Abstract
Recent evidence suggests that visual attention does not operate continuously over time, but that attention rhythmically highlights task relevant locations, objects, or features, at a rate of between 4 to 12 Hz.
So far, this attentional sampling mechanism has been demonstrated with regard to external stimuli. In the present study, we investigated whether a similar mechanism might apply to the internal visual representations that guide human vision. Specifically, we were interested in whether two visual target-orientation representations are continuously maintained in working memory at a constant fidelity, or whether the representations are activated alternatingly, similar to attended stimuli in our external environment.
We used a matching-to-sample task. At the beginning of each trial, two bar-shaped orientation stimuli were presented as a template sequentially in direct succession, for 200 ms each. These template bars were oriented at 0°, 45°, 90°, or 135°, and indicated which target stimulus the participants had to respond to. Following a variable inter-stimulus interval of between 500 to 1,700 ms, a target Gabor patch was presented, and participants had to perform a speeded response if its orientation matched one of the templates. Task difficulty was continuously adjusted to produce around 66% correct responses, by changing the contrast of the Gabor.
We analysed the individual hit-rate as a function of the interval between the second template and the target, separately for targets with orientations corresponding to the first and second template. Using permutation statistics, our data exhibit significant oscillatory patterns at 9 Hz, both for targets matching the first- and the second template. This suggests that the internal representations of visual templates fluctuate over time.