Visual Short-Term Memory Tasks & How They Can be Used to Detect Early Alzheimer's Disease

  The visual system perceives a high amount of detail in our environment in a flawless execution. Our behavior is consistently guided by the cues and information retrieved from our visual system. The visual information we receive from our environment is temporarily stored in a buffer known as visual short-term memory (VSTM). VSTM has long been thought of as being a limited-capacity system. It can only hold a limited amount of information in a state in which it can be extracted and manipulated. The capacity limitations of VSTM are often studied using a change detection paradigm. In this method, a memory array consisting of a small set of objects is briefly presented before a second long blank retention interval. A test array is then presented and participants must decide whether the test and the memory arrays are identical or not. These tests and memory arrays have provided us with the knowledge that the estimated capacity of VSTM is about three or four items. 

Recent work by Sligte et al., however, has debunked (or attempted to debunk) the belief of a limited-capacity VSTM, reporting that after early removal of a memory array, a presented cue allowed participants to access a “fragile, high-capacity stage of VSTM that is distinct from iconic memory.” Iconic memory is described as the persistence of information following a visual event that is characterized by high-capacity storage and rapid decay. Only a very small fraction of the information stored in iconic memory can be consolidated into VSTM. This finding intrigued Michi Matsukura and Andrew Hollingworth as they sought to determine whether or not Sligte et al.’s claim was true. They performed four experiments; the first three consisted of replications of Sligte et al.’s method and the last was an examination of the possibility of practice influencing the results of Sligte et al.. Matsukura et al. found that although there was a significant increase in performance with a valid cue as compared to a neutral one (Sligte et al. used all valid cues), the valid cue conditions of the three orientation experiments still yielded a capacity estimate that varied from 3.2 to 4.1 items; the exact range for VSTM capacity. They were unable to replicate the capacity estimate of 5.5 items that Sligte et al. achieved. No evidence was found that supported the hypothesis that separating orientations into two different perceptual groups would improve the change-detection accuracy. It was determined that grouping by orientation did not contribute to the extremely high capacity estimates of 5-7 items made by Sligte et al. Matsukura et al. concluded that their results failed to ratify the Sligte et al. “discovery” of an early, high capacity stage of VSTM. It was concluded that Sligte et al.’s 16-item capacity observed in their Experiment 1 was due to the fact that individual items were arranged in a way in which they formed larger figural groups. Overall, to this date, there is no evidence of an early-stage VSTM that is characterized by high-capacity. 

Outside of the laboratory, VSTM and the change detection paradigm are used as clinical tools to examine the performance of typically affected brain areas in patients developing neurodegenerative memory disorders, such as dementia. Clinicians and researchers utilize feature binding tasks and the brain’s ability to bind object features together during a VSTM task. To understand this, we must understand that the human visual system is organized in a special way in which different features, such as color, shape, name, and location, of an object are processed in different areas of the brain. Mechanisms in the brain bind an object’s visual and location features together to create a unified perception of a unified object. These mechanisms are important in our everyday lives, allowing us to remember the location and visual properties of objects around us. Our ability to do this during a VSTM task is thought to be within the hippocampus and surrounding brain regions. These structures deteriorate with age and are affected early in the progression of Alzheimer’s disease (AD). Mild cognitive impairment (MCI) is a condition that is considered the preclinical stage that differentiates between the deterioration of cognitive function that is associated with normal aging, and that which is associated with dementia. These individuals complain of memory problems that both they and those around them notice. Patients with MCI are at a higher risk of developing dementia of AD, and therefore researchers Sapkota et al. sought to determine whether people with MCI would show changes in VSTM unimodal binding and crossmodal binding. VSTM unimodal binding is considered the binding of object and location, whereas crossmodal binding is the binding of an auditorily presented object name and location. They aimed to compare VSTM binding performance for object names, locations, and identities between the MCI group and healthily aging controls. Stimuli consisted of 180 line drawings of real-world objects. The researchers found that the performance of the MCI participants was selectively and substantially lower than that of the healthy controls. They concluded that tasks that measure unimodal and crossmodal binding performance could be successful tools in the detection of early cognitive changes in patients who are at risk of developing dementia due to AD. 

Overall, the VSTM is a very interesting system and concept. Although Sligte et al. was unsuccessful in proving that the VSTM has an earlier, high-capacity stage, and were debunked by Matsukura et al., many will probably still continue to attempt to discover a high-capacity VSTM system. Afterall, there is always plenty to discover in neuroscience. Furthermore, VSTM can be applied in clinical situations, in which VSTM tasks, such as the change detection paradigm, are used in order to detect early cognitive changes in patients who are at risk of developing dementia due to Alzheimer’s disease. Visual short-term memory is a very interesting topic, and I cannot wait to see what more research derives from it. 

References

Matsukura M., Hollingworth A. (2011). Does visual short-term memory of a high capacity stage? Psychon Bull Rev; 18:1098-1104. doi: 10.3758/s13423-011-0153-2

Sapkota R. P., van der Linde I., Lamichhane N., Upadhyaya T., Pardhan S. Patients with Mild Cognitive Impairment Show Lower Visual Short-Term Memory Performance in Feature Binding Tasks. Dement Geriatr Cogn Disord Extra; 7:74-86. doi: 10.1159/000455831 https://www.karger.com/Article/FullText/455831#