Does spatiotemporal integration occur with single empty time intervals instead of two neighboring intervals in the visual modality?

The kappa effect is an illusion involving spatiotemporal integration in the cognitive process and is demonstrated with three successive signals delimiting two neighboring empty time intervals. The present experiment was conducted with single time intervals delimited by two signals, instead of three, to examine whether perceived duration would be modulated by space. Each of the two flashes was delivered from the left or right side in one session (horizontal direction), while each was delivered from the upper or bottom side in the other session (vertical direction). Empty time intervals were perceived as longer when two flashes were delivered from different locations than when delivered from an identical location, but only when the flashes were presented in the horizontal direction. Given that the kappa effect can occur when three signals are presented vertically, spatiotemporal integration seemed difficult to occur with single time intervals compared to two neighboring intervals.