Now you see it...

Unconscious perception does not exist.
12 November 2015

Interview with 

Megan Peters, University of California Los Angeles


Human observers have optimal introspective access to perceptual processes even for visually masked stimuli


Can information sneak into our brains unconsciously and influence our decisions without us being aware of it? Previous studies had speculated that there are two different thresholds: some stimuli we are immediately consciously aware of; others, might bias thinking without us knowing why. But it turns out this is just wishful thinking, as Megan Peters explained to Chris Smith...

Megan - For researchers studying consciousness and conscious perception, we want to be able to separate how well you can do on a task from your ability to consciously see things that the task is asking you to do.  The problem with this is that many times, when you make something on a computer screen so dim that you can't actually consciously see it, you also remove the ability to completely do the task at all, and so it takes a careful experimental design to create a stimulus that you cannot see but that you might still be able to do some tasks with.  Because then, we can separate your ability to do some task from the conscious experience of that task, and that allows us to do things like neuro-imaging studies or brain scans where you can say, okay well we held task performance the same, you can do the task just as well but you say you are not conscious of it.  So then we go and look for what makes something conscious or not.

Chris - There are, therefore, different thresholds.  There's a threshold at which you are consciously aware of something and a threshold at which your brain, nonetheless, knows it's there.

Megan - That's the theory, but what we are showing here with this study is that those two thresholds are actually one and the same.  In this type of context there isn't a threshold between when your brain becomes aware of something and when you consciously become aware of something, and this is very surprising and flies in the face of a large amount of research that's been done up until this point.

Chris - Very, very tricky to unpick that.  How did you manage to prise apart those two and prove they are effectively one and the same?

Megan - The previous way of doing this type of research was to show someone a stimulus on a computer screen, or have them listen to a sound stimulus and  have them do some task and then report "did you see it or not?" or "how sure are you that the answer that you just gave was correct?"  But there's a really bit problems with this, which is just because I tell you that I don't see the thing on the screen, doesn't mean I actually didn't see it all.  Maybe it was just slightly dimmer than whatever I count as a threshold for reporting that I saw it.  We wanted to get around this threshold problem by designing a task where you do two judgements to two different stimuli and then you indicate which of the two judgements you are more confident about. So this way it doesn't matter whether I am very confident overall or not very confident overall, I still have to choose one.  One to say I'm more confident about this decision than the other

Chris - Because one act's as its own control in that way, doesn't it? You're comparing one against the other so they are literally cancelling the noise in both, which is very elegant.

Megan - That's the goal, and the slightly clever thing that we did with this type of tasks is we had participants again make two judgements about two different stimuli, but one of the stimuli actually wasn't even there.  So we showed them this stimulus on the screen as a grey and white stripy patch, and it's very brief, and it's masked by other colourful images that flash before and after it, and the task is to say, are the stripes tilted to the left or the right of vertical and then we show you another patch, and we ask you to do the same task, left or right of vertical.  But this time we turn the brightness, the contrast down all the way to zero, so there's actually nothing there on the screen, and then we say, which of these two decisions are you more confident in, and if you can do the task where you report left or right, better than you would if you were just totally guessing, but then you can't tell us which one you're more confident in, then you really can't tell the difference being able to do the task and being able to just guess randomly at nothing there on the screen.  So in this way you have no introspective access to the information that contributed to your decision about that stimulus and so in this way we would say, okay if that occurs, that is unconscious perception and it turns out it doesn't exist at all.


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