The primate sensory and motor brain enables the organism to model the external environment with an internal representation of that environment. This in turn enables it to better predict and control events.
Mammals also need to monitor and maintain their own internal environment – e.g. blood pressure, temperature, biochemical equilibrium etc.
More complex animals like primates may be able to manipulate those external representations in the course of "thinking." Kohler's idea of "insight" in chimps is an example of this where an animal can forsee in mind the consequences of future actions.
The five basic senses – vision, hearing, smell, touch, and taste largely report the external environment to the brain. Internal body senses – balance, proprioception, and monitoring levels of oxygen, glucose, etc. – report directly to the brain or other organs. These internal senses are far less observed by consciousness -- consciousness is too slow!
Proprioception, actually a complex sense based on several sources of information, has often been called the sixth sense as it is so important yet so unobtrusive in our daily life – unless it is disrupted.
(Watch/Read about Ian Waterman, who lost all proprioception and could not move, but then learned to use visual feedback to know his body's location and was then able to move.) See below for more...
Something is there. In all of these functions, there is typically a degree of sensory integration in which info from several senses combines seamlessly to form a "gestalt" perceptual object not exclusively based on just one source.
(This would include localizing one's self in a cognitive map of your environment. )
This would include oneself, kin, conspecifics, dangers, food, possible tools, etc.
This is one of the most fundamental roles for the five basic senses, whether vision, hearing, olfaction, touch, or taste. Without a means to find appropriate mates, evolution could never work.
In humans this communication has evolved into our unique referential language.
Grooming and vocalization are necessary elements in non-human primate social life.
Faces are critical in most primate species life and we are probably adapted to notice and identify faces (and other body parts) for sexual and social relationships. (cf. prospagnosia)
(The genetic basis for this function has been studied in fruit flies: "Courtship in Drosophila involves a complex series of stereotyped behaviors that include numerous exchanges of multimodal sensory information over time..**" I expect soon to read about similar issues in primates.
These and other receptors maintain the internal environment as well delineate the 'self.'
Senses keep out irrelevant noise and detect potentially important stuff without much costly brain involvement.
During early brain development, sensory filtering may play a significant role in the organization of brain circuits.
All senses appear to be built from similar receptor neuron types. All these neurons respond to their particular stimuli and produce an electrical output code. Some respond to particular molecules, others to pressure, movement, or temperature changes, and yet others to varying frequencies and intensities of light.
Body and external receptors report to the brain and are given the brain space relevant to their importance. For example, 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 less than half of the space that the human hand gets. Again think of Limber's puppet metaphor-- how many strings does it take to get Pinochio walking vs tying his shoes or playing a piano. Here is a function model of the motor cortex.
Comparative anatomy and recent DNA comparisons show much similarity of receptors across many species. The brain circuits, including sensory-motor mappings -- even basic receptors -- are also very similar though with some species modifications. For example all mammals share a small set of basic taste receptors ( sweet, bitter, sour, salty and umami (the taste of monosodium glutamate). These, in combination, serve to provide critical information about recognition and quality of food. In general, our conscious experiences, for example of taste, color, sound, or pain, is a complex process based on a complex of receptors and brain processes.
Without verbal report available to humans, other methods must be employed in non-humans.
Some aspects are genetically determined; others develop via the organism's personal experience. Several ideas are important.
Humans are born before their brain is mature; it continues to develop neurons until up to 200% of the adult numbers are present. Then those numbers are reduced in the process of forming the adult neural circuits. Timing varies depending brain regions, earlier maturity in visual cortex, later for PFC. There is a lso the issue of developing new synapses, either genetically or experientially.
Sensory receptors may compete for brain space in the sensory-motor cortex. For example, early blind humans may come to use regions normally devoted to vision for processing auditory information. Strange phenomena associated with "phantom limbs" , synesthesia, and even sexual feelings associated with the foot stimulation possibly result from input from one receptor "overflowing" into another receptor's brain space.
Cell death may occur in several ways, natural 'apoptosis', lack of use, or damage.
Each sensory system has its own developmental process and timing; these vary by species as well. In primates, vision matures in the first year but in humans the auditory cortex may not be myelinated fully until around puberty.
This unconscious but critical sense informs us of the position of our limbs. Loss (in rare cases of infections) results in an almost indescribable state of body out of control. Amazingly a few individuals have regained control to a limited extent using other senses, especially vision. That is, they seem to have figured out how to determine the position of their body parts in a different way. Of course when the lights go out, they are screwed and collapse. (See Cole's book on Waterman.) An infection had destroyed his proprioceptive feedback system.
This sense is mediated by muscle and joint receptors, aided by surface skin receptors. It informs the brain, without consciousness, of the location of the body in space. (Consciousness often costs time!)
