Locally correlated activity in the goldfish tectum in the absence of optic innervation

Despite a substantial literature on the role of correlated presynaptic activity in the patterning of nerve connections during synaptogenesis in the central nervous system, few studies have focuses on postsynaptic activity during this process. To address the possibility that the target exhibits correlated activity independently of presynaptic activity, extracellular activity was recorded from the main optic innervation layer stratum fibrosum et griseum superficiale (SFGS) in goldfish in which the optic nerve was crushed or the eye removed. At about 2 weeks after denervation, multiunit recordings revealed phasic temporally correlated discharge between different tectal units. Auto-correlation analysis of these trains showed a broad peak 75-100 ms wide confirming temporal correlation. Using cross-correlation analysis of two simultaneous recordings at different distances across tectum, this correlation was shown to be local. Strong positive correlations were detected over about 200 micron and decrease with greater distances disappearing beyond about 400 micron. These correlograms showed a broad symmetrical peak about 75-100 ms wide. This pattern of activity persisted from the day following nerve crush into the period of activity dependent reinnervation at 1 month. When the eye was removed, the pattern could be demonstrated for up to 3 months of denervation indicating the circuitry responsible for the correlated activity was quite stable in the absence of optic innervation. We conclude that tectal elements are capable of locally correlated discharge independently of optic innervation. We propose that locally correlated discharge represents cooperative groups of tectal cells and that these groups, rather than single cells, are the target of the activity dependent synaptic rearrangement such as ocular dominance columns which occurs during synaptogenesis.