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

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

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

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

The change in theta power during the non-RFTs during each learning stage of the negative patterning task.

<p>Panel A shows the change in hippocampal theta activity along a time course during non-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 non-RFT of the negative patterning task (*: <i>p</i> < 0.05). 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 non-RFT of the simple discrimination task.</p