COHERENT SPATIOTEMPORAL PATTERNS OF ONGOING ACTIVITY REVEALED BY REAL-TIME OPTICAL IMAGING COUPLED WITH SINGLE-UNIT RECORDING IN THE CAT VISUAL-CORTEX

1. We examined the spatiotemporal organization of ongoing activity in cat visual areas 17 and 18, in relation to the spontaneous activity of individual neurons. To search for coherent activity, voltage-sensitiv e dye signals were correlated with the activity of single neurons by t he use of spike-triggered averaging. In each recording session an area of at least 2 X 2 mm of cortex was imaged, with 124 diodes. In additi on, electrical recordings from two isolated units, the local field pot ential (LFP) from the same microelectrodes, and the surface electroenc ephalogram (EEG) were recorded simultaneously. 2. The optical signals recorded from the dye were similar to the LFP recorded from the same s ite. Optical signals recorded from different cortical sites exhibited a different time course. Therefore real-time optical imaging provides information that is equivalent in many ways to multiple-site LFP recor dings. 3. The spontaneous firing of single neurons was highly correlat ed with the optical signals and with the LFP. In 88% of the neurons re corded during spontaneous activity, a significant correlation was foun d between the occurrence of a spike and the optical signal recorded in a large cortical region surrounding the recording site. This result i ndicates that spontaneous activity of single neurons is not an indepen dent process but is time locked to the firing or to the synaptic input s from numerous neurons, all activated in a coherent fashion even with out a sensory input. 4. For the cases showing correlation with the opt ical signal, 27-36% of the optical signal during spike occurrence was directly related to the occurrence of spontaneous spikes in a single n euron, over an area of 2 X 2 mm. In the same cortical area, 43-55% of the activity was directly related to the visual stimulus. 5. Surprisin gly, we found that the amplitude of this coherent ongoing activity, re corded optically, was often almost as large as the activity evoked by optimal visual stimulation. The amplitude of the ongoing activity that was directly and reproducibly related to the spontaneous spikes of a single neuron was, on average, as high as 54% of the amplitude of the visually evoked response that was directly related to optimal sensory stimulation, recorded optically. 6. Coherent activity was detected eve n at distant cortical sites up to 6 mm apart. 7. The spontaneous activ ity of adjacent neurons, isolated by the same electrode and sharing th e same orientation preference, was often correlated with different spa tiotemporal patterns of coherent activity, suggesting that adjacent ne urons in the same orientation column can belong to different neuronal assemblies. 8. The averaged optical signal consisted of several tempor al components, mostly in the frequency range of 0-14 Hz, as expected f rom the anesthetized state. It contained only a small amount of fast s ignals in the 15- to 60-Hz range normally found in alert states. 9. Th e spatiotemporal patterns of coherent activity in areas 17 and 18 were explored along the representation of the vertical meridian. The coher ent activity in both areas was time locked to the spontaneous firing o f single neurons over distances exceeding 6 mm. This activity changed in an organized manner as a function of time, but the two cortical reg ions tended to reach peaks or troughs of coherent activity at differen t times. 10. Temporal Fourier analysis of the spatiotemporal patterns imaged in these two visual areas revealed that activity at different c ortical loci consisted of different frequencies. Within a given functi onal area, however, the activity tended to contain the same pattern of frequencies.