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Recent studies have found that non-consciously perceived information can be retained for several seconds, a feat that has been attributed to non-conscious working memory processes. However, these studies have mainly relied on subjective measures of visual experience, and the neural processes responsible for non-conscious short-term retention remains unclear. Here we used continuous flash suppression to render stimuli non-conscious in a delayed match-to-sample task together with fMRI to investigate the neural correlates of non-conscious short-term (5-15 s) retention. The participants' behavioral performance was at chance level when they reported no visual experience of the sample stimulus. Critically, multivariate pattern analyses of BOLD signal during the delay phase could classify presence versus absence of sample stimuli based on signal patterns in frontal cortex, and its spatial position based on signal patterns in occipital cortex. In addition, univariate analyses revealed increased BOLD signal change in prefrontal regions during memory recognition. Thus, our findings demonstrate short-term maintenance of information presented non-consciously, defined by chance performance behaviorally. This non-consciously retained information seems to rely on persistent neural activity in frontal and occipital cortex, and may engage further cognitive control processes during memory recognition.

Background: Despite our distinct impression to the contrary, we are only conscious of a fraction of all the neural activity underlying our thoughts and behavior. Most neural processes occur non-consciously, and in parallel with our conscious experience. However, it is still unclear what the limits of non-conscious processes are in terms of higher cognitive functions. Many recent studies have shown that increasingly more advanced functions can operate non-consciously, but non-conscious information is still thought to be fleeting and undetectable within 500 milliseconds. Here we used various techniques to render information non-conscious, together with functional magnetic resonance imaging (fMRI), to investigate if non-consciously presented information can be retained for several seconds, what the neural substrates of such retention are, and if it is consistent with working memory maintenance.

Results: In Study I we used an attentional blink paradigm to render stimuli (single letters) non-conscious, and a variable delay period (5 – 15 s) prior to memory test. It was found that non-conscious memory performance was above chance after all delay durations, and showed no signs of decline over time. Univariate fMRI analysis showed that the durable retention was associated with sustained BOLD signal change in the prefrontal cortex and cerebellum during the delay period. In Study II we used continuous flash suppression (CFS) to render stimuli (faces and tools) non-conscious, and a variable delay period (5 or 15 s) prior to memory test. The durable retention of up to 15 s was replicated, and it was found that stimuli identity and spatial position was retained until prospective use. In Study III we used CFS to render tools non-conscious, and a variable delay period (5 – 15 s) prior to memory test. It was found that memory performance was not better than chance. However, by using multi-voxel pattern analysis it was nonetheless possible to detect the presence vs. absence of non-conscious stimuli in the frontal cortex,and their spatial position (left vs. right) in the occipital cortex during the delay.

Conclusions: Overall these findings suggest that non-consciously presented information (identity and/or position) can be retained for several seconds,and is associated with BOLD signal in frontal and posterior regions. These findings are consistent with working memory maintenance of non-consciously presented information, and thereby constrain models of working memory and theories of consciousness.

A crucial role for working memory in temporary information processing and guidance of complex behavior has been recognized for many decades. There is emerging consensus that working-memory maintenance results from the interactions among long-term memory representations and basic processes, including attention, that are instantiated as reentrant loops between frontal and posterior cortical areas, as well as sub-cortical structures. The nature of such interactions can account for capacity limitations, lifespan changes, and restricted transfer after working-memory training. Recent data and models indicate that working memory may also be based on synaptic plasticity and that working memory can operate on non-consciously perceived information.

Although non-consciously perceived information has previously been assumed to be short-lived (<500 ms), recent findings show that non-consciously perceived information can be maintained for at least 15s Such findings can be explained as working memory without a conscious experience of the information to be retained. However, whether or not working memory can operate on non-consciously perceived information remains controversial, and little is known about the nature of such non-conscious visual short-term memory (VSTM). Here we used continuous flash suppression to render stimuli non-conscious, to investigate the properties of non-consciously perceived representations in delayed match-to-sample (DMS) tasks. In Experiment I we used variable delays (5 or 15s) and found that performance was significantly better than chance and was unaffected by delay duration, thereby replicating previous findings. In Experiment II the DMS task required participants to combine information of spatial position and object identity on a trial-by-trial basis to successfully solve the task. We found that the conjunction of spatial position and object identity was retained, thereby verifying that non-conscious, trial-specific information can be maintained for prospective use. We conclude that our results are consistent with a working memory interpretation, but that more research is needed to verify this interpretation.

Conscious processing is generally seen as required for flexible and willful actions, as well as for tasks that require durable information maintenance. Here we present research that questions the assumption that only consciously perceived information is durable (>500 ms). Using the attentional blink (AB) phenomenon, we rendered otherwise relatively clearly perceived letters non conscious. In a first experiment we systematically manipulated the delay between stimulus presentation and response, for the purpose of estimating the durability of non-conscious perceptual representations. For items reported not seen, we found that behavioral performance was better than chance across intervals up to 15 s. In a second experiment we used fMRI to investigate the neural correlates underlying the maintenance of non conscious perceptual representations. Critically, the relatively long delay period demonstrated in experiment 1 enabled isolation of the signal change specifically related to the maintenance period, separate from stimulus presentation and response. We found sustained BOLD signal change in the right mid-lateral prefrontal cortex, orbitofrontal cortex, and crus II of the cerebellum during maintenance of non consciously perceived information. These findings are consistent with the controversial claim that working-memory mechanisms are involved in the short-term maintenance of non-conscious perceptual representations.