2. Senses

1. functions of the SENSES to organism

A. senses give information/meaning about environment

The combined sensory-brain enables the organism to model that environment with an internal representation. This in turn enables prediction and control of events.
Some basic functions
1 alerting function
2 localization
of self in environment

of other objects

3 recognition (self, kin, others, enemies)
4 social/sexual/reproductive communication
In humans, this has evolved into our complex referential language.

B. Senses give information about internal states and "self"

pressure, tension, glucose, oxygen, orientation, balance
These not only maintain the internal "environment" but also serve in part to define or delineate the "self" along with vision.

C Senses serve as a selective filter of information

They keep out irrelevant noise and detect important information without much brain involvement.

During early brain development, sensory information may play a very important role in the organization of brain circuits.

D. Three questions concerning human senses

Why poor smell?

Is hearing specialized for speech?

Is our human world seen and felt differently than a chimp world?

2. methods of study

A. anatomical

1 types of sensory cells
1 chemical, mechanical, photo sensitive
2 cell architecture
3 interconnecting pathways
4 single cell recording & neural mapping
5 lesioned/normal comparisons
6 sensory "maps" in the neocortex
Recall Penfield's work on conscious humans undergoing brain surgery.

the sensory-motor homonculus for humans

B. discrimination tasks

matching

e.g. recovery from habituation

e.g. operant conditioning (conditional responses)

C. ecological inference-relation between senses and life-style

e.g. nocturnal, mouth foodhandler, ground dwelling organisms

e.g. diurnal, claw, tree-dwelling organisms

e.g. visual, vocal, bipedal, neotenous big-brained grasping tool-users!

D. development

phylogeny: mosaic evolution-heterochrony

ontogeny: neural overproduction and pruning-- which is sensitive to experience

the role of sensory-motor feedback in structuring the central nervous system (hand/limb examples)

3. specific input senses

smell--recognition of molecules

(show 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)

taste
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.

balance
vision (depth, acuity, color, recognition)

Basic structures, Fig. 2.9 include receptors (rods, cones, converging eyes and large binocular overlap of forward facing eyes. (See comparison figures.)
color
Diurnal primates developed a fovea with high acuity and color vision. Foraging for fruit, flowers, mates is enhanced by color.

Old world monkeys are attracted to orange and yellow fruits, whereas birds go for red and purple fruits. Since most mammals, even many vertebrates (e.g. fish) have at least dichromatic color vision, lack of color vision in primates is probably a suppressed feature in noctural species and trichromatic color may have evolved in concert with fruit coloration. Trichromatic vision also is utilized in recognition of various emotional and sexual states in some primates, brightly colored themselves. (The first experimental study showing the similarity of trichromatic vision in chimpanzees to humans was published by Kohts (192x) using her home-raised chimpanzee, Joni.)

Mollon, J. D. (1989). "Tho' she kneel'd in that place where they grew...": The uses and origins of primate color vision. Journal of experimental biology, 146, 21-38.

One can imagine that a lack of color vision would have serious implications for primates abilties. A rare genetic variant among humans produces an eye without the color receptor cones. These achromatics only have rods; hence their acuity is low, color vision lacking, and they are extremely sensitive to light. Knut Nordby, a Norwegian vision scientist, is one such achromatic and has written an interesting account of his experiences. Excepts from that autobiography are revealing:

"...it became very obvious to me that my vision was different from that of other children. They could see things that I could not see; such as recognizing each other at a distance, spotting ripe berries on bushes and trees , reading cars' licence plates at a distance, etc. They could also take part in activities and sports, especially ball-games, that I could not. Hitting a ball with a bat, or catching a ball thrown towards me, was next to impossible for me, except under the most optimal light-conditions, such as in twilight....

Picking berries has always been a big problem for me. I usually have to grope around among the leaves with my fingers, feeling for the berries by their shape, except in the shade or in the evenings when light levels are low. Then I can usually see the berries; the red ones as small "black' spheres among the "grey" leaves."

depth perception
Binocular depth perception has obvious advantages for tree-dwellers! It is also of value to predators, who tend to have forward facing eyes in contrast to their prey, who value peripheral vision.

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

(include other somatosensory, e.g. pressure and temperature) See video notes.

Information from touch receptors--like other sensory receptors -- is relayed to the neocortex. Body surfaces are represented there; the extent jof the representation is related to the importance to the animal of information from that region of the body. (p.34)

Not surprisingly, monkeys with prehensile tails like spider monkeys have five times the corresponding neocortex "tail" surface of a macaque. (HP, p. 34)

The human foot gets only one half the space that the hand gets (and that is probably an underestimate.)

cross-modal meanings

We humans are very good at integrating across different modalities. We hear a sound and reach out to touch it's source. We touch something in the dark and can recognize it visually. We can recognize letters and simple words "written" on our backs by touch! This does not appear to be a function of having language to mediate between senses (though it may facilitate the process).