Atypical Balance between Occipital and Fronto-Parietal Activation for Visual Shape Extraction in Dyslexia

Reading requires the extraction of letter shapes from a complex background of text, and an impairment in visual shape extraction would cause difficulty in reading. To investigate the neural mechanisms of visual shape extraction in dyslexia, we used functional magnetic resonance imaging (fMRI) to examine brain activation while adults with or without dyslexia responded to the change of an arrow’s direction in a complex, relative to a simple, visual background. In comparison to adults with typical reading ability, adults with dyslexia exhibited opposite patterns of atypical activation: decreased activation in occipital visual areas associated with visual perception, and increased activation in frontal and parietal regions associated with visual attention. These findings indicate that dyslexia involves atypical brain organization for fundamental processes of visual shape extraction even when reading is not involved. Overengagement in higher-order association cortices, required to compensate for underengagment in lower-order visual cortices, may result in competition for top-down attentional resources helpful for fluent reading.Ellison Medical FoundationMartin Richmond Memorial FundNational Institutes of Health (U.S.). (Grant UL1RR025758)National Institutes of Health (U.S.). (Grant F32EY014750-01)MIT Class of 1976 (Funds for Dyslexia Research

Locomotion modulates specific functional cell types in the mouse visual thalamus

The visual system is composed of diverse cell types that encode distinct aspects of the visual scene and may form separate processing channels. Here we present further evidence for that hypothesis whereby functional cell groups in the dorsal lateral geniculate nucleus (dLGN) are differentially modulated during behavior. Using simultaneous multi-electrode recordings in dLGN and primary visual cortex (V1) of behaving mice, we characterized the impact of locomotor activity on response amplitude, variability, correlation and spatiotemporal tuning. Locomotion strongly impacts the amplitudes of dLGN and V1 responses but the effects on variability and correlations are relatively minor. With regards to tunings, locomotion enhances dLGN responses to high temporal frequencies, preferentially affecting ON transient cells and neurons with nonlinear responses to high spatial frequencies. Channel specific modulations may serve to highlight particular visual inputs during active behaviors

Gaze fixation improves the stability of expert juggling

Novice and expert jugglers employ different visuomotor strategies: whereas novices look at the balls around their zeniths, experts tend to fixate their gaze at a central location within the pattern (so-called gaze-through). A gaze-through strategy may reflect visuomotor parsimony, i.e., the use of simpler visuomotor (oculomotor and/or attentional) strategies as afforded by superior tossing accuracy and error corrections. In addition, the more stable gaze during a gaze-through strategy may result in more accurate movement planning by providing a stable base for gaze-centered neural coding of ball motion and movement plans or for shifts in attention. To determine whether a stable gaze might indeed have such beneficial effects on juggling, we examined juggling variability during 3-ball cascade juggling with and without constrained gaze fixation (at various depths) in expert performers (n = 5). Novice jugglers were included (n = 5) for comparison, even though our predictions pertained specifically to expert juggling. We indeed observed that experts, but not novices, juggled significantly less variable when fixating, compared to unconstrained viewing. Thus, while visuomotor parsimony might still contribute to the emergence of a gaze-through strategy, this study highlights an additional role for improved movement planning. This role may be engendered by gaze-centered coding and/or attentional control mechanisms in the brain

Interaction between Attention and Bottom-Up Saliency Mediates the Representation of Foreground and Background in an Auditory Scene

Bottom-up (stimulus-driven) and top-down (attentional) processes interact when a complex acoustic scene is parsed. Both modulate the neural representation of the target in a manner strongly correlated with behavioral performance

A Probabilistic, Distributed, Recursive Mechanism for Decision-making in the Brain

Decision formation recruits many brain regions, but the procedure they jointly execute is unknown. Here we characterize its essential composition, using as a framework a novel recursive Bayesian algorithm that makes decisions based on spike-trains with the statistics of those in sensory cortex (MT). Using it to simulate the random-dot-motion task, we demonstrate it quantitatively replicates the choice behaviour of monkeys, whilst predicting losses of otherwise usable information from MT. Its architecture maps to the recurrent cortico-basal-ganglia-thalamo-cortical loops, whose components are all implicated in decision-making. We show that the dynamics of its mapped computations match those of neural activity in the sensorimotor cortex and striatum during decisions, and forecast those of basal ganglia output and thalamus. This also predicts which aspects of neural dynamics are and are not part of inference. Our single-equation algorithm is probabilistic, distributed, recursive, and parallel. Its success at capturing anatomy, behaviour, and electrophysiology suggests that the mechanism implemented by the brain has these same characteristics

