Higher functions of the nervous system

HIGHER FUNCTIONS OF THE NERVOUS SYSTEM
Introduction: The association cortex:
The sensory deficits after partial or total ablation of primary cortical receiving areas can in large
measure predictable on the basis of the modality specificity and the topologic organisation of the
projections they receive from the periphery via the specific thalamic relay nuclei.
The primate cortical areas are located within the parietopreoccipital convexity (the classic sensory
association areas) and include, in addition, the prefrontal area occupying the anterior pole of the
frontal lobe (the classic frontal association area). These intrinsic cortical areas are phylogenetically
more recent and become myelinated later in development than the sensory and motor areas. There is
some regional specificity for different sense modalities and functions: the parietal − occipital portion is
mainly concerned with the somesthetic sphere of perception and behaviour, the anterior temporal
region is involved in auditory discrimination and the anterior temporal convolution is important for
vision. There is also evidence that some functions transcend any single modality and involve the
capability of dealing with the spatial arrangement of the objects of perception and with the temporal
sequencing of behaviour.
Posterior parietal area
Damage of the parietal lobe in the right (minor) hemisphere of right − handed subjects leads to
difficulties in recognising spatial relationships in the contralateral half of the body and extracorporal
space: the patient may act as though the limbs on the side opposite to the lesion did not exist; he may
not use the affected hand, yet deny abnormality in it; or he may claim that a contralateral limb belongs
to another person. He may disregard events taking place in the side of the environment that is
contralateral to the lesion, and he will tend to give naive and irrational explanations of these
aberrations. He may deny the existence of the contralateral body half altogether or deny illnesses
present in it. The patient may fail to complete that portion of drawings or to correctly construct that
side of a scene that is contrary to his lesion. The latter symptom is known as constructional aphasia.
Disorders caused by parietal lobe lesions in the left (dominant) hemisphere differs from those
attributable to the minor hemisphere by characteristically involving both sides of body and
extracorporeal space. These disorders, which involve disturbances related to classification and naming
and to manipulating the symbol of a particular class of percepts, are traditionally subsumed under the
term agnosias.
The distinction between left and right parietal lobe function does not imply that the dominant
hemisphere lacks the capability for morphosyntesis; rather, it appears that this function is overlaid,
and its defects obscured, by a process of higher abstraction that encompasses the perception of the
entire body and extracorporeal space in a unitary, indivisible manner that is one important aspect of
cerebral dominance: purely contralateral disturbances of morphosyntesis are therefore rarely seen
after parietal lesions in the dominant hemisphere.
Haptic sensitivity: tactic apprehension of object quality and shape
Identification of objects through the sense of touch presupposes the combined and co−ordinated
contribution of two submodalities − cutaneous touch and kinesthesis. Together, these two sources
contain the information needed to specify the layout of the surfaces of an object being manipulated.
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Supramodal mechanism in stereognosis
When the inability to recognise the form of objects by touch, known as stereognosis, occurs in the
absence of any overt signs of sensory deficits as measured by the conventional tests of somatic sensation,
the concept of tactual agnosia has traditionally been invoked and associated with damage to the
posterior parietal area. While this aspect of the posterior parietal lobe function is well established, there
is now also evidence that brain lesions that spare this and primary sensory and motor areas can also
produce some impairment of shape discrimination.
Hemispheric asymmetry in sensory − motor function
Specialisation of hemispheric function does not appear limited to those manifestations commonly
implied by the concept of cerebral dominance. This concept may even be misleading, for it suggests
superiority of the dominant over the minor hemisphere, whereas it is now understood that the
hemispheres differ with respect to the particular function for which they are specialised, each
performing its assigned role in the sense of a reciprocal specialisation rather than a hierarchy of
importance.
Associative learning of tasks in somesthesis
There is firm evidence that the posterior parietal lobe lesions in macaques produce well delineated
deficiencies in the acquisition and performance of tasks involving discrimination of tactual stimuli;
deficiencies of a nature that excludes the involvement of the ataxic component of the parietal lobe
syndrome.
