, Bowdoin, ME, USA; shank diameter, 500 μm; impedance,
2–4 MΩ at 1 kHz). A guide tube positioned at 5–10 mm distance from the recording electrode—touching but not penetrating the dura—served as the reference. We used a Plexon data acquisition system (MAP) to record and store spike and LFP data simultaneously (Plexon Inc., Dallas). The electrode signal was passed through a headstage with unit gain and then split into the spike and the LFP components. Selleckchem INK1197 For spike recordings the signal was filtered between 250 and 8000 Hz, amplified, and digitized at 40 kHz. Single-unit spiking activity was then isolated using Plexon online and offline sorting software. Analysis of spike data (firing rates) and statistical tests were performed using MATLAB (MathWorks, Natick, MA, USA). The activity/response of each neuron was plotted as a spike density function, generated by convolving a spike train with a function that resembled a postsynaptic potential, i.e., the time constant of the growth phase and the time constant of the decay phase were 1 and 20 ms, respectively (Murthy et al., 2007).
The mean firing rate at different stages of the task was analyzed by computing the mean number of action potentials over a given epoch in repeated presentations (trials). Where indicated, firing rates of each neuron were normalized to the mean activity within a 300 ms time window prior to color-change onset during task trials. Typically, we obtained 12–20 trials per condition during the main task, six to ten during the fixation task, Histamine H2 receptor and six during mapping. In order to examine target selection selleck products as a function of ordinal distance, we grouped trials according to the distance between target and distracter colors and to the stimulus position. This resulted in six groups: target at the preferred location-distance 1, 2, and 3; and distracter at the preferred location-distance 1, 2, and 3 (the preferred location is defined as the position where the target produced the strongest response).
Because distance 1 comprised more color combinations than distances 2 and 3, we corrected for the number of trials through a randomization procedure (i.e., if one condition had 20 trials and the other 15, we randomly chose 15 trials of the former condition to match the number in the latter). All statistical tests were evaluated at p < 0.05. We performed a one-way ANOVA to test for possible effects of task period in the main task. Firing rates within four different task periods were computed: during 200 ms before stimulus onset while the animal was fixating (baseline); during 400 ms preceding the color-change onset when both RDPs were white; and during two 400-ms windows—one starting 50 ms, and the other 550 ms after color-change onset. If the ANOVA revealed a significant main effect of task period, a post hoc test for multiple comparisons was performed.