Sensation

 The first subject with which we must begin our exploration of the interior of the human mind is sensation. Sensation may be defined as the detection, by specialized physical structures in or on the surface of the human body, of the various energies and chemical compounds that reach the sensory apparatus. In short, the senses are the means by which a human experiences various qualia. In our evolutionary line sensation was primary and fundamental, that which existed prior to the emergence of consciousness. Traditionally, humans have designated five senses—touch/feeling, sight, hearing, smell, and taste—as the chief means by which a human gathers information about the world around them, or information about their internal status. But as we will see, there are researchers who now see the senses more broadly.

None of these senses, as we have already discussed, is in direct contact with the free-standing “thing-in-itself” reality, inasmuch as the human nervous system is the product of a very particular and specific evolutionary process, as we have seen. This process has created a very specific (and narrow) frame of reference. And all sensory data received by the senses is converted into electrical impulses which are then processed in the brain, the phenomenon known as sensory transduction. So, in many ways, the senses give us indirect, moderated information about reality.

However, we are compelled to say that each one of our senses is in some sort of relationship with the “real” reality, and that some sort of stimulus from that reality (or some aspect of that reality which makes this stimulus possible) must be present in order for a sensation to be perceived. There must be some aspect of the “real” reality that produces, for example, the phenomena that we perceive as the energies that produce light or sound.  Our internal reality is constructed from an amalgam of these senses, and the set of reactions we have had in previous times in our lives to something that has been experienced through them.

The Nature of Sensation

Physically, sensation is based on phenomena known as event related potentials. Two researchers who are specialists in this field define them as follows:

Event-related potentials (ERPs) are very small voltages generated in the brain structures in response to specific events or stimuli…Event-related potentials can be elicited by a wide variety of sensory, cognitive or motor events. They are thought to reflect the summed activity of postsynaptic potentials produced when a large number of similarly oriented cortical pyramidal neurons (in the order of thousands or millions) fire in synchrony while processing information...ERPs in humans can be divided into 2 categories. The early waves, or components peaking roughly within the first 100 milliseconds after stimulus, are termed ‘sensory’ or ‘exogenous’ as they depend largely on the physical parameters of the stimulus. In contrast, ERPs generated in later parts reflect the manner in which the subject evaluates the stimulus and are termed ‘cognitive’ or ‘endogenous’ ERPs as they examine information processing. The waveforms are described according to latency and amplitude.¹

 We have already discussed the nature of conscious experience, but now we can zero in on the nature of sensation specifically. Nicholas Humphrey has laid out what he believes are its five primary features:

1.    Sensations belong to the subject. They are part of the definition of “what is happening to me”.

2.  Sensations have a bodily location, which is to say a location immediately known to the subject experiencing the sensations.

3.   Sensations are of a specific kind (what Humphrey calls modality specific). They are “tactile, visual, auditory, olfactory, gustatory, or a sub-modality of one of these”.

4.   Sensations exist in the present.

5. Sensations are self-characterizing, which is to say that sensation is primary and precedes perception.²           

Other researchers give sensations attributes such as intensity (and changes in the intensity of a sensation), duration, and most significantly, sensations acting in interaction with each other, a situation in which different modalities can be the most prominent sensation detected at different times. The intermixture of sensations can also create syntheses of sensation. We will discuss sensations in interaction in greater detail below.

Sensations are processed in the sensory cortices of the brain. As we noted in the chapter on the anatomy and physiology of the brain, the sensory cortices consist of the visual, auditory, and somatosensory cortices. In a sense, the somatosensory cortex forms a representational map based on the nerve signals coming into it (signals known as afferent inputs). This map, usually referred to by researchers as the sensory homunculus (a miniature human-like form), is meant to represent the areas of the somatosensory cortex receiving specific kinds of afferent input.³

Sensations are not only processed in the brain, they change the brain itself, which is the heart of the concept of brain plasticity. Two experts in the field explain it this way:

The somatotopic arrangement of the sensory cortex and much of the other parts of the brain are subject to the concept of plasticity. The brain adapts to the sensory needs of the body and changes in shape based on usage. Thus the cortical area that corresponds to the sensation of different parts of the body depends on the amount of sensory input that area of the body receives relative to other areas. Even though the amount of cortical space dedicated to sensation can change, the overall distribution of the homunculus is conserved.⁴

Ranges and Capacities of the Human Senses

The major organs of human sensation and perception are remarkably close to one another. The head is the location of four of the five conventionally defined major sensory systems. Only touch is generalized throughout the body (although, oddly perhaps, the brain as a physical organ itself feels nothing, and can be operated on without the use of any anesthesia specifically meant for it). The proximity of the senses to each other is useful. The ears are permanently open as a defense mechanism. Taste is strongly related to smell. The sense of smell and the sense of taste taken together comprise the chemosensory system, or the chemical senses. And vision, of course, processes myriad information about the world for a human. Vision occupies a serious percentage of the total cortex, since it was superior vision that contributed so strongly to human evolutionary success. The exact percentage is in dispute, but there can be no doubt that it is formidable.

