Biomedical subjects

Christos Constantinidis

Publications and source records attributed to Christos Constantinidis.

9 recordsLinked to original sources

PubMed2007

Temporal properties of posterior parietal neuron discharges during working memory and passive viewing.

Working memory is mediated by the discharges of neurons in a distributed network of brain areas. It was recently suggested that enhanced rhythmicity in neuronal activity may be critical for sustaining remembered information. To test whether working memory is characterized by unique temporal discharge patterns, we analyzed the autocorrelograms and power spectra of spike trains recorded from the posterior parietal cortex of monkeys performing a visuospatial working-memory task. We compared the intervals of active memory maintenance and fixation and repeated the same analysis in spike trains from monkeys never trained to perform any kind of memory task. The most salient effect we observed was a decrease of power in the 5- to 10-Hz frequency range during the presentation of visual stimuli. This pattern was observed both in the working-memory condition and the control condition, although it was more prominent in the former, where it persisted after cue presentation when the monkeys actively remembered the spatial location of the stimulus. Low-frequency power suppression resulted from relative refractory periods that were significantly longer in the working-memory condition and presumably emerged from local-circuit inhibition. We also detected a spectral peak in the 15- to 20-Hz range, although this was more prominent during fixation than during the stimulus and working-memory periods. Our results are in line with previous reports in prefrontal cortex and indicate that unique temporal patterns of single-neuron firing characterize persistent delay activity, although these do not involve the appearance of enhanced oscillations.

Action Potentials↗

PubMed2006

Posterior parietal mechanisms of visual attention.

The posterior parietal cortex plays an important role in visual-spatial attention. Imaging studies reveal consistent parietal activation in attention tasks, while lesions to this area produce significant perceptual deficits. Non-human primate models have been particularly informative in investigating the neurophysiology of attention. The activity of posterior parietal neurons to the same operant stimuli is modulated by whether they are attended or not based on prior information, or whether they attract attention by virtue of their saliency. Posterior parietal neuronal activity appears to provide critical information for shifting attention between stimuli and spatial locations.

Animals↗

PubMed2005

A software solution for the control of visual behavioral experimentation.

Psychophysical and neurophysiological research requires precise control of experimental devices for the purpose of delivering stimuli and monitoring behavioral and neural responses. This has previously been accomplished by complex, often proprietary, programmable systems, interfacing with a limited range of hardware. We have developed a software solution entirely within the Matlab environment that can achieve high-speed control of experimental and behavioral variables. We make this Wake-Forest Visual Experimentation (WaVE) software freely available under the GNU public license, and demonstrate how to customize it to individual laboratory needs. WaVE takes advantage of existing Matlab libraries and toolboxes to present visual stimuli, collect experimental data, update behavioral variables, and communicate with other computers. Although we have developed it for use in a Windows-based Personal Computer, the portability of the Matlab code makes possible its customization for use in a variety of other systems. We present simulation results showing sub-millisecond sampling rate and updating precision, running on single-processor, desktop PCs. The WaVE software offers a simple, flexible and powerful solution that compares favorably with many of its costly alternatives.

Animals↗

PubMed2005

Posterior parietal cortex automatically encodes the location of salient stimuli.

We examined the responses of neurons in posterior parietal area 7a to salient stimuli appearing alone or within multiple-stimulus displays in monkeys trained only to maintain fixation. Discharges in a population of parietal neurons encoded the location of the salient stimulus, although the latter had no task significance for the monkey. Neuronal selectivity for the location of the salient stimulus depended solely on its intrinsic difference from the background elements in the array and not on the color of the stimulus per se. These results were similar to those reported in monkeys trained to actively locate a salient stimulus in a multiple-stimulus display. A lower percentage of neurons with significant selectivity for the salient stimulus was observed in the fixation-only animals. These neurons took longer for the selective responses to emerge and showed a lower power of discrimination. The findings suggest that the posterior parietal cortex automatically detects and encodes the location of salient stimuli even when they are unrelated to the behavioral task.

Animals↗

PubMed2004

A neural circuit basis for spatial working memory.

The maintenance of a mental image in memory over a time scale of seconds is mediated by the persistent discharges of neurons in a distributed brain network. The representation of the spatial location of a remembered visual stimulus has been studied most extensively and provides the best-understood model of how mnemonic information is encoded in the brain. Neural correlates of spatial working memory are manifested in multiple brain areas, including the prefrontal and parietal association cortices. Spatial working memory ability is severely compromised in schizophrenia, a condition that has been linked to prefrontal cortical malfunction. Recent computational modeling work, in interplay with physiological studies of behaving monkeys, has begun to identify microcircuit properties and neural dynamics that are sufficient to generate memory-related persistent activity in a recurrent network of excitatory and inhibitory neurons during spatial working memory. This review summarizes recent results and discusses issues of current debate. It is argued that understanding collective neural dynamics in a recurrent microcircuit provides a key step in bridging the gap between network memory function and its underlying cellular mechanisms. Progress in this direction will shed fundamental insights into the neural basis of spatial working memory impairment associated with mental disorders.

