Exploring Attention in Depth: Event-Related and Steady-State Visual Evoked Potentials During Attentional Shifts Between Depth Planes in a Novel Stimulation Setup

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study is very interesting and relevant as it deepens our understanding of depth perception using electrophysiological measures, specifically SSVEP (Steady-State Visual Evoked Potentials) and ERP (Event-Related Potentials). It is quite original as it explores 3D measures in a more ecologically valid way compared to the predominantly 2D-focused studies.

The research uses a 3D experimental paradigm and examines the temporal neural dynamics related to attentional shifts in 3D space. Results indicate that while SSVEPs are reliable, there are no significant modulations following attentional shift cues, confirming subjects' ability to discriminate between different depths without relying on flicker frequency.

The methodology is well-described, enhancing the study's replicability. The results are clearly presented, leading to an intriguing discussion in the context of human and animal vision models regarding the allocation of attentional resources in early visual processing tasks. The proposed hypothesis is fascinating, suggesting that the observed delayed stimulus suppression is driven by a feedback mechanism originating from cortical areas with larger receptive fields, aiding in attentional stimulus selection in the early visual cortex.

Author Response

Thank you for the well-meaning comment!

Reviewer 2 Report

Comments and Suggestions for Authors

The current study examines the effects of shifting attention to stimuli at different depth planes on both SSVEP and ERP activity. Predicted modulations in SN and P3b components of the ERP for cued vs. uncued depth planes were demonstrated. However, no significant differences were found in relative SSVEP for cued vs. uncued depth planes. The authors posit that this is because attentional processing based on depth is happening in higher levels of the visual system that do not respond in the same cyclical fashion to flickered stimuli as earlier areas (such as V1).

The manuscript is clear, thorough, and logical. I have only two minor suggestions for a final draft:

1. Figures: Some of the text in the figures is relatively small. Can all text be made to be at least as large as the main text of the paper?
2. Discussion, Lines 407-420: Are there SSVEP differences for left vs. right stimuli of the same color? What about different colors at different depth planes? If these have been tested, a brief mention should be made of the results and how they relate to your interpretation of the lack of SSVEP differences for depth planes. If they have not, they should be mentioned at least briefly as possible future directions/controls.

Author Response

1. Figures: Some of the text in the figures is relatively small. Can all text be made to be at least as large as the main text of the paper?

All figures have been updated to match the font size of the main text.

2. Discussion, Lines 407-420: Are there SSVEP differences for left vs. right stimuli of the same color? What about different colors at different depth planes? If these have been tested, a brief mention should be made of the results and how they relate to your interpretation of the lack of SSVEP differences for depth planes. If they have not, they should be mentioned at least briefly as possible future directions/controls.

Spatial selective attention has been studied before using SSVEPs, and we now added the results of those studies in the discussion. Usually, the results show only an amplification of the to-be-attended stimulus, but no suppression of the to-be-ignored stimuli. This is most likely due to spatial separation of the stimuli prohibiting biased competition effects to occur. Biased competition occurs strongest when to-be-attended and to-be-ignored stimuli are closest in space. An explanation concerning this has been added in the revised manuscript.

Different colors in different depth planes have not yet been studied to the best of our knowledge. However, if we understand the reviewer’s comment correctly, this would mean confounding two variable manipulations, namely color and stereoscopic depth. If assuming a successful attentional selection of depth planes, one would expect significantly different SSVEP amplitudes for the two flicker frequencies. One could, however, never exclude the explanation that participants selected by color rather than depth. Thus, in our opinion, this experimental design does not suffice.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript investigates the mechanism of visual attention shifts in the depth dimension. The researchers employed a novel experimental paradigm in which participants were instructed to attend to random-dot kinematograms (RDKs) presented at different stereoscopic depth planes. These RDKs flickered at different frequencies, eliciting steady-state visual evoked potentials (SSVEPs). Event-related potentials (ERPs) were analyzed to examine the effects of attention on target events, such as changes in motion coherence. However, I still have the following suggestions for this manuscript.

1.The study investigates attentional shifts in 3D space, but it would benefit from a more explicit description of why it is important to study depth-based attentional shifts.

2.Although the paper provides certain statistical analysis methods, such as Bayesian statistics, is there any potential bias in the selection of the SSVEP time window and filtering parameters? It is recommended to provide more justification for the chosen parameter settings, such as why the flicker frequencies of 15Hz and 18Hz were selected instead of other frequencies.

3.The control experiment is insightful, but further explanation of the hypothesis and expected outcomes would clarify its importance. How does this experiment tie into the larger findings of the main experiment?

Author Response

1. The study investigates attentional shifts in 3D space, but it would benefit from a more explicit description of why it is important to study depth-based attentional shifts.

An explanation has been added in the introduction. The study of this phenomenon is interesting because of ecological validity reasons and in terms of helping future VR and AR technology to improve. Furthermore, the question of which neuronal processes are associated with spatial attentional selection in depth and at which level this selection occurs, is not a trivial one at all. Despite this, in the common literature regarding visual attention, no differentiation is made between 2D attentional processes, which have been studied for a long time and extensively, and 3D processes, which have not been studied nearly as well.

2. Although the paper provides certain statistical analysis methods, such as Bayesian statistics, is there any potential bias in the selection of the SSVEP time window and filtering parameters? It is recommended to provide more justification for the chosen parameter settings, such as why the flicker frequencies of 15Hz and 18Hz were selected instead of other frequencies.

Justifications for parameter settings have been added for the chosen frequencies. In essence, the selection of flicker frequencies is arbitrary, but it is advised to steer clear of the alpha band range (8-12 Hz). The chosen frequencies have been proven useful earlier in our SSVEP studies. The SSVEP time window was chosen post-hoc, but since there is no difference between conditions at any time point during the whole post-cue time course, any other window size or placement would suffice. A justification for the Gabor filter parameter selection has been added.

3. The control experiment is insightful, but further explanation of the hypothesis and expected outcomes would clarify its importance. How does this experiment tie into the larger findings of the main experiment?

The control experiment proves that RDK flicker frequencies were not used solely or in addition to disparity to aid stimulus identification/ differentiation. This addresses the common critique that flicker frequency of course is confounded with disparity throughout the experiment and thus could, in theory, act as an additional cue. Furthermore, it also shows a high performance level and thus corroborates the behavioral findings of the main experiment. An explanation of expected outcomes has been added together with more thorough discussion of the results in the discussion part of the article.