A technician explains measurements whilst a scientist explains observations.

The Phi Phenomenon and the half second gap

The phi phenomenon occurs when two sequentially flashed lights or presented images are accompanied by a vision of movement between the two.  An example is shown below:

The moving red bar is actually a 150ms flash of a red bar on the left, a gap of 120ms with no bars shown then a 150ms flash of red bar on the right.  Muckli et al (2005), Sterzer etal (2006) and  Larsen et al (2006) have shown that the path of the motion is drawn on the visual cortex.

The "colour phi phenomenon" (see Kolers and Von Grunau 1976) occurs when the two stimuli have different colours:

The transition from dark to light blue occurs somewhere between the two extreme positions. Combining this with the discovery of Larsen et al, that the bar is drawn on the visual cortex, suggests that there will be a transition on the cortex from one colour to another. There must be some mechanism in the brain that draws the bar across the cortex and to fit the bar in the right position and at the correct rate this mechanism must only begin to draw the moving bar once the second flash has occurred. This suggests that the appearance of the bar on the cortex is delayed by the time gap from one flash to another.

The phi phenomenon disappears if the gap between the flashes is over about 500ms:

The flashing bars above are presented for 150ms with a 600ms white space gap.  This possibility of a half second gap between a physical stimulus and experience was also found to occur in the case of direct brain stimulation by Libet et al 1979.  It is also commonly observed in studies of Event Related Potentials in the EEG in which cortical activity is strongest more than 300ms post stimulus, conscious experience being related in some way to electrical events 400ms or more post stimulus.


The brain is quite clever at modelling motion. The moving bar above contains two phi phenomena and shows the colours washing into one another at intermediate positions. This particular effect is enhanced by viewing with one eye. Notice that the bar makes two excursions to the left and then two excursions to the right, it does not make false starts to left or right, again suggesting that the final position of the second flash is available before modelling begins.  If you cover the flashes at the left and right with your fingers and then expose one of them it suddenly releases a motion, again showing that the second flash must be available before modelling begins. The motion is interpolated, not extrapolated.

The half second delay is consistent with what is known about visual experience, our eyes dodge everywhere (performing "saccades") and our visual experience is a synthetic phenomenon based on this succession of short duration images. If we see a door close we tend to glance about as it happens, the brain's modelling capacity fills in the gaps to create a smoothly moving door.

The phi phenomenon is an illusion but the experience exists (see The use of the word illusion).

Primitive materialists, who only believe in nineteenth century ideas of physics and geometry, dismiss the phi effect, arguing that it is evidence that experience does not exist - how could we be "seeing" our brains?  As if something material actually had to flow between the object of sight in our experience and the centre of our experience! We need not be so constrained by faith and can accept our observations then propose a theory to account for them (see Time and conscious experience).  One thing is certain, the object of sight in our experience is some peculiar field of events in our brains and not the pixels on this screen.

There are other types of motion modelling performed by the brain, for instance the beta phenomenon:

Courtesy Wikpedia







Kolers, P., & Von Grunau, M. (1976). Shape and color in apparent motion. Vision Research. 16, 329-335.

Muckli L, Kohler A, Kriegeskorte N, Singer W. (2005). Primary visual cortex activity along the apparent-motion trace reflects illusory perception. PLoS Biol. 2005 Aug;3(8):e265. Epub 2005 Jul 19.

Larsen, A., Madsen, K.H., Lund, T.E., and Bundesen, C. (2006). Images of Illusory Motion in Primary Visual Cortex. Journal of Cognitive Neuroscience. 2006;18:1174-1180.

Libet, B., Wright, E. W., Jr., Feinstein, B., and Pearl, D.(1979). Subjective referral of the timing for a conscious sensory experience. Brain, 102, pp. 192-224.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=427530&dopt=Abstract

Sterzer, P and Haynes, JD and Rees, G (2006) Primary visual cortex activation on the path of apparent motion is mediated by feedback from hMT+/V5. NEUROIMAGE , 32 (3) 1308 - 1316. 10.1016/j.neuroimage.2006.05.029.

Eagleman, D. M. (2001). Visual Illusions and Neurobiology. Nature Reviews Neuroscience. 2(12), 920-6. http://neuro.bcm.edu/eagleman/papers/Eagleman.NatureRevNeuro.Illusions.pdf

1 comment:

  1. "The motion is interpolated, not extrapolated."

    A concise and cogent way of putting it.

    ReplyDelete