Within-Hemifield Competition in Early Visual Areas Limits the Ability to Track Multiple Objects with Attention

Abstract

It is much easier to divide attention across the left and right visual hemifields than within the same visual hemifield. Here we investigate whether this benefit of dividing attention across separate visual fields is evident at early cortical processing stages. We measured the steady-state visual evoked potential, an oscillatory response of the visual cortex elicited by flickering stimuli, of moving targets and distractors while human observers performed a tracking task. The amplitude of responses at the target frequencies was larger than that of the distractor frequencies when participants tracked two targets in separate hemifields, indicating that attention can modulate early visual processing when it is divided across hemifields. However, these attentional modulations disappeared when both targets were tracked within the same hemifield. These effects were not due to differences in task performance, because accuracy was matched across the tracking conditions by adjusting target speed (with control conditions ruling out effects due to speed alone). To investigate later processing stages, we examined the P3 component over central-parietal scalp sites that was elicited by the test probe at the end of the trial. The P3 amplitude was larger for probes on targets than on distractors, regardless of whether attention was divided across or within a hemifield, indicating that these higher-level processes were not constrained by visual hemifield. These results suggest that modulating early processing stages enables more efficient target tracking, and that within-hemifield competition limits the ability to modulate multiple target representations within the hemifield maps of the early visual cortex.

Sustained Splits of Attention within versus across Visual Hemifields Produce Distinct Spatial Gain Profiles

“…In accord with hypotheses as well as with earlier studies on the different-hemifield advantage using fMRI (Kraft et al, 2011) and SSVEPs (Störmer et al, 2014;Walter et al, 2014), we found a pronounced attention effect for the two to-be-attended positions versus the intermediate to-be-ignored positions only when participants divided their attention across the left and right visual hemifields. There was no evidence for this gain effect in "within-hemifield" conditions.…”

“…If, however, attended LED pairs were located in one visual hemifield, we expected a smaller difference between processing at attended and intermediate ignored positions given the greater competition for processing capacity within a single hemisphere. In addition, superior behavioral performance (Alvarez et al, 2012;Alvarez & Cavanagh, 2005;Sereno & Kosslyn, 1991) and greater gain effects for attended stimuli during "across-hemifield" conditions (Störmer et al, 2014;Walter et al, 2014) should indicate the different-hemifield advantage, thus replicating earlier findings.…”

“…However, a recent study, employing frequency tagging, showed that increased perceptual load-which in turn increases the effort necessary to deal with a task-does not affect and modulate SSVEPs (Hindi Attar & Müller, 2012). Moreover, although thoroughly matching task difficulties between "across-hemifield" and "within-hemifield" conditions, Störmer et al (2014) reported a difference in gain effects that nevertheless closely resembled ours. Finally, only event-free trials entered SSVEP analyses, which likely separated task-related from stimulus-related neural processes to some extent.…”

“…Similar results have been obtained while employing frequency tagging in a motion object tracking (MOT) paradigm (Störmer, Alvarez, & Cavanagh, 2014). MOT tasks typically require attending to one or more stimuli that traverse the visual field concurrently but on different trajectories (Atmaca et al, 2013;Alvarez & Cavanagh, 2005).…”

“…MOT tasks typically require attending to one or more stimuli that traverse the visual field concurrently but on different trajectories (Atmaca et al, 2013;Alvarez & Cavanagh, 2005). Störmer et al (2014) adapted the MOT paradigm to allow for a comparison between attentional resource allocation during the tracking of two initially cued stimuli (out of 8; two moving within each quadrant) that were positioned either within or across visual hemifields. The authors reported that attended stimuli received substantial gain effects only during "across-hemifield" conditions even after they reduced object speed in (more difficult) "withinhemifield" conditions to match task performance between conditions.…”