What are the physical limits within which the senses function? What are their sensitivities? What information are they conveying to us—and what are they incapable of?

Vision: What are the parameters of human vision? And how do we sense light?

The visible light spectrum is the segment of the electromagnetic spectrum that the human eye can view. More simply, this range of wavelengths is called visible light. Typically, the human eye can detect wavelengths from 380 to 700 nanometers.⁵

Humans therefore see only a very narrow part of the spectrum of electromagnetic energy. Other forms of light have wavelengths either too long or too short for humans to detect. But within that limited range of vision humans display a remarkable ability to discern colors (if they have normally functioning eyes) and an excellent ability to focus.

Light enters the eye through the transparent front part of the organ, the cornea. The colored part of the eye is known as the iris, and it circles the pupil, which expands or contracts to adjust to different intensities of light. The lens, behind the pupil, focuses light on the back of the eye. About 70% of the eye’s ability to focus is due to the cornea and 30% is due to the lens. Light passes through the vitreous humor, the gelatin-like material that fills the vitreous cavity (found between the lens and the back of the eye) and reaches the retina, at the back of the eye. The retina, as we’ve seen already, is actually a part of the brain. A very small region of the retina, the macula, gives a human their central vision. The peripheral retina makes up the rest of the retina’s structure. Peripheral vision, of course, is what makes us aware of visual stimuli to our sides, outside the central focus of our vision. The loss of peripheral vision, known as tunnel vision, can put a human at a significant disadvantage. The retina possesses specialized cells known as photoreceptors. It is these cells that transduce light into the electrical signals that are sent to the brain. Photoreceptors fall into two categories: rods and cones. Rods enable us to see in very low light and perceive black and white. Cones are the color-perceiving receptors and give a human detailed vision. Electrical impulses from the retina are sent via the optic nerve, a bundle of millions of nerve fibers, to the brain’s visual cortex.⁶

Most primates, including humans, have trichromatic vision, which is to say the broadest range, one capable of perceiving reds, blues, and greens. This trait apparently evolved first in Old World primates, the line from which humans are descended.⁷ Prosimians tend to be dichromatic, which is to say they can only perceive blues and greens. Some New World monkeys are dichromatic as well. And there is a species of monkey that is monochromatic, seeing only black, white, and shades of gray. Between 6% and 8% of humans are red-green color blind, primarily males.⁸ Blue-yellow color vision is found in fewer than 1 in 10,000 people worldwide, affecting males and females at the same rate. The rarest condition is called blue cone monochromacy. It affects about 1 in 100,000 people worldwide, the great majority of whom are male.⁹

The origin and utility of color vision (and acute primate vision in general) are matters of some dispute among physical anthropologists and others studying the issue. We touched on this subject in Volume One (pp. 295-297). To briefly recapitulate, the emerging view is that primate visual acuity stemmed from a synthesis of causes. The ability to see insect prey was crucial, as was the necessity of having depth perception in an environment which had to be navigated largely by brachiation. The radiation of angiosperms (flowering plants) helped feed the members of the evolving primate order, who had to have good vision to fully exploit the fruit and leaves such plants provided.

Interestingly enough, recent research has found that humans throughout the world associate particular colors with specific emotions in somewhat similar ways, although there are local differences in such associations. This would seem to indicate that certain reactions to color are “wired into” us. In cultures throughout the world, for example, the color red is associated both with both love and anger. But different cultures sometimes associate different colors with sorrow or mourning. While in the West people often associate black with grief, in China the color of mourning is white.¹⁰

There are regions of the visual field called blind spots. Normally functioning eyes all possess such areas. Two specialists in eye physiology explain their origin and scope:

The natural blind spot occurs where axons passing over the front of the retina converge to form the head of the optic nerve, and where retinal blood vessels enter and exit the eyeball, resulting in a hole in the photoreceptor mosaic. Natural blind spots are present only in the eyes of vertebrates…

