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The first detailed parallel model of visual processing of multi-element displays was the independent channels model proposed by Eriksen and his colleagues (e.g. In a parallel model of attention, several stimuli can be attended at the same time. Thus, the rate of increase in mean positive response time as a function of N equals Δ t/2 (see for experimental evidence of serial processing in a behavioural task). Because the order in which items are scanned is independent of their status as targets or distractors, the number of items processed before a positive response is made varies at random between 1 and N with a mean of (1 + N)/2. A positive response is made as soon as a target is found. Furthermore, the rate of increase in mean negative response time as a function of N equals the mean time taken to process one item, Δ t. Thus, the number of items processed before a negative response is made equals N. A negative response is made when all items have been scanned and classified as distractors. The order in which items are scanned is independent of their status as targets versus distractors. When an item is scanned, it is classified as a target or a distractor. In a simple serial model, the N items are scanned one at a time. The method of analysis was laid out by Sternberg and further developed by Schneider & Shiffrin. Positive (target present) and negative (target absent) mean response times are analysed as functions of the display set size N (the number of items in the display). In typical experiments on visual search, the task of the observer is to indicate as quickly as possible if a certain type of target is present in a display. Behavioural methods for distinguishing between serial and parallel visual search Thus, it is possible to follow attentional shifts in individual neurons, that would otherwise be averaged out if these do not happen simultaneously in the entire neuron population.ġ.1. In particular, we do not average over trials or neurons, but model each spike train individually within a larger model, to allow the finer dynamics and interactions to reveal themselves. This provides alternative means to quantify parallel versus serial visual processing. Thus, at a given time, attention is divided if all neurons are not responding to the same stimulus. In this article, we define attention to a stimulus on a cellular level as the stimulus a specific neuron is responding to. Point process modelling is a natural mathematical framework for addressing such phenomena, and we embed this into models of visual attention. A spike train is a sequence of recorded times at which a neuron fires an action potential, and it is believed that spike times are the primary way to transmit information in the nervous system. The method is based on analysis of spike trains of simultaneously recorded single neurons measured in prefrontal cortex of rhesus monkeys while being exposed to a pair of stimuli, which the animal should detect and later respond to with a saccade towards a target object, first presented in. In this article, we briefly review extant empirical methods and their results and then present and exemplify a new method for distinguishing between serial and parallel visual search. The question of serial versus parallel search has been extensively investigated by behavioural methods in cognitive psychology, but it is still highly controversial. In serial search, only one stimulus is attended at a time, whereas in parallel search, several stimuli are attended at the same time. 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.Ī fundamental question in theories of visual search is whether the process is serial or parallel for given types of stimulus material (for comprehensive reviews, see ). 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. The same model can explain both serial and parallel processing by adopting different parameter regimes. We combine mathematical models describing neuronal attention and point process models for spike trains. 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. Serial processing allows only one object at a time to be processed, whereas parallel processing assumes that various objects are processed simultaneously. Serial and parallel processing in visual search have been long debated in psychology, but the processing mechanism remains an open issue.
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