1. Introduction
When attention is directed towards space or time, both affect the perception of being stimulated by a target [
1,
2]. Posner first used the spatial cue goal paradigm to distinguish between two types of attentional orientation: endogenous and exogenous attention [
3]. Studies show that endogenous attention to spatial location can enhance perceptual processing, as manifested by faster and more accurate responses to objects in the location being attended to [
4]. Exogenous attention is usually guided by emergent stimuli in the peripheral space. When the time interval between the cue and the target is approximately 200-300 ms, participants respond more slowly to the attended target, which is called inhibition of return (IOR) [
5], this phenomenon has been observed in detection [
6,
7] and discrimination tasks [
8,
9].
Temporal attention is the term used to describe time-based expectations of events. Several types of temporal structures, such as rhythms, can guide temporal attention [
10]. Recurrent temporal structures can enable individuals to actively prepare for anticipated events, and repetitive rhythmic stimuli are often used in studies of auditory, visual, and tactile attention channels [
11,
12,
13]. At the same time, the number of rhythmic stimuli also affects participants‘ responses. Jones adopted four and five repeated tactile cues, and found that the response was faster with five repeated tactile cues [
11]. A similar pattern was found with more repetitions of rhythmic stimuli [
14,
15]. Similar to spatial attention, temporal attention can be divided into exogenous and endogenous attention [
16]. Endogenous temporal attention can be studied using a temporal cue task employing a Posner-like cue-goal paradigm, in which temporal cues can be used to direct attention to a moment in time to predict an upcoming goal. Endogenous temporal attention can shorten reaction times and enhance the power of the alpha frequency band of the attended time [
17,
18]. Exogenous temporal attention is often triggered by stimuli in the environment, such as rhythms [
19], which can trigger exogenous temporal attention without the need to pay attention to rhythmic events [
20].
Dynamic attending theory (DAT) suggests that rhythms cause periodic fluctuations in attention, thereby regulating the gain of sensory input [
21]. However, time anticipation can occur through the passage of time, even in the absence of external stimuli. The likelihood of an event occurring may change over time (e.g., the longer one waits, the more likely it is that a traffic light will change from red to green). This increasing expectation over time is called the hazard function [
22]. The longer the time interval between the cue and the target, the faster the response, which is also known as the foreperiod effect. It is not clear how the time between the cue and target works [
23]. Jones et al. found that when presented with regular tones, the accuracy of pitch judgments followed an inverted U-shaped pattern, with participants judging the expected tones with rhythm the most accurately, and the tones presented earlier or later than the rhythm the least accurately [
24]. Herrmann et al. conducted a study using magnetoencephalography (MEG) to present participants with pitch sequences containing an intensity bias, and found that temporal attention could adjust the target detection threshold; that is, a stimulus presented in sync with the rhythm had a higher detection rate [
25]. Jones adopted rhythmic tactile stimulation and found that the later the stimuli appeared, the shorter the stimulus response time, following the previous effect [
11].
Spatial and temporal attention have been explored separately. However, studies on the relationship between the two have been inconsistent [
2,
26,
27,
28]. Studies on the relationship between temporal and spatial attention, originally observed by Doherty et al., manipulated temporal and spatial expectations by presenting a ball moving from left to right across an obscured area and found shorter reaction times when the ball reappeared behind the obscured area in sync with the previous rhythm [
29]. Similarly, shorter reaction times were found when the ball reappeared in a spatial location predicted by the trajectory of the ball. When temporal and spatial attention were matched, the reaction time was the shortest, but there was no interaction between temporal and spatial attention in terms of accuracy and reaction time. Hence, the effects were independent. Rohenkohl et al. (2014) used arrow direction to indicate the target position and arrow color to indicate the time at which the target appeared, and found that temporal and spatial attention had a strong interaction in enhancing visual discrimination (d′), indicating that the effect of temporal attention depended on spatial position [
30]. Using tactile cues, Jones (2019) found that there was no interaction between exogenous temporal and endogenous spatial attention, although there was an interaction between exogenous temporal and exogenous spatial attention [
11]. Jones (2015) used the Posner cue task to explore stimulation with visual, auditory, and cross-modal audiovisual rhythms, and found that exogenous temporal attention and endogenous spatial attention did not interact [
14]. Jones et al. found that temporal attention can improve recognition memory for spatially noticed objects [
31].