This sense, located in our inner ear and closely connected with vision, maintains coordination between eye, head,and body movements. Apes and humans have a vestibular-ocular reflex keeping eyes focused over a wide range of head and body movements. (See discussion in Kellogg video notes.)
Proprioception, vestibular (inner ear) information, and vision generally work together in primates to enable them to move rapidly through space, yet maintaining sharp vison on a target branch, prey, etc (Recent evidence indicates Neanderthal's had less sensitive inner ear functions (smaller semicircular canals).
Proprioception itself is a result of information from surface skin receptors, receptors deeper in muscles and joints.
This information is shared with the hippocampus which is responsible for "cognitive maps" in mammals.
I personally observed these organs in operation in several experiences -- motion after effects e.g. (coming off a boat and feeling the ground sway like waves), dizziness after spinning around in a chair, seasickness watching objects sway in closed bunkroom on board, and gradually losing balance taking a shower with my eyes closed. This is immediately and automaticallly corrected simply by touching the shower wall --try it!.
It is no surprise there are few noctural primates, given the importance of vision to spatial location of self. This is probably the explanation of why the slow loris is really slow!
Cross-modal perception also refers to a kind of sensory integration. For example upon hearing a noise, the sensory systems turn the head and eyes reflexively toward the source of sound -- localizing that source in space relative to the hearer. I say this is reflexive as it is reported blind children may move their eyes to the source. Hearing and vision are also integrated in speech. We take both visual and auditory input into account when perceiving syllables-- often in a surprising way (See McGurk effect) .
We can also recognize a shape by feeling it (haptic sense) and then matching a visual stimulus to the felt image. There are many studies evaluating primate abilities to do cross-modal matching.
Gomex (2004) reviews some of this research and says "The conclusion of these studies with human and other primates appears to be that one crucial, and probably hard-wired feature of the primate brain is the ability to put together information from different modalities into cross-modal representations of objects and events....an extension of a trend already present in mammalian brains. p. 53 "
We have discussed primates' binocular vision, giving depth perception based on two overlapping images from their forward-facing eyes. The above sketch shows how visual information is critical for body control. (recall the Ian Waterman story.). However humans have relatively smaller amounts of their larger brains devoted to vision than other primates -- presumably language and other cognitive abilities are using this space.
We also know how important color vision is to monkeys and apes as well as humans. The color receptors, cones, are located in the fovea and serve also to produce great visual acuity which we use for reading, etc. (Recall or read the achromat Nordby's story of life without color vision.).
Many primates have color cues for sexual attraction. Even though humans and chimps share many characteristics, how many human males are attracted to a female chimp's pink swellings! This suggests some specialized perceptual circuitry, perhaps linking visual and olfactory cues has evolved in each species.
For humans, and possibly chimps, faces are particularly important in attraction and other behaviors. Face blindness (prosopagnosia) is a striking demonstration of this -- as we have seen in several videos illustrating the effects of this disorder including the right hemisphere's contribution to the perception of upright faces.
From prosimians to monkeys and apes there is a loss of the more efficient 'wet' nose. This doesn't of course mean these later species have lost their olfactory ability. The usual suggestion is that there has been a trade-off to improve vision. Yet olfaction plays a role in both foraging and mating for many species – perhaps including humans where the role of pheremones is under debate. Recent genetic analysis suggests for humans, apes, and some monkeys the role of pheromones in reduced.
Proc Natl Acad Sci U S A. 2003 March 18; 100(6): 3328Ð3332. Published online 2003 March 11. doi: 10.1073/pnas.0636123100.
Who really knows? Do we really know that much about anyone else? But from guessing from senses – maybe not much difference moment to moment. But what about the effects of language?
All mammals that I know of have a larynx and good hearing. These function in common ways accross species -- maternal-infant communication, social/sexual relations, territorial claims, etc. As mammal brains evolved, auditory connections with neocortex increaased, improving high frequency hearing. Humans have a somewhat larger auditory cortex than expected for a primate brain our size.
Recent genetic evidence indicates significant differences in genes (e.g. FOX2P in news) between chimps and humans in regions relating to vocalization and hearing. Myelination of auditory cortex is slow, suggesting a recently evolved component.
These last two are probably the most important evolutionary questions -- how did human language and related cognitive skills develop that enabled humans to begin to accumulate knowledge -- human culture. We see the larger neocortex, left hemisphere specialization, and larger, richly connected temporal lobes that support these functions. That doesn't tell us much about evolution nor the actual processes that produce the capacities.
Devanand S. Manolia, Geoffrey W. Meissnera and Bruce S. Baker
Blueprints for behavior: genetic specification of neural circuitry for innate behaviors
Trends in Neurosciences
Volume 29, Issue 8 , August 2006, Pages 444-451