Simultaneity in the millisecond range as a requirement for effective shape recognition

Abstract

Neurons of the visual system are capable of firing with millisecond precision, and synchrony of firing may provide a mechanism for "binding" stimulus elements in the image for purposes of recognition. While the neurophysiology is suggestive, there has been relatively little behavioral work to support the proposition that synchrony contributes to object recognition. The present experiments examined this issue by briefly flashing dots that were positioned at the outer boundary of namable objects, similar to silhouettes. Display of a given dot lasted only 0.1 ms, and temporal proximity of dot pairs, and among dot pairs, was varied as subjects were asked to name each object. In Exp 1, where the display of dots pairs was essentially simultaneous (0.2 ms to show both), there was a linear decline in recognition of the shapes as the interval between pairs increased from 0 ms to 6 ms. Compared with performance at 0 ms of delay, even the 2 ms interval between pairs produced a significant decrease in recognition. In Exp 2 the interval between pairs was constant at 3 ms, and the interval between pair members was varied. Here also a linear decline was observed as the interval between pair members increased from 0 ms to 1.5 ms, with the difference between 0 ms and 0.5 ms being significant. Thus minimal transient discrete cues can be integrated for purposes of shape recognition to the extent that they are synchronously displayed, and coincidence in the millisecond and even submillisecond range is needed for effective encoding of image data.

Figure 1

The average shape displayed 57 dots, this being every 4th dot from the full inventory of dots in the boundary. The full inventory of boundary dots for a shape that matches this average is shown in panel A. At B the method for choosing the display set is illustrated. To select this set for a given subject, a random dot was chosen as the starting point, here indicated by an arrow. Then, counting clockwise, every Nth dot was marked for inclusion in the set of dots to be displayed (every fourth dot for this example). The full complement of dots that would be shown, i.e., the display set, is provided in panel C.

Figure 2

Adjacent dots from the display set were formed into pairs. The members of a given pair were shown sequentially, but the order of pair presentation was chosen at random. The left panels show the successive display of four pairs from the display set, and this sampling would continue until all pairs were shown. The right panels illustrate that the pairs would be seen as brief flashes of light at the specified positions within the array of LEDs. Dot size is not to scale for purposes of illustration.

Figure 3

In Exp. 1, each dot of the display set was flashed for a duration of 0.1 ms, and there was no temporal separation between members of each pair. The temporal separation of pairs was varied from 0 to 8 ms. The time line has been expanded for the illustration of Exp. 2, most of it being used to illustrate the alternative intervals at which the second member of a given pair would be positioned. In this example, the pair formed by dots 33 and 34 are separated for an interval that varied between 0 and 1.5 ms, and the spacing between one pair and the successive pair (dots 71 and 72 in this illustration) was a constant 3 ms for all pairs in the display set.

Figure 4

In Exp. 1, with 8 subjects and 150 shapes, each dot pair was presented within a 0.2 ms interval, and the T3 interval between pairs was varied. The hit rate declined across the 8 ms range for T3, and relative to hit rates at T3 = 0, the decline in recognition was significant even with a T3 of 2 ms. The right scale shows the logit values that were the basis for statistical analysis, and the error bars (+/- SEMs) should be interpreted against this scale. The ordinate on the left shows the corresponding levels of percent recognition.

Figure 5

For Exp. 2 (14 subjects, 150 shapes), dot pairs were separated by a constant T3 interval of 3 ms, and T2 – the interval between members of each pair – was varied. Hit rates declined significantly with as little as 0.5 ms of separation between the pair members, and the decline in recognition was linear across the T2 intervals that were tested.