Using functional imaging, John-Dylan Haynes and Geraint Rees at University College London first showed that some parts of the brain involved in early visual processing are activated more than others, depending on the particular angular orientation (e.g. / or \ ) of the visual pattern being looked at. That some brain cells are organised according to sensitivity to different orientations is well-known from the direct recording of individual neurons in monkeys, but this is the first time it has been shown in humans using brain scanning.
Next, Haynes and Rees presented participants with ‘invisible’ visual patterns. The patterns were invisible because they were presented so briefly, and because each one was followed by a visual mask – a second stimulus that interferes with processing of the first. This meant that when asked, a participant couldn’t say what orientation a pattern had – they could only guess. But although a pattern was invisible to a participant, its orientation still left a signature trace of activity in their brain. This meant the researchers could discern the orientation of the pattern by observing the distribution of activity in the participant’s brain (in their early visual cortex, called ‘V1’). So the researchers could read information in a participant’s brain that was inaccessible to the participant herself. “Human V1 can represent information about the orientation of visual stimuli that cannot be used by participants to make a simple behavioural discrimination”, the authors concluded.
Haynes, J-D. & Rees, G. (2005). Predicting the orientation of invisible stimuli from activity in human primary visual cortex. Nature Neuroscience, 8, 686-691.
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