The role of visual short term memory load in visual sensory detection

In this thesis I established the role of Visual Short-Term Memory (VSTM) load in visual detection while comparing to the roles of perceptual load and Working Memory (WM) cognitive control load. Participants performed a short-term memory task combined with a visual detection task (as well as attention task, Chapter 2) during the memory delay. The level and type of load was varied (perceptual load, VSTM load or WM cognitive control load). Measures of detection sensitivity demonstrated that increased VSTM load and perceptual load have both resulted in impaired detection sensitivity that was of equivalent magnitude. In contrast, increased WM cognitive control load had either no effect on detection or under some conditions (when the detection task was combined with an attention task of higher priority) resulted in enhanced detection sensitivity, the opposite effect to VSTM load. The contrasting effects of different types of memory load rule out alternative accounts in terms of general task difficulty. Other interpretations in terms of changes in attention deployment, response bias, task priorities, verbal strategies, were also ruled out. These VSTM load effects lasted over delays of 4 seconds, were generalized to foveal, parafoveal and peripheral stimuli, and were shown to be predicted from estimates of the effects of load on VSTM capacity. fMRI results (Chapter 4) showed that high VSTM load reduces retinotopic V1 responses to the detection stimulus and psychophysics experiments (Chapter 5) showed that high VSTM load resulted in reduced effective contrast of the detection stimulus. These results in this thesis clarify the distinct roles of WM maintenance processes from those of WM cognitive control process in visual detection. These findings provide further support to the sensory recruitment hypothesis of VSTM, clarify previous discrepancies in WM research and extend load theory to account for the effects of VSTM load on visual detection