There are two different aspects to the role of the posterior parietal lobe in associative learning of
somesthetic stimulus discriminations. In the firs place, in tests involving the discrimination of roughness
of textures, the deficit induced by posterior parietal ablations consists in a decrement of the capacity to
resolve fine differences between stimuli, while the performance remains unimpaired when the
difference between the stimuli is great. The second aspect concerns the selection of the strategy for
handling multiple − object, problem − solving tasks.
Inferotemporal cortex
The recognition of the role of the temporal lobe in vision originated one of the most startling defects
following bilateral temporal lobotomy in macaques was their inability to recognise objects by vision
alone. Discrimination of two − dimensional patterns is the most severely impaired visual ability, but
there is also some deficiency in the discrimination of object quality in terms of colour, size, and
brightness. Subsequent studies led to a progressively more precise delineation of a circumscribed region
in the temporal lobe, ablation of which abolished the monkey's ability to discriminate between pair of
visual patterns that he was successful in discriminating preoperatively. The area in question is the
inferior temporal convexity, comprising the middle and inferior temporal convolutions.
Visual deficits after temporal lobe damage in man
Among these visual deficits are disturbances in tachistoscopic recognition and dot numerosity, in
classification of pictures of faces in which normal contour lines have been eliminated, and in the
detection of small differences in complex patterns.
Middle temporal area: Auditory discrimination
The evidence for modality − specific auditory discrimination deficits after temporal lobe lesions is less
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definite than for tactile and visual deficits with regard to their respective intrinsic cortical areas. To
some extent this is related to the fact that monkeys, the preferred species for studies of this general type,
have great difficulty in acquiring behaviour guided by auditory stimuli.
Auditory perception deficiencies in man
In its most dramatic form the auditory deficit entails a failure to comprehend spoken language. Most
patients with such a condition also have severe expression disorders; they produce a rush of speechlike
but incomprehensible sounds, often without appreciating their own disability.
Information processing in perception
Information − processing theories of perceptual processes in problem solving, including those
employing computer simulation as their means of formalisation and analysis, have in recent years
generated valuable insights. It is now generally agreed that problem solving can be represented as
searching through a large set of possibilities until a satisfactory solution is found.
In solving problems that require actions as out come this process of pattern conception becomes an
integral part of the problem − solving process itself. Evidence for the character of the initial perceptual
activity comes largely from situations where problems are presented to subjects in visual form. The
evidence takes two forms: a record of the subject's eye movements during the first few seconds after
problems are presented to them, showing the succession of fixations during this time, and the subject's
ability to retain information about complex visual displays after brief exposure.
Frontal intrinsic sector
In the studies of Jacobsen the delayed response task proved to be a selective index of primate frontal
lobe injury: after bilateral orbital − frontal lesions, delays as short as 5 sec between seeing the reward
and selecting its location reduces success in the choice to a matter of chance; the animal is simply at a
loss in choosing the container that conceals the food.
It was also suggested that locomotor hyperactivity occurring after frontal surgery in the monkey might
interfere with the utilisation of mnemonic devices adopted to help bridge the interval between stimulus
exposure and response opportunity. It appears as though the deficit in an animal with a frontal lobe
lesion implies an inability to cope with situations in which an element of remoteness and ambiguity is
introduced into the relation between stimulus and success of response.
Sensory − motor co−ordination
The example of the relation between eye movements, perception, and planning an activity that was
discussed earlier emphasises the intimate interrelations between sensory and motor processes in the
information transactions of the CNS. Another example is that active movements of the eyes leave the
perceived world stable, while passive movements make the visual scene jump.
The significance of the interrelations between sensory and motor systems has become apparent from
experiments on restricted rearing, in which a normal adult was fitted with distorting spectacles. The
prisms imposed tilts and curvatures upon contours and displaced the visual scene.
