Conditional discrimination learning and negative patterning in rats with neonatal hippocampal lesion induced by ionizing radiation.

This study was undertaken to investigate the associative process underlying serial feature positive conditional discrimination learning (X-->A+/A-) and the role of the hippocampus in the solution of tasks demanding a configural association strategy such as the negative patterning discrimination (XA-/X+/A+). It has been suggested that the hippocampus is essential for the learning of complex tasks, so, it is expected that hippocampal lesions would prove equally detrimental to performance in both tasks, but would not interfere with simple discrimination learning. Hippocampal lesions were made with X-radiation exposure to neonate rats after completion of a parametric study 'J. Neurosci. Methods 75 (1997) 41' that established the best radiation parameters to selectively lesion the hippocampal dentate gyrus. When adults, rats were submitted to a serial feature positive conditional discrimination task with the trials 'House light/Tone: water (H-->T+)', 'Tone: no water (T-)', and two simple discrimination with the trials 'Clicker: water (C+)' and 'Noise: no water (N-)' in Experiment I. In Experiment II, adult rats, irradiated and control, were submitted to the negative patterning task with the trials 'House light/Tone: no water (HT-)', 'House light: water (H+)', 'Tone: water (T+)', and to the non-conditional discrimination with the trial Noise: no water (N-)'. In contrast to the expectation of impaired performance in these tasks by lesioned rats, animals with damage to the hippocampal dentate gyrus learned the complex and the simple tasks as well as control subjects. These results suggest that the dentate gyrus does not participate directly in the modulation of acquisition of tasks demanding a complex strategy of occasion setting in procedures of serial conditional discrimination or a configural strategy, important for the negative patterning discrimination solution.