NBC appeared to be in great shape only a few months ago when it was quickly selling out its 67 spots for $3 million each for Sunday's Super Bowl between the Arizona Cardinals and the Pittsburgh Steelers.

That's no longer so with the U.S. economy in the tank.

NBC is having a tough time selling off the final few units, and as it turns out, advertisers have good reason to wonder just how effective their ads might be, airing before a nation struggling with layoffs, sinking home values and fears that it could get a lot worse.

The Super Bowl may indeed be the big television event of the year, with a huge audience and tons of media coverage over the ads that will air, but research shows that viewers don't pay as much attention to Super Bowl commercials when the economy is tanking.

The research, by Gallup & Robinson, a marketing research firm in New Jersey, finds that during down economies recall for Super Bowl commercials dips about 11 percent compared to years when the economy is seeing normal growth. During good times, recall shoots up 39 percent over average years.

“The relationship we found between these two measures turned out to be a relatively strong one,” says Scott Purvis, president of G&R, which has been tracking recall data for the past dozen Super Bowls.

“It showed that the less confidence there is in the economy, the less people tend to recall the commercials.”

There are many factors that affect recall, including the quality of the commercials, but that doesn't alter the essential findings. As Purvis notes, the Super Bowl tends to attract a similar audience each year and the commercials are usually high-quality productions, making year-to-year comparisons easy to make.

“Recall is influenced by a lot of factors but with the Super Bowl those tend to be constant,” he says. “It’s a football game, it’s on Sunday, and the audience is homogenous in terms of their psychographics.”

Just why recall dips is not hard to understand.

Viewers who are feeling economically squeezed have less reason to take notice of ads, since they're that much less likely to spend on anything that's not an essential, whether a fancy new car or a night out at Olive Garden, for that matter.

Does low recall translate directly into lower sales? Not necessarily, but it does give advertisers cause for worry, particularly when they're spending $3 million for one 30-second spot, this year's asking price.

NBC says that as of late last week it had sold more than 90 percent of its Super Bowl ads, which is typical for this time of the year, a week before the game.

It's those last ads that are hardest to unload during down times, and consultants say the network will likely end up selling some of the remaining units at discounted prices, perhaps to advertisers that have already purchased time.

Meanwhile, there’s some concern that ratings for this year’s Super Bowl may not be as strong as some sports analysts were predicting a few weeks ago.

The NFC Championship game on Fox pulled a 21.9 household rating, down 24 percent from last year. The AFC Championship game on CBS pulled a 22 rating, down 14 percent.

Still, sports consultants like Neal Pilson say ratings will be strong if the game is competitive.

Last year’s Super Bowl generated a 43.1 rating when the New York Giants beat the New England Patriots.

“The matchup is part of the rating, but the quality of the game in my view is a more important factor,” says Pilson. “A weak matchup with a very strong game going into the final minutes has the same ability to get a strong rating as a strong matchup in a blowout game.”