Topography of cortical projections to the dorsolateral neostriatum in rats: multiple overlapping sensorimotor pathways.

In rodents, the whisker representation in primary somatosensory (SI) cortex projects to the dorsolateral neostriatum, but the location of these projections has never been characterized with respect to layer IV barrels and their intervening septa. To address this issue, we injected a retrograde tracer into the dorsolateral neostriatum and then reconstructed the location of the labeled corticostriatal neurons with respect to the cytochrome oxidase (CO)-labeled barrels in SI. When the tracer was restricted to a small focal site in the neostriatum, the retrogradely labeled neurons formed elongated strips that were parallel to the curvilinear orientation of layer IV barrel rows. After larger tracer injections, labeled neurons were distributed uniformly across layer V and were aligned with both the barrel and septal compartments. Labeled projections from the contralateral SI barrel cortex, however, were much fewer in number and were disproportionately associated with the septal compartments. A comparison of the labeling patterns in the ipsilateral and contralateral hemispheres revealed symmetric, mirror-image distributions that extended across primary motor cortex (MI) and multiple somatosensory cortical regions, including the secondary somatosensory (SII) cortex, the parietal ventral (PV) and parietal rhinal (PR) areas, and the posteromedial (PM) region. Examination of the thalamus revealed labeled neurons in the intralaminar nuclei, in the medial part of the posterior nucleus (POm), and in the ventrobasal complex. These results indicate that the dorsolateral neostriatum integrates sensorimotor information from multiple sensorimotor representations in the thalamus and cortex.

Effect of auditory cortex lesions on the discrimination of frequency-modulated tones in rats.

The lateralization of functions to individual hemispheres of the mammalian brain remains, with the exception of the human brain, unresolved. The aim of this work was to investigate the ability to discriminate between falling and rising frequency-modulated (FM) stimuli in rats with unilateral or bilateral lesions of the auditory cortex (AC). Using an avoidance conditioning procedure, thirsty rats were trained to drink in the presence of a rising FM tone and to stop drinking when a falling FM tone was presented. Rats with a lesion of the AC were able to learn to discriminate between rising and falling FM tones; however, they performed significantly worse than did control rats. A greater deficit in the ability to discriminate the direction of frequency modulation was observed in rats with a right or bilateral AC lesion. The discrimination performance (DP) in these rats was significantly worse than the DP in rats with a left AC lesion. Animals with a right or bilateral AC lesion improved their DP mainly by recognizing the pitch at the beginning of the stimuli. The lesioning of the AC in trained animals caused a significant decrease in DP, down to chance levels. Retraining resulted in a significant increase in DP in rats with a left AC lesion; animals with a right lesion improved only slightly. The results demonstrate a hemispheric asymmetry of the rat AC in the recognition of FM stimuli and indicate the dominance of the right AC in the discrimination of the direction of frequency modulation.