Controlling precedence in sequential stimulus presentation with Euler tours

It is important to control for stimulus history in experiments probing responses to and similarity between sequentially presented stimuli. We present a method for stimulus order randomisation that guarantees identical precedence across stimuli. Generating sequences through sampling Euler tours allows for perfectly uniform stimulus history. This deconfounds the stimulus history from the present stimulus and maximises sensitivity to stimulus history effects including repetition suppression.

Filled-Duration Illusions

Data relevant to the ‘filled-duration illusion’, the claim that filled intervals appear to last longer than unfilled ones of the same real duration, are reviewed. A distinction is made between divided-time studies (where an empty interval has one or more than one brief dividing stimulus inside it) and filled-duration studies (where the filled intervals are filled with some continuous event). Divided durations appear to last longer than empty ones, and the effect grows with the number of dividers, although it may be restricted to short durations. The best current explanation appears to involve the weighted summation of the different subintervals of which the total duration is composed. When intervals with simple fillers are contrasted with empty ones, they are usually judged as longer, and the effect may grow as the intervals lengthen, at least over short duration ranges. When complex fillers are used, fillers usually have no effect on perceived duration or shorten it. A pacemaker-counter approach can account for some simple filler effects, and division of attention for complex filler effects. Although there are some exceptions, ‘filled-interval illusions’ of all these types are normally found, but some problems, such as questions about the relative perceived variability of filled and unfilled intervals, or stimulus order effects, merit further research.

Encoding of Predictable and Unpredictable Stimuli by Inferior Temporal Cortical Neurons

Animals and humans learn statistical regularities that are embedded in sequences of stimuli. The neural mechanisms of such statistical learning are still poorly understood. Previous work in macaque inferior temporal (IT) cortex demonstrated suppressed spiking activity to visual images of a sequence in which the stimulus order was defined by transitional probabilities (labeled as “standard” sequence), compared with a sequence in which the stimulus order was random (“random” sequence). Here, we asked whether IT neurons encode the images of the standard sequence more accurately compared with images of the random sequence. Previous human fMRI studies in different sensory modalities also found a suppressed response to expected relative to unexpected stimuli but obtained various results regarding the effect of expectation on encoding, with one study reporting an improved classification accuracy of expected stimuli despite the reduced activation level. We employed a linear classifier to decode image identity from the spiking responses of the recorded IT neurons. We found a greater decoding accuracy for images of the standard compared with the random sequence during the early part of the stimulus presentation, but further analyses suggested that this reflected the sustained, stimulus-selective activity from the previous stimulus of the sequence, which is typical for IT neurons. However, the peak decoding accuracy was lower for the standard compared with the random sequence, in line with the reduced response to the former compared with the latter images. These data suggest that macaque IT neurons represent less accurately predictable compared with unpredictable images.