Animals↗

PubMed2004

The primate working memory networks.

Working memory has long been associated with the prefrontal cortex, since damage to this brain area can critically impair the ability to maintain and update mnemonic information. Anatomical and physiological evidence suggests, however, that the prefrontal cortex is part of a broader network of interconnected brain areas involved in working memory. These include the parietal and temporal association areas of the cerebral cortex, cingulate and limbic areas, and subcortical structures such as the mediodorsal thalamus and the basal ganglia. Neurophysiological studies in primates confirm the involvement of areas beyond the frontal lobe and illustrate that working memory involves parallel, distributed neuronal networks. In this article, we review the current understanding of the anatomical organization of networks mediating working memory and the neural correlates of memory manifested in each of their nodes. The neural mechanisms of memory maintenance and the integrative role of the prefrontal cortex are also discussed.

Animals↗

PubMed2003

Temporally irregular mnemonic persistent activity in prefrontal neurons of monkeys during a delayed response task.

An important question in neuroscience is whether and how temporal patterns and fluctuations in neuronal spike trains contribute to information processing in the cortex. We have addressed this issue in the memory-related circuits of the prefrontal cortex by analyzing spike trains from a database of 229 neurons recorded in the dorsolateral prefrontal cortex of 4 macaque monkeys during the performance of an oculomotor delayed-response task. For each task epoch, we have estimated their power spectrum together with interspike interval histograms and autocorrelograms. We find that 1). the properties of most (about 60%) neurons approximated the characteristics of a Poisson process. For about 25% of cells, with characteristics typical of interneurons, the power spectrum showed a trough at low frequencies (<20 Hz) and the autocorrelogram a dip near zero time lag. About 15% of neurons had a peak at <20 Hz in the power spectrum, associated with the burstiness of the spike train; 2). a small but significant task dependency of spike-train temporal structure: delay responses to preferred locations were characterized not only by elevated firing, but also by suppressed power at low (<20 Hz) frequencies; and 3). the variability of interspike intervals is typically higher during the mnemonic delay period than during the fixation period, regardless of the remembered cue. The high irregularity of neural persistent activity during the delay period is likely to be a characteristic signature of recurrent prefrontal network dynamics underlying working memory.

Action Potentials↗

PubMed2002

A role for inhibition in shaping the temporal flow of information in prefrontal cortex.

The prefrontal cortex is important in guiding or inhibiting future responses, which requires the temporal integration of events and which provides continuity to the thought process. No cellular mechanism has been proposed to explain how the mental representation of a response or idea is linked to the next. Using simultaneous recordings in monkeys, we revealed inhibitory interactions between neurons active at different time points relative to the cue presentation, delay interval and response period of a working memory task. These findings suggest an important role of inhibition in the cerebral cortex-controlling the timing of neuronal activities during cognitive operations and thereby shaping the temporal flow of information.

Animals↗

PubMed2002

Correlated discharges among putative pyramidal neurons and interneurons in the primate prefrontal cortex.

Neurophysiological recordings have revealed that the discharges of nearby cortical cells are positively correlated in time scales that range from millisecond synchronization of action potentials to much slower firing rate co-variations, evident in rates averaged over hundreds of milliseconds. The presence of correlated firing can offer insights into the patterns of connectivity between neurons; however, few models of population coding have taken account of the neuronal diversity present in cerebral cortex, notably a distinction between inhibitory and excitatory cells. We addressed this question in the monkey dorsolateral prefrontal cortex by recording neuronal activity from multiple micro-electrodes, typically spaced 0.2-0.3 mm apart. Putative excitatory and inhibitory neurons were distinguished based on their action potential waveform and baseline discharge rate. We tested each pair of simultaneously recorded neurons for presence of significant cross-correlation peaks and measured the correlation of their averaged firing rates in successive trials. When observed, cross-correlation peaks were centered at time 0, indicating synchronous firing consistent with two neurons receiving common input. Discharges in pairs of putative inhibitory interneurons were found to be significantly more strongly correlated than in pairs of putative excitatory cells. The degree of correlated firing was also higher for neurons with similar spatial receptive fields and neurons active in the same epochs of the behavioral task. These factors were important in predicting the strength of both short time scale (<5 ms) correlations and of trial-to-trial discharge rate covariations. Correlated firing was only marginally accounted for by motor and behavioral variations between trials. Our findings suggest that nearby inhibitory neurons are more tightly synchronized than excitatory ones and account for much of the correlated discharges commonly observed in undifferentiated cortical networks. In contrast, the discharge of pyramidal neurons, the sole projection cells of the cerebral cortex, appears largely independent, suggesting that correlated firing may be a property confined within local circuits and only to a lesser degree propagated to distant cortical areas and modules.

Animals↗