Dorsal hippocampus is necessary for visual categorization in rats

Abstract

The hippocampus may play a role in categorization because of the need to differentiate stimulus categories (pattern separation) and to recognize category membership of stimuli from partial information (pattern completion). We hypothesized that the hippocampus would be more crucial for categorization of low-density (few relevant features) stimuli-due to the higher demand on pattern separation and pattern completion-than for categorization of high-density (many relevant features) stimuli. Using a touchscreen apparatus, rats were trained to categorize multiple abstract stimuli into two different categories. Each stimulus was a pentagonal configuration of five visual features; some of the visual features were relevant for defining the category whereas others were irrelevant. Two groups of rats were trained with either a high (dense, n = 8) or low (sparse, n = 8) number of category-relevant features. Upon reaching criterion discrimination (≥75% correct, on 2 consecutive days), bilateral cannulas were implanted in the dorsal hippocampus. The rats were then given either vehicle or muscimol infusions into the hippocampus just prior to various testing sessions. They were tested with: the previously trained stimuli (trained), novel stimuli involving new irrelevant features (novel), stimuli involving relocated features (relocation), and a single relevant feature (singleton). In training, the dense group reached criterion faster than the sparse group, indicating that the sparse task was more difficult than the dense task. In testing, accuracy of both groups was equally high for trained and novel stimuli. However, both groups showed impaired accuracy in the relocation and singleton conditions, with a greater deficit in the sparse group. The testing data indicate that rats encode both the relevant features and the spatial locations of the features. Hippocampal inactivation impaired visual categorization regardless of the density of the category-relevant features for the trained, novel, relocation, and singleton stimuli. Hippocampus-mediated pattern completion and pattern separation mechanisms may be necessary for visual categorization involving overlapping irrelevant features.

Selective attention in rat visual category learning

“…rate; however, both tasks were learned very quickly, especially compared to other category stimuli (Wasserman et al 2012;Brooks et al 2013;Kim et al 2018). Although this result would conventionally be interpreted as a deficit or absence of a declarative system, an alternative interpretation is that the rats learned the II tasks faster than expected, as they were able to quickly switch to the optimal II strategy.…”

“…Experiment 2 was conducted to rule out the possibility that no differences were observed in learning rate between RB and II tasks because of a ceiling effect. The rats in Experiment 1 reached the learning criterion very quickly, especially compared to other categorization tasks using the same trial procedures (Brooks et al 2013;Kim et al 2018). This may have made it difficult to detect differences in learning between the task types.…”

“…Training sessions continued until the rat reached a learning criterion (75% accuracy in both categories, on two consecutive sessions). These procedures have been used effectively for category and discrimination learning in rats (Brooks et al 2013;Kim et al 2018).…”

“…On the one hand, one might predict that rats would show a learning advantage for the RB tasks, as classic behavioral paradigms have demonstrated that rat cognition supports the executive functions described by COVIS's declarative system, including working memory and selective attention (Bruin et al 1994;Horst and Laubach 2009). Furthermore, rats have learned visual categories that require attention to single relevant features among irrelevant information, suggesting that selective attention can be used in category learning (Wasserman et al 2012;Brooks et al 2013;Vinken et al 2014;Kim et al 2018).…”

Visual learning and its underlying neural substrate in two species of teleost fish (Zebrafish and Ambon damselfish)

“…However, not only the cortex is involved in visual learning and discrimination. Recently, some evidence has come to light showing that the mammalian hippocampus is also involved in visual learning and discrimination (Knutson et al, 2012;Hales et al, 2015;Liu et al, 2017;Kim et al, 2018). The Dl of the teleost pallium has been described as a possible homologue of the mammalian hippocampus (Mueller et al, 2011), and it receives visual information (Vanegas and Ito, 1983;Demski, 2003;Northcutt, 2006;Yamamoto and Ito, 2008).…”