Increased phase precision of spatial light modulators using irrational slopes: application to attosecond metrology.

The ability of spatial light modulators (SLMs) to modify the amplitude and phase of light has proved them invaluable to the optics and photonics community. In many applications, the bit-depth of SLMs is a major limiting factor dictated by a digital processor. As a result, there is usually a compromise between refresh speed and bit-depth. Here, we present a method to increase the effective bit-depth of SLMs, which utilizes a linear slope, as is commonly applied to deal with the zeroth-order effect. This technique was tested using two interferometric transient absorption spectroscopy setups. Through the high harmonic generation in gases producing a train of attosecond pulses and harmonics from solids in the ultraviolet, two pulses are generated that interfere in the far field providing a measurement of the optical phase. An increase in the precision far beyond the limit dictated by the digital processor in the bit-depth was found.

Design of an optically-locked interferometer for attosecond pump-probe setups.

We present the design and performance of an active stabilization system for attosecond pump-probe setups based on a Mach-Zehnder interferometer configuration. The system employs a CW laser propagating coaxially with the pump and probe beams in the interferometer. The stabilization is achieved with a standalone feedback controller that adjusts the length of one of its arms to maintain a constant relative phase between the CW beams. With this system, the time delay between the pump and probe beams is stabilized within 10 as rms over several hours. The system is easy to operate and only requires a few minutes to set up before any pump/probe measurements.

A neuropsychological approach to differentiating cannabis-induced and primary psychotic disorders.

AIM: Rates of cannabis use are elevated in early psychosis populations, rendering it difficult to determine if an episode of psychosis is related to cannabis use (e.g., cannabis-induced psychosis), or if substance use is co-occurring with a primary psychotic disorder (e.g., schizophrenia). Clinical presentations of these disorders are often indistinguishable, hindering assessment and treatment. Despite substantial research identifying cognitive deficits, eye movement abnormalities and speech impairment associated with primary psychotic disorders, these neuropsychological features have not been explored as targets for diagnostic differentiation in early psychosis. METHODS: Eighteen participants with cannabis-induced psychosis (Mage  = 21.9, SDage  = 4.25, 14 male) and 19 participants with primary psychosis (Mage  = 29.2, SDage  = 7.65, 17 male) were recruited from early intervention programs. Diagnoses were ascertained by primary treatment teams after a minimum of 6 months in the program. Participants completed tasks assessing cognitive performance, saccadic eye movements and speech. Clinical symptoms, trauma, substance use, premorbid functioning and illness insight were also assessed. RESULTS: Relative to individuals with primary psychosis, individuals with cannabis-induced psychosis demonstrated significantly better performance on the pro-saccade task, faster RT on pro- and anti-saccade tasks, better premorbid adjustment, and a higher degree of insight into their illness. There were no significant differences between groups on psychiatric symptoms, premorbid intellectual functioning, or problems related to cannabis use. CONCLUSIONS: In early stages of illness, reliance on traditional diagnostic tools or clinical interviews may be insufficient to distinguish between cannabis-induced and primary psychosis. Future research should continue to explore neuropsychological differences between these diagnoses to improve diagnostic accuracy.

A plano-convex thick-lens velocity map imaging apparatus for direct, high resolution 3D momentum measurements of photoelectrons with ion time-of-flight coincidence.

Since their inception, velocity map imaging (VMI) techniques have received continued interest in their expansion from 2D to 3D momentum measurements through either reconstructive or direct methods. Recently, much work has been devoted to the latter of these by relating electron time-of-flight (TOF) to the third momentum component. The challenge is having a timing resolution sufficient to resolve the structure in the narrow (<10 ns) electron TOF spread. Here, we build upon the work in VMI lens design and 3D VMI measurement by using a plano-convex thick-lens (PCTL) VMI in conjunction with an event-driven camera (TPX3CAM) providing TOF information for high resolution 3D electron momentum measurements. We perform simulations to show that, with the addition of a mesh electrode to the thick-lens geometry, the resulting plano-convex electrostatic field extends the detectable electron cutoff energy range while retaining the high resolution. This design also extends the electron TOF range, allowing for a better momentum resolution along this axis. We experimentally demonstrate these capabilities by examining above-threshold ionization in xenon, where the apparatus is shown to collect electrons of energy up to ∼7 eV with a TOF spread of ∼30 ns, both of which are improved compared to a previous work by factors of ∼1.4 and ∼3.75, respectively. Finally, the PCTL-VMI is equipped with a coincident ion TOF spectrometer, which is shown to effectively extract unique 3D momentum distributions for different ionic species in a gas mixture. These techniques have the potential to lend themselves to more advanced measurements involving systems where the electron momentum distributions possess non-trivial symmetries.

