The Effect of Visual Similarity on Short-term Memory for Spatial Location: Implications for the Capacity of Visual Short-term Memory

Evidence from the recency effect suggests that visual short-term memory is limited to preserving information about a single pattern. Three experiments explored the capacity of visual short-term memory using a task which involved presenting a series of patterns in a random spatio-temporal sequence and probing memory for the spatial location of one of them. Experiment 1 used sequences of quasi-random block patterns which were either visually similar or dissimilar. Serial position curves showed a single-item recency effect. However, there was also a deleterious effect of visual similarity for the earlier patterns, indicating that visual short-term memory was supporting their retention. Experiment 2 showed that performance was unaffected by articulatory suppression, confirming that phonological coding is not an important factor in the localisation task. Experiment 3 switched to letter stimuli and, because of the known tendency for this type of stimulus to be phonologically recoded, required articulatory suppression. Letters were shown in a customised form in sequences which were either visually similar or visually dissimilar. The results replicated experiment 1 in showing restricted recency combined with an effect of visual similarity extending across earlier list items. When the same letters were shown in standard form, the recency effect remained but the similarity effect disappeared, ruling out possible interpretations in terms of semantic as well as phonological coding. It is concluded that the recency effect gives a misleading indication of the capacity of visual short-term memory. The visual similarity effect suggests that it is capable of supporting the retention of several patterns.