In recent years, an increasing number of researchers have focused on temporal and spatial attention; however, the relationship between the two remains controversial. In conclusion, both the quantity and type of cues affect the relationship between temporal and spatial attention. Therefore, this study aimed to explore the relationship of temporal attention induced by rhythm with endogenous and exogenous spatial attention. It consisted of two experiments using the adapted Posner spatial cue target paradigm. Notably, this study did not explicitly manipulate endogenous temporal attention. Exogenous temporal attention used in this study included both the influence of rhythm, which can automatically generate the prediction of stimulus time, and the foreperiod effect, which is automatically driven by the passage of time. Experiment 1 explored the relationship between temporal and endogenous spatial attention. The clues were indicated by arrows, and the target appeared earlier, synchronously, or later than the rhythm cues. Experiment 2 explored the relationship between temporal and exogenous spatial attention. The cues were highlighted with a peripheral box, and the same target appeared earlier, synchronously, or later than the rhythm clues.
4. Discussion
This study explored the relationship between temporal attention induced by rhythm and endogenous and exogenous spatial attention. The results showed that the interaction between rhythmic temporal attention and endogenous spatial attention was not significant, but the interaction between rhythmic temporal attention and exogenous spatial attention was significant.
Experiment 1 found that the main effect of spatial attention was significant and the reaction time under the attended condition was significantly shorter than that under the unattended condition, consistent with previous studies on endogenous spatial attention [
14,
33,
34]. The main effect of the number of repetitive rhythms was significant, and the response time with four repetitions was significantly longer than that with five repetitions, indicating that participants responded faster to more likely targets, which is consistent with relevant studies using multiple repetitive rhythm numbers [
11,
17]. The main effect of temporal attention was significant, and the longer the interval time, the shorter the reaction time, which was consistent with the foreperiod effect; that is, the longer the time between the cue and target, the faster the response. The results of this study did not find an advantage of synchronous presentation in response time, which is consistent with some studies [
35,
36], but inconsistent with others [
15,
37,
38]. The different numbers of repetitions in this study may have disrupted this rhythm, and thus, the foreperiod effect. The interaction between endogenous spatial attention and rhythmic temporal attention was not significant, which is consistent with studies on rhythm cues using visual, auditory, cross-modal audiovisual, and tactile stimuli, suggesting that endogenous spatial attention and exogenous temporal attention can function independently [
11,
14,
29,
39,
40]. Consistent with this, Coull et al. found that different neural regions were involved [
41]. However, some researchers have found an interaction between endogenous spatial and exogenous temporal attention, and this interaction is regulated by the power and phase of alpha, possibly due to the different experimental paradigms, that is, the researcher used a masking paradigm to detect threshold stimuli [
42].