Each eye has a surprisingly large blind region, about 4° of visual angle, the width across your four fingers held at arm's length. Luckily, they are in different locations in each eye, the one in the left eye is about 10° (two hand widths at arm's length) to the left of central vision, and the one in the right eye, equally far out on the other side. Surprisingly, we are normally unaware of these natural blind spots. They are either filled in perceptually (a remarkable phenomenon) or they are ignored and so not seen.¹¹

Hearing: The ears are evolved to gather in sound waves. Sound waves are measured in a unit called the hertz. A hertz is an oscillation, which is to say a periodic motion. The number of oscillations, or cycles per second, of sound waves, determines the frequency of a sound. The range of frequencies normally functioning human ears can sense is from somewhere around 20 Hz at the lowest end of our range of hearing, to 20 kHz at the high end. Anything lower than 20 Hz or higher than 20kHz is undetectable to normally functioning adult human ears. Not all mammals detect the same range of frequencies we do.¹²

While not the most ancient sense, hearing does go back deep into evolutionary history. An expert on the ear puts it this way:

Take a second and think about what an ear is: an organ that senses the changes in pressure of molecules. We tend to imagine ears hearing music or car horns, but what they are really noticing is vibration. Early vertebrates used vibration sensitivity for two different purposes. One was to monitor changes in fluid flow right around their bodies, using what is called a lateral line system, still found in almost all fish and larval amphibians still around today. The second was used to monitor shifts in internal fluid flow in specialized organs located on each side of their heads. These structures had no specialized organs for picking up airborne sounds, since back then everyone still lived in the seas, but they were used to detect angular and linear acceleration of the animal’s head.

He goes on to say that the physical structures that detected internal vibration allowed marine animals as far back as 350 million ybp to sense their acceleration. From such early anatomical features ears evolved, and with them the ability to gather and sense sound waves.¹³

So the ear transduces vibrations, the oscillations of what humans call sound waves, into an electrical signal the brain can interpret. By what process is this done? The pinna, the cartilaginous structure that actually stands out from the side of the head, captures sound waves. Its shape means it captures sounds in front of a person before sounds coming from the back, which helps a human identify the direction from which a sound is coming (as does the position of the ears on opposite sides of the head). Sound goes through the external auditory canal until it reaches the tympanic membrane, otherwise known as the eardrum. (The middle ear is connected to the back of the nose by a small tunnel known as the Eustachian tube.) Three small bones convey sound from the tympanic membrane to the inner ear. Within the inner ear is the cochlea, a small but complex, coiled structure. It contains around 30,000 hair cells which transduce the vibrations coming into it into nerve signals. Around 19,000 nerve fibers then convey signals to and from the brain.¹⁴

Smell: As noted above, the sense of smell is one of the chemical senses. From an evolutionary standpoint, the chemical senses are the oldest ones.¹⁵ The sense of smell is vital to both human safety and the human enjoyment of life. The ability to detect rotten food, noxious gases, and smoke was a vital part of our species’ survival. As one team of specialists puts it,

Olfaction, the sense of smell, has evolved to detect signals from the chemical environment, which contains clues about where to move, what to eat, when to reproduce, and which stimuli to remember as rewarding or dangerous. How an animal responds to chemical cues is in part learned and in part innate, depending on how the olfactory nervous system has been shaped by both experience across a lifetime and evolution across generations.¹⁶

What is the mechanism by which humans detect odors? In a small area in the upper reaches of the nose’s interior there are olfactory sensory neurons. These neurons are connected to the brain. A single odor receptor is in each olfactory neuron. The molecules of various substances in our environment stimulate these receptors, which in turn causes a signal to be sent to the brain. The number of possible smells is greater than the number of receptors, and a given molecule may stimulate an array of different receptors, which causes the brain to interpret this combination as a specific smell. Smells reach these neurons either through the nostrils or the passageway that links the roof of the throat to the nose. Viral infections can block the throat-to-nose passageway, affecting the ability to taste food, since smell and taste are so closely linked.¹⁷

A team of researchers centered largely but not exclusively at the University of California San Diego makes the classic case for the relative inferiority of human olfactory abilities compared to other animals:

Genetic evidence indicates humans have lost the function of all but 388 of over 1,000 olfactory receptors encoded in the genome. These non-functional genes, called pseudogenes, appear to have accumulated faster and be in higher abundance in the human genome compared to their putative orthologs in chimpanzee, gorilla, orangutan, and rhesus macaque genomes. This loss in the repertoire and function of olfactory componentry indicates reduced evolutionary selection pressure and may reflect reliance on other sensory systems. In another study, a comparison of the human genome to olfactory receptor gene orthologs in the chimpanzee genome indicates the number of intact olfactory receptor genes in both humans and chimpanzees is shrinking, indicating a diminished importance in both species from relaxed positive selection constraints.

