THE RF-CINEMATOGRAM - A CROSS-CORRELATION TECHNIQUE FOR MAPPING SEVERAL VISUAL RECEPTIVE-FIELDS AT ONCE

We present a spike-triggered averaging method capable of mapping the v isual receptive fields of several neurons simultaneously. The stimulat ion is general and the mapping proceeds automatically without the need to match the stimulation to the cells' preference for position, orien tation, direction, etc. The maps are spatiotemporal; receptive field ( RF) structures are quantitatively determined in three dimensions: the two dimensions of visuotopic space, and time. The method presented is one of a family of ''reverse correlation'' or ''spike-triggered averag ing'' techniques (DeBoer and Kuyper 1968) capable of revealing linear aspects of stimulus-response coupling. The formal relationship of thes e methods to stimulus-response cross-correlation is shown. The analysi s is extended to provide some second-order axis-of-motion information (''direction marks''). The stimulus is a constantly illuminated, rando mly jumping bright or dark spot, not an elongated bar. Spot diameters between one-third to 1 x RF width are effective. The method ascertains for each recorded action potential or ''spike'' the prior visual fiel d position of the spot. The average or most probable spot positions de fine the receptive field spatially. Repeating the process for a succes sion of times prior to observed spikes defines the field temporally, p resented here as a succession of spatial maps. We term this portrayal a receptive field cinematogram, RFc or cine. The RFc reveals and econo mically portrays the spread of excitability and suppression across the receptive field, culminating in the generation of a spike. RFcs for L GN neurons and for simple cells recorded in cat cortical areas 17 and 18 are presented and interpreted in terms of classic ON/OFF regions. T he availability of temporal information permits the separation of an e xcitatory exit response, generated when a moving bright spot leaves an OFF region, from an excitatory entrance response occurring when a bri ght spot enters an ON region, because these responses occur at differe nt times (exit responses earlier). Spike emission remains coupled to ( cross-correlated with) stimulus events over time periods as long as 96 ms, implying that some stimulus drive or afferent visual input is del ayed by as much as 96 ms more than other input. This is a striking ins tance of temporal dispersion in the visual system. In some cells, said to be ''spatiotemporally inseparable'', the delay (latency) varies sy stematically across the visual field; i.e., the place for optimal stim ulation varies with the time prior to spike emission. In these cells, the RFc shows receptive field structures which move across the visual field over trajectories equal to approximately twice the total convent ional RF width. Exit and entrance responses, on the other hand, arise in a simple way from separated ON and OFF RF subregions. ON/OFF mechan isms thus appear unrelated to spatiotemporal inseparability. The RFc m ethod is easily automated, efficient, and characterizes multiple RFs s imultaneously, as required in work with multiple electrode arrays.