No main effect of spatial attention was found in Experiment 2, and no return on inhibition was found in previous studies [
11,
43,
44,
45]. Previous studies found that spatial attention exists in the form of IOR and responds more slowly to cued targets than to uncued ones. In this study, a repetitive time structure was used, which may be a rhythmic cue that affects the IOR phenomenon. There was a significant difference in spatial attention only under the early condition; that is, there was an exogenous spatial attention IOR under the early condition, but the IOR was no longer effective under the synchronous and late conditions. The main effect of the number of repetitions was significant and was significantly longer with four than with five repetitions. The main effect of temporal attention was significant and reaction times were significantly longer in the early than synchronous and late conditions. No difference was found between the synchronous and late conditions, possibly because synchronous exogenous temporal attention was balanced by the foreperiod effect. However, the overall trend was that reaction time decreased, which partially supports the foreperiod effect. As time passes, the participants’ perception of time changes [
46,
47], and when stimuli are presented rhythmically, this effect will be weakened or even disappear. This can be explained as entrainment by the rhythm, which will further affect the behavioral response [
41,
48]. The interaction between temporal attention and the number of repeated rhythms was significant, and the overall trend was that the difference between four and five repetitions was less significant the later the time, and the rate of decline in the reaction for five repetitions was not as fast as that for four repetitions, indicating that the effect of rhythm declined in the late period. The interaction between exogenous spatial attention and exogenous temporal attention was significant, which is consistent with some studies [
11,
30,
49], which suggests that the effects of temporal attention depend on other coexisting properties, such as spatial location. The difference from Experiment 1 is that spatial attention is exogenous; while some studies have shown that endogenous attention involves more higher cortical brain regions, requires more cognitive resources, and the process is relatively slow [
50,
51,
52], exogenous attention is more rapid and automated [
53], which may be caused by task resource requirements.
In Experiments 1 and 2, the target interval time was fixed, but was consistent with the foreperiod effect, which could also indirectly prove that temporal rhythm guided exogenous temporal attention. However, both the dynamic attention theory and foreperiod effect predicted the longest early target response time, which was confirmed in Experiments 1 and 2. Neither Experiment 1 nor 2 found a U-shaped pattern predicted by dynamic attention theory. This is contradicted by some studies [
25,
38], although other studies have found the opposite result; that is, the synchronous reaction time is the longest [
54,
55]. Therefore, rhythmic stimuli influence behavior, although different task settings may influence the effects of dynamic attention theory [
56]. Rhythm not only causes exogenous temporal attention but can also be used as an endogenous time cue to accelerate target detection. When the stimulus is out of sync with the rhythm, a foreperiod effect will appear. In both Experiments 1 and 2, the main effect of the repetitive rhythm number was significant, and the response with four repetitions was significantly longer than that with five repetitions. Owing to the influence of rhythm in the task, the response with five repetitions was more conducive to the formation of a regular time rhythm by the participants, which was consistent with previous studies [
11,
14]. At the same time, there was a higher likelihood of rhythm entrainment under the condition of five repetitions; therefore, the response time was shorter. It is also possible that rhythm produces some form of habituation [
57]. Similar results have been obtained in auditory studies [
14,
58]. The interaction between temporal attention and number of repeated rhythms was significant. In Experiment 1, there was a significant difference between four and five repetitions under the early, synchronous, and late conditions, and in Experiment 2, there was a significant difference between four and five repetitions under the early and synchronous conditions, which also indicated that the role of rhythm was weakened. Experiment 2 found that spatial attention had a significant difference only under the early four repetitions, and the early stage was the time window of return of inhibition, while the intensity of exogenous and endogenous temporal attention was small; therefore, the response time was longer for the attended than for the unattended condition. Under the condition of five repetitions, exogenous temporal attention was enhanced, and synchronous, late exogenous, and endogenous temporal attention were enhanced, which could offset the IOR; therefore, the difference was not significant.
Although repetitive rhythm is not directly related to the task, it still contains information that the participants may indirectly use as a processing strategy, which can be seen from the main effect of the number of repetitive rhythms in Experiments 1 and 2. Therefore, the addition of endogenous temporal attention cannot be ruled out. Even though the separation of the two has been studied [
41], these inconsistencies in previous studies may be attributed to the addition of endogenous temporal attention. In future, attention should be paid to the distinction between endogenous and exogenous temporal attention. The experimental paradigm needs to be improved, and further research should be conducted from a neuroscience perspective. Possible reasons for the dynamic change in temporal attention, such as the difference between endogenous and exogenous temporal attention and the number of repeated rhythms or habituation, require further research.