**3. Results**

### *3.1. Analysis of Title Terms*

The numbers of documents published in the last 120 years containing the keywords of interest in the title (TI = ("vividness" or "liveliness")/TI = "mental imag\*"/TI = "consciousness") are shown in Figure 1. Overall, 22,909 documents contained consciousness in the title were published, followed by 1381 for mental imag\* (which includes "mental image", "mental images", "mental imaging", etc.) and 393 documents on "vividness or liveliness". We can assume that if mentioned in the title, that the articles focus on the respective concepts. When the concepts appear in the abstract or keyword field, they might still be relevant but can be considered less central to the article overall.

**Figure 1.** Annual publication rates in Web of Science (WoS) for documents with search terms in the title.

The first publications mentioning "vividness" in the title were both published in the *Journal of Experimental Psychology* in 1929 [7] and 1932 [8]. With the exception of five other early works in in the 1940s and 1950s, the concept only starts to become relevant in the late 1960s. With some declines in 1985 and 1995, the number of publications increased overall, particularly in the mid-2000s. The number of annual publications focusing on vividness peaked in 2017 at 30 documents.

Several features of this figure deserve to be highlighted. The publication rates for each of the search terms from about 1966 onward show similar growth trends. A particular increase can be observed from the mid-2000s onwards. The increase is similar to growth rates in these same fields (Psychology/Neurosciences) for publications using cognate terms such as "mind" and "mental" (data not shown). We note, however, that there is a time lag between the times at which these concepts reached a critical threshold of relative publication rates. The terms "vividness/liveliness" only reached 10 or more publications per year (roughly 30% of the peak of 30 publications per year in 2017 after 2008) whereas "mental imag\*" reached that 30% from its peak per year in 2002 and "consciousness" reached that threshold in 1975.

Looking at the overlap between documents that mention more than one of the concepts in their titles (Figure 2), it becomes apparent that none of the documents address all three concepts together. The size of each circle is proportional to the total number of documents and the size of the intersections is proportional to their co-occurrence in the literature. There are also no documents that mention both vividness (or liveliness) and consciousness in the title. Moreover, the overlap between vividness and mental imag\* is low: 32 documents address both concepts in their titles, this corresponds to 8.1% of 393 vividness or 2.3% of mental imaging publications. The overlap between consciousness and mental imag\* is even lower at nine documents.

**Figure 2.** Venn diagram of the co-occurrence of terms in the title field (TI).

On the level of the disciplinary distribution of publications, almost half (48%) of the 393 articles with "vividness" in the title were published in Psychology journals, followed by Neuroscience and Neurology (6%), Psychiatry (5%), Business and Economics (4%) and Literature (3%). On the journal level, articles were much more evenly distributed. As shown in Figure 3, the most common publication venues were the *British Journal of Psychology* (10 documents; 2.5%), *Journal of Behavior Therapy and Experimental Psychiatry* (9; 2.3%), *International Journal of Clinical and Experimental Hypnosis* (7; 1.8%) and *Memory* (6; 1.5%).

The co-occurrence network in Figure 4 shows that in the whole network of relationships there seem to be many connections between the main vividness node and the main nodes for concepts related to imagery, memory and consciousness. However, these connections are scattered and preponderantly weak. The most frequent noun phrases (as displayed by node size) were *vividness*, *liveliness*, *<sup>e</sup>*ff*ect*, *imagery*, *imagery vividness*, *mental imagery*, *relationship*, *VVIQ*, *visual imagery*, *memory*, *individual di*ff*erence*, *emotionality*, *eye movement*, *memory vividness* and *vividness <sup>e</sup>*ff*ect*. The 1065 title terms were grouped into 67 clusters according to their co-occurrence similarity. The largest cluster (red) grouped the terms *memory* (10 document titles), *emotionality* (9), *eye movement* (9) and *memory vividness* (9). It was followed by cluster 2 (green), which contained the terms *vividness* (200), *animated image* (3), *divine power* (3), *naturalism* (3) and *roman theory* (3). Cluster 3 (blue) contained the terms *function* (6), *implication* (6) and *measure* (5). The title terms *individual di*ff*erence* (10), *moderating role* (3) and *multisensory imagery vividness* (3) were the most frequent noun phrases in cluster 4 (yellow). Cluster 5 contained *impact* (5), *study* (5), *comparison* (3), *fMRI* (3) and *visual imagery vividness* (3), while *liveliness* (44) together with *privacy* (2) was assigned to cluster 6 (turquoise).

### *3.2. Analysis of Title and Abstract Terms*

Expanding the title search to titles, abstracts and keywords (WoS topic search (TS field tag)) includes additional documents which refer to the concepts of vividness, mental imagery and consciousness, but these concepts might not be central to the respective publications. The expansion of the queries increases the number of relevant publications to 2186 for "vividness or liveliness", 5203 for "mental imag\*" and 76,920 to "consciousness". We note here that there is proportionally more work published on *vividness* and *mental imagery* than there is on either *consciousness* and *mental imagery* or *consciousness* and *vividness*.

Taken together over time, the number of publications in the intersection of the search terms that co-occur in the title, abstract or keyword fields is shown in the Venn diagram in Figure 5. As little as 15 documents (0.7% of vividness publications) mention all three concepts in the title, abstract or keyword fields. These documents are listed in Table 1 in descending order of citations received. These documents were published in 14 journals between 1999 and 2019 with seven of them published since 2016, which suggests that the combination of the three concepts is gaining traction in recent years. However, the scattering of articles across 14 journals suggests that there is no "natural habitat" for these kinds of publications.

**Figure 5.** Venn diagram of the co-occurrence of terms in the title, abstract or keyword (TS) fields.

We note also that a subset of these papers argue that the link between these concepts has been neglected over the years. In [3] for example, one of the co-authors of this article argued that a systematic account of mental imagery that integrates its cognitive, affective, neural and phenomenological aspects, including vividness and consciousness is still lacking.




**Table 1.** *Cont*.

A tree-map (Figure 6) of the top 15 most frequent disciplines, in which journal articles containing "vividness or liveliness" were published as a topic, shows that the reach of this concept far exceeds the disciplinary boundaries of Psychology. Articles containing "vividness" have been exported to fields such as Computer Science, Business, Engineering and Education. The treemap, where the size of each rectangle represents the number of articles, is based on WoS categories, which are assigned at the journal level.


**Figure 6.** Top 15 disciplines of publications containing the term "vividness" in the title, abstract or keyword fields.