Loss of olfactory receptors is linked to the evolutionary appearance of a heightened trichromatic visual system in humans; similarly, primates with trichromatic color vision have a larger fraction of pseudogenes than other mammals, suggesting olfaction is less important in species with trichromatic vision.¹⁸

The human sense of smell is, indeed, usually portrayed as inferior to that of most other mammals (and that was my own view for a very long time) but there is now evidence that the human sense of smell is more powerful than most of us have been taught. There is research demonstrating that humans can detect one trillion different combinations of 128 odors, odors which represent a wide range of possible smells. It should be understood that there are vast differences in the olfactory sensitivity of individuals. Some can distinguish no more than 80 million smells while others can perceive far more than a trillion. Experts point out that there may not be one trillion smells in the natural world. Also, research is proceeding on how combinations of smells are processed by the brain.¹⁹

And there are other researchers vigorously pushing back against the notion that the human sense of smell is inferior to that of most other animals. Some argue that even though the number of genes in the human genome that specialize in olfaction has declined relative to other mammals, that doesn’t necessarily mean that the human sense of smell isn’t as acute as that of many other animals.

Another type of study has tested smell perception in primates, and has shown that, despite their reduced olfactory receptor gene repertoire, primates, including humans, have surprisingly good senses of smell Comparing the data on smell detection thresholds shows that humans not only perform as well or better than other primates, they also perform as well or better than other mammals.

A third type of study demonstrating human olfactory abilities shows that in tests of odor detection, humans outperform the most sensitive measuring instruments such as the gas chromatograph.

These results indicate that humans are not poor smellers (a condition technically called microsmats), but rather are relatively good, perhaps even excellent, smellers (macrosmats)…

Humans even seem to detect certain smells better than dogs. ²⁰

The sense of smell, can of course be lost in several different ways, and there is the phenomenon of phantosmia, which is best of thought of as an olfactory hallucination. One researcher explains it like this:

Phantosmias (those unstimulated perceptions of odor that last longer than a few minutes) are also thought to have either peripheral or central causation, or a combination of the two. Peripherally, `rogue' neurons that emit abnormal signals to the brain or a loss of inhibitory cells to normally functioning olfactory neurons could be at fault…Centrally an area of hyper-functioning brain cells could generate this odor perception.²¹

The olfactory environment is just as real and distinct as the visual, auditory, and tactile environments are. The key difference is that humans have a harder time (in general) describing what they smell as opposed to their experience of sight, sound, and touch. There are researchers who contend that smell has played a greater role in human interaction than we might suspect. They point out that most modern Western societies are “deodorized” so to speak and that smells have great social importance in many cultures and settings. “These social effects may well reach a zenith around puberty, as surges of sex hormones lead to sexual maturation and concurrent activation of sexual interest, as well as activation of sebaceous and apocrine glandular activity that produces sex-typical adult body odour.”²²

Taste: the chemical sensory partner of smell, taste, appears to have evolved as a mechanism for determining the safety of food being ingested. Taste is referred to, among researchers, as gustation. Gustation is not a “stand alone” sense, since the smell of food and the feel of its texture are such key elements in a human’s judgment of what tastes good and what doesn’t.

What is the mechanism of taste? The tongue is the chief part of the body involved in gustation. Taste buds arrange themselves in columns that encircle the tongue. Taste buds are chemosensory receptors. The receptors that actually sense taste are known as microvilli. Specialized nerve fibers in turn transmit taste impulses from these microvilli to the brain. Although the great majority of taste buds are on the tongue, there are some in other areas of the oral cavity and the upper part of the esophagus.²³

It's important to remember, however, that a human decides to swallow or reject food based on more than just taste. As we have noted, the smell and texture of food play a part, as does its temperature. Furthermore, taste itself is subject to changes based on hormones in the gastrointestinal tract and glucose levels in the blood.²⁴

Touch: We have already discussed the anatomy and physiology of the nervous system in the human animal. But now we need to focus on the channels through which sensation flows.

