Hippocampal Theta Activity During Stimulus Discrimination Task

The configural association theory and conflict resolution model both propose that hippocampal function plays role in the solving a negative patterning task but not simple discrimination task. Some hippocampal lesion study showed that inactivity of rats’ hippocampal CA1 area induced impairment of performance of a negative patterning task. Other previous studies, however, showed that the lesion did not affect the performance of the task. Thus, it did not reveal whether hippocampal function was important for solving the negative patterning task. Our recent research using an electrophysiological approach showed that the hippocampal theta power decreased with a compound stimulus of a negative patterning task, and that the hippocampal theta power was decreased by a compound stimulus of a feature negative task. These results indicate that a decrease in hippocampal theta activity is elicited by behavioral inhibition for conflict stimuli with overlapping elements. This finding strongly supports the conflict resolution model and suggests a hippocampal role in learning behavioral inhibition for conflict stimuli during nonspatial stimulus discrimination tasks

Electrophysiological study of the hippocampus on stimulus discrimination tasks in the rat <Summaries of the Doctoral Theses>

Sutherland & Rudy (1989) proposed configural association theory that the hippocampus plays a role in configuration of compound stimuli. This theory assumed that the hippocampus is necessarily for solving non-linear task but is not necessarily for solving linear task. This study examined the relationship hippocampal theta and non-linear and linear task. The chapter 1 presents a review of previous research. The chapter 2 showed that the hippocampal theta power during negative patterning task (A+, B+, AB-) as non-linear task was higher than simple discrimination task (A+, B-) as linear task. The chapter 3 showed that the hippocampal theta power during positive patterning task (A-, B-, AB+) as non-linear task was not higher than simple discrimination task (A+, B-) as linear task. The chapter 4 reanalyzed the data of chapter 2 and 3 by time-frequency analysis. The chapter 5 compared the hippocampal theta power during negative patterning (A+, B+, AB-), simultaneous feature negative (A+, AB -) and simple discrimination task (A+, B-). The result revealed that the hippocampal theta was the highest in negative patterning task, followed by simultaneous feature negative and simple discrimination task. In the chapter 6, I discussed these result of this study and proposed the new hippocampal function theory that the hippocampus is important for the modulation of expectation value of reinforcement for compound stimulus

Analysis of error responses in the negative patterning task in rats

This experiment studied the learning phenomena by three behavioral indicators. The configural association theory insists that the hippocampus is required to solve configural discrimination learning tasks in rats. The theory identifies the negative patterning discrimination task as a typical example. We used this task to examine three behavioral indicators, i.e., the number of responses during the inter-trial interval, reaction time, and response rate. The results showed the serial changes of these indicators depend on learning progress. At first, number of responses during inter-trial intervals was decreased. Next, reaction time of error responses became longer. Finally, response rate decreased in nonreinforced stimuli. Therefore, these three indicators can be classified into three phases; early, middle, and late phases of learning from different aspects. These error indexes showed clearly that the three phases of learning depend on the negative patterning task

Transient decline in hippocampal theta activity during the acquisition process of the negative patterning task.

Hippocampal function is important in the acquisition of negative patterning but not of simple discrimination. This study examined rat hippocampal theta activity during the acquisition stages (early, middle, and late) of the negative patterning task (A+, B+, AB-). The results showed that hippocampal theta activity began to decline transiently (for 500 ms after non-reinforced stimulus presentation) during the late stage of learning in the negative patterning task. In addition, this transient decline in hippocampal theta activity in the late stage was lower in the negative patterning task than in the simple discrimination task. This transient decline during the late stage of task acquisition may be related to a learning process distinctive of the negative patterning task but not the simple discrimination task. We propose that the transient decline of hippocampal theta activity reflects inhibitory learning and/or response inhibition after the presentation of a compound stimulus specific to the negative patterning task

Negative patterning and simple discrimination paradigms.

<p>In the negative patterning task, lever presses were reinforced following either of the stimulus elements (Tone +, Light +), but not following the compound stimulus (Compound -; panel A). In the simple discrimination task, for 1 group, lever responses were rewarded when the tone stimulus was presented (Tone +), but not when the light stimulus was presented (Light -; panel B). For the other group, the relationship between cue modality and availability of reinforcement was reversed (Light +, Tone -).</p

Electrode placements in the rat brains.

<p>This figure was modified from “the rat brain in stereotaxic coordinates” of Paxinos and Watson (1997). Black circles indicate the placement of the electrode tips in each rat (<i>n</i> = 12).</p

A comparison of the mean relative hippocampal theta activity between tasks.

<p>Panel A shows the relative hippocampal theta power during the 250-ms and Panel B does during 500-ms epochs between the negative patterning and simple discrimination task groups. A group (negative patterning task and simple discrimination task groups) × stage (early, middle, and late) ANOVA for hippocampal theta activity during a 250-ms epoch in the non-RFTs showed a significant interaction (<i>F</i>_(2,20) = 5.18, <i>p</i> < 0.05). Multiple comparisons revealed that hippocampal theta power increased during the early stage in the negative patterning task compared to the simple discrimination task group (<i>p</i> < 0.05). A group (negative patterning task and simple discrimination task groups) × stage (early, middle, and late) ANOVA for hippocampal theta activity during a 500-ms epoch in the non-RFT showed a significant interaction (<i>F</i>_(2,20) = 6.12, <i>p</i> < 0.05). Multiple comparisons revealed that hippocampal theta power decreased during the late stage in the negative patterning task compared to the simple discrimination task group (<i>p</i> < 0.05; *: <i>p</i> < 0.05). Hippocampal theta power during the 500 ms non-RFT correlated with the discrimination rate in the negative patterning task (<i>r</i> = -0.70, <i>p</i> < 0.05; panel C), but not the simple discrimination task (<i>r</i> = -0.06, <i>p</i> = <i>n.s</i>; panel D).</p

The change in theta power during the RFTs duringeach learning stage of the negative patterning task.

<p>Panel A shows the change in hippocampal theta activity along a time course during RFTs on the early stage, panel B shows theta activity on the middle stage and panel C shows theta activity on late stage of negative patterning task. The x-axis is time (ms) and the y-axis is frequency (Hz). In each panel, the period is from 500 ms before stimulus onset to 4000 ms after stimulus onset. The period was divided into 19 sub-periods of 250 ms each. The mean hippocampal theta power during 500 ms before stimulus onset was counted as the -500-ms period (no stimuli were present and no rats pressed the lever during this period) and the relative theta power calculated for each period was normalized to that during the -500-ms period (relative theta activity of each period = theta power of each period/theta power at the -500-ms period). Panel D contains a comparison of the mean (± S.E.M.) relative hippocampal theta activity at 6–12 Hz among each learning stage (early, middle, and late) throughout the time course of the experiment during RFT of the negative patterning task. Panel E contains a comparison of the mean (± S.E.M.) relative hippocampal theta activity at 6–12 Hz among each learning stage (early, middle, and late) throughout the time course of the experiment during RFT of the simple discrimination task.</p

Comparison of hippocampal theta power between correct-response and incorrect-response trials.

<p>This figure shows the hippocampal theta power between trials with correct lever press response for RFT and incorrect lever press responses for non-RFTs during the late stage of the negative patterning task. The 0 period was lever press timing. The analysis period from 1250 ms before lever press to 1500 ms after lever press was divided into 11 250-ms epochs. The 250-ms period from -1250 to -1000 ms was used as the baseline, and the relative theta activity for each period was calculated as follows: relative theta activity of each period = theta activity of each period/theta activity during the baseline period.</p