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However, inconsistencies in the definition and implementation of ALFF have led to confusion in the field. In this study, we provide a mathematical clarification of ALFF and fALFF by introducing two variants: the arithmetic mean\u2010defined ALFF\/fALFF (amALFF\/amfALFF) and the quadratic mean\u2010defined ALFF\/fALFF (qmALFF\/qmfALFF). We examine the relationships between mean BOLD intensity (MBI), amALFF, and qmALFF across both subjects and voxels using two independent datasets mapped onto different brain templates. Additionally, we investigate the impact of\n                    <jats:italic>z<\/jats:italic>\n                    \u2010scoring the original BOLD signal on ALFF and fALFF metrics. Finally, we evaluate the validity and test\u2013retest reliability of (f)ALFF using a dataset with two runs at voxel, parcellation, and cortical level. Our key findings include: (1) MBI is positively correlated with both amALFF and qmALFF, highlighting the need for normalization to subject\u2010level means; (2) normalized qmALFF and qmfALFF are highly correlated with normalized amALFF and amfALFF, respectively, at both the subject and voxel levels; (3)\n                    <jats:italic>z<\/jats:italic>\n                    \u2010scoring the BOLD signal does not affect amfALFF or qmfALFF, but it substantially alters amALFF and qmALFF; (4) ALFF exhibits higher reliability than fALFF and both perform best at the parcellation level compared to voxel and cortical (subject) levels. Based on these findings, we present a comprehensive flowchart of the (f)ALFF algorithm implemented in the temporal domain. The full procedure is implemented in R, and the corresponding script is available at:\n                    <jats:ext-link xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" xlink:href=\"https:\/\/github.com\/lejianhuang\/ALFF\">https:\/\/github.com\/lejianhuang\/ALFF<\/jats:ext-link>\n                    .\n                  <\/jats:p>","DOI":"10.1002\/hbm.70506","type":"journal-article","created":{"date-parts":[[2026,3,23]],"date-time":"2026-03-23T08:46:56Z","timestamp":1774255616000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Revisiting Amplitude of Low\u2010Frequency Fluctuations (\n                    <scp>ALFF<\/scp>\n                    ) in Resting\u2010State\n                    <scp>fMRI<\/scp>\n                    : Clarifications and Improvements"],"prefix":"10.1002","volume":"47","author":[{"given":"Lejian","family":"Huang","sequence":"first","affiliation":[{"name":"Department of Neuroscience, Feinberg School of Medicine Northwestern University  Chicago Illinois USA"},{"name":"Center for Translational Pain Research, Feinberg School of Medicine, Northwestern University  Chicago Illinois USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Rami","family":"Jabakhanji","sequence":"additional","affiliation":[{"name":"Department of Neuroscience, Feinberg School of Medicine Northwestern University  Chicago Illinois USA"},{"name":"Center for Translational Pain Research, Feinberg School of Medicine, Northwestern University  Chicago Illinois USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Andrew D.","family":"Vigotsky","sequence":"additional","affiliation":[{"name":"Departments of Biomedical Engineering and Statistics and Data Science Northwestern University  Evanston Illinois USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Paulo","family":"Branco","sequence":"additional","affiliation":[{"name":"Center for Translational Pain Research, Feinberg School of Medicine, Northwestern University  Chicago Illinois USA"},{"name":"Department of Anesthesiology, Feinberg School of Medicine Northwestern University  Chicago Illinois USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Marwan N.","family":"Baliki","sequence":"additional","affiliation":[{"name":"Center for Translational Pain Research, Feinberg School of Medicine, Northwestern University  Chicago Illinois USA"},{"name":"Department of Physical Medicine and Rehabilitation Northwestern University  Chicago Illinois USA"},{"name":"Shirley Ryan AbilityLab  Chicago Illinois USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"A. Vania","family":"Apkarian","sequence":"additional","affiliation":[{"name":"Department of Neuroscience, Feinberg School of Medicine Northwestern University  Chicago Illinois USA"},{"name":"Center for Translational Pain Research, Feinberg School of Medicine, Northwestern University  Chicago Illinois USA"},{"name":"Department of Anesthesiology, Feinberg School of Medicine Northwestern University  Chicago Illinois USA"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"311","published-online":{"date-parts":[[2026,3,23]]},"reference":[{"key":"e_1_2_12_2_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.media.2007.06.004"},{"key":"e_1_2_12_3_1","doi-asserted-by":"publisher","DOI":"10.2466\/pr0.1966.19.1.3"},{"key":"e_1_2_12_4_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.neuroimage.2007.04.042"},{"key":"e_1_2_12_5_1","doi-asserted-by":"publisher","DOI":"10.1002\/mrm.1910340409"},{"key":"e_1_2_12_6_1","doi-asserted-by":"publisher","DOI":"10.1007\/s00406-024-01838-4"},{"key":"e_1_2_12_7_1","doi-asserted-by":"publisher","DOI":"10.1109\/BIBM66473.2025.11357007"},{"issue":"7","key":"e_1_2_12_8_1","first-page":"1326","article-title":"Frequencies Contributing to Functional Connectivity in the Cerebral Cortex in \u201cResting\u2010State\u201d Data","volume":"22","author":"Cordes D.","year":"2001","journal-title":"AJNR. 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