The problem with our sensory world - this "blooming, buzzing confusion" of sights, sounds and smells - is that we put so much faith in it. We believe that the world we experience the world as it is, and that our sensations are an accurate summary of reality.

But that's a convenient illusion. In fact, it is the one illusion that makes every other perceptual illusion possible. Although we're convinced that we're living in an Ingres canvas - full of exquisite detail and verisimilitude - we actually inhabit a post-impressionist painting, rife with empty spaces and abstraction. It's a world so full of ambiguities that it requires constant interpretation.

I'm most interested in the practical consequences of our sensory flaws. Let's begin with this clever paper, published earlier this year in Cognition. The study was led by Lars Hall, at Lund University. It was inspired by a 2005 study, led by Petter Johansson, that showed male subjects a pair of female faces. The subjects were asked to choose the face that they found more attractive. Then, the mischievous scientists used a "card trick" to reverse the outcome of the choice. Here's where the results get a little sad: Less than 30 percent of subjects noticed that their choice had been changed. Our eyes might have preferences, but this doesn't mean our mind can remember them.

In this latest study, Hall and colleagues sought to extend this phenomenon - it's known as choice blindness - to the world of smell and taste. (The paper is called "Magic at the marketplace: Choice blindness for the taste of jam and the smell of tea".) They asked 180 consumers at a supermarket to participate in a quick little experiment. (The scientists pretended to be "independent consultants con- tracted to survey the quality of the jam and tea assortment" in the retail store.) The consumers were told to focus on the taste of the jam and the smell of the tea, and were asked to pick their preferred product when given a variety of different samples. For instance, a participant might be asked to choose between Ginger and Lime jam, or Cinnamon-Apple and Grapefruit. If they were smelling teas, then they might be given a choice between Apple Pie versus Honey, or Pernod versus Mango.

Here's where things get tricky. I'll let the scientists describe their method, in which they slyly reversed the preferences of the hapless consumers:

In a manipulated trial, the participants were presented with the two prepared jars. After tasting a spoon of jam from the first jar, or taking in the smell of the tea, they were asked to indicate how much they liked the sample on a 10-point scale from ‘not at all good’ to ‘very good’. While Experimenter 1 solicited the preference judgment, and interacted with the participants, Experimenter 2 screwed the lid back on the container that was used, and surreptitiously turned it upside down. After the participants had indicated how much they preferred the first option, they were offered the second sample, and once again rated how much they liked it. As with the first sample, Experimenter 2 covertly flipped the jar upside down while returning it to the table. Immediately after the participants completed their second rating, we then asked which alternative they preferred, and asked them to sample it a second time, and to verbally motivate [explain] why they liked this jam or tea better than the other one.

At first glance, this seems like a ridiculous experiment. It's hard to believe that, when asked to choose between Cinnamon-Apple and Grapefruit jam, I wouldn't notice the difference. Or that, after choosing Mango tea over Pernod, I would fail to realize that I was actually being given Pernod.

And yet, that's exactly what happened. According to the scientists, less than a third of participants realized at any point during the experiment that their preferences had been switched. In other words, the vast majority of consumers failed to notice any difference between their intended decision ("I really want Cinnamon-Apple jam") and the actual outcome of their decision (getting bitter grapefruit jam instead).* We spend so much time obsessing over our consumer choices - I just spent ten minutes debating the merits of Guatemalan coffee beans versus Indonesian beans - but this experiment suggests that all this analysis is an enormous waste of mental energy. I could have just gotten Sanka: My olfactory system is too stupid to notice the difference.

What's most unsettling, however, is that we are completely ignorant of how fallible our perceptions are. In this study, for instance, the consumers were convinced that it was extremely easy to distinguish between these pairs of jam and tea. They insisted that they would always be able to tell grapefruit jam and cinnamon-apple jam apart. But they were wrong, just as I'm wrong to believe that I would be able to reliably pick out the difference between all these different coffee beans. We are all blind to our own choice blindness.

*In the paper, the scientists nicely rebut a variety of counter-explanations, such as that the subjects were too polite or intimidated to say that their choices had been switched.

Image: 99 Cent Store, by Andreas Gurksy