Abstract
The present study is part of a series of experiments, where we analyze why and how damage of the rat’s dorsal hippocampus (dHC) can enhance performance in a sequential reaction time task (SRTT). In this task, sequences of distinct visual stimulus presentations are food-rewarded in a fixed-ratio-13-schedule. Our previous study (Busse and Schwarting, 2016) had shown that rats with lesions of the dHC show substantially shorter session times and post-reinforcement pauses (PRPs) than controls, which allows for more practice when daily training is kept constant. Since sequential behavior is based on instrumental performance, a sequential benefit might be secondary to that. In order to test this hypothesis in the present study, we performed two experiments, where pseudorandom rather than sequential stimulus presentation was used in rats with excitotoxic dorsal hippocampal lesions. Again, we found enhanced performance in the lesion-group in terms of shorter session times and PRPs. During the sessions we found that the lesion-group spent less time with non-instrumental behavior (i.e., grooming, sniffing, and rearing) after prolonged instrumental training. Also, such rats showed moderate evidence for an extinction impairment under devalued food reward conditions and significant deficits in a response-outcome (R-O)-discrimination task in comparison to a control-group. These findings suggest that facilitatory effects on instrumental performance after dorsal hippocampal lesions may be primarily a result of complex behavioral changes, i.e., reductions of behavioral flexibility and/or alterations in motivation, which then result in enhanced instrumental learning.
Highlights
It is our current understanding that memory can be divided into two main categories: declarative and non-declarative memory
In order to provide a translation of this task for rodents, we developed a sequential reaction time task (SRTT) in rats, where sequential instrumental nose-poking is reinforced under fixedratio conditions by food pellets (Domenger and Schwarting, 2006, 2007; for review, see Schwarting, 2009), using series of sequential stimuli which are identical to those used in typical research with humans (Nissen and Bullemer, 1987; Schwarting, 2009)
SRTT Statistical analysis revealed that reaction times (RTs) of the correct pokes decreased over days and became asymptotic in both groups (factor days: F(5.713,79.981) = 27.726; p < 0.001)
Summary
It is our current understanding that memory can be divided into two main categories: declarative (memory of ‘‘what’’) and non-declarative memory (memory of ‘‘how’’). Further division leads to distinct sub-categories of memory, e.g., procedural memory, which is a certain type of non-declarative memory This psychological classification is paralleled by a neurobiological one, since declarative memory is linked to structures like the hippocampus, Hippocampal Lesions Impair Behavioral Flexibility whereas procedural memory is linked, among others, to basal ganglia function (McDonald and White, 1994). Both types of memory require and partly share neocortical mechanisms. The same animals had the expected deficits in an object-place recognition task That is, they were impaired in a so-called episodic memory task, which is usually attributed to hippocampal mechanisms, but showed improved performance in a procedural task thought to reflect striatal function. This was achieved by a specific method of perforant pathway stimulation, a paradigm that produces hippocampal granule cell discharges over a prolonged period of subsequent electrical stimulation (Norwood et al, 2010)
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