U.S. stocks held slight gains at midday on Friday as investors looked toward a speech from Federal Reserve Chair Janet Yellen this afternoon. It's unlikely she will say anything explicit about interest rates, though investors will be on the lookout for hints. Consumer sentiment rebounded in May from April, though worries over the presidential election overshadowed improving confidence in the economy. The University of Michigan reading was slightly weaker than expected. FEI (FEIC) surged after Thermo Fisher (TMO) - Get Report agreed to purchase the microscopy tech developer in an all-cash deal worth $4.2 billion. The acquisition is expected to close early 2017. Palo Alto Networks (PANW) - Get Report slumped after forecasting slower growth in its fourth quarter. TheStreet's Keris Lahiff reports from Wall Street.