Organophosphate pesticides are the most widely used insecticides in the world; they are also (according to WHO), one of the most hazardous pesticides to vertebrate animals. While the toxic effects of high levels of organophosphates are well established, the effects of long-term low-level exposure are still controversial.
Reports on cognitive decline with age have, over the years, come out with two general findings: older adults do significantly worse than younger adults; older adults are just as good as younger adults. Part of the problem is that there are two different approaches to studying this, each with their own specific bias. You can keep testing the same group of people as they get older — the problem with this is that they get more and more practiced, which mitigates the effects of age.
A review of 35 studies published between 1991 and 2007 has found that depression does not always lead to cognitive impairment. Part of the variability in findings may be due to inconsistent measurement and diagnosis of depression. Processing speed was found to be the cognitive function most consistently affected by depression. Processing speed deficits can be helped by decreasing the amount of information to process at one time.
Amnestic mild cognitive impairment often leads to Alzheimer's disease, but what predicts aMCI? A study involving 94 older adults has revealed that lower performance on tests measuring learning, in conjunction with either slower visuomotor processing speed or depressive symptoms, predicted the development of aMCI a year later with an accuracy of 80-100%. It is worth emphasizing that poor learning alone was not predictive in that time-frame, although one learning measure was predictive of aMCI two years later.