Distinguishing between parallel and serial processing in visual attention from neurobiological data

Serial and parallel processing in visual search have been long debated in psychology but the processing mechanism remains an open issue. Serial processing allows only one object at a time to be processed, whereas parallel processing assumes that various objects are processed simultaneously. Here we present novel neural models for the two types of processing mechanisms based on analysis of simultaneously recorded spike trains using electrophysiological data from prefrontal cortex of rhesus monkeys while processing task-relevant visual displays. We combine mathematical models describing neuronal attention and point process models for spike trains. The same model can explain both serial and parallel processing by adopting different parameter regimes. We present statistical methods to distinguish between serial and parallel processing based on both maximum likelihood estimates and decoding the momentary focus of attention when two stimuli are presented simultaneously. Results show that both processing mechanisms are in play for the simultaneously recorded neurons, but neurons tend to follow parallel processing in the beginning after the onset of the stimulus pair, whereas they tend to serial processing later on. This could be explained by parallel processing being related to sensory bottom-up signals or feedforward processing, which typically occur in the beginning after stimulus onset, whereas top-down signals related to cognitive modulatory influences guiding attentional effects in recurrent feedback connections occur after a small delay, and is related to serial processing, where all processing capacities are being directed towards the attended object.\n\nAuthor summaryA fundamental question concerning processing of visual objects in our brain is how a population of cortical cells respond when presented with more than a single object in their receptive fields. Is one object processed at a time (serial processing), or are all objects processed simultaneously (parallel processing)? Inferring the dynamics of attentional states in simultaneously recorded spike trains from sensory neurons while being exposed to a pair of visual stimuli is key to advance our understanding of visual cognition. We propose novel statistical models and measures to quantify and follow the time evolution of the visual cognition processes right after stimulus onset. We find that in the beginning processing appears to be predominantly parallel, which develops into serial processing 150 - 200 ms after stimulus onset in prefrontal cortex of rhesus monkeys.