Hippocampal Th Eta Wave

Hippocampal theta cells can be distinguished from theta interneurons in freely moving rats on the basis of their electrophysiological properties (Wiebe and Staubli, 2001). The hippocampal theta wave is called theta rhythm or rhythmic slow activity (RSA). Theta rhythm is a local field potential known as a sinusoidal-like EEG signal occurring at frequencies within the bandwidth from 6 to 12 Hz in rats. There are many studies showing a strong correlation between theta rhythm and voluntary motor behaviors such as running, rearing, and jumping (e.g., Bland, 1986; Whishaw and Vanderwolf, 1973). The correlation between the temporal dynamics of theta frequency and nonsteady wheel-running speed within a single trial tends to be positive (Shin and Talnov, 2001). A few studies have reported that learning-related theta cell firing correlates well with performance in simple perceptual discrimination tasks (e.g., Deadwyler et al., 1996), as well as task-related theta frequency changes associated with performance in an auditory discrimination (e.g., Brankack et al., 1996).

13.3.1 Analysis of Hippocampal Theta

The hippocampal theta has long been considered to be a reflection of the neural processing occurring in the structures of the hippocampus. In the analysis of the ongoing hippocampal theta, there are two ways to conduct a time series analysis; one is to remove random variation by averaging and to derive the ERPs. These potentials can be recorded in front and back of the stimulus presentation point from specific brain regions during task performance in rats. ERPs are extracted from the ongoing EEG by means of filtering and signal averaging. ERPs can be evaluated in both frequency and time domains at the point of stimulus presentation. The other method is to evaluate the time series by a power frequency analysis. In this analysis, it is customary to subdivide the EEG signals into quasi-stationary epochs and to characterize them by a number of statistical parameters, such as frequency spectra. The power spectrum gives the distribution of the squared amplitude of different frequency components. These spectral distributions are calculated using the Fast Fourier transform.

13.3.2 Hippocampal Theta and Interval Timing

It has been proposed that the hippocampus serves as a working memory buffer for many types of information, including temporal information (e.g., Meck et al., 1984). We recorded hippocampal theta in both the T-task and C-task. The rat had to evaluate the duration of auditory stimuli in the former task, either 2- or 8-sec durations, and choose to respond to one of the two levers. On the other hand, the rat does not need to attend to the duration of the stimulus in the latter task; only a 2-sec tone is presented, and the rat is only required to respond on one lever. No significant differences were observed between reaction times in the T-task and the C-task (724 ± 70 and 757 ± 122 msec, respectively; F(1, 8) = 0.13, not significant (n.s.)). Those

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Frequency

FIGURE 13.4 Mean power spectra of the hippocampal CA1. T refers to the temporal discrimination task, and C indicates the simple reaction time task used as a control. Onset, 1-sec duration after tone onset; offset, 1-sec duration after tone offset.

5 10 15 Hz

Frequency

FIGURE 13.4 Mean power spectra of the hippocampal CA1. T refers to the temporal discrimination task, and C indicates the simple reaction time task used as a control. Onset, 1-sec duration after tone onset; offset, 1-sec duration after tone offset.

spectral distributions of hippocampal theta were calculated by fast Fourier transforms (FFT) to compare the involvement of working memory in both tasks. We removed the data for one animal because of contamination by artifacts. We also calculated hippocampal power spectra in three conditions: onset of the auditory stimuli, offset of tones, and intertrial intervals (ITIs). These results are shown in Figure 13.4.

The data show a clear peak in power at a frequency range between 6 and 12 Hz. In the statistical analysis, there were two main effects. First, the hippocampal theta power increases more in the T-task than in the C-task at the onset of the auditory stimuli (F(1, 7) = 17.8, P < .01). Second, the peak frequency shifted for higher ranges in the T-task than in the C-task (F(2, 14) = 5.90, P < .05). There were no significant differences among onset, offset, and ITIs in the C-task (F(2, 14) = 1.89, n.s.).

13.3.3 Function of the Hippocampus in Models of Interval Timing

There have been many theories of hippocampal function, timing, and theta rhythm presented in the literature (e.g., Grossberg and Schmajuk, 1989; Kesner, 2002; Meck, 2002a, 2002b; Schmajuk, 1990). The hippocampal system is related to the formation of episodic memory of space, time, objects, etc. The main function of the hippocampus appears to be some form of memory storage or consolidation. In the information-processing model of interval timing, the hippocampus is thought to be primarily involved in working memory (e.g., Meck et al., 1984). As indicated above, Gibbon et al. (1984) proposed three main processing stages in their model: clock, memory, and decision stages. The clock stage is hypothesized to consist of a pacemaker that emits pulses that are transferred to an integrator through a switch.

The brain areas proposed to be involved in this clock stage have included the striatum (Mattel and Meck, 2000; Matell et al., this volume) and the cerebellum (Diedrichsen et al., this volume; Gibbon et al., 1997; Ivry, 1996; Ivry and Richardson, 2002). The memory stage is as a proposed mechanism for clock input to be transferred to reference memory. In the transfer of temporal information to memory, the encoding process first needs to reset the working memory at the beginning of the trial. The final stage is the comparison stage, which makes the decision to respond. In the T-task of this experiment, the decision occurs at a point in time after the offset of auditory stimuli. Rats must make a decision to choose either the left or the right response lever. After the 2-sec stimulus presentation, rats will make a judgment at the offset of the signal. However, in the case of the 8-sec stimulus, rats will make a judgment sometime during the stimulus presentation after the 2-sec criterion has been passed. Consequently, we should compare ERPs following the 2-sec stimulus in both the T-task and C-task. We can evaluate the differences between these two tasks by using the offset ERP.

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