SINGLE neurons can signal subtle changes in the sensory environment with surprising fidelity, often matching the perceptual sensitivity of trained psychophysical observers1–10. This similarity poses an intriguing puzzle: why is psychophysical sensitivity not greater than that of single neurons? Pooling responses across neurons should average out noise in the activity of single cells, leading to substantially improved psychophysical performance. If, however, noise is correlated among these neurons, the beneficial effects of pooling would be diminished10–12. To assess correlation within a pool, the responses of pairs of neurons were recorded simultaneously during repeated stimulus presentations. We report here that the observed covariation in spike count was relatively weak, the correlation coefficient averaging 0.12. A theoretical analysis revealed, however, that weak correlation can limit substantially the signalling capacity of the pool. In addition, theory suggests a relationship between neuronal responses and psychophysical decisions which may prove useful for identifying cell populations underlying specific perceptual capacities.

Correlated neuronal discharge rate and its implications for psychophysical performance

http://hardcarve.com/wikipic/GrayCM1995_tetrodes.pdf

Tetrodes markedly improve the reliability and yield of multiple single-unit isolation from multi-unit recordings in cat striate cortex.

Closed-Loop Deep Brain Stimulation Is Superior in Ameliorating Parkinsonism

1. To test the mode of functional connectivity in the basal ganglia circuitry, we studied the activity of simultaneously recorded neurons in the globus pallidus (GP) of a behaving rhesus monkey. The cross-correlograms of pairs of neurons in the GP were compared with those of neurons in the thalamus and frontal cortex and to the cross-correlograms of pallidal pairs after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment. 2. In contrast with cortical and thalamic neuronal activity, almost all pairs (n = 76/81 pairs; 93.8%, 1,629/1,651 histograms; 98.7%) of GP neurons in the normal monkey were not driven by a common input. 3. The monkey was systemically treated with MPTP until the appearance of parkinsonian signs and an intermittent 7- to 11-Hz action/postural tremor. After the MPTP treatment, many pallidal neurons (49/140; 35%) became oscillatory, and 19% (n = 31/162) of pallidal pairs had oscillatory cross-correlograms. 4. These results support the model of parallel processing in the basal ganglia of normal monkeys and suggest a breakdown of the independent activity in the parkinsonian state.

/pdf/neurons-in-the-globus-pallidus-do-not-show-correlated-1vlklnc2sl.pdf

Neurons in the globus pallidus do not show correlated activity in the normal monkey, but phase-locked oscillations appear in the MPTP model of parkinsonism

To investigate the role of the basal ganglia in parkinsonian tremor, we recorded hand tremor and simultaneous activity of several neurons in the external and internal segments of the globus pallidus (GPe and GPi) in two vervet monkeys, before and after systemic treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and development of parkinsonism with tremor of 5 and 11 Hz. In healthy monkeys, only 11% (20/174) of the GPe cells and 3% (1/29) of the GPi cells displayed significant 3–19 Hz oscillations. After MPTP treatment, 39% (107/271) of the GPe cells and 43% (26/61) of the GPi cells developed significant oscillations. Oscillation frequencies of single cells after MPTP treatment were bimodally distributed around 7 and 13 Hz. For 10% of the oscillatory cells that were recorded during tremor periods, there was a significant tendency for the tremor and neuronal oscillations to appear simultaneously. Cross-correlation analysis revealed a very low level of correlated activity between pallidal neurons in the normal state; 95.6% (477/499) of the pairs were not correlated, and oscillatory cross-correlograms were found in only 1% (5/499) of the pairs. After MPTP treatment, the correlations increased dramatically, and 40% (432/1080) of the cross-correlograms had significant oscillations, centered around 13–14 Hz. Phase shifts of the cross-correlograms of GPe pairs, but not of GPi, were clustered around 0°. The results illustrate that MPTP treatment changes the pattern of activity and synchronization in the GPe and GPi. These changes are related to the symptoms of Parkinson9s disease and especially to the parkinsonian tremor.

https://www.jneurosci.org/content/jneuro/20/22/8559.full.pdf

Firing patterns and correlations of spontaneous discharge of pallidal neurons in the normal and the tremulous 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine vervet model of parkinsonism.

Florentin Wörgötter

Papers

An on-line spike form discriminator for extracellular recordings based on an analog correlation technique.