Rat psychomotor vigilance task with fast response times using a conditioned lick behavior

Abstract

Investigations into the physiological mechanisms of sleep control require an animal psychomotor vigilance task (PVT) with fast response times (<300 ms). Rats provide a good PVT model since whisker stimulation produces a rapid and robust cortical evoked response, and animals can be trained to lick following stimulation. Our prior experiments used deprivation-based approaches to maximize motivation for operant conditioned responses. However, deprivation can influence physiological and neurobehavioral effects. In order to maintain motivation without water deprivation, we conditioned rats for immobilization and head restraint, then trained them to lick for a 10% sucrose solution in response to whisker stimulation. After approximately 8 training sessions, animals produced greater than 80% correct hits to the stimulus. Over the course of training, reaction times became faster and correct hits increased. Performance in the PVT was examined after 3, 6 and 12 h of sleep deprivation achieved by gentle handling. A significant decrease in percent correct hits occurred following 6 and 12 h of sleep deprivation and reaction times increased significantly following 12 h of sleep deprivation. While behaviorally the animals appeared to be awake, we observed significant increases in EEG delta power prior to misses. The rat PVT with fast response times allows investigation of sleep deprivation effects, time-on-task and pharmacological agents. Fast response times also allow closer parallel studies to ongoing human protocols.

Figure 1

Two pictures show an animal in our custom designed wrap during different stages of wrapping. A) The rat was first wrapped in felt, covering the front paws with Velcro forming the internal collar around the neck to restrict/limit head movement. B) Denim fabric was wrapped around the rat and secured with Velcro to prevent body movements.

Figure 2

A schematic of electrode placement shows the 64 channel array used to map the somatosensory cortex. The skull was removed over the right hemisphere to implant the 64 (8 × 8) channel electrode array over the intact dura. Stainless steel screws were used to record frontal and parietal EEG with two additional screws behind lamda serving as a ground reference. Two stainless steel wires were guided under the skin across the rib cage to record EKG and two wires in the neck muscles recorded EMG activity.

Figure 3

The experimental setup includes a sipper tube (connected to the force transducer) placed in front of the rat’s face to measure licks and deliver fluid. The rat’s body was restrained using the wrap described in Figure 1, and the head was held steady by inserting earbars into the hollow tube on the back of the rat’s head stage. Amplifier boards were connected to the rat’s head stage and electrode array to record evoked responses when the whiskers were stimulated. The whisker was placed in the hook at the end of the whisker stimulator for stimulation. The rat was trained to lick in response to the whisker stimulation to receive 100 µl of a sucrose solution.

Figure 4

Bar graphs illustrate the mean (+S.E.M.) percent correct hits to the PVT under the three conditions of: 1) the whisker in the stimulator and receiving stimulation (WIN) 2) the whisker in the stimulator with no stimulation (WNS) and 3) the whisker out of the stimulator and the stimulator operating (WO) (* = p < 0.001 when compared to other conditions). Test was done with 80 dB background white noise. Any licks within 2s of the stimulus during WNS and WO conditions were most likely random lick events.

Figure 5

Mean (± S.E.M.) percent correct and reaction times over the twenty training sessions show that the animals significantly improved their accuracy over the training sessions (p < 0.001), reaching the criterion of 80% correct for 3 consecutive days by the eighth session. The animals performed significantly faster over the training sessions (p < 0.001).

Figure 6

Example averaged evoked responses from all animals are shown during the baseline before licking (BBL, black trace), during licking (DL, thick gray trace), and the baseline after licking (BAL, light gray trace). The licking behavior during the DL period is shown as a solid black line below the evoked responses in the form of an event histogram. No licks could be recorded during baseline conditions because the sipper tube was not available. An increase in P1/N1 amplitude was seen during conditioned learning when comparing baseline averages (whiskers were stimulated, but the sipper tube was removed and no licking occurred) to those during active licking (p < 0.05).

Figure 7

Mean (+ S.E.M.) percent correct hits during baseline and sleep deprivation sessions are shown in the upper panel (A). The animal’s performance significantly decreased during 6 and 12 hours of sleep deprivation (p < 0.005). Mean (+ S.E.M.) reaction time during baseline and sleep deprivation sessions are shown in the lower panel (B). The reaction time significantly increased during 12 hours of sleep deprivation (p < 0.05).

Figure 8

The average reaction times during each 5 minute bin (1–5 min, 5–10 min, 10–15 min, 15–20 min, and 20–25 min) of the 25 minute session were analyzed using a regression analysis. The top panel shows the mean (+S.E.M.) reaction times for each 5 minute bin during the training and baseline sessions. The regression analysis revealed a linear relationship (p < 0.01, r² = 0.96 and r² = 0.92, respectively) such that reaction times decreased during the session but not during the sleep deprivation sessions (p > 0.05, r² = 0.02 for 3 hours, r² = 0.05 for 6 hours, and r² = 0.00 for 12 hours of deprivation) as seen in the lower panel.

Figure 9

The mean (+ S.E.M.) delta, EMG and gamma power (% change) during sleep deprivation sessions show delta power was significantly higher when the animal did not respond to the stimulus (miss) (* = p < 0.05). There was a trend for EMG and EEG gamma power to be lower when the animal did not respond to the stimulus but did not reach significance (p < 0.15).