A gradual transition from veridical to categorical representations along the visual hierarchy during working memory, but not perception.

The ability to stably maintain visual information over brief delays is central to cognitive functioning. One possible way to achieve robust working memory maintenance is by having multiple concurrent mnemonic representations across multiple cortical loci. For example, early visual cortex might contribute to storage by representing information in a "sensory-like" format, while intraparietal sulcus uses a format transformed away from sensory driven responses. As an explicit test of mnemonic code transformations along the visual hierarchy, we quantitatively modeled the progression of veridical-to-categorical orientation representations in human participants. Participants directly viewed, or held in mind, an oriented grating pattern, and the similarity between fMRI activation patterns for different orientations was calculated throughout retinotopic cortex. During direct perception, similarity was clustered around cardinal orientations, while during working memory the obliques were represented more similarly. We modeled these similarity patterns based on the known distribution of orientation information in the natural world: The "veridical" model uses an efficient coding framework to capture hypothesized representations during visual perception. The "categorical" model assumes that different "psychological distances" between orientations result in orientation categorization relative to cardinal axes. During direct perception, the veridical model explained the data well in early visual areas, while the categorical model did worse. During working memory, the veridical model only explained some of the data, while the categorical model gradually gained explanatory power for increasingly anterior retinotopic regions. These findings suggest that directly viewed images are represented veridically, but once visual information is no longer tethered to the sensory world, there is a gradual progression to more categorical mnemonic formats along the visual hierarchy.

Vertical Aerosol Distribution and Mesospheric Clouds From ExoMars UVIS.

The vertical opacity structure of the martian atmosphere is important for understanding the distribution of ice (water and carbon dioxide) and dust. We present a new data set of extinction opacity profiles from the NOMAD/UVIS spectrometer aboard the ExoMars Trace Gas Orbiter, covering one and a half Mars Years (MY) including the MY 34 Global Dust Storm and several regional dust storms. We discuss specific mesospheric cloud features and compare with existing literature and a Mars Global Climate Model (MGCM) run with data assimilation. Mesospheric opacity features, interpreted to be water ice, were present during the global and regional dust events and correlate with an elevated hygropause in the MGCM, providing evidence that regional dust storms can boost transport of vapor to mesospheric altitudes (with potential implications for atmospheric escape). The season of the dust storms also had an apparent impact on the resulting lifetime of the cloud features, with events earlier in the dusty season correlating with longer-lasting mesospheric cloud layers. Mesospheric opacity features were also present during the dusty season even in the absence of regional dust storms, and interpreted to be water ice based on previous literature. The assimilated MGCM temperature structure agreed well with the UVIS opacities, but the MGCM opacity field struggled to reproduce mesospheric ice features, suggesting a need for further development of water ice parameterizations. The UVIS opacity data set offers opportunities for further research into the vertical aerosol structure of the martian atmosphere, and for validation of how this is represented in numerical models.

The dynamic atmospheric and aeolian environment of Jezero crater, Mars.

Despite the importance of sand and dust to Mars geomorphology, weather, and exploration, the processes that move sand and that raise dust to maintain Mars' ubiquitous dust haze and to produce dust storms have not been well quantified in situ, with missions lacking either the necessary sensors or a sufficiently active aeolian environment. Perseverance rover's novel environmental sensors and Jezero crater's dusty environment remedy this. In Perseverance's first 216 sols, four convective vortices raised dust locally, while, on average, four passed the rover daily, over 25% of which were significantly dusty ("dust devils"). More rarely, dust lifting by nonvortex wind gusts was produced by daytime convection cells advected over the crater by strong regional daytime upslope winds, which also control aeolian surface features. One such event covered 10 times more area than the largest dust devil, suggesting that dust devils and wind gusts could raise equal amounts of dust under nonstorm conditions.

Mars Perihelion Cloud Trails as revealed by MARCI: Mesoscale Topographically Focussed Updrafts and Gravity Wave Forcing of High Altitude Clouds.

