Recorded spike waveforms were sorted into separate units using an automated cluster analysis method referred to as the KlustaKwik algorithm (Harris et al., 2000), which applied principal component analysis of the waveforms. Neurons with significant elevation of firing rate during the
presentation of visual stimuli were identified by comparing the firing rate in the 0.5-s (0.3-s in the reaction-time task) interval of a stimulus presentation with the 0.5-s interval of fixation (paired t-test; P < 0.05). The spatial tuning of visually responsive neurons was determined by comparing the firing rates during the presentation of cue stimulus of either color (level 1 difficulty) at the different RG7420 cell line locations. Neurons with spatial selectivity for the location of the single stimulus, demonstrated by a significant main effect of stimulus location (two-way anova; P < 0.05), were included in analysis. Neuronal time of target discrimination was computed by comparing population firing rates of the salient stimulus in receptive fields and the distractor in receptive fields. Significance of firing rate difference was determined for 10-ms bins stepped by 1 ms (paired t-test, P < 0.05). Target discrimination time was identified as the time point of the first of 10 consecutive
bins with significantly greater responses to a salient stimulus than to distractors selleck chemicals llc (Katsuki & Constantinidis, 2012a). In order to quantify the trial-to-trial association between perceptual choice and neuronal activity, we analysed trials that resulted in correct choices and incorrect choices in the delayed match-to-sample task and the reaction-time task using the choice probability analysis based on signal detection theory (Britten et al., 1996). We first identified the stimulus location with
the highest firing rate for each neuron. Firing rates of correct and error trials when the identical stimulus appeared at this location were pooled separately. A receiver operating characteristic learn more (ROC) curve was computed from these two distributions of firing rates. The choice probability, a measurement of correlation between the behavioral choice and neuronal activity, was defined as the area under the ROC curve. A choice probability value of 1 indicates that there is a perfect correlation between the behavioral choices and the neuronal discharge rates; a value of 0.5 indicates a random correlation between the two. Time-resolved choice probabilities were computed from the spikes in 250-ms time windows, stepped by 50-ms intervals. The choice of bin size was dictated by the discharge rate of the population of neurons and number of trials available in each condition, particularly error trials. To obtain a sufficient number of error trials and spikes to analyse, we only used the trials with most difficult stimulus level (Level 3 in Fig. 1D) and relied on neurons with at least three error trials for this condition.