how does either an increase in the overall indices affect the stock exchange prices and a decrease affects too.
If there is an effect, I think it should be small.
Foreign exchange is determined by flows of money into and out of a country, and there are many factors, including balance of payments, perceived real estate value, level of savings of the nationals, and fiscal policy of the government.
If there is any effect, then I would expect an increasing stock market index to attract foreign money into the stocks, thus increasing demand for the currency and helping to raise the level of the currency. US Fed Chairman Bernanke may be trying to do this.
Best of success.