Effects of frontal cortex lesions on action sequence learning in the rat

To understand the role of frontal cortex in motor sequence learning we compared the effects of motor (MI), premotor (M2) and midline frontal (MFr) cortical lesions on rats making nose-pokes guided by luminance cues. Organizational demands were manipulated by varying the number (1 vs. 5) and predictability (random vs. repeated) of nose-pokes in a response. Learning was studied by comparing sessions with random or repeated cues. All cortical lesions increased reaction time (RT) during response initiation. These effects were larger for nose-pokes initiating sequential responses but spared RT for nose-pokes completing them. Repetition learning had significant effects on the speed and accuracy of single nose-poke responses that were unaffected by any of the cortical lesions. Repetition learning had more complex effects on sequential responding. RTs increased for nose-pokes initiating sequences over several sessions of continuous repetition and then decreased or leveled off. RTs decreased incrementally across all repetition sessions for subsequent nose-pokes in repeated sequences, following a time-course consistent with habit learning. Lesions involving M2 and MFr cortex exacerbated the increase in RT during initiation without affecting the incremental decrease in RT for nose-pokes completing repeated sequences. These results were confirmed by analyses of interference effects when training shifted from repeated (learned) to random (novel) sequences or to a new repeated sequence. These results implicate dorsomedial frontal cortex in organizational aspects of sensory-guided responding and motor sequence learning reflected in RT during response initiation.