Hart has come under fire in recent weeks after mistakes have started to creep into his game.
He was blamed for Birmingham's first equaliser in their 2-2 draw with City on Wednesday night which many feel effectively signalled the end of their Premier League title bid.
But, while Hart acknowledges the right of the doubters to have their say, he does not believe they are accurate.
"People are entitled to their opinions and can say what they want," said Hart, who was speaking at a `Strike A Balance' event at Eastlands to promote healthy eating among children. "But when it comes to football, I focus.
"I don't know what else there is to think about when you are in goal. What else I could be considered to be concentrating on? It is not as if there is a movie going on in the background or you are doing your homework and a fly comes into the room and you go after it. There is a ball. That is it."