ULTIMATE COMPUTING - Quantum Consciousness Studies

74 From Brain to Cytoskeleton
period of time on the order of 45 seconds (Cherkin and Harroun, 1971). Some
regard the two processes as independent parallel functions. If brain activity is
markedly altered by seizures, trauma resulting in unconsciousness, or by general
anesthesia the phenomenon of amnesia may occur in which the subject cannot
remember events that occurred immediately before the disruption. Events farther
in the past are not forgotten by such interventions. One explanation is that short
term memory depends on some dynamic process such as the continued circulation
of impulses in a pattern which, when disrupted, is erased. According to this
scheme, long term memories are stored by an enduring change in the neurons or
in connections between them which would be unaffected by such upheavals. An
analogy to computer jargon would suggest that a software program becomes
“hardwired.” Other models of memory process describe more of a continuum in
the consolidation process divided into three or five stages, or entirely different
mechanisms with varying methods of entry, storage capacities, lifespan and
accessibility. Still other models regard all memory functions as the same basic
mechanism differing only in secondary characteristics.
Learning is linked to memory; learning a complex task probably involves
generating a pattern suitable for memory storage. For example, the image of a
person’s face may be recalled by exciting neurons in a pattern similar to the one
generated when that face was actually perceived. The ability to swing a tennis
racket implies the existence of a program or pattern for activating muscles in the
proper sequence and degree. Acquiring such neuronal programs has been
attributed to changes in the functioning of synapses between specific neurons
(Figure 4.3). Structural changes which lead to alteration in the function and
sensitivity of interneuronal synaptic connections are the cornerstone of current
concepts of learning and memory. Classifications of the types of plastic changes
that could occur in synapses include habituation, long term potentiation, and
heterosynaptic potentiation.
Habituation to a response is the simplest form of learning. When an animal
hears a new sound it may respond by perking its ears showing some form of
attention. If that sound is repeated continuously, the animal learns that the sound
or stimulus is neither threatening nor interesting and becomes “habituated” to it.
Habituation is different from other forms of decreased response such as synaptic
fatigue or desensitization and is specific for the stimulus and its intensity. If the
habituating stimulus is withheld for a period of time and presented again, the
response reappears (“dishabituation”). Habituation to noxious stimuli does not
occur and when non-noxious and noxious stimuli are paired there is no
habituation to either of the two. The mechanism of habituation has been studied in
detail in the marine organism aplysia, or sea slug. The sea slug has a “gill
withdrawal reflex” which is convenient for study. When the skin of the slug’s
syphon is stimulated, the animal withdraws its gill. This response shows
habituation, dishabituation and other features typical of more complex
mammalian responses. The neuronal network mediating this reflex response has
been extensively mapped and the participating synapses studied electrically. The
habituation of gill withdrawal might have been the result of many processes
including synaptic inhibition, but in fact has been shown to be the result of
decreased output of excitatory neurotransmitter at the presynaptic axon terminals
mediating the withdrawal reflex. Neuroscientist Eric Kandel (1976) of Columbia
University has shown that the habituation is different from ordinary fatigue, does
not involve exhaustion of available transmitter, and is related to decreased flux of
calcium ions through presynaptic calcium channels. Thus a behavioral response
has been elegantly related to molecular level events. The activity of presynaptic
calcium channels and other neural proteins have been viewed as “allosteric”-an