Summary: Science, 286:2231, 1999.
Filtering Reveals Form in Temporally Structured Displays
In a recent report, Lee and Blake (1) asked whether the visual system could use temporal microstructure to bind
image regions into unified objects, as has been proposed in some neural models (2). They presented two regions of
dynamic texture. The elements of the target region changed in synchrony according to a random sequence, while
the elements of the background region changed at independent times. The stimulus was designed in an attempt
to remove all classical form-giving cues such as luminance, contrast, or motion, so that timing itself would provide
the only cue. Subjects were readily able to distinguish the shape of the target region. Lee and Blake posited the
existence of new visual mechanisms "exquisitely sensitive to the rich temporal structure contained in these high-
order stochastic events." The results have already generated much excitement (3).
We believe the effects can be explained with well-known mechanisms. The filtering properties of early vision can
convert the task into a simple static or dynamic texture discrimination problem. A sustained cell (temporal lowpass)
will emphasize static texture through the mechanisms of visual persistence; a transient cell (temporal bandpass) will
emphasize texture that is flickering or moving.
We simulated a lowpass mechanism to see what would emerge. Lee and Blake's stimuli were composed of randomly
oriented Gabor elements, where the Gabor phase shifted forward or backward on each frame according to a coin-flip.
We downloaded one such movie from their web site and ran it through a temporal lowpass filter (4). An input frame
is shown in Figure 1(a). A filtered output frame is shown in Figure 1(b). At the particular moment shown, the target
region has a lower effective contrast than the background, providing a strong form cue. At other momentsthe target's
contrast may be above or below the background's contrast, due to statistical fluctuations in the reversal sequences. If