Subliminal perception has long been a hot topic. The idea that something (generally an image) could appear and disappear before us so quickly that it escapes conscious perception, and yet affect us subconsciously, is a fascinating (and scary) one. Psychologists and neuroscientists are fairly skeptical of any grand or sinister claims for the power of subliminal advertising or propaganda, but on the other hand, many of them use the technique as a research tool. So what's the absolute speed limit of the brain? What's the minimum time that a stimulus needs to appear in order to trigger a measurable brain response? In a new study, Swiss researchers Holger Sperdin and colleagues say that they've detected neural activity in response to images presented for just 250 microseconds - that's 1/4 of a millisecond, or 1/4000-th of a second. Sperdin et al. say that these ultra-brief stimuli are undetectable on a conscious level, yet still evoke a brain response - albeit a small one. Here's the paper: Submillisecond Unmasked Subliminal Visual Stimuli Evoke Electrical Brain Responses. The authors recorded brain activity using EEG and presented the brief stimuli using a device they invented called the LCD Tachistoscope.
It's built around two LCD monitors and a mirror. Sperdin et al. published the design last year. A normal screen just isn't fast enough to present stimuli at microsecond durations. (Other
tachistoscopes exist.)
While the apparatus was complex, the stimulus Sperdin et al. used was a simple checkerboard pattern of black and white squares. The researchers presented the stimuli for 250, 500 and 1000 microseconds. These are, respectively: consciously undetectable, only sometimes detectable, and easy to spot. Here's what happened. The black lines are the visual evoked EEG signal in response to the checkerboard; the red lines were a control stimulus:
Both the 500 and 1000 microsecond checkerboards clearly evoked a neural response which began roughly 80 milliseconds after the checkerboard appeared. The 250 microsecond stimulus did so too, albeit marginally. (Note that the graph's scale varies.) The bottom half of the diagram shows the statistical significance of the difference between the brain responses to the checkerboard and the control stimuli. For the 250 microsecond stimuli, only a few short periods were significant. Still, even if the effect of a 250 microsecond stimulus on the brain isn't huge, I find it pretty impressive that the 500 microsecond stimuli can evoke such marked neural responses. The brain is able to detect a flash of black and white that occupies just one two thousandth part of one second. This doesn't mean, of course, that we could process a message that only appeared for such a brief time, but still, it's rather cool.
Sperdin HF, Spierer L, Becker R, Michel CM, & Landis T (2014). Submillisecond unmasked subliminal visual stimuli evoke electrical brain responses. Human Brain Mapping PMID: 25487054
