The Operation of Memory Systems in the Brain - Computational Neuroscience: A Comprehensive Approach - page 487

representation of places that is present in the rat hippocampus, how these spatial rep-
resentations are in a form that could be implemented by a continuous attractor which
could be updated in the dark by idiothetic inputs, and how a unified attractor theory
of hippocampal function can be formulated using the concept of mixed attractors.
The visual output from the inferior temporal visual cortex may be used to provide
the perirhinal and hippocampal systems with information about objects that is useful
in visual recognition memory, in episodic memory of where objects are seen, and for
building spatial representations of visual scenes. Before summarizing the computa-
tional approaches to these issues, we first summarize some of the empirical evidence
that needs to be accounted for in computational models.

16.2.1 Effects of damage to the hippocampus and connected struc-

tures on object-place and episodic memory

Partly because of the evidence that in humans with bilateral damage to the hippocam-
pus and nearby parts of the temporal lobe, anterograde amnesia is produced [100],
there is continuing great interest in how the hippocampus and connected structures
operate in memory. The effects of damage to the hippocampus indicate that the very
long-term storage of at least some types of information is not in the hippocampus,
at least in humans. On the other hand, the hippocampus does appear to be necessary
to learn certain types of information, that have been characterized as declarative, or
knowing that, as contrasted with procedural, or knowing how, which is spared in
amnesia. Declarative memory includes what can be declared or brought to mind as
a proposition or an image. Declarative memory includes episodic memory (memory
for particular episodes), and semantic memory (memory for facts) [100].
In monkeys, damage to the hippocampus or to some of its connections such as
the fornix produces deficits in learning about where objects are and where responses
must be made (see [12]) and [76]. For example, macaques and humans with damage
to the hippocampus or fornix are impaired in object-place memory tasks in which not
only the objects seen, but where they were seen, must be remembered [28, 60, 99].
Such object-place tasks require a whole-scene or snapshot-like memory [25]. Also,
fornix lesions impair conditional left-right discrimination learning, in which the vi-
sual appearance of an object specifies whether a response is to be made to the left or
the right [94]. A comparable deficit is found in humans [61]. Fornix sectioned mon-
keys are also impaired in learning on the basis of a spatial cue which object to choose
(e.g., if two objects are on the left, choose object A, but if the two objects are on the
right, choose object B) [26]. Further, monkeys with fornix damage are also impaired
in using information about their place in an environment. For example, [27] found
learning impairments when the position of the monkey in the room determined which
of two or more objects the monkey had to choose. Rats with hippocampal lesions are
impaired in using environmental spatial cues to remember particular places [35, 45],
and it has been argued that the necessity to utilize allocentric spatial cues [14], to
utilize spatial cues or bridge delays [34, 37], or to perform relational operations on
remembered material [19], may be characteristic of the deficits.
One way of relating the impairment of spatial processing to other aspects of hip-