Research seems to indicate that eating excess soy product brings on brain aging and alzheimers disease. Soy is a key component in most processed foods. While Japanese and Chinese diets only include around 2 Tbsp of soy a day as a condiment, western consumers eat many times that in the processed food products typical to their diet.

Dr. Lon White of Hawaii was interested in how tofu eating affected the men in his area. He traced a group of men from 1965 to 1993. Dr. White found "a significant link between tofu consumption during midlife and loss of mental ability and even loss of brain weight."
Hawaii Research

Research into this has found the connection. First, healthy brain cells are measured by a brain-derived neurotrophic factor (BDNF). A low BDNF causes brain atrophy and is linked with Alzheimer's. In a study published in "Neuroscience Letters", they found that soy intake lowered the BDNF in rats.
More about the BDNF Study

One issue with soy and the brain is that soy contains "phytates" which block zinc absorption. While you're ingesting soy - found in most processed foods - your body is not absorbing zinc. Zinc is critical for brain and nervous system functioning. It affects perception, motor coordination, memory, reasoning, and much more.
Research on Zinc Deficiency and Brain Impairment

In research reported on brain.com, a site that talks about the brain, it turns out that every plant has its means of defense. Roses have thorns, some plants taste awful. The soy plant's defense is that it puts out an estrogen-like substance. This causes males who eat it to become less fertile, in essence lessening the likelihood that the males will create baby "soy eaters" to keep eating the plant. This estrogen-like substance interferes with how natural estrogens are received in the brain - and compromises brain efficiency.
The brain.com full report on soy

In the end, the best diet is one that is full of fish (omega oils) and healthy vegetables. If you are eating well and taking your vitamins, there should be little need for the processed foods that are full of soy and lining the supermarket shelves. Eat fresh foods :) This is the way nature intended you to eat, and is the healthiest for you!

Soy Consumption in China and Japan
Soy Products - Safe or Harmful?
Soy and Fertility Problems

Myth - Your Brain needs Carbohydrates to Function
Vitamins for Healthy Brain Functioning