## 3 Results and Discussion
### Co-Occurrence Keywords Network  
Leading edges in scientific research over a specific period are easily predicted using the author's keywords (DE) and keywords plus (ID) [76]. Journal editors always require these at the submission stage of a manuscript for peer review and subsequent publication. Between six and eight of these words, which help us understand the major areas covered by the study, are often mandatorily provided before a successful manuscript submission [31]. Figures 6 and 7 reveal some top keywords and keywords plus used in this field of study between 2011 and 2020. The top keywords used by several authors as revealed by this study are neurotoxicity (n = 46), developmental neurotoxicity (n = 30), oxidative stress (n = 21), Parkinson's disease (n = 20), pesticides  
Figure 4: Network of collaboration among institutions from 2011 to 2020. (n = 16), lead (n = 14), neurodevelopment (n = 14), chlorpyrifos (n = 11), neurotoxicants (n = 11), and zebrafish (n = 11). Similarly, the first 10 most relevant keywords plus as provided by this bibliometric study are the following: exposure (n = 59), in vitro (n = 41), oxidative stress (n = 36), Parkinson's disease (n = 31), brain (n = 30), toxicity (n = 29), developmental neurotoxicity (n = 28), neurotoxicity (n = 28), central nervous system (n = 20), and expression (n = 20). Noticeably, six of those DE and ID in this study were generally found to be uniquely common to both groups, including neurotoxicity, developmental neurotoxicity, oxidative stress, Parkinson's disease, children, and methylmercury. It is obvious that keywords with the highest number of occurrences (n > 15) were \"neurooxicity\", \"developmental neurotoxicity\", \"oxidative stress\", \"Parkinson's disease\", and \"pesticides\". However, all the keywords plus showed occurrences that were \(\geq\)15 except \"disease\", \"methylmercury\", and \"model\" with n = 14 each. As the term \"neurooxicity\" addresses harm caused to the brain or peripheral nervous system due to human exposure to some natural or synthetic toxic materials, the above-mentioned keywords and keywords plus showed pesticides (including chlorpyrifos), lead, and methylmercury as the top neurotoxicants. In addition, the huge effects of these toxic chemicals on \"children\", especially unborn babies, are noteworthy. Lead (Pb) has been reported to have possible impairment on the intelligence quotient (IQ) of these young fellows [8]. Besides, high concentrations of polychlorinated biphenyls (PCBs) in pregnant women's blood could reduce both the weights and IQs of unborn babies, as well as trigger many other neurological conditions [2]. Hence, \"developmental neurotoxicity\", defined as the damage done to the \"developing brain and nervous system\" of living  
Figure 5: Authors’ co-citation analysis between 2011 and 2020.  
organisms by some identified neurotoxicants or neurotoxins, has attracted the attention of scientists often [77], even though the available information regarding humans is very scarce. Zebrafish, medaka, and some other fish species have been used as model animals in the developmental neurotoxicological and ecotoxicological studies of many neurotoxicants, with an emphasis on their developmental stages (early life) [3; 78; 79]. Key pesticides with neurotoxic tendencies are organophosphates, which have been found to be useful for boosting agricultural production of fibers and food [6]. The effects of these on humans include memory loss, unclear speech, sleep disorder, atax, shudders, confused mental state, paraesthesia, and schizophrenia [8]. The killing of pests using pesticides is most often achieved using neurotoxic mechanisms. Although they are commonly found in very low concentrations as complex mixtures in the aquatic environment, their neurotoxic effects on the population of many species in the system are enormous [80; 81].