts for variation in the data that is not explained by the TMS conditions (e.g. each participant could respond to stimulation differently). They compared differences in reaction times following TMS to the IPS and the FEF in each hemisphere and to the sham condition. The authors also computed distractor cost as the difference in reaction times between the distractor-present and distractor-absent conditions. Finally, to examine the effect of distractors on the preceding trial, they calculated distractor cost for trials that were preceded by a distractor-absent compared to a distractor-present trial. They hypothesized that in the sham condition, distractor cost would be smaller if the previous trial contained a distractor, as participants would be relatively more prepared to suppress subsequent distraction.
What did they find?
In general, participants were slower and less accurate when a distractor was present than when it was absent. In the sham condition, participants were faster on distractor trials if the distractor was also present on the preceding trial, versus when it was absent. Reaction times did not differ when TMS was applied to the FEF, IPS, or the sham region in the left hemisphere. However, distractor costs were smaller after right FEF stimulation than after left FEF stimulation, but there was no difference in distractor costs following left and right IPS or sham stimulation, suggesting that the FEF is functionally lateralized during visual attention, at least in relation to distractor inhibition. Moreover, the reduction in distractor costs following right FEF stimulation was driven by faster reaction times on distractor-present trials, particularly when the preceding trial did not contain a distractor. When a distractor was present on the previous trial, there was no difference in distractor cost in the right FEF and sham stimulation conditions, likely reflecting the attentional preparation carried over from the previous trial. These findings indicate that the dorsal attention network – particularly the FEF but not the IPS – is right-lateralized in tasks requiring top-down control of distractor suppression.
What's the impact?
This study highlights a causal role of the right frontal eye fields, but not the intraparietal sulcus, in exerting top-down control over visual attention and distractor suppression. These findings also suggest that in addition to single-trial effects, stimulation of prefrontal regions like the FEF prolong attentional control across multiple trials, providing further insight into their role in modulating visual attention in a sustained manner.
Lega et al. Probing the neural mechanisms for distractor filtering and their history-contingent modulation by means of TMS. Journal of Neuroscience (2019). Access the original scientific publication here.