There is evidence that fetuses can sense their surroundings as early as eight weeks into gestation, and by sixteen weeks will begin exploring the womb through touch.²⁵ (But it should be noted that fetal pain perception probably does not exist prior to the third trimester of gestation)²⁶ Infants do a great deal of tactile exploration with the mouth, using their mouths to test the shape or texture of various objects. Tactile exploration by young children then shifts to the hands.²⁷

The current view of the sensory homunculus is one that is a modification the original work started by neurosurgeon Wilder Penfield in the 1930s. As we noted above, the homunculus was meant to show the relative size of the areas in the somatosensory cortex devoted to different areas of the human body. By far the largest regions are those devoted to the hands (the fingers in particular), the mouth, and the face in general.²⁸

The ability of the fingers to sense the presence of even the smallest physical objects is nothing less than astonishing:

The human finger is exquisitely sensitive in perceiving different materials, but the question remains as to what length scales are capable of being distinguished in active touch. We combine material science with psychophysics to manufacture and haptically explore a series of topographically patterned surfaces of controlled wavelength, but identical chemistry. Strain-induced surface wrinkling and subsequent templating produced 16 surfaces with wrinkle wavelengths ranging from 300 nm to 90 μm and amplitudes between 7 nm and 4.5 μm. Perceived similarities of these surfaces (and two blanks) were pairwise scaled by participants and interdistances among all stimuli were determined by individual differences scaling (INDSCAL). The tactile space thus generated and its two perceptual dimensions were directly linked to surface physical properties – the finger friction coefficient and the wrinkle wavelength. Finally, the lowest amplitude of the wrinkles so distinguished was approximately 10 nm, demonstrating that human tactile discrimination extends to the nanoscale.²⁹

There is also the phenomenon of stereognosis. This is the ability to discern an object’s physical properties by touch alone. Stereognosis allows humans to judge such properties as an object’s dimensions and shape. Specialized structures within the somatosensory cortices interpret input from the extremities. The sensitivity of the fingers means this input can be very detailed. There is a condition known as astereognosis which is the inability to deduce an object’s properties through tactile stimulation.³⁰

Proprioception and Kinesthesia

As a human moves through space, their brain is constantly making adjustments based on the sense of where the body is in relation to the objects found in the physical environment. Calculations are made on the sense of where the boundary between “me” and “not me” lies, the speed at which the body is moving through the environment, an estimate of the ever-changing position of objects encountered in the environment, and so on. The sense of where one is in space-time and an awareness of one’s movements through it is known as proprioception. Proponents of the idea that there are more than the traditionally designated five senses often count proprioception as a separate sensory modality.

There is a distinction between kinesthesia and proprioception, although the terms are sometimes conflated. One source explains it in this manner:

Kinesthesia is the awareness of the position and movement of the parts of the body using sensory organs, which are known as proprioceptors, in joints and muscles. Kinesthesia is a key component in muscle memory and hand-eye coordination. The discovery of kinesthesia served as a precursor to the study of proprioception. While the terms proprioception and kinesthesia are often used interchangeably, they actually have many different components. Often the kinesthetic sense is differentiated from proprioception by excluding the sense of equilibrium or balance from kinesthesia…kinesthesia focuses on the body’s motion or movements, while proprioception focuses more on the body’s awareness of its movements and behaviors. This has led to the notion that kinesthesia is more behavioral, and proprioception is more cognitive.³¹

Unusual Aspects of Sensation

An odd phenomenon that occurs somewhat rarely in humans is known as synesthesia. Synesthesia occurs when the stimulation of one sense causes a response in another. People with this condition can, for example, hear colors. The anomalous sensing tends to be very consistent, with the same stimulus producing the same sensation. Estimates of the prevalence of synesthesia vary from one in 20,000 people to one in 200.³²

Some humans are capable of using echolocation to find their way through the world. Some blind humans have developed echolocation as a method of navigation, and this can be used to a certain extent by sighted persons if they’re trained for it. Echolocation uses echoes to judge the size of a room, for example. The number of vocalizations a human uses to echolocate, as well as their variety, governs its effectiveness. Measures of brain activity during human echolocation show it has certain similarities to the echolocation used by bats.³³

Phantom Limb Syndrome (also known as Phantom Limb Pain) is the sensation of pain or discomfort experienced by those who have lost an arm or leg through disease, accident, or violence. The pain tends to occur in the area of the amputation. The origins of this condition are not entirely understood, and the condition affects 60% to 85% of all amputees.³⁴

Are there more than five senses?

