Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

Self and Conscious Experience 171

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

8.4.1 Discrete mode

Discrete sensations from the outside world start as stimuli striking
the sense organs: retina of the eye; auditory hair cells of the inner ear;
olfactory cells of the nose; taste buds of the tongue; skin receptors for
touch, pressure, and vibration; cutaneous nerve endings for pain and
temperature; and vestibular cells for movement and gravitational sense.
From the sense organs the messages are transduced into nerve impulses
that travel to the specific sensory areas in the cerebral cortex, by way of
a relay station in the thalamus.^15 Neurophysiologic correlates of discrete
sensations can be detected by sensory evoked potentials recorded from
the respective areas in the brain.
Cortical areas for discrete sensations include: visual cortex in the
occipital lobe, auditory cortex in the temporal lobe, and somatosensory
cortex (for touch, pressure, pain and temperature) in the parietal lobe.
(For locations of these structures refer to Fig. 7.10 in Chapter 7). However,
perception is an active neural construct that involves more than activa-
tion of specific sensory areas. Take hearing for example. Electrical signals
reaching the auditory cortex in the temporal lobe are projected to the
parietal and frontal cortices, from where feedback signals are sent down
to the temporal cortex. The reverberation of neural signals distinguishes
perception from simple sensation, suggesting that conscious awareness of
the environment requires recurrent processing of the incoming sense data
in the higher-order association areas.^16 Studies on visual perception show
that a human subject needs 300 milliseconds of stimulation for the process
to reach the conscious state (to become visible). Concomitant electrical
recording correlates the conscious state to a widespread, sustained acti-
vation of the fronto-parieto-temporal network, involving multiple recur-
rent cortical loops, without which only the occipital area (primary visual
cortex) is stimulated to a subliminal degree.^17 Further evidence supports
the^ top-down modulation of visual sensory processing, when it was shown
that neurons from the cingulate region of the frontal cortex send fibers to
the primary visual cortex to modulate discrimination of visual input. This