Neuroanatomy of Object-Recognition Memory

Monkey Model of Object-Recognition Amnesia: The Delayed Nonmatching-to-Sample Test

Monkeys with bilateral medial temporal lobe lesions had deficits in the delayed nonmatching-to-sample test (Figs. 11.9 and 11.10).

These deficits were similar to those of HM’s (explicit, episodic memories).

Monkeys could show short-term memories, but had difficulty consolidating them into long-term memories (Fig. 11.10).

In fact, humans with bilateral medial temporal lobe lesions behaved similarly in the delayed nonmatching to sample tests.

One problem with monkeys is that lesions aimed at hippocampus also lesioned the rhinal cortex (cortex adjacent to the hippocampus) (Figs. 11.11 and 11.12) .

Therefore, researchers could not be sure whether memory deficits in monkeys were due to lesion of the hippocampus or rhinal cortex.

The Delayed Nonmatching-to-Sample Test for Rats

However, the rat model presented the advantage that lesions could be restricted to the hippocampus (Fig. 11.12).

The version of the delayed nonmatching-to-sample task in rats is the Mumby box (Fig. 11.13). This box has 3 compartments.

In one compartment, the rat is exposed to the sample object concealing the food.

In the middle compartment, the rat is made to wait through the delay.

In the third compartment, the rat was presented with the sample object that had to be rejected in favor of the new one that now conceals the food.

Rats performed as well as monkeys with delays up to one minute.

Neuroanatomical Basis of the Object-Recognition Deficits Resulting from Medial Temporal Lobectomy

The rodent model revealed that the hippocampus and amygdala were not involved in deficits of object-recognition memory tested with delayed nonmatching-to-sample tasks.

Instead, these experiments found that the rhinal cortex was important for object recognition memory (Fig. 11.15).

The hippocampus and memory for spatial location.

The hippocampus does play a key role in memory for spatial location.

Hippocampal Lesions Disrupt Spatial Memory

Damage of the hippocampus result in severe deficits in spatial memory tested in mazes like the Morris water maze and the radial arm maze tests.

In the radial arm maze, each day rats quickly learn the position of the arms with food (reference memory for the general principles and skills needed in the task) and refrain to visit an arm more than once during the day (working memory: ability to maintain relevant memories while a task is being performed). Both reference and working memories are deficient after lesion of the hippocampus.

Hippocampal Place Cells

When rats familiarize themselves with the environment, many cells in the hippocampus acquire a place field, that is, they fire when the rats is in a particular place in the environment.

Comparative Studies of the Hippocampus and Spatial Memory

Species of birds that remember where they store seeds have larger hippocampuses than birds that do not store seeds, supporting the idea that hippocampus is important for spatial memory in many, if not all, species.

Experiment with humans in virtual-reality towns (show activity in hippocampus using positron emission tomography, PET) and with taxi drivers (bigger hippocampuses measured with magnetic resonance imaging, MRI) also support this idea.

Theories of Hippocampal Function

The hippocampus may use sensory input to form an allocentric map of the environment (space represented by the relation between external landmarks).

The hippocampus may be important for recognizing spatial arrangements of objects (such as furniture, pictures, etc, in a familiar room).

 

Where are memories stored?

In addition to the structures damaged in patients with medial temporal amnesia (hippocampus, amygdala and adjacent cortex (rhinal cortex)), other brain nuclei also play a role in memory.

Some of these structures, such as the mediodorsal nucleus of the thalamus, and the basal forebrain (a midline area, rich in Ach, located just above the hypothalamus) (Fig. 11.17) appear to be damaged in patients suffering Korsakoff syndrome (often due to alcoholism, as we will see later in the course) and Alzheimer syndrome (a terminal condition including progressive amnesia and dementia).

Other areas implicated in memory are (Fig. 11.17)

The inferotemporal cortex stores memories of visual patterns.

The amygdala plays a role in memory for the emotional significance of experiences. Rats with amigdala lesion fail to associate shocks with fear.

The prefrontal cortex stores the temporal order of events, affecting jobs that require a series of responses, such as cooking.

The cerebellum stores implicit sensorimotor tasks such as the eyeblink response.

The striatum, part of the basal ganglia stores implicit sensorimotor tasks such as habit formation that develop incrementally after many trials.