sensory systems

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?

What are their functions?

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.

How do they work?

common sense

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.

individual senses

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.

receptors: chemical, light, mechanical
localization
perception and recognition
a sense of time?

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.

How do they differ among the primates, especially humans?

There are three general questions

different functions?

It seems unlikely that there is any major functional differences among the sensory systems of primates.

different structures?

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.

different phenomenal experience?

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.

For humans, we have three specific questions:

loss of sensitivity of smell?
smell--recognition of molecules

(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)


adaptations for speech perception?
hearing

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.

how does our consciousness or phenomenal experience differ from a chimp's?

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!

How and why did they evolve that way?

external

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!

hearing
vision (depth, acuity, color, recognition)

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).

touch
smell
taste

time

circadian

(see "evolution notes" on comparative analysis of circadian rhythms)

interval

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 )

consciousness?

(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.)