If you can threshold you can look for the lowest index in the binary image. Here's one of many ways
binaryImage = grayImage < someThreshold;
[rows, columns] = find(binaryImage);
topRow = min(rows);
leftColumn = min(columns);
Another way
verticalProfile = mean(binaryImage, 2);
topRow = find(verticalProfile, 1, 'first');
horizontalProfile = mean(binaryImage, 1);
leftColumn = find(horizontalProfile, 1, 'first');
