The stimulus at any point in the visual field is rarely static during normal viewing: observer and object movement conspire to produce a continually changing series of stimuli. Our aim was to study both the short- and long-term interactions between responses to a series of stimuli presented at a single visual location. We used rapid serial visual presentation (RSVP) in which the stimuli were randomly oriented gratings delivered at the rate of 30 per second. Human subjects pressed a key whenever they saw a target orientation, for example horizontal. The results were analyzed by finding two orientations before each key-press. The first preceded the key-press by the reaction time, and the second preceded the first by an interval of variable duration. There were two main findings. First, the subject was more likely to press the key when the target was immediately preceded by a grating of similar orientation. This facilitation presumably results from the summation of sub-threshold inputs. Second, a key-press was reduced in probability when a target orientation was preceded by a similar orientation with an interstimulus interval of 100–400 ms. The time course of this suppression is similar to that seen in attentional blink experiments.

^{2}). Left and right eye stimuli were presented on the left and right sides, respectively, of the monitor screen and viewed through a mirror stereoscope. Each half of the screen showed a black fusion circle with an inner diameter of 3° and a width of 0.25°, and subjects adjusted the stereoscope to fuse the two circles. Each fusion circle contained a sinusoidal grating with a spatial frequency of 2 cycles/deg and a contrast of 1. Contrast was calculated by subtracting background luminance from peak luminance and dividing the difference by background luminance. Each grating had one of 10 orientations spaced 18° apart and one of four spatial phases spaced a quarter of a cycle apart. There were therefore 40 possible gratings. Each monocular stimulus consisted of a series of gratings, as shown in Figure 1, with a new grating chosen every 33.3 ms (2 video frames). The choice was made from the 40 available gratings with equal probability. Each experimental run lasted 60 s. On any run the series for one eye was independent of that chosen for the other eye, and both series were independent of those presented in other runs.

_{ i}is the number of key-presses preceded by orientation

*i*. Chi-square is shown on the vertical axis versus time prior to a key-press on the horizontal axis, for the five subjects in this experiment. The resulting curves peak at differing times across subjects, but the peaks are clustered around 400 ms. Figure 2C shows the same data as in Figure 2B but at an expanded vertical scale in order to show the significance level for chi-square (9 degrees of freedom,

*α*= 0.05) obtained with a goodness-of-fit test. This graph therefore shows the range of times prior to a key-press at which the probability densities differed significantly from flatness.

*producing*a key-press.

*θ*

_{1}, is preceded by another,

*θ*

_{2}. To demonstrate the effect of

*θ*

_{2}on the response to

*θ*

_{1}we plot in Figure 3A the probability that a key-press is preceded by

*θ*

_{1}and, immediately before that,

*θ*

_{2}. The interstimulus interval here is 33 ms, orientation

*θ*

_{1}is given on the horizontal axis and

*θ*

_{2}on the vertical axis. The gray level at each point codes the probability,

*p*

_{obs}(

*θ*

_{1},

*θ*

_{2}), that a key-press is preceded by the combination (

*θ*

_{1},

*θ*

_{2}), and the scale at the right of the plot shows the relationship between probability and gray level.

*θ*

_{1}and

*θ*

_{2}contribute to a key-press independently of each other. This density was calculated from the observed probabilities in three steps. First, probabilities were summed across

*θ*

_{2}to find the marginal density for

*θ*

_{1},

*p*

_{1}(

*θ*

_{1}). Second, probabilities were summed across

*θ*

_{1}to give the other marginal density,

*p*

_{2}(

*θ*

_{2}). Third, the two marginal densities were multiplied to find the independence model:

*θ*

_{1}and

*θ*

_{2}in producing a key-press:

*θ*

_{2}facilitates

*θ*

_{1}in producing a key-press. Elsewhere in the plot the probability differences are negative, producing dark areas. This means that

*θ*

_{2}has a suppressive effect, making it less likely that

*θ*

_{1}will evoke a key-press.

*θ*

_{1}was presented to one eye and

*θ*

_{2}to the other eye. Again there are two cases (

*θ*

_{1}to the left or right eye), and the mean is shown. The interstimulus interval, the time by which

*θ*

_{2}preceded

*θ*

_{1}, is shown to the right of each row. There is no intraocular plot for an interstimulus interval of 0 because only one orientation was presented to each eye at any given time.

*t*is time,

*b*is the pattern of interactions at the briefest interstimulus interval (33 ms for the intraocular case and 0 ms for the interocular case), and

*l*is the pattern of interactions averaged over the longer interstimulus intervals (the bottom pattern in Figure 4). This analysis has an advantage over that in parts A and B of the figure: it uses all the data in the interaction plots, not just those data within a contour. The model in Equation 4 was fitted to the observations using least-squares regression, and the resulting time courses,

*f*(

*t*) and

*s*(

*t*), are shown in Figure 5C labeled

*Facilitation*and

*Suppression,*respectively. For both the intraocular and interocular cases, model-fitting reveals a rapid decline in the facilitatory component and a suppressive component that takes 100–200 ms to reach its trough, followed by a long decay period. A one-tailed

*t*-test on the unsmoothed data showed that the suppression differed significantly from zero at the 5% level for interstimulus intervals of 100–400 ms for the intraocular case, and 67–400 ms for the interocular. The General discussion section takes up the mechanisms that may underlie these time courses.

*t*-test showed that the thresholds at the longest two intervals did not differ significantly at the 5% level, indicating that they together represented the asymptotic level expected in the absence of masking. There are two clear differences between the intraocular and interocular data. The first is that intraocular masking is of greater magnitude, as can be seen from the differing vertical scales in Figure 7A. Second, Figure 7B shows that the time course differs. While intraocular masking descends smoothly from its maximum at short interstimulus interval, interocular masking appears to occur in two steps with a plateau phase between them. A one-tailed

*t*-test showed that the threshold elevation was significant at the 5% level for interstimulus intervals up to 50 ms in the intraocular case but not at 100 ms. For the interocular case the threshold elevation at 100 ms was also significant.

*b*stands for

*brief,*and for the suppression it was a gamma density

*l*stands for

*long-term, t*is the interstimulus interval,

*τ*

_{ b}and

*τ*

_{ l}are time constants, and

*k*

_{1}and

*k*

_{2}are constants.

*k*s may differ from those used in Equations 5 and 6. The result of fitting this model to the mean over subjects in Experiment 2 is shown in Figure 8B. The goodness of fit, while not perfect, suggests that the model is a useful characterization of the measured time courses. Time constants are provided in Table 1. As expected from electrophysiological experiments (Shapley & Enroth-Cugell, 1984), the time constants obtained with the strong stimuli used in Experiment 2 tend to be lower than those obtained with the weak stimuli of Experiment 1.

Experiment | Stimulation | Time constant (ms) | |
---|---|---|---|

Brief process, τ _{ b} | Long-term process, τ _{ l} | ||

Rapid serial visual presentation | Intraocular | 25.9 | 134 |

Interocular | 32.8 | 118 | |

Forward masking | Intraocular | 30.5 | Not used |

Interocular | 15.6 | 47.5 |