Daily, global wide angle imaging of Mars clouds in MARCI (MARs Color Imager, (Malin et al., 2008)) ultraviolet and visible bands reveals the spatial/seasonal distributions and physical characteristics of perihelion cloud trails (PCT); a class of high altitude (40-50 km), horizontally extended (200-1000 km, trending W to WSW) water ice clouds formed over specific southern low-to-mid latitude (5S-40S), mesoscale (~50 km) locations during the Mars perihelion, southern summer season. PCT were first reported in association with rim regions of Valles Marineris (Clancy et al., 2009). The current study employs MARCI 2007-2011 imaging to sample the broader distributions and properties of PCT; and indicates several distinct locations of peak occurrences, including SW Arsia Mons, elevated regions of Syria, Solis, and Thaumasia Planitia, along Valles Marineris margins, and the NE rim of Hellas Basin. PCT are present over Mars solar longitudes (L S ) of 210-310°, in late morning to mid afternoon hours (10am-3pm), and are among the brightest and most distinctive clouds exhibited during the perihelion portion of the Mars orbit. Their locations (i.e., eastern margin origins) correspond to strong local elevation gradients, and their timing to peak solar heating conditions (perihelion, subsolar latitudes and midday local times). They occur approximately on a daily basis among all locations identified (i.e., not daily at a single location). Based on cloud surface shadow analyses, PCT form at 40-50 km aeroid altitudes, where water vapor is generally at near-saturation conditions in this perihelion period (e.g. Millour et al., 2014). They exhibited notable absences during periods of planet encircling and regional dust storm activity in 2007 and 2009, respectively, presumably due to reduced water saturation conditions above 35-40 km altitudes associated with increased dust heating over the vertically extended atmosphere (e.g., Neary et al., 2019). PCT exhibit smaller particle sizes (R eff =0.2-0.5µm) than typically exhibited in the lower atmosphere, and incorporate significant fractions of available water vapor at these altitudes. PCT ice particles are inferred to form continuously (over ~4 hours) at their PCT eastern origins, associated with localized updrafts, and are entrained in upper level zonal/meridional winds (towards W or WSW with ~50 m/sec speeds at 40-50 km altitudes) to create long, linear cloud trails. PCT cloud formation is apparently forced in the lower atmosphere (≤10-15 km) by strong updrafts associated with distinctive topographic gradients, such as simulated in mesoscale studies (e.g., Tyler and Barnes, 2015) and indicated by the surface-specific PCT locations. These lower scale height updrafts are proposed to generate vertically propagating gravity waves (GW), leading to PCT formation above ~40 km altitudes where water vapor saturation conditions promote vigorous cloud ice formation. Recent mapping of GW amplitudes at ~25 km altitudes, from Mars Climate Sounder 15 µm radiance variations (Heavens et al., 2020), in fact demonstrates close correspondences to the detailed spatial distributions of observed PCT, relative to other potential factors such as surface albedo and surface elevation (or related boundary layer depths).

Drifting codes within a stable coding scheme for working memory.

Working memory (WM) is important to maintain information over short time periods to provide some stability in a constantly changing environment. However, brain activity is inherently dynamic, raising a challenge for maintaining stable mental states. To investigate the relationship between WM stability and neural dynamics, we used electroencephalography to measure the neural response to impulse stimuli during a WM delay. Multivariate pattern analysis revealed representations were both stable and dynamic: there was a clear difference in neural states between time-specific impulse responses, reflecting dynamic changes, yet the coding scheme for memorised orientations was stable. This suggests that a stable subcomponent in WM enables stable maintenance within a dynamic system. A stable coding scheme simplifies readout for WM-guided behaviour, whereas the low-dimensional dynamic component could provide additional temporal information. Despite having a stable subspace, WM is clearly not perfect-memory performance still degrades over time. Indeed, we find that even within the stable coding scheme, memories drift during maintenance. When averaged across trials, such drift contributes to the width of the error distribution.

