Forwards Matej Vydra and James Wilson and winger Ikechi Anya joined during the international break.
All are available when the Rams face Newcastle United in the Championship at home on Saturday.
"We talked about the implications of missing out on opportunities to gain promotion so personalities have a big impact," Pearson told BBC Radio Derby.
"You also get players who want fresh starts so they bring positive aspects into a dressing room that has qualities already.
"The idea of bringing in new players was two-fold - to freshen up the dressing room, which is something I have been mindful of, and also to compliment what we already have."
Pearson reluctantly allowed Jeff Hendrick to join Premier League side Burnley, while Chris Martin left on a loan deal to Fulham.
"Jeff is a player I was not overly keen on losing but the player made it clear he wanted to leave," said Pearson, whose side are 18th in the table and have scored only one goal in their five league games.
"It's never easy allow players who have great potential to go but the driving force was Jeff and the fee that was agreed gives us a very good return."
Although happy with his side's business, former Leicester manager Pearson described the summer transfer window as an "absolute nonsense".
He added: "The final week becomes a bit of a circus. It's farcical that even the biggest clubs in the world get sucked in to what has become a sideshow - or bigger than a sideshow and what has now become a huge part of the industry.
"I don't see that transfer windows are healthy being in place when the season is already started."
Derby County boss Nigel Pearson says it was vital to "freshen up" the dressing room during the summer transfer window.