Mona Spiridon

How distributed is visual category information in human occipito-temporal cortex? An fMRI study.

We used fMRI to study the distribution of object category information in the ventral visual pathway. Extending the findings of, we find that categories of stimuli can be distinguished by the pattern of activation they elicit across this entire pathway, even when the stimuli within a category differ in viewpoint, exemplar, or image format. However, regions within the ventral visual pathway are neither interchangeable nor equipotential. Although the FFA and PPA permit excellent discrimination between preferred versus nonpreferred stimuli (e.g., faces-bottles and houses-bottles, respectively), we find that neither region alone permits accurate discrimination between pairs of nonpreferred stimuli (e.g., bottles-shoes). These findings indicate that the ventral visual pathway is not homogeneous, but contains some regions (including FFA and PPA) that are primarily involved in the analysis of a single class of stimulus.

Location and spatial profile of category-specific regions in human extrastriate cortex.

Subjects were scanned in a single functional MRI (fMRI) experiment that enabled us to localize cortical regions in each subject in the occipital and temporal lobes that responded significantly in a variety of contrasts: faces > objects, body parts > objects, scenes > objects, objects > scrambled objects, and moving > stationary stimuli. The resulting activation maps were coregistered across subjects using spherical surface coordinates [Fischl et al., Hum Brain Mapp 1999;8:272–284] to produce a “percentage overlap map” indicating the percentage of subjects who showed a significant response for each contrast at each point on the surface. Prominent among the overlapping activations in these contrasts were the fusiform face area (FFA), extrastriate body area (EBA), parahippocampal place area (PPA), lateral occipital complex (LOC), and MT+/V5; only a few other areas responded consistently across subjects in these contrasts. Another analysis showed that the spatial profile of the selective response drops off quite sharply outside the standard borders of the FFA and PPA (less so for the EBA and MT+/V5), indicating that these regions are not simply peaks of very broad selectivities spanning centimeters of cortex, but fairly discrete regions of cortex with distinctive functional profiles. The data also yielded a surprise that challenges our understanding of the function of area MT+: a higher response to body parts than to objects. The anatomical consistency of each of our functionally defined regions across subjects and the spatial sharpness of their activation profiles within subjects highlight the fact that these regions constitute replicable and distinctive landmarks in the functional organization of the human brain. Hum Brain Mapp, 2005.

Modulation by zinc of the glutamate transporters in glial cells and cones isolated from the tiger salamander retina

1 Zinc may be released from some presynaptic glutamatergic neurons, including hippocampal mossy fibres and retinal photoreceptors. We whole‐cell‐clamped glial (Müller) cells isolated from the salamander retina to investigate the effect of zinc on glutamate transporters in these cells. Glutamate‐evoked currents in these cells are generated largely by carriers homologous to the mammalian GLAST/EAAT1 transporter. 2 Zinc inhibited both glutamate uptake into the cells, and glutamate release by reversal of the uptake process. The IC50 for inhibition of uptake (< 1 μM) was similar to or below the values for zinc modulating NMDA, α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate (AMPA) and GABA receptors, and 100‐fold less than the calculated value for the rise in extracellular zinc concentration evoked by depolarization with potassium in area CA3 of the hippocampus. 3 Although zinc altered the apparent affinity of the transporter for glutamate and Na+, it did not act simply by binding competitively to the glutamate‐, Na+‐, K+‐ or H+‐binding sites on the transporter. Zinc inhibited both forward and reversed glutamate transport from the outside of the cell membrane, but not from the inside. The inhibitory action of zinc on uptake was voltage independent, indicating a zinc‐binding site outside the membrane field. 4 As well as inhibiting glutamate transport, zinc potentiated activation of the anion conductance in the Müller cell glutamate transporter. However, zinc reduced the current mediated by the anion conductance in the cone synaptic terminal glutamate transporter (homologous to the mammalian EAAT5), indicating that zinc has different actions on different glutamate transporter subtypes. 5 By acting on glutamate transporters, zinc may have a neuromodulatory role during synaptic transmission and a neuroprotective role during transient ischaemia.

On the wave transmission and reflection properties of stenoses

This study is concerned with the wave reflection properties of arterial stenoses. Two theoretical models have been developed for deriving the reflection coefficient: a linear model resulting from the linearization of the pressure drop-flow equation and an indirect, quasi-nonlinear model, based on the separation of pressure waves into their forward and backward running components proximal and distal to the stenosis. The linear method gave consistently lower values for the reflection coefficient when compared to the quasi-nonlinear model. In vitro experiments in elastic tubes showed that the reflection coefficient is strongly dependent on stenosis severity, mean flowrate, and the elastic properties of the proximal unobstructed artery. For critical stenoses the reflection coefficient is frequency and pulsatility independent. The results suggest that hemodynamically nonsevere stenoses may cause significant wave reflections.

Discriminative Analysis for Image-Based Studies

In this paper, we present a methodology for performing statistical analysis for image-based studies of differences between populations and describe our experience applying the technique in several different population comparison experiments. Unlike traditional analysis tools, we consider all features simultaneously, thus accounting for potential correlations between the features. The result of the analysis is a classifier function that can be used for labeling new examples and a map over the original features indicating the degree to which each feature participates in estimating the label for any given example. Our experiments include shape analysis of subcortical structures in schizophrenia, cortical thinning in healthy aging and Alzheimers disease and comparisons of fMRI activations in response to different visual stimuli.

Noise spectrum and signal transmission through a population of spiking neurons

How reliably can a population of spiking neurons transmit a continuous-time signal? We study the noise spectrum of a fully connected population of spiking neurons with relative and absolute refractoriness. Spikes are generated stochastically with a rate that depends on the postsynaptic potential. The analytical solution of the noise spectrum of the population activity is compared with simulations. We find that strong inhibitory couplings can considerably reduce the noise level in a certain frequency band. This allows the population to reliably transmit signals at frequencies close to or even above the single-neuron firing rate.

Frequency Spectrum of Coupled Stochastic Neurons with Refractoriness

Biological neurons are noisy and integrate their inputs over an extended period of time. On the other hand, the nervous system is able to transmit temporally-changing signals reliably. We address this problem with a homogeneous, fully connected model network. Neurons in our model have an absolute refractory period and fire with a stochastic rate that depends on the input they receive. An analytical solution of the noise spectrum of a population of such neurons is derived, which is in good agreement with simulations. We find that strong inhibitory couplings can considerably reduce the noise level in a certain frequency band. This allows the system to transmit signals with very low noise at frequencies close to the single-neuron mean firing rate.

The accuracy of the population vector estimate in networks of integrate-and-fire type neurons using stationary and transient stimuli

Abstract We study the accuracy of a population code using the population vector method. We consider a network of spiking neurons where each neuron responds preferentially to a particular direction of the stimulus. We show that the accuracy of the population vector depends on the coupling weights between neurons in the population. Some types of interactions produce more accurate estimations for stationary stimuli whereas other types are better for transient stimuli.

Noise Modulation by Stochastic Neurons of the Integrate-and Fire Type

We study the noise properties of a population of spiking neurons of the integrate-and-fire type. We derive an analytical solution for homogeneous network of neurons with relative refractoriness. We show that anticorrelations in the firings decrease the noise in a frequency band that can extend beyond the single neuron frequency.