Neural evidence for advantaged representation of first items in memory.

Visual areas activated during perception can retain specific information held in memory without the presence of physical stimuli via distributed activity patterns. Neuroimaging studies have shown that the delay-period representation of information in visual areas is modulated by factors such as memory load and task demands, raising the possibility of serial position as another potential modulator. Specifically, enhanced representation of first items during the post-encoding delay period may serve as a mechanism underlying the well-established but not well-understood primacy effect - the mnemonic advantage of first items. To test this hypothesis, 13 males and 16 females performed a human fMRI task, wherein each trial consisted of the sequential encoding of two stimuli (a famous face and landscape, order counterbalanced), followed by a distracting task, a delay period, and then a cued recall of one of the items. Participants exhibited the expected behavioral primacy effect, manifested as faster recall of the first items. In order to elucidate the still debated neural underpinnings of this effect, using multivariate decoding, a classifier was trained on data collected during encoding to differentiate stimulus categories (i.e., faces vs. landscapes) and tested on data collected during the post-encoding period. Greater reactivation of first versus second items was observed in the ventral occipito-temporal cortex during the entire post-encoding period but not during encoding. Moreover, trial-level analyses revealed that the degree of first-item neural advantage during the post-encoding delay predicted the behavioral primacy effect. These findings highlight the role of item reinstatement in ventral occipito-temporal cortex in the primacy effect and are discussed in the context of the uniqueness of the very first item and event boundaries, illuminating putative neural mechanisms underlying the effect.

Inhibitory and excitatory mechanisms in the human cingulate-cortex support reinforcement learning: A functional Proton Magnetic Resonance Spectroscopy study.

The dorsal anterior cingulate cortex (dACC) is crucial for motivation, reward- and error-guided decision-making, yet its excitatory and inhibitory mechanisms remain poorly explored in humans. In particular, the balance between excitation and inhibition (E/I), demonstrated to play a role in animal studies, is difficult to measure in behaving humans. Here, we used functional magnetic-resonance-spectroscopy (1H-fMRS) to measure the brain's major inhibitory (GABA) and excitatory (Glutamate) neurotransmitters during reinforcement learning with three different conditions: high cognitive load (uncertainty); probabilistic discrimination learning; and a control null-condition. Participants learned to prefer the gain option in the discrimination phase and had no preference in the other conditions. We found increased GABA levels during the uncertainty condition, potentially reflecting recruitment of inhibitory systems during high cognitive load when trying to learn. Further, higher GABA levels during the null (baseline) condition correlated with improved discrimination learning. Finally, glutamate and GABA levels were correlated during high cognitive load. These results suggest that availability of dACC inhibitory resources enables successful learning. Our approach helps elucidate the potential contribution of the balance between excitation and inhibition to learning and motivation in behaving humans.

WAIS Information Subtest as an indicator of crystallized cognitive abilities and brain reserve among highly educated older adults: A three-year longitudinal study.

The Wechsler Adult Intelligence Scale-Third Edition (WAIS-III) Information Subtest (IS) is known as a neuropsychological "Hold" test that is relatively resistant to decline with aging. We administered neuropsychological tests among highly educated healthy older adults once a year for three subsequent years. Results showed highly stable performances on the IS across years (Mean Z score: T0 = 1.39, SD = 0.60; T1 = 1.37, SD = 0.77; T2 = 1.50, SD = 0.66; T3 = 1.48, SD = 0.66), that were significantly higher than zero (T0: t = 12.08; T1: t = 9.29; T2: t = 11.71; T3: t = 11.68; for all, p < 0.0001). In contrast, other neuropsychological tests showed differences in performance across time; some performances significantly declined (Rey Osterrieth Complex Figure test-copy, Rey-Auditory Verbal Learning Test, and Montreal Cognitive Assessment test [MoCA]), whereas others were improved, possibly due to practice effects (Rey Osterrieth Complex Figure test- delayed, Rey-Auditory Verbal Learning Test- delayed, and Trail Making Test- part A). Correlation with whole brain volumetric analysis revealed a positive correlation between gray matter volumes and IS scores (r = 0.46, p < 0.05) even when controlling for age and education (partial correlations: r = 0.43; r = 0.45, for both p < 0.05). No significant correlations were found between gray matter and other test scores. Therefore, the WAIS-III Information subtest appears to be an adequate neuropsychological measurement of crystallized ability in highly educated older adults and may be considered as a proxy measure of brain reserve.

Altered responses to social chemosignals in autism spectrum disorder.

Autism spectrum disorder (ASD) is characterized by impaired social communication, often attributed to misreading of emotional cues. Why individuals with ASD misread emotions remains unclear. Given that terrestrial mammals rely on their sense of smell to read conspecific emotions, we hypothesized that misreading of emotional cues in ASD partially reflects altered social chemosignaling. We found no difference between typically developed (TD) and cognitively able adults with ASD at explicit detection and perception of social chemosignals. Nevertheless, TD and ASD participants dissociated in their responses to subliminal presentation of these same compounds: the undetected 'smell of fear' (skydiver sweat) increased physiological arousal and reduced explicit and implicit measures of trust in TD but acted opposite in ASD participants. Moreover, two different undetected synthetic putative social chemosignals increased or decreased arousal in TD but acted opposite in ASD participants. These results implicate social chemosignaling as a sensory substrate of social impairment in ASD.