Interhemisferic integrations
This topic directs attention immediately to the large forebrain commissures, of which the corpus
callosum is the most prominent structure. Sectioning the optic chiasm in a mammal with crossed optic
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fibbers leaves the major part of the visual field intact, but each eye then projects only to its homolateral
hemisphere. Cats with sectioned chiasms trained to discriminate visual patterns through one eye while
the second eye is covered with a mask, can also execute the same task through the other, previously
covered eye. Thus there is intercommunication between the two brain halves. However, this cross −
availability of information between the two hemispheres can be abolished by sectioning the corpus
callosum. Accordingly with both optic chiasm and corpus callosum sectioned, the cat can learn a
different, even opposite, discrimination task with each eye without mutual interference.
Mechanisms of learning and memory
The process of learning is concerned with establishing a relation between a present and a past event;
furthermore, when successive stimuli can be compared and a response that is contingent on that
comparison can be made, we speak of memory. The implication is that the original event must leave
some change in the nervous system that permits the present event to be related to it.
Natural history of the memory trace
The original theory of consolidation was that information is initially stored as some form of persistent,
reverberating neural activity that is then gradually converted into structural changes at synapses. In
more general terms the implications is that there are at least two kinds of traces: one that is short −
lasting and susceptible to disrupting treatments and one that is more stable and long − lasting. There is
a suggestive parallel between the distinction between these types of traces and the concepts of a more
immediate short − term and more indirect long − term memory, based on observations in man.
Storage versus accessibility
The retrograde amnesia that occurs after ECS does not necessarily imply that this treatment interferes
with consolidation. Experimental as well as clinical evidence indicates improvement of retention with
time after the disruptive treatment. This suggests that the interfering trauma does not cause total trace
destruction but that the retrieval of traces may be temporarily interfered with.
Neurophysiologic mechanisms related to learning
A first approach to the study of neural mechanisms that may be involved in the formation of the
memory trace is to study the EEG changes during acquisition of new behaviour. In the nonattentive,
relaxed state the EEG exhibits the characteristic 8 to 13 Hz oscillations of the alpha rhythm. Any novel
and unexpected stimulus results in an arrest or blocking of this alpha activity and the electric record
assumes the form of low − voltage, fast activity.
Human memory processes: a conceptual scheme
In the first place, there is evidence for a sensory memory of limited capacity that holds raw data of
sensory events for a period of a few hundred milliseconds for attention, scanning, and further
processing. This may occur in two different forms: a verbal label may be attached to the content of the
sensory memory, which is then thought to be transferred to another limited − capacity storage system,
the primary memory. An alternate route, if verbal recoding does not take place, consists in transfer
from sensory to secondary memory, a large and more permanent storage system.
Neural mechanisms of verbal behaviour
During the second half of the 19th century, neurologists became increasingly impressed by observations
suggesting that disorders of language resulting from brain damage can occur in the absence of
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generalised motor and sensory deficits of intellectual impairment; similarly a deficiency in
comprehension may occur for spoken or written words in the absence of auditory or visual deficits for
non−verbal material.
Another aphasic syndrome, known as isolation syndrome, can also readily be explained. Language
comprehension is completely lacking and the patient is totally unable to spontaneously produce
propositional speech; however, the patient can repeat sentences perfectly, with normal articulating, and
he can complete sentences after having been told the beginning.
Psycholinguistic considerations in the study of aphasia
In addition to the classic neurologic approach to the study of aphasias, attempts have also been made to
characterise aphasic impairments in terms of linguistic theory. The production of linguistic expressions
starts with the selection of constituents that are to be combined and integrated into a context: this
process may be thought of as encoding, with the selection process being the antecedent and the building
of the context the consequent. Conversely, aphasias of decoding consists in primary impairment of the
word selection and identification. Thus the overriding symptoms are the impoverishment of the variety
of means available, a tendency to supplant them by generalised substitutes, and an inability to furnish
synonyms and antonyms.
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