Remembering more than you can say: Re-examining "amnesia" of attended attributes.

Attribute amnesia (AA) describes a phenomenon whereby observers fail a surprise memory test which asks them to report an attribute they had just attended and used to fulfil a task goal. This finding has cast doubt on the prominent theory that attention results in encoding into working memory (WM), to which two competing explanations have been proposed: (1) task demands dictate whether attended information is encoded into WM, and (2) attended information is encoded in a weak state that does not survive the demands of the surprise memory test. To address this debate our study circumvented the limitations of a surprise memory test by embedding a second search task within a typical color-based AA search task. The search task was modified so that the attended attribute would reappear in the second search as either the target, a distractor, or not at all. Critically, our results support encoding of the attended attribute in WM though to a weaker extent than the attribute that is required for report. A second experiment confirmed that WM encoding only occurs for the attended attribute, though distractor attributes produce a bias consistent with negative priming. Our data provide novel support for a theory of memory consolidation that links the strength of a memory's representation with expectations for how it will be used in a task. Implications for the utility of this procedure in future investigations previously limited by single trial data (i.e., surprise question methodology) are discussed.

Attention bias modification in depression: A randomized trial using a novel, reward-based, eye-tracking approach.

BACKGROUND AND OBJECTIVES: Biased attention to negative information is a mechanism for risk and relapse in depression. Attentional bias modification (ABM) paradigms manipulate attention away from negative information to reduce this bias. ABM results have been mixed due to inconsistent methodologies and stimuli design. This randomized controlled trial used a novel approach to modifying attentional bias. METHODS: An eye tracker manipulated stimuli in response to participants' fixations to preferentially reward attention to positive stimuli by obscuring or enhancing image quality of negative and positive stimuli, respectively. Participants with major depressive disorder completed three 35-min sessions of active (n = 20) or sham (n = 20) ABM training. Attentional bias, memory for emotional words, and mood were assessed pre- and post-training. RESULTS: Training reduced negative attentional bias; relative to sham, active training participants focused significantly more on positive compared to negative stimuli in a free-viewing eye-tracker task (p = .038, ηp2 = 0.109) and, at trend, disengaged from sad information more quickly in a computerized task (p = .052, ηp2 = 0.096). Active training participants remembered more happy than sad words in an emotional word learning task, indicating a distal transfer of training to emotional memory (p = .036, ηp2 = 0.11). Training did not significantly affect mood in the one-week trial. LIMITATIONS: Future studies should build on this proof-of-principle study with larger sample sizes and more intensive treatment to explore which mechanisms of training may lead to improvements in mood. CONCLUSIONS: Attention biases in depression are modifiable through reward-based, eye-tracking training. These data suggest generalizability of training to other cognitive faculties - recall for affective information.

Significance of interictal occipital epileptiform discharges in children.

OBJECTIVE: Interictal occipital epileptiform abnormalities have not been well characterized. The objective of this pilot study was to assess their significance in children. METHODS: A search was performed on the EEG database for the keywords "occipital", "spike", "sharp wave" and "epileptiform". Patients were divided into two groups based on the absence of all (group 1) or presence of any (group 2) of the following criteria: mental retardation, cerebral palsy, neurological deficits, abnormal MRI and/or intractable epilepsy. Special attention was given to the spike/sharp wave amplitude/duration and background slowing. RESULTS: A total of 44 children (eight months to 15 years) were studied. Groups 1 and 2 were each composed of 22 children. Background slowing was more frequent in group 2 (10/22, 45%) compared to group 1 (1/22, 4.5%; p = 0.002). In group 2, 8/22 (36%) had spikes or sharp waves with amplitudes below 50 microV or above 150 microV with a positive predictive value of 89%, and a negative predictive value of 39%. Only 1/22 (4.5%) in group 1 had epileptiform activity outside of the 50-150 microV range. CONCLUSIONS: The presence of very high or low-amplitude occipital epileptiform abnormalities or background slowing may be indicative of encephalopathy.

In support of selective rehearsal: Double-item presentation in item-method directed forgetting.

