The effect of medial thalamic lesions on acquisition of a go, no-go, tone-light discrimination task

Acoustic discrimination in the grey bamboo shark Chiloscyllium griseum

Cognitive abilities of sharks are well developed and comparable to teleosts and other vertebrates. Most studies exploring elasmobranch cognitive abilities have used visual stimuli, assessing a wide range of discrimination tasks, memory retention and spatial learning abilities. Some studies using acoustic stimuli in a cognitive context have been conducted, but a basic understanding of sound induced behavioural changes and the underlying mechanisms involved are still lacking. This study explored the acoustic discrimination abilities of seven juvenile grey bamboo sharks (Chiloscyllium griseum) using a Go/No-Go method, which so far had never been tested in sharks before. After this, the smallest frequency difference leading to a change in behaviour in the sharks was studied using a series of transfer tests. Our results show that grey bamboo sharks can learn a Go/No-Go task using both visual and acoustic stimuli. Transfer tests elucidated that, when both stimulus types were presented, both were used. Within the tested range of 90–210 Hz, a frequency difference of 20–30 Hz is sufficient to discriminate the two sounds, which is comparable to results previously collected for sharks and teleosts. Currently, there is still a substantial lack of knowledge concerning the acoustic abilities and sound induced behaviours of sharks while anthropogenic noise is constantly on the rise. New insights into shark sound recognition, detection and use are therefore of the utmost importance and will aid in management and conservation efforts of sharks.

Medial dorsal thalamic lesions and working memory in the rat

Pigmented rats of the DA strain with either radiofrequency or ibotenic acid lesions of the thalamic nucleus medialis dorsalis were postoperatively given nonspatial and spatial tests of working memory. In the nonspatial task, delayed nonmatching-to-sample, rats with both types of thalamic lesions showed acquisition impairments. The subgroup of rats with nucleus medialis dorsalis lesions that were able to reach the acquisition criterion did, however, perform normally when the retention interval was extended to 60 s. In the spatial task, delayed forced-alternation, rats were tested with differing retention intervals and with both spaced and massed trials. Damage to nucleus medialis dorsalis had no effect on acquisition or on spaced trials, but a slight deficit was found in the animals with radiofrequency lesions under the massed trial condition. Much clearer deficits were, however, present in those animals in which the lesion extended appreciably into the anterior thalamic nuclei. The findings indicate that while cellular damage to nucleus medialis dorsalis may disrupt learning, some impairments in tests of spatial working memory attributed to this nucleus may reflect damage to the adjacent anterior thalamic nuclei.

Multiple-unit activity of the prefrontal cortex and mediodorsal thalamic nucleus during acquisition of discriminative avoidance behavior in rabbits

Multiple-unit activity of the prefrontal cortex (PFC) and the mediodorsal (MD) thalamic nucleus was recorded during discriminative conditioning of a locomotory avoidance response in rabbits. A major objective was to compare the results with those previously obtained from the posterior limbic (cingulate) cortex, and the anteroventral (AV) thalamic nucleus. The results indicated the development, with training, of discriminative neuronal activity, i.e., greater neuronal discharges to presentation of the positive conditional stimulus (CS+, a tone paired with a footshock unconditional stimulus), relative to the negative conditional stimulus (CS-, a tone not paired with the footshock). The rostral-sulcal subfield of the PFC developed the discriminative activity during the first session of conditioning, prior to the acquisition of discriminative behavioral responding. This effect persisted throughout training to the asymptote of behavioral acquisition. The medial subdivision of the MD nucleus, with which the rostral-sulcal PFC is interconnected, did not develop discriminative effects. The caudal (supragenual) subfield of the PFC also developed discriminative activity in the first session of training but the effect declined progressively in the remaining acquisition sessions. The lateral subdivision of the MD nucleus, with which the caudal PFC is interconnected, manifested robust discriminative activity in the late acquisition sessions, concomitant with acquisition of significant behavioral discrimination. Thus, as in the cingulate cortical AV nuclear system, a corticothalamic sequential progression of discriminative activity occurred in the PFC-MD nuclear system during behavioral acquisition. However, the effect was confined to one subsystem: the caudal subfield of the PFC and the lateral subdivision of the MD nucleus. Also, discriminative activity developed more rapidly in the PFC than in the cingulate cortex, and it developed more rapidly in the MD nucleus than in the AV nucleus. Implications are considered concerning the contributions to learning and memory processes, of the PFC-MD thalamic system.

Parallel and serial processes of the prefrontal and cingulate cortical systems during behavioral learning

Multiple unit activity of the prefrontal cortex (PFC) and the mediodorsal (MD) nucleus of the thalamus was recorded during differential conditioning of a locomotory (wheel-rotation) avoidance response. The positive and negative conditional stimuli (CS+ and CS-) were pure tones (l kHz and 8 kHz), and the unconditional stimulus was a footshock (1.5-2.0 mA) delivered through the grid floor of the wheel apparatus. A principal objective was to compare the development during training of CS-related neuronal activity in the PFC and MD nucleus, to that obtained in past studies of the adjacent limbic mesocorticothalamic system (the cingulate cortex and the anteroventral (AV) thalamic nucleus). The results indicated that these systems perform associative processing of task-related stimuli in parallel: as in the cingulate cortex, discriminative neuronal discharges (greater discharges to the CS+ than to the CS-) developed in the PFC in the earliest training stage, before incipient discriminative behavior, and as in the AV nucleus, discriminative discharges in the MD nucleus developed in a later training stage. Nevertheless the possibilities of serial processing and intercausality in these systems was suggested by the finding that discriminative activity in the PFC and MD nucleus developed at briefer latencies and earlier in the course of trials than in the cingulate cortex and the AV nucleus.