Electroencephalogram And Interval Timing Behavior

Many studies have been conducted in the past decade to measure the electroencephalogram (EEG) during discrimination task performance in rats (e.g., Ehlers et al., 1999; Givens and Olton, 1994; Sato and Sakata, 1999). Virtually all of these studies have involved simple auditory or visual memory tasks or discriminative learning tasks not involving temporal discrimination or timing. Only a few studies have investigated interval timing behavior and EEG correlations in animals (e.g., Onoda and Sakata, submitted). The event-related potentials (ERPs) are voltage fluctuations that are associated in time with some physiological or mental occurrence (e.g., Picton et al., 2000). In human studies, the timing of stimuli is a very important factor affecting ERPs, even more so for animal studies. Temporal discrimination involves a variety of complex mental operations in animals. Consequently, it is useful to conduct ERP analysis for studying the brain mechanisms involved in timing and time perception.

13.2.1 How to Measure the EEG during Interval Timing Tasks in Rats

There are three major methods to study time perception in animals: temporal discrimination such as the temporal bisection procedure, temporal differentiation procedures, and peak-interval reproduction procedures (Catania, 1970). The temporal bisection procedure deals with the concept of time using short and long anchor durations. The bisection point refers to the point of subjective equality in timescale, which indicates the durations that subjects classify as equally short or long. The temporal differentiation procedure deals with the decision to judge the appropriate point in time to make a response. It requires that a subject suppress its response until a criterion time has been reached. This procedure has both strengths and weaknesses for the study of interval timing behavior; one strength is that it is easy to train, but one weakness is that it is difficult to evaluate the cause of premature responses. The peak-interval procedure is perhaps the best method for studying interval timing behavior in animals. The behavioral responses (e.g., lever presses) occur more naturally and reflect the temporal expectancy of the availability of reinforcement.

EEG is reflected by the brain activity associated with certain decision processes. Typically, it is used in conjunction with the study of stimulus discrimination. The stimulus comes to have meaning for the animal with repeated experience. Changes in brain responsiveness to an external stimulus can be investigated by the measurement of evoked potentials. The evoked potential will increase its amplitude by presenting the stimulus in combination with certain experiences (e.g., Takeuchi et al., 2000). In the same way, an internal stimulus such as the judgment of a temporal criterion will evoke an ERP. Unfortunately, because of the variety of response artifacts, our method for recording ERPs is more easily adapted to a simple temporal discrimination procedure than to the peak-interval procedure. Consequently, we trained rats to make a left response following a 2-sec duration and a right response following an 8-sec duration. We then used this temporal discrimination method to obtain the correlation with ERPs.

In order to record ERPs, one needs to surgically implant chronically indwelling microelectrodes that are targeted as specific regions in the brain. We selected the target regions based on the proposal that the frontal cortex, hippocampus, and the cerebellum are involved in timing (see Meck, 1996, this volume). After sufficient training of the temporal discrimination task, we implanted the electrodes (0.2 mm in diameter) in rats. The stimuli were 2- or 8-sec 2000-Hz, 80-dB tones. The EEG sampling phases were the onset and offset of the stimulus, as shown in Figure 13.2.

In this study we trained nine male rats. All rats were trained on the same temporal discrimination task (T-task) in the first stage of training. After the rats maintained over 85% of correct responses in three consecutive sessions, rats were implanted with electrodes. The EEG was recorded during the T-task in two consecutive sessions, and then rats were trained a simple reaction time control task (C-task). In the C-task, only a 2-sec tone was presented and only one lever was inserted into the box. This simple reaction time task was used as a control for both timing and motor response. The mean ERP for the nine rats is presented in Figure 13.3.

Temporal Discrimination Task


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Lever Presses

Control Task

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Lever Presses

Temporal Discrimination Task n

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