BACKGROUND: V1 is known to contain neurons tuned to visual low-level features such as spatial frequencies and orientation. Psychophysical studies using delayed discrimination experiments have shown that these features can be retained in memory with high fidelity. According to the sensory recruitment model of working memory, visual working memory recruits the perceptual areas involved in discriminating the features that is to be retained. If this is the case, V1-neurons tuned to spatial frequencies should be recruited when information along this dimension is retained in memory. We tested this hypothesis taking advantage of the memory masking effect.
METHOD: We used fMRI to measure BOLD responses while participants performed a delayed discrimination task for spatial frequencies. While performing the discrimination task, the participants had to retain spatial frequency information about an irrelevant masker stimulus. The mask differentially interfered with discrimination accuracy on the main task, and this memory masking effect was used to probe early visual areas for differential BOLD modulation related to the masking effect. In one experiment we presented the mask and the sample stimulus to be remembered in the same retinotopic position, and in a second experiment the mask and sample were spatially separated, allowing us to investigate the spatial extent of the low-level memory representation. Early visual areas were identified using a retinotopic mapping procedure, and ROIs retinotopically coding stimuli positions were defined for each visual area in an independent localizer session.
RESULTS: When the mask and sample stimulus were presented to the same retinotopic position, the mask impaired discrimination accuracy when it differed in spatial frequency from the sample stimulus. This memory masking effect was observed as a reduced BOLD response in V1. When spatially separating the mask and sample stimulus, we found no significant decrease in BOLD activation in V1. CONCLUSIONS: Our results indicate that neurons in V1 involved in the perceptual coding of spatial frequencies are recruited during memory of the same information, in accordance with the sensory recruitment model of visual working memory. The memory masking effect is proposed to result from cross-channel inhibition, and is a local process in the retinotopically organized visual cortex.