We scarcely need to be reminded about the importance of the basic human sensory
processes --taste, touch, smell, vision, and hearing. With a little
reflection, we also recognize the necessity of various internal "senses"
monitoring our balance, body position, blood pressure, and other biochemical
body processes.
We can ask several fundamental questions about these sensory systems. What
exactly do they do? How do they work? To what extent are they shared by other
primates? How did they evolve?
Fig. x.1 Four functions of the senses
1. define self
2. localize self in relation to others
3. inform self about internal and external environment
4. enable conscious experience (vs. subliminal and automatic responses)
The senses in conjunction with the central nervous system define and
localize the self in space and time in relation to other objects. They
provide information that enables us to predict and control our
relationship to that environment. The senses serve as a "filter" that
calls attention to salient aspects of a species environment.
Our experience of consciousness also derives from sensory processes.
There are several layers to this question. All the external senses work
together informing us about objects --their location, characteristics, and
identity.
Our immediate sensory experiences integrated with memories of past experiences
define our "self."
The receptors, working with our nervous systems, inform all of our adaptive
behavior.
The prominent senses of touch, taste, sight, and hearing and the many other
"internal" senses (see Fig. x.2) share a common set of receptor processes.
These receptors serve to translate chemical, light, or movement information
into the code or language of the nervous system.
Primates share with other vertebrates a basic sense of time. This manifests
itself in two ways: light controlled Circadian rhythms governing sleep and
hormone levels and more precise "interval timers" that are necessary to govern
movements ranging from hourly egg incubation activity in ring doves to the
millisecond timing of human speech.
There are three general questions
It seems unlikely that there is any major functional differences among the
sensory systems of primates.
Again it does not appear there are great differences in the sensory structures
themselves. There are however, species differences in the proportions of
neocortex utilizing sensory information. Moreover, there are very likely some
important differences in the organization of central nervous systems reflecting
species specific characteristics--notably human language.
Very similar receptors suggest very similar experiences across species. This
presupposes that phenomenal experience is not affected by such factors as
interest, affect, or importance --an important and questionable assumption.
(See figure of nasal cavity & olfactory system comparison from Passingham,
p.25)
Human smell results from primate loss of "wet nose", relatively small olfactory
bulb, diurnal visual lifestyle (less need for nocturnal senses and greater need
for visual brain)
Mammals evolved the inner ear, using pieces of bone from the jaw, to provide
very sensitive hearing at relatively high frequencies.
Humans are most sensitive in the region of speech frequencies but not
remarkably different from chimps and perhaps other apes.
Humans appear to have no special ability to discriminate speech sounds than
some other mammals though much is unknown about human speech processing.
Surely there were changes in circuitry enabling rapid processing of auditory
information into meaningful speech.
Since our receptors are almost exactly the same, at one level that experience
might be expected to be "almost exactly the same."
However at a more important level of the meaning and value of sensory
information, there may be great differences. Consider two different types of
experience: language and sex.
We surely experience the vocal movements made by someone talking to us in our
language differently than someone who doesn't share our language.
Similarly as an adult human male, I surely experience the sexual swellings and
scents of a female chimpanzee quite differently than does an adult chimpanzee
male!
Filter effects of the sensory systems can be noted in communication where, for
example, the auditory system is most sensitive generally in the range of
significant vocalization of the species. Color vision likely serves to call
attention to ripe fruits or suitable mates. Specific adaptations may also
occur to sense predators who of course would love to be filtered out of their
prey's experience!
Basic structures, HP Fig. 2.9 include receptors (rods, cones, converging eyes
and large
binocular
overlap of forward facing eyes. (See comparison figures.)
Diurnal primates developed a fovea with high acuity and color vision. Foraging
for fruit, flowers, mates is enhanced by color.
Binocular depth perception has obvious advantages for tree-dwellers!
The human visual system is largely similar to other primates. While our
ancestors used foveal vision for eye-hand coordination in both arboreal
locomotion and foraging, we use it for precision movements in tool use such as
sewing, throwing, watchmaking, artistic endeavors, and reading.
Thus it comes as no real surprise to learn that recent high resolution fMRI
human visual cortex area maps reveal even greater emphasis on central vision
areas than in macaque or owl monkeys. These are the regions that overlap
several areas by written words. (Serano et al (1995).
(see "evolution notes" on comparative analysis of circadian rhythms)
One structure involved is the basal ganglia which coordinates muscle movements.
Research suggests that a complex neural circuit involving levels of the
neurotransmitter dopamine in the basal ganglia and its neural connection to the
frontal cortex determine interval timing. Humans with Parkinson's disease,
which destroys dopamine producing cells, show timing deficits which is restored
when levels of dopamine are increased . (Morrel, V. Setting a biological
stopwatch. Science, 271, pp.905-6 )
(See Humphrey, N. (1992). A history of the mind. New York: Simon &
Schuster. for one story on consciousness as an evolutionary holdover from
early primitive stimulus-response organisms. Consciousness is the sensory
cortex response to sensory stimulation in this theory.)
