Abstract
A fundamental property of brain function is that the spiking activity of cortical neurons is variable and that some of this variability is correlated between neurons. Correlated activity not due to the stimulus arises from shared input but the neuronal circuit mechanisms that result in these noise correlations are not fully understood. Here we tested in the visual system if correlated variability in mid-level area V4 of visual cortex is altered following extensive lesions of primary visual cortex (V1). To this end we recorded longitudinally the neuronal correlations in area V4 of two behaving macaque monkeys before and after a V1 lesion while the monkeys fixated a grey screen. We found that the correlations of neuronal activity survived the lesions in both monkeys. In one monkey, the correlation of multi-unit spiking signals was strongly increased in the first week post-lesion, while in the second monkey, correlated activity was slightly increased, but not greater than some week-by-week fluctuations observed. The typical drop-off of inter-neuronal correlations with cortical distance was preserved after the lesion. Therefore, as V4 noise correlations remain without feedforward input from V1, these results suggest instead that local and/or feedback input seem to be necessary for correlated activity.
Highlights
From this analysis we can conclude that correlated neuronal activity in V4 was present in the absence of feedforward input from V1, but that it displayed a surprising increase in at least one monkey compared to conditions with intact V1 input during the first week after lesioning V1
In this study we have shown that a lesion to the primary visual cortex, through which almost all visual information enters cortex, and which projects directly and indirectly to V4 among other areas[28], leads to an increase in correlated noise in V4, in one monkey and no change or a small increase in another
The increase was found in multi-unit activity (MUA) at both long and short timescales and in single unit activity (SUA) for different firing rate ranges, and without an increase in the overall spike rate magnitude and Fano factor pre to post lesion, i.e., effects that could cause correlation increase[12,22,29]
Summary
The average multi-unit activity (MUA) rate for each trial[24] (see Methods) was calculated for each electrode (Fig. 1). Correlations increased from pre to post lesion for spike count correlations between single-unit pairs assessed in monkey B (see Supplementary Fig. S3).
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