Unimodal and Bimodal Access to Sensory Working Memories by Auditory and Visual Impulses.

It is unclear to what extent sensory processing areas are involved in the maintenance of sensory information in working memory (WM). Previous studies have thus far relied on finding neural activity in the corresponding sensory cortices, neglecting potential activity-silent mechanisms, such as connectivity-dependent encoding. It has recently been found that visual stimulation during visual WM maintenance reveals WM-dependent changes through a bottom-up neural response. Here, we test whether this impulse response is uniquely visual and sensory-specific. Human participants (both sexes) completed visual and auditory WM tasks while electroencephalography was recorded. During the maintenance period, the WM network was perturbed serially with fixed and task-neutral auditory and visual stimuli. We show that a neutral auditory impulse-stimulus presented during the maintenance of a pure tone resulted in a WM-dependent neural response, providing evidence for the auditory counterpart to the visual WM findings reported previously. Interestingly, visual stimulation also resulted in an auditory WM-dependent impulse response, implicating the visual cortex in the maintenance of auditory information, either directly or indirectly, as a pathway to the neural auditory WM representations elsewhere. In contrast, during visual WM maintenance, only the impulse response to visual stimulation was content-specific, suggesting that visual information is maintained in a sensory-specific neural network, separated from auditory processing areas.SIGNIFICANCE STATEMENT Working memory is a crucial component of intelligent, adaptive behavior. Our understanding of the neural mechanisms that support it has recently shifted: rather than being dependent on an unbroken chain of neural activity, working memory may rely on transient changes in neuronal connectivity, which can be maintained efficiently in activity-silent brain states. Previous work using a visual impulse stimulus to perturb the memory network has implicated such silent states in the retention of line orientations in visual working memory. Here, we show that auditory working memory similarly retains auditory information. We also observed a sensory-specific impulse response in visual working memory, while auditory memory responded bimodally to both visual and auditory impulses, possibly reflecting visual dominance of working memory.

Dynamic hidden states underlying working-memory-guided behavior.

Recent theoretical models propose that working memory is mediated by rapid transitions in 'activity-silent' neural states (for example, short-term synaptic plasticity). According to the dynamic coding framework, such hidden state transitions flexibly configure memory networks for memory-guided behavior and dissolve them equally fast to allow forgetting. We developed a perturbation approach to measure mnemonic hidden states in an electroencephalogram. By 'pinging' the brain during maintenance, we show that memory-item-specific information is decodable from the impulse response, even in the absence of attention and lingering delay activity. Moreover, hidden memories are remarkably flexible: an instruction cue that directs people to forget one item is sufficient to wipe the corresponding trace from the hidden state. In contrast, temporarily unattended items remain robustly coded in the hidden state, decoupling attentional focus from cue-directed forgetting. Finally, the strength of hidden-state coding predicts the accuracy of working-memory-guided behavior, including memory precision.

Extended temporal integration in rapid serial visual presentation: Attentional control at Lag 1 and beyond.

In the perception of target stimuli in rapid serial visual presentations, the process of temporal integration plays an important role when two targets are presented in direct succession (at Lag 1), causing them to be perceived as a singular episodic event. This has been associated with increased reversals of target order report and elevated task performance in classic paradigms. Yet, most current models of temporal attention do not incorporate a mechanism of temporal integration and it is currently an open question whether temporal integration is a factor in attentional processing: It might be an independent process, perhaps little more than a sensory sampling rate parameter, isolated to Lag 1, where it leaves the attentional dynamics otherwise unaffected. In the present study, these boundary conditions were tested. Temporal target integration was observed across sequences of three targets spanning an interval of 240ms. Integration rates furthermore depended strongly on bottom-up attentional filtering, and to a lesser degree on top-down control. The results support the idea that temporal integration is an adaptive process that is part of, or at least interacts with, the attentional system. Implications for current models of temporal attention are discussed.

Decoding Rich Spatial Information with High Temporal Resolution.

