One fact about the world is that the most famous memory researchers did most of their work on verbal memory. Alan Baddeley and George Miller both come to mind — and I doubt anybody can think of more famous memory researchers in the last 50 years.
Another fact about the world is that many researchers — not necessarily Baddeley or Miller — have assumed that anything discovered using memory tests involving words should apply to other forms of memory as well. To pick unfairly on one person, Cowan notes in his masterful paper “The magical number 4 in short-term memory” that out of several related experiments, one has results that diverge from the others. Cowan attempts an explanation but basically throws up his hands. He doesn’t notice that of all the experiments discussed in that section, the divergent one was the only one to use visual rather than verbal stimuli.
Similarly, a reviewer of my paper which just came out complained that the results reported in the paper only “told us things we already knew.” As evidence, the reviewer cited a number of other papers, all of which had investigated verbal rather than visual short-term memory.
As it happens, the results in this case were very similar to what had been reported previously for verbal memory. But it could have come out differently. That was the point of doing the experiment.
Partly because of this bias in favor of verbal materials, not enough is known about visual memory, though this has been changing in recent years, thanks in part to folks like Steve Luck, George Alvarez, Yuhong Jiang, Edward Vogel and several others.
Cowan, N. (2001). The magical number 4 in short-term memory: a reconsideration of mental storage capacity. Behavioral and brain sciences, 24, 87-185.
Hartshorne, J.K. (2008). Visual working memory capacity and proactive interference. Public Library of Science One
Miller, G.A. (1956). The magical number seven, plus or minus two: some limits on our capacity for processing information. Psychological Review, 63, 81-97.