How Transcranial Magnetic Stimulation over Early Visual Cortex impacts short-term memory precision and guess rate

Neuroimaging studies have demonstrated that activity patterns in early visual areas predict stimulus properties actively maintained in visual short-term memory. Yet, the mechanisms by which such information is represented remain largely unknown. In this study, observers remembered the orientations of 4 briefly presented gratings, one in each quadrant of the visual field. A 10Hz Transcranial Magnetic Stimulation (TMS) triplet was applied directly at stimulus offset, or midway through a 2-second delay, targeting early visual cortex corresponding retinotopically to a sample item in the lower hemifield. Memory for one of the four gratings was probed at random, and participants reported this orientation via method of adjustment. Replication errors were smaller when the visual field location targeted by TMS overlapped with that of the cued memory item, compared to errors for stimuli probed diagonally to TMS. This implied topographic storage of orientation information, and a memory-enhancing effect at the targeted location. Furthermore, early pulses impaired performance at all four locations, compared to late pulses. Next, response errors were fit empirically using a mixture model analysis to characterize memory precision and guess rates. Memory was more precise for items proximal to the pulse location, irrespective of pulse timing. Guesses were more probable with early TMS pulses, regardless of stimulus location. Thus, whereas TMS administered at the offset of the stimulus array might disrupt early-phase consolidation in a topographically unspecific manner, TMS also boosts the precise representation of an item at its targeted retinotopic location, perhaps by increasing attentional resources or by injecting a beneficial amount of noise.