Individual variability of neural computations underlying flexible decisions

In this study rats were trained to perform a task requiring context-dependent selection and accumulation of sensory evidence. The task was performed in a behavioural box consisting of three straight walls and one curved wall with three nose ports. Each nose port was equipped with an LED to deliver visual stimuli, and with an infrared beam to detect the rat's nose when entering the port. In addition, above the two side ports there were speakers to deliver sound stimuli, and water cannulas to deliver a water reward. At the beginning of each trial, rats were presented with an audiovisual cue indicating the context of the current trial, either prevalent location (LOC) context or prevalent frequency (FRQ) context. The context cues consisted of 1-s-long, clearly distinguishable frequency modulated sounds, and in addition the LOC context was signalled by turning on the LEDs of all three ports, whereas in the FRQ context only the centre LED was turned on. After the end of the context cue, the rats were required to place their nose into the centre port. While maintaining fixation in the centre port, rats were presented with a 1.3-s-long train of randomly timed auditory pulses. Each pulse was played either from the speaker to the rat's left or from the speaker to their right, and each pulse a 5-ms pure tone with either low frequency (6.5 kHz) or high frequency (14 kHz). The pulse trains were generated by Poisson processes with different underlying rates. The strength of the location evidence was manipulated by varying the relative rate of right versus left pulses, and the strength of the frequency evidence was manipulated by varying the relative rate of high versus low pulses. The overall pulse rate was kept constant at 40 Hz. In the LOC context, rats were rewarded if they turned, at the end of the stimulus, towards the side that had played the greater total number of pulses, ignoring the frequency of the pulses. In blocks of frequency trials, rats were rewarded for orienting left if the total number of low-frequency pulses was higher than the total number of high-frequency pulses, and orienting right otherwise, ignoring the location of the pulses. The context was kept constant in blocks of trials, and block switches occurred after a minimum of 30 trials per block, and when a local estimate of performance reached a threshold of 80% correct.