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Memory load effect in auditory–verbal short-term memory task: EEG fractal and spectral analysis

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Abstract

The objective of this preliminary study was to quantify changes in complexity of EEG using fractal dimension (FD) alongside linear methods of spectral power, event-related spectral perturbations, coherence, and source localization of EEG generators for theta (4–7 Hz), alpha (8–12 Hz), and beta (13–23 Hz) frequency bands due to a memory load effect in an auditory–verbal short-term memory (AVSTM) task for words. We examined 20 healthy individuals using the Sternberg’s paradigm with increasing memory load (three, five, and seven words). The stimuli were four-letter words. Artifact-free 5-s EEG segments during retention period were analyzed. The most significant finding was the increase in FD with the increase in memory load in temporal regions T3 and T4, and in parietal region Pz, while decrease in FD with increase in memory load was registered in frontal midline region Fz. Results point to increase in frontal midline (Fz) theta spectral power, decrease in alpha spectral power in parietal region—Pz, and increase in beta spectral power in T3 and T4 region with increase in memory load. Decrease in theta coherence within right hemisphere due to memory load was obtained. Alpha coherence increased in posterior regions with anterior decrease. Beta coherence increased in fronto–temporal regions. Source localization delineated theta activity increase in frontal midline region, alpha decrease in superior parietal region, and beta increase in superior temporal gyrus with increase in memory load. In conclusion, FD as a nonlinear measure may serve as a sensitive index for quantifying dynamical changes in EEG signals during AVSTM tasks.

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Acknowledgments

This work was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia under Projects OI 178027 and III 43011.

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Authors and Affiliations

  1. Life Activities Advancement Center, Gospodar Jovanova 35, 11 000, Belgrade, Serbia

    Miodrag Stokić & Vanja Nenadović

  2. Institute for Experimental Phonetics and Speech Pathology, Belgrade, Serbia

    Miodrag Stokić & Vanja Nenadović

  3. School of Electrical Engineering, University of Belgrade, Kralja Aleksandra 73, Belgrade, Serbia

    Dragan Milovanović

  4. College of Medicine and Health Sciences, UAE University, P. O. Box 17666, Al Ain, United Arab Emirates

    Miloš R. Ljubisavljević

  5. Biomedical Center, Torlak Institute, Vojvode Stepe 458, 11 000, Belgrade, Serbia

    Milena Čukić

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  1. Miodrag Stokić
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Correspondence to Miodrag Stokić.

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Stokić, M., Milovanović, D., Ljubisavljević, M.R. et al. Memory load effect in auditory–verbal short-term memory task: EEG fractal and spectral analysis. Exp Brain Res 233, 3023–3038 (2015). https://doi.org/10.1007/s00221-015-4372-z

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