New research suggests that magnetoencephalography (MEG) contains rich spatial information for decoding neural states. Even small differences in the angle of neighbouring dipoles generate subtle, but statistically separable field patterns. This implies MEG (and electroencephalography: EEG) is ideal for decoding neural states with high-temporal resolution in the human brain.

Revealing hidden states in visual working memory using electroencephalography.

It is often assumed that information in visual working memory (vWM) is maintained via persistent activity. However, recent evidence indicates that information in vWM could be maintained in an effectively "activity-silent" neural state. Silent vWM is consistent with recent cognitive and neural models, but poses an important experimental problem: how can we study these silent states using conventional measures of brain activity? We propose a novel approach that is analogous to echolocation: using a high-contrast visual stimulus, it may be possible to drive brain activity during vWM maintenance and measure the vWM-dependent impulse response. We recorded electroencephalography (EEG) while participants performed a vWM task in which a randomly oriented grating was remembered. Crucially, a high-contrast, task-irrelevant stimulus was shown in the maintenance period in half of the trials. The electrophysiological response from posterior channels was used to decode the orientations of the gratings. While orientations could be decoded during and shortly after stimulus presentation, decoding accuracy dropped back close to baseline in the delay. However, the visual evoked response from the task-irrelevant stimulus resulted in a clear re-emergence in decodability. This result provides important proof-of-concept for a promising and relatively simple approach to decode "activity-silent" vWM content using non-invasive EEG.

Two visual targets for the price of one? Pupil dilation shows reduced mental effort through temporal integration.

In dynamic sensory environments, successive stimuli may be combined perceptually and represented as a single, comprehensive event by means of temporal integration. Such perceptual segmentation across time is intuitively plausible. However, the possible costs and benefits of temporal integration in perception remain underspecified. In the present study pupil dilation was analyzed as a measure of mental effort. Observers viewed either one or two successive targets amidst distractors in rapid serial visual presentation, which they were asked to identify. Pupil dilation was examined dependent on participants' report: dilation associated with the report of a single target, of two targets, and of an integrated percept consisting of the features of both targets. There was a clear distinction between dilation observed for single-target reports and integrations on the one side, and two-target reports on the other. Regardless of report order, two-target reports produced increased pupil dilation, reflecting increased mental effort. The results thus suggested that temporal integration reduces mental effort and may thereby facilitate perceptual processing.

Visual selective attention is equally functional for individuals with low and high working memory capacity: evidence from accuracy and eye movements.

Selective attention and working memory capacity (WMC) are related constructs, but debate about the manner in which they are related remains active. One elegant explanation of variance in WMC is that the efficiency of filtering irrelevant information is the crucial determining factor, rather than differences in capacity per se. We examined this hypothesis by relating WMC (as measured by complex span tasks) to accuracy and eye movements during visual change detection tasks with different degrees of attentional filtering and allocation requirements. Our results did not indicate strong filtering differences between high- and low-WMC groups, and where differences were observed, they were counter to those predicted by the strongest attentional filtering hypothesis. Bayes factors indicated evidence favoring positive or null relationships between WMC and correct responses to unemphasized information, as well as between WMC and the time spent looking at unemphasized information. These findings are consistent with the hypothesis that individual differences in storage capacity, not only filtering efficiency, underlie individual differences in working memory.

First Atmospheric Science Results from the Mars Exploration Rovers Mini-TES.

Thermal infrared spectra of the martian atmosphere taken by the Miniature Thermal Emission Spectrometer (Mini-TES) were used to determine the atmospheric temperatures in the planetary boundary layer and the column-integrated optical depth of aerosols. Mini-TES observations show the diurnal variation of the martian boundary layer thermal structure, including a near-surface superadiabatic layer during the afternoon and an inversion layer at night. Upward-looking Mini-TES observations show warm and cool parcels of air moving through the Mini-TES field of view on a time scale of 30 seconds. The retrieved dust optical depth shows a downward trend at both sites.