For any perceived image, observers analyze spatial properties, but they can also simultaneously process presentation duration. The way these two aspects interact is still relatively unknown. Research on time perception has not yet clarified precisely how people judge duration (see Merchant, Harrington, & Meck
2013, for a recent discussion). Cognitive psychology has applied an information-processing framework, which proposes an internal clock as a central mechanism to estimate stimulus duration (Gibbon, Church, & Meck,
1984; Meck,
2003; Wearden,
2003). According to Scalar Expectancy Theory (Gibbon et al.,
1984) duration is processed by a pacemaker-accumulator clock in which pulses are transferred from the pacemaker to the accumulator via a switch between the two. When attention is paid to time, the switch is closed and accumulation occurs; when no attention is paid to time, the switch is open and accumulation ceases. Duration judgments are based on the number of accumulated pulses.
Two main factors are known to influence subjective duration: arousal and attention. Changes in arousal following drug administration (Meck,
1983), temperature change (Wearden & Penton-Voak,
1995), emotional stimulation (Droit-Volet, Brunot, & Niedenthal,
2004), and repetitive prestimulation (Penton-Voak, Edwards, Percival, & Wearden,
1996) have been found to alter subjective duration by changing pacemaker speed. Increases in arousal increase the rate at which pulses are emitted from the pacemaker; thus, more pulses are accumulated within the same physical time unit, and subjective time lengthens. Although many studies have attributed increases in subjective duration to increases in arousal, arousal is not precisely defined within the timing literature (Angrilli, Cherubini, Pavese, & Manfredini,
1997; Droit-Volet,
2003; Droit-Volet & Gil,
2009; Fox, Bradbury, Hampton, & Legg,
1967; Rose & Summers,
1995; Tse, Intriligator, Rivest, & Cavanagh,
2004).
It is also known that when attention is diverted away from timing, duration is underestimated (Zakay & Block,
1997). Processing other stimulus attributes (i.e., nontemporal elements of the to-be-timed stimulus) or additional stimuli (i.e., secondary tasks) diverts attention away from the internal clock. This causes the connector between the pacemaker and the accumulator to open; consequently, there is a loss of time units, and perceived duration is underestimated (Buhusi & Meck,
2006). Evidence for this comes from dual-task paradigms in which participants simultaneously complete timing and nontiming tasks (Brown,
2006,
2008; Ogden, Salominaite, Jones, Fisk, & Montgomery,
2011; Zakay,
1998) and from comparisons of the perceived duration of relatively more or less attention-grabbing stimuli (Gil, Rousset, & Droit-Volet,
2009).
The complexity of the stimulus being timed can also divert attention away from timing and thereby alter subjective duration (Cantor & Thomas,
1977; Folta-Schoofs et al.,
2014; Hogan,
1975). Hogan (
1975) used color slides of line drawings and abstract paintings. The number of each drawing's interior angles defined five levels of complexity. He applied a time interval estimation procedure in which he first presented a standard slide (moderate complexity; 15 s), followed by a test slide (more or less complex than the standard one; 15 s). Participants indicated whether the time interval of the test slide was shorter, equal to, or longer than the interval of the standard slide. The result was that stimuli that are both the least and most complex were experienced as lasting for more time than stimuli of moderate complexity.
Thomas and Cantor (
1975) found that perceived duration increased with the size of circles. Cantor and Thomas (
1977) examined both the area and the perimeter of checkerboard patterns (see
Figure 1). They found that perceived duration increased with an increase in area but decreased with an increase in perimeter. Cardaci et al. (
2006) developed a fuzzy model of complexity based on local and global spatial features extraction defined by an entropic distance function. The authors used perceived time as an indirect measure of complexity. In line with the attentional model, they found that paintings (an illustration of the stimuli used by Cardaci et al.,
2006, is provided in
Figure 1) with a high entropic complexity level generated shorter estimations of perceived time. Some years later, the same authors confirmed a relationship between the visual complexity of high-semantic heterogeneous paintings (selected from the stimuli range of the previous study) and their perceived duration (Cardaci et al.,
2009). Visual complexity for this class of stimuli was computed by objective local features algorithms, which extract information about edges and symmetries. However, the authors did not focus on subjective evaluation of complexity. In line with their previous work, they found that paintings with high complexity levels were perceived as being exposed for a shorter period of time. Similarly, Folta-Schoofs et al. (
2014) observed that high-complexity distractors, presented during the reproduction of a previously learned duration, lengthened reproductions to a greater extent than low-complexity distractors. Here complexity was defined by subjective ratings of the amount of detail or intricacy of lines in the picture. Folta-Schoofs et al. suggest that high-complexity distractors detracted attention away from timing, affecting switch operation and reducing the accumulation of pulses from the pacemaker.
In summary, arousal and attention are associated with opposite effects on perceived duration. Arousal elongates subjective duration, and visual complexity takes attention away from timing and shortens subjective duration. So far, this has been found mainly with stimuli containing a certain amount of semantic content and therefore possible memory associations or in dual tasks (i.e., judging the intensity and the duration of a stimulus). One exception is the recent work by Aaen-Stockdale, Hotchkiss, Heron, and Whitaker (
2011). The authors tested the role of spatial frequency on estimated durations using an oddball paradigm. Typically, the duration of unexpected oddball stimuli is overestimated relative to the expected or standard stimuli (Tse et al.,
2004). Aaen-Stockdale et al. showed a standard stimulus for 320 ms followed by a blank screen of variable interstimulus interval and then the oddball stimulus ranging between 260 and 380 ms at incremental steps of 20 ms. They found midrange spatial frequencies (2 c/°) of the oddball stimulus to be judged as longer in duration than high (8 c/°) or low (0.5 c/°) frequencies, and this was irrespective of oddball-related temporal expansion. This study suggests that the relationship between spatial frequency and perceived duration may not be linear and that low-level visual properties can affect perceived duration.
The question of which aspect of visual complexity modulates perceived duration is still open. In relation to the internal clock theory, an important question is whether the number of time units being accumulated depends upon the visual properties of the stimuli. This is because, even in a single-task condition, we cannot exclude the possibility that nontemporal information-processing load, resulting from the visual complexity of the stimuli, may interfere with attention to time. In principle, different amounts of visual complexity could modulate the short-term description of the information or “attentional template” (Duncan & Humphreys,
1989) and, to a certain degree, visual working memory (Baddeley,
1986). The question as to whether such an influence on attention would be enough to generate a bias in perceived duration has not been tested. Similarly, it is unclear whether nonsemantic image properties alone are sufficient to detract attention from timing. This is relevant because it would distinguish perceptual processes that have an effect on attention, at least at the point of bias temporal processing, and those that do not have it. Moreover, any study of complexity faces the problem of selecting a definition and a measure of visual complexity. The current work tried to unravel this research issue by analyzing both subjective complexity and subjective duration.