**3.2 Theta oscillation underlies hippocampal novelty detection and learning**

Although brain imaging has given important functional information about brain learning and memory, it cannot reveal how the brain works at level of individual neurons.

However, understanding of object recognition and its neural basis, it necessarily means to focus on a first-order question: how individual neurons represent individual memories. This has led to theoretical models of short-term memory such as sustained spiking activity by single neurons that typically reflects a single memorandum [Fuster and Alexander, 1971; Fuster and Jervey, 1982; Hopfield, 1995]. In other words, there is increasing evidence that information encoding may also depend on the temporal dynamics between neurons; for example, from relative spikes to rhythmic activity across the neural population generating local field potential (LFP) [Metha et al., 2002; Ninokura et al., 2003; Warden and Miller, 2007; Siegel et al., 2009; Kayser et al., 2009; Warden and Miller, 2010].

It is well documented that central cholinergic system plays a crucial role in cognitive functions; therefore, from an electrophysiological and neurochemical point of view, the integrity of the frontal cortex and hippocampus circuitry is essential for brain cognitive processes. In fact, it is well known that neuronal loss, following basal cholinergic degeneration, shows a close correlation with neuronal death in another vulnerable region of the brain such as the hippocampus.

Hippocampus is another brain area important for learning and memory and it exhibits relevant theta (4-7 Hz) frequency oscillations *in vivo* during behavioural activity. In fact, neural action can vary during different cognitive processes, becoming rhythmic during such a brain activity; in particular, in brain, hippocampal theta rhythmicity could contribute to learning and memory [Lee et al., 2005]. In rat, spatial memory is supported by interaction between hippocampus and cortical areas, frontal cortex mainly, which is critically involved in attention and learning [O'Keefe and Recce , 1993; Morris, 2001; Monosov et al., 2010].

Different studies indicate that hippocampus plays an essential role in novelty detection. These researches show that an important electrophysiological mechanism, by which hippocampus learn and discriminate objects in novelty detection, is the hippocampal theta activity [König et al., 1995]. Recently, new findings offer an insight into the mechanisms underlying hippocampal novelty detection stimulating new questions within the debate: theta peak or theta power?

It was proposed a link between the hippocampal theta and the detection of novel contexts. Some authors reported that in rats, exposed to familiar and novel environments, the peak

Neural Basis of Object Recognition 9

In conclusion, some indications can be given. Prefrontal cortex regions are involved in shortterm memory and object discrimination. Cholinergic signaling, coming from basal forebrain to frontal cortex, septum and hippocampus, are implicated in short-term memory; in addition, the hippocampus could be important for discrimination processes in cognition.

In laboratory, cognitive tasks have shown a good reliability in many experimental models of human neurodegenerative diseases. Specifically, a lot of laboratory studies have shown that the object recognition task in rodents is highly sensitive to psychoactive drug. For example, this is the case of drugs such as acetylcholinesterase inhibitors (AChEIs) which can improve object memory performance in rats [Prickaerts et al., 2002; Hornick et al., 2008; Goh et al., 2009]. In fact, in rats these ACh enhancers can reverse drug-induced memory impairments [Bejar et al., 1999; van der Staay and Bouger, 2005; Yamada et al., 2005]. This has encouraged researchers that such drugs may also be useful in treating memory impairments in patients with dementia. On the other hand, to date, clear evidence for a reliable memory enhancing effect of these drugs in humans is lacking and controversial [Snyder et al., 2005; Wezenberg et al., 2005]; that might probably be related to the discrepancy between the large numbers of animal studies and only a limited number of human studies showing memory enhancing

Object discrimination requires the integrity of cortical cholinergic system; in rodents the cortex-hippocampus circuitry consents to distinguish individual objects such as different

The novel object test or object recognition test (ORT) was first described by Ennaceur and Delacour (1988). Rats or mice are exposed first to two identical objects and then one of the objects is replaced by a new object. The time spent exploring each of the objects is measured. The test has become popular for assessing the effects of amnesic drugs in rodents in general and, after that, to test new compounds enhancing attention and memory [Bartolini et al., 1996]. The test is based on spontaneous behavior with no reinforcement such as food or shock. Non-amnesic animals will spend more time exploring the novel object than the familiar one. An absence of any difference in exploration time can be interpreted as a

Although the novel object recognition task has shown high sensibility and it can be a simple approach to test new potential antidementia drugs, researchers need a stronger experimental tools to test *in vivo* pharmacological activity before clinical trials. From our point of view, an electrophysiological approach together with novel object recognition task

Discovering the cause of Alzheimer's disease should imply the ultimate hope of developing

Most researches on working memory, carried out in experimental models of AD, have been

memory defect or, in case an amnesic drug is tested, a non-effective drug.

safe and effective pharmacological treatments [Francis et al., 1999].

modelled on those conducted in physiological studies of monkeys.

**3.4 The object recognition test** 

effects of these drugs.

shapes [Hauser et al., 2009].

can probably be useful.

**3.5 An experimental model of AD** 

hippocampal theta frequency dropped (by about 0.6 Hz) when the rats were tested in a novel environment [Jeewajee et al., 2008]. This change in theta frequency might function as a novelty signal because hippocampal theta frequency is the same in the whole hippocampus [Buzsaki, 2002], and they suggested that the reduction in theta frequency would have implications for memory encoding. The authors speculate that novelty leads a lowfrequency theta depending on acetylcholine release. In fact, it is well known that new experience and novel environment induces in brain increase in cholinergic input to the hippocampus and increase in ACh release which affects hippocampal theta activity [Givens and Olton, 1994, 1995; Podol'skii et al., 2001]. On the other hand, other authors did not find any change in peak theta frequency when animals were stimulated by a novel environment; they instead reported a change in theta power that differentiated active from passive behavior, with novelty increasing power at both levels of activity [Sambeth et al., 2009].

Nevertheless, taken together both findings suggest that theta oscillations in hippocampus are affected by novelty, and that this probably gives reasons for hippocampal learning.
