Academic Report on “Contralateral Delay Activity and Alpha Lateralization Reflect Retinotopic and Screen-Centered Reference Frames in Visual Memory”

Introduction

The visual system is organized in a lateralized manner, with the left and right visual fields processed by the contralateral cerebral cortex. This organization applies not only to perception but also affects cognitive processes, particularly short-term memory of visual information. In visual short-term memory (VSTM), attention focused on lateral positions mainly regulates the activity of the contralateral visual cortex. Recent studies have shown that objects in VSTM are stored in the contralateral hemisphere, with several studies indicating that neural indices of storage capacity are mainly located in the contralateral hemisphere. However, frequent eye movements cause positional changes of memorized objects. It is still unclear whether memory is maintained based on the initial retinotopic or the updated screen-centered positions after eye movements.

Source

This study was conducted by scholars Wanja A. Mössing, Svea C.Y. Schroeder, Anna Lena Biel, and Niko A. Busch, from the Institute of Psychology, Westfälische Wilhelms-Universität Münster, and the Otto Creutzfeldt Center for Behavioral Neuroscience, both in Germany. The paper was published in 2024 in “Progress in Neurobiology” as an open-access article.

Research Workflow

Overview of Research Methods

The study included two experiments aiming to test the effect of eye movements on visual short-term memory. In the experiments, participants first memorized the color of target objects and then performed lateral saccades during the retention period, causing positional changes of target objects on the retina. Electrophysiological methods were used to evaluate the contralateral delay activity (CDA) and alpha-band lateralization before and after saccades.

Experiment 1

Participants: 30 subjects (17 female, aged 19-35).

Procedure: 1. Preparation Phase: Participants initiated the experiment by fixating on a central diamond marker. 2. Encoding Display: The target display lasted for 1000 ms, showing four target objects of random colors, which participants were required to memorize. 3. Pre-Saccade Delay Period: Lasted for 1000 ms, during which participants maintained fixation. 4. Inter-Saccade Period: A lateral saccade cue line appeared within 500 ms, instructing participants to execute a quick saccade within 1000 ms. 5. Post-Saccade Delay Period: Lasted for 1000 ms, during which participants maintained fixation at the new position, with a total interval of 3000 ms. 6. Probe Stimulus: The probe object appeared at the new fixation position, and participants performed a color change localization task.

Data Collection and Analysis: EEG was used to record brain signals, processed and analyzed using MATLAB to transform waveforms and spectra. Small EEG artifacts were corrected using ICA, and differences between the left and right hemispheres were calculated to evaluate lateralization effects.

Experiment 2

Participants: 35 subjects (27 female, aged 18-27).

Additional Elements: An introduced “no memory” control condition had probe stimuli for a color discrimination task, with color differences adjusted via a staircase method to ensure 90% accuracy. An independent evaluation of visual short-term memory capacity was also conducted.

Summary of Results

Main Results of Experiment 1

1. Behavioral Performance: The accuracy rate of color change localization was 83%, unaffected by the position or direction of the memorized objects.
2. CDA Effect: Structured prior to the scan, increased contralateral negative activity. The CDA effect post-saccade displayed a more complex directionality but still maintained the initial retinotopic position.
3. Alpha-Band Lateralization: During the initial delay, alpha-band power decreased in the memory-related contralateral hemisphere, with significant effects on alpha-band lateralization pre- and post-saccade.

Results of Experiment 2

1. Behavioral Performance: Accuracy rates for localized memory objects were 76% and 75%, respectively, while the no memory condition was 87%. The color changes used for memory were significantly higher than the no memory control condition.
2. CDA Effect: Continued results from Experiment 1, with significant memory-related CDA effects both pre- and post-saccade.
3. Alpha-Band Lateralization: Post-saccade lateralization was apparent but independent of the memory condition, contrary to the expected spatial coordinates update, leaning more toward central position bias.

Conclusion and Significance

This study clarifies the reference frames for encoding in visual short-term memory, emphasizing memory retention after saccades primarily relies on retinotopic rather than spatial reference frames. Although alpha-band lateralization is usually associated with memory retention, this study suggests post-saccade alpha-band lateralization is more related to central position attention bias rather than specific memory object spatial updates.

Research Highlights

1. Continuity of Retinotopic Reference Frame: The experiment confirmed contralateral delay activity’s continued reliance on retinotopic reference frames for memorized objects pre- and post-saccade.
2. Unique Post-Saccade Alpha-Band Performance: This study first pointed out that post-saccade alpha-band lateralization might reflect attention bias toward central position rather than spatial reference frame updates.

This research provides new insights into the neural mechanisms of visual short-term memory, particularly the effect of eye movements on memory retention, offering valuable references for future research and applications.