Despite the substantial evidence highlighting the role of selective rehearsal in item-method directed forgetting, recent work has suggested that forgetting may occur as a function of an active inhibitory mechanism that is more effortful than elaborative rehearsal processes. In the present work, we test this hypothesis by implementing a double-item presentation within the item-method directed forgetting paradigm. Participants studied two unrelated items at a time. Some words were followed by the same cue, and participants were instructed to remember or forget both items (pure condition). On other trials, participants were to remember one but forget the other word (mixed condition). Selective rehearsal and inhibition accounts make distinct predictions regarding memory performance in the double-item presentation. In Experiment 1, we compared recognition performance in the pure and mixed conditions, while in Experiment 2, we included a neutral baseline condition to further distinguish between the selective rehearsal and inhibition accounts. Contrary to the inhibition account but consistent with selective rehearsal, we found for both remember and forget items that recognition was greater in the mixed than in the pure condition. Recognition for forget items also did not differ from neutral items. We conclude that selective rehearsal, not inhibition, is responsible for item-method directed forgetting.

Temporal trimming: Evidence that common-onset masking shortens perceptual sampling of conscious object representations.

Common-onset masking (COM) refers to a methodology where a mask can impair awareness of an object if the mask's offset is delayed relative to the offset of the object. This method has classically been used to understand how discontinuities in visual input lead to the discrete removal of object representations before they reach conscious awareness. However, COM has recently been shown to reduce the precision of conscious object representations (Harrison, Rajsic, & Wilson, Psychonomic Bulletin & Review, 23(1), 180-186, 2016). As a result, Harrison et al. proposed that COM shortens the temporal window for perceptual sampling of an object's representation, an account consistent with interruption-based theories of masking. In the present study we modified the standard COM methodology to assess the impact of a delayed mask offset on the temporal perception of an object's representation. Across two experiments we provide novel evidence that a delayed mask offset can impair temporal perception of a conscious percept, such that it reduces the percept's perceived duration (Experiment 1), and prematurely terminates updating of the percept's dynamic orientation (Experiment 2). We refer to these results as temporal trimming, and suggest that the mechanism responsible for COM operates during the sustained perception of an object.

Achieving sleep state during EEG in children; sequence of activation procedures.

OBJECTIVE: Interictal epileptiform discharges are often enhanced with attainment of sleep during an EEG. Hyperventilation (HV) during an EEG is often followed by drowsiness. However, the effect of intermittent photic stimulation (IPS) on awake/sleep state is unclear. The current study was conducted to determine if the sequence of activation procedures has an impact on acquisition of sleep during a routine, non-sleep deprived EEG. METHODS: Children, scheduled between 9 and 11 a.m., who were not sleep deprived and able to hyperventilate, were included in a 3-month prospective study undertaken at the Neurophysiology Laboratory of a tertiary children's hospital. In the first 48 children evaluated, IPS was started 5 min into the EEG recording and HV was elicited at the end of the session (group I). In the next 48 children, HV was started 5 min into the EEG recording and IPS was presented at the end of the EEG (group II). A third group served as a control group and received both activating procedures at the end of the recording (group III). All 3 groups were assessed for the presence or absence of sleep during their EEG recording. RESULTS: Only 3 of the 48 (6.2%) children in group I versus 17/48 (35.5%) in group II (P<0.0001), OR=8.68, 95% CI 2.34-32.22) and 11/48 (23%) in group III (P<0.05) attained sleep. The difference between the 3 groups was statistically significant (P=0.02). The children who were hyperventilated at the beginning of the session had an 8-fold increased chance of attaining sleep in comparison with children who received IPS at the beginning of the recording followed by HV at the end. CONCLUSIONS: We recommend that when performing routine non-sleep deprived EEGs in children, HV be performed first with IPS at the end of the EEG in order to maximize the yield of attaining sleep recording during the study.

Object-substitution masking degrades the quality of conscious object representations.

Object-substitution masking (OSM) is a unique paradigm for the examination of object updating processes. However, existing models of OSM are underspecified with respect to the impact of object updating on the quality of target representations. Using two paradigms of OSM combined with a mixture model analysis we examine the impact of post-perceptual processes on a target's representational quality within conscious awareness. We conclude that object updating processes responsible for OSM cause degradation in the precision of object representations. These findings contribute to a growing body of research advocating for the application of mixture model analysis to the study of how cognitive processes impact the quality (i.e., precision) of object representations.

A conserved RNA structure within the HCV IRES eIF3-binding site.

The hepatitis C virus (HCV) internal ribosome entry site (IRES) is recognized specifically by the small ribosomal subunit and eukaryotic initiation factor 3 (eIF3) before viral translation initiation. Using extensive mutagenesis and structure probing analysis, we show that the eIF3-binding domain of the HCV IRES contains an internal loop structure (loop IIIb) and an adjacent mismatched helix that are important for IRES-dependent initiation of translation. NMR studies reveal a unique three-dimensional structure for this internal loop that is conserved between viral isolates of varying primary sequence in this region. These data indicate that internal loop IIIb may be an attractive target for structure-based design of new antiviral agents.