Oscillatory firing and interneuronal correlations in squirrel monkey striate cortex.

1. This work explores a mechanism that the brain may use for linking related percepts. It has been proposed that temporal relationships in the firing of neurons may be important in indicating how the stimuli that activate those neurons are related in the external world. Such temporal relationships cannot be seen with conventional receptive field mapping but require cross-correlation and auto-correlation analysis. 2. In the cat and the macaque monkey, cells with similar receptive field properties show correlated firing even when their receptive fields do not overlap. Here I report that in the squirrel monkey, as in the cat, pairs of cells < or = 5 mm apart can show correlated firing, and these correlations between pairs of cells are often stronger when they are stimulated by a single contour. This suggests that the correlations reflect not only permanent connections between cells with similar receptive fields, but in addition may encode information that the activating stimuli are continuous or part of a single object. I also find that, as in the cat, and contrary to some other reports on experiments in monkeys, the correlated firing is often rhythmic. These recordings further indicate that periods of rhythmicity are associated with stronger interneuronal synchrony, which is consistent with the hypothesis that recurrent feedback loops are involved in generating both. 3. Pairs of cells in the same cortical column, but at different depths also showed correlated firing, but with several milliseconds difference in timing between layers. This was true for cells at different depths within layer 2/3 and for pairs of cells in different layers (2/3 vs. 4B or 4C alpha), providing evidence for cross-talk between the magno- and parvocellular streams.