IT'S reassuring to know that some TV viewers bring a critical eye - let's even say a cynical one - to bear on the medium. Networks tend to think of us as a collective mental sponge soaking up signals, direct and implicit, that flow through prime-time entertainment. But there is a gimlet-eyed group out there, many of them academics, who practice and teach a form of psychological self-defense - whistle-blowers of the great electronic message field.One of them is Professor Brian Stonehill, a walking antidote to the syndrome of viewer passivity. When he was in Boston not long ago, he told me something about the courses he teaches at California's Pomona College. There's one I'd be inclined to call "How To Remember What Broadcast TV Is Really All About" but actually carries a more dignified title: "The Arts of Persuasion." It constructs an early-warning system in the trusting minds of students, alerting them to the way that nearly everything happening on TV - from soap operas to news programs - can have a political and ideological effect. It's an old concern, of course, but teaching a course in it calls for an insightful and conscientious scholar like Stonehill. What gives me pause, though, is the premise: "There's absolutely no social or informational benefit built into TV," Stonehill claims. "You can't expect a commercial medium to necessarily inform us well, or to ennoble us, or enrich our lives visually or culturally. What happens on TV between commercials is just to get people to watch the commercials." One of the more sinister results, he says, is something he dubs the "ghettoization" of TV. "It's an economic hierarchy being imposed on what used to be the mass culture of television," he says. "First, you have the most expensive cable services like HBO and Cinemax and Showtime and the Movie Channel, which can afford not to have any commercials, because they get enough from subscriptions to play top-quality, first-cable-release movies.... Then you have high-quality culture services like Bravo, which also has no commercials and gets its funding from subscription. Below you have services like Arts & Entertainment which have commercials but also features you don't get on broadcast TV." Finally, he says, you get to the bottom layer: broadcast channels. "They have the most widely appealing programs," he says, "the greatest common denominator. You have the sitcoms The Cosby Show,The Simpsons,' and so forth. McDonald's and Burger King advertise on the broadcast channels; BMW, Apple, and IBM on A&E and TBS and the high-priced cable channels." The only trouble with this theory is that commercial motivation has a cow-catcher effect. The network content that results is a complex mix in which certain good ingredients can be identified among the bad. Take shows with largely black casts. Some analysts say they are being relegated to network "ghettos" in order to appeal to low-income black viewers who cannot afford cable services. Yet isn't it ironic to find the presence of blacks on the small screen being construed as "ghettoization." TV used to be considered a possible way out of demeaning stereotypes. I remember how glad I was to see the great Nat King Cole get his own music show back in the '50s, even if some southern affiliates - to their viewers' loss - did refuse to carry it. The noted black TV-film producer Topper Carew once told me he thought the old NBC-TV series "Julia" (1968-1971) achieved a lot for blacks in those sti ll-unreconstructed days, even if its black star, Diahann Carroll, did have to play a maid. At their best, the networks have been a crude ladder helping viewers out of provincial attitudes about race, gender, and the disabled. Stonehill explains all this by saying, "It takes a lot to interest us in watching the commercials" and concedes "you can see sublimely beautiful things on TV." Actually I can't think of many, but the point is that when they appear, their commercial origins don't really matter. The goal may be to get us to watch commercials, but the networks have found - perhaps in spite of themselves - that they end up offering some good stuff in the bargain.