Coding of spatial attention priorities and object features in the macaque lateral intraparietal cortex

Primate posterior parietal cortex (PPC) is known to be involved in controlling spatial attention. Neurons in one part of the PPC, the lateral intraparietal area (LIP), show enhanced responses to objects at attended locations. Although many are selective for object features, such as the orientation of a visual stimulus, it is not clear how LIP circuits integrate feature-selective information when providing attentional feedback about behaviorally relevant locations to the visual cortex. We studied the relationship between object feature and spatial attention properties of LIP cells in two macaques by measuring the cells' orientation selectivity and the degree of attentional enhancement while performing a delayed match-to-sample task. Monkeys had to match both the location and orientation of two visual gratings presented separately in time. We found a wide range in orientation selectivity and degree of attentional enhancement among LIP neurons. However, cells with significant attentional enhancement had much less orientation selectivity in their response than cells which showed no significant modulation by attention. Additionally, orientation-selective cells showed working memory activity for their preferred orientation, whereas cells showing attentional enhancement also synchronized with local neuronal activity. These results are consistent with models of selective attention incorporating two stages, where an initial feature-selective process guides a second stage of focal spatial attention. We suggest that LIP contributes to both stages, where the first stage involves orientation-selective LIP cells that support working memory of the relevant feature, and the second stage involves attention-enhanced LIP cells that synchronize to provide feedback on spatial priorities

Pregnane steroids and short-term neural plasticity

Gamma-aminobutyric acid (GABA) is the major inhibitory transmitter in the brain, and its fast effects are mediated by the GABA-A receptor. It is well known, from pharmacological manipulations, that many exogenous agents alter the efficacy of GABA-A receptors. For example, benzodiazepines increase the effect of GABA and some β-carbolines reduce the effect of GABA at these receptors. Increasing the strength of neuronal inhibition can prevent seizures, reduce anxiety and be neuroprotective. There are also endogenous mechanisms that increase efficacy. For example, more GABA-A receptors can be synthesized and inserted into synapses, but this requires up to 1 h or more. On a shorter timescale, GABAergic inhibition can be potentiated by steroids, e.g., allopregnanolone, synthesized de novo in neural tissue or derived from peripheral endocrine organs. The widespread distribution of these neuroactive steroids across the brain suggests an extensive role in short-term neural plasticity. © 2008 Springer Netherlands

Cellular distribution of the GABAA receptor-modulating 3 α-hydroxy, 5 α-reduced pregnane steroids in the adult rat brain

The 3 α-hydroxy,5 α-reduced pregnane steroids, allopregnanolone and allotetrahydrodeoxycorticosterone, are the most potent endogenous positive modulators of GABAA receptor-mediated inhibition. This study presents the first immunohistochemical examination of the cellular distribution of 3 α-hydroxy,5 α-reduced pregnane steroids across the brain. We found a widespread distribution in the adult rat, with dense immunolabelling in the olfactory bulb, striatum and cerebral cortex, and lower density labelling in the brainstem reticular formation. In general terms, this distribution accords with the regional concentrations of 3 α-hydroxy,5 α-reduced steroids determined, in other laboratories, by brain region sampling and either gas chromatography-mass fragmentography or radioimmunoassay. However, immunohistochemistry allowed for a more detailed examination of regional distribution and cellular specificity. All immunoreactivity was confined to the cell bodies and thick dendrites of neurones; no identifiable glia were labelled. In most brain areas, the location and morphology of labelled cells identified them as excitatory neurones. In addition, cell populations known to be projecting GABAergic neurones (e.g. cerebellar Purkinje cells) were immunoreactive, whereas local inhibitory neurones generally were not. The cellular distribution of 3 α-hydroxy,5 α-reduced steroids suggests that sensory, motor, limbic and homeostatic systems can be influenced by neurosteroids at multiple stages of processing