The researchers explain that "processing loops," in which visual
information flows from the retina to the "top" of the visual processing
string in the brain and then back down again for more processing, were
previously thought to be necessary in identifying specific scenes or
objects.
As such, they expected to observe a substantial decline in performance
around the 50-millisecond mark, the time other studies have suggested it
takes for visual information to complete such a loop.
But they found that the participants still continued to perform
better than chance when the image exposure time dropped from 80
milliseconds to 53, 40, then 27 and finally 13 milliseconds, which was
the fastest possible speed their computer monitor could manage.
"This didn't really fit with the scientific literature we were familiar
with, or with some common assumptions my colleagues and I have had for
what you can see," says Prof. Potter.
As the images were progressively shown faster, she believes the
subjects' better performance may be attributable to the practice they
had in fast detection.
'Feedforward processing' enough for brain to remember concepts
Prof. Potter says that she and her team were able to "show more evidence
of knowledge than in previous experiments where people hadn't really
been expecting to find success, and didn't look very hard for it."
The study shows evidence that "feedforward processing," which is
the one-directional flow of information from the retina through visual
processing centers in the brain, is sufficient for the brain to
recognize concepts without needing to do more feedback processing.
Additionally, the researchers add that although the images were only
seen for 13 milliseconds before the next image appeared, the results
suggest that part of the brain continues processing the images for
longer.
Prof. Potter says that in some cases, participants were not asked about
the presence of a specific image until after they had seen the whole
sequence, adding:
"If images were wiped out after 13 milliseconds, people would never be
able to respond positively after the sequence. There has to be something
in the brain that has maintained that information at least that long."
She notes that the ability to identify images seen for such a short span
of time may help the brain decide where to focus the eyes.
The team is now conducting research on how long visual information
presented for a brief period can be held in the brain, as well as
investigating - via magnetoencephalography scanners - which brain
regions are active when a person completes the identification task.
Medical News Today recently reported on a study that suggested directing ultrasound at the human brain improves sensory perception.