The effects of lesions to the postsubiculum or the anterior dorsal nucleus of the thalamus on spatial learning in rats.

Abstract

To investigate the role of the head direction (HD) cell circuit in spatial navigation, rats with bilateral, neurotoxic lesions to the postsubiculum (PoS; Experiment 1) or the anterior dorsal nucleus of the thalamus (ADN; Experiment 2) were compared to sham controls on 2 tasks that could be solved using directional heading. Rats were first trained on a direction problem in a water T maze where they learned to travel either east or west from 2 locations in the experimental room. ADN lesioned rats were impaired relative to sham controls on the first block of 8 trials, but not on the total trials taken to reach criterion. This transient deficit was not observed in rats with lesions to the PoS. In the food-foraging task, rats were trained to leave a home cage at the periphery of a circular table, find food in the center of the table, and return to the home cage. Both PoS and ADN lesioned rats showed impairments on this task relative to sham rats, making more errors on the return component of the foraging trip. The spatial deficits produced by lesions to the PoS and the ADN, downstream structures in the HD cell circuit, are not as severe as those observed in earlier studies in rats with lesions to the dorsal tegmental nucleus.

Do the anterior and lateral thalamic nuclei make distinct contributions to spatial representation and memory?

“…While control animals typically swam toward the relative location within the pool, animals with an inactivated anterodorsal thalamus showed an overwhelming preference for the absolute location. In a complementary study using a water T-maze, Peckford et al (2014) confirmed that directional navigation was impaired after anterodorsal thalamic lesions. These findings collectively suggest that the anterodorsal thalamus is involved in processing information regarding the direction of movements, but the retrieval of the absolute location of the platform in relation to the distal cues was seemingly maintained after inactivation.…”

The Head-Direction Signal Plays a Functional Role as a Neural Compass during Navigation

“…However, a major limitation to interpreting these studies as demonstrating that HD cells are used in navigation comes from the absence of specificity afforded by a lesion. For example, lesions to areas such as the mammillary bodies [21] or anterior thalamus [18,20] likely have broader effects on learning and memory in addition to their effects on HD cells [37,38]. In addition, if the effects that these lesions have on behavior are due to knocking out the HD signal, it is unclear why lesions further upstream in the network have greater effects than lesions in, for example, downstream cortical regions [20].…”

“…The other approach to explore the role of HD cells in behavior has shown that lesions of areas involved in the HD signal leads to various deficits in spatial ability [18][19][20][21]. However, a major limitation to interpreting these studies as demonstrating that HD cells are used in navigation comes from the absence of specificity afforded by a lesion.…”

“…For example, lesions to areas such as the mammillary bodies [21] or anterior thalamus [18,20] likely have broader effects on learning and memory in addition to their effects on HD cells [37,38]. In addition, if the effects that these lesions have on behavior are due to knocking out the HD signal, it is unclear why lesions further upstream in the network have greater effects than lesions in, for example, downstream cortical regions [20]. In contrast to this previous work, by using the temporal and anatomically specific inactivation afforded by optogenetics, we were able to address the role of HD cells in behavior much more directly by targeting a well-defined input to the HD network.…”

“…Correlations between place cell firing and spatial behavior, especially in relation to goaldriven behavior [16], have prompted recent work showing that there is a causal link between place cells and an animal's sense of location [17]. However, despite numerous lesion studies showing indirect evidence that the HD circuit might be involved broadly in spatial processing [18][19][20][21], a direct causal role of the HD signal as a neural compass that guides directional behavior has not been demonstrated yet.…”