Many researchers now believe there are, but the number of additional purported senses varies from source to source. The ones most commonly cited are as follows:

--The Vestibular System, located in the inner ear, which helps regulate our sense of balance, the ability of our eyes to stay focused on an object even when we’re moving, and our sense of where we are in space.³⁵

--Proprioception, as we noted above.

--Regulation of temperature

--The sense of heat or cold (which seem to me to be aspects of tactile sensing)

--Pain perception (although I would argue that nociception, the technical term for this, is an aspect of the sense of touch).

In addition, there are researchers who count as many as 21 senses, and one that contends the true number is 53.³⁶

In my view, it is possible that a number of purported senses that go beyond our traditional definitions may be subtle combinations of the five basic senses, interacting in ways that are not necessarily well understood.

The Interaction of the Senses

When a human is experiencing an event through several senses, we say that the experience is multimodal or multisensory. The senses, of course, interact with each other in myriad combinations. The nature of such interactions is inextricably tied to perception, which we will address in some detail in the next chapter.

Sensory interaction is a subject of intense inquiry by researchers in several fields. The chief areas of investigation involve such issues as:

-- How one modality conditions or modulates others.

-- Whether individual sensory modalities can be clearly identified in multimodal experiences or whether the interaction of these modalities creates a synthesis which must be understood in a holistic sense.

-- The degree to which learning, previous multimodal experiences, expectations, emotions, and other variables affect multimodal experiences.

-- How to classify the very numerous and complex sensory interactions that humans commonly experience, and how our various explanatory schemes affect such classifications.³⁷

The brain uses input from the various senses to create a representation of the outer environment. The ways in which this can done are enormous in number and affect the perceptions of an experience profoundly. Moreover, not all senses are equally represented in a multimodal experience. One particular modality may be dominant. A particular human may be trained to react to a specific input during a multimodal experience, and a human may be unfamiliar with a particular sensation or sensory combination.

It should also be noted that in the complex interactions of the senses some senses can play very subtle roles, exerting influence that is not always noticeable. And when senses seem to conflict with each other, the results can be unexpected. For example, a clash between what we see and what we hear can cause us to change what we hear.³⁸ (We will explore this phenomenon in more detail in a chapter called Brain Tricks and Cognitive Biases, located in this section.)

The Effect of Emotions on Sensation

Can emotional states affect the quality and/or duration of sensory experiences? There is some evidence that tactile sensation is, contrary to expectation, reduced by fear and unaffected by anger.³⁹ There is research that demonstrates that our emotions can have an effect on our perception of whether a facial expression is neutral or friendly.⁴⁰ More broadly, researchers are delving into the issue of emotional memory and its effect on our senses:

Our emotions and sensory cortices can impact one another in both directions. A review by Vuilleumier (2005) explained that emotions provide a boost to our sensory cortices. Neuroimaging showed that in response [to emotion], our sensory cortices have increased activation. Vuilleumier (2005) hypothesized that this is due to learning from the sensory characteristics of emotional situations. Similar findings were present in the research of fear memory. Using fear conditioning, Sacco and Sacchetti (2010) found that sensory cortices affect emotional memory. Rats were trained to associate visual, auditory, or olfactory cues with an aversive stimulus. When the respective secondary cortex was lesioned, the cues that were previously learned were lost. This means that there is some storage in the secondary sensory cortices when it comes to emotional memory. ⁴¹

Recent research tends to recognize that sensations, perception, and cognition interact with each other regularly, and they should be studied holistically rather than in isolation. This research has confirmed the hypothesis that emotion very often influences what we sense, particularly in the area of vision. Objects that have emotional significance to humans may appear larger, for example, and fear makes us see possible threats more readily.⁴²

So the senses feed information to the brain. The purpose of a brain, as we have already seen, is to organize the stimuli entering an animal’s sensory apparatus in such a way that effective action can be taken by the animal to ensure its survival. The “objective” of this survival is the enhancement of reproductive chances. In this regard, therefore, the human brain is the most “useful” of any in the animal kingdom. No other animal comprehends its own situation as fully as the typical human does. No other animal possesses as many behavioral options as a human does.

This is not to say, of course, that the human sensory apparatus is superior to that of all other animals—far from it. There are animals with a superior sense of smell. Many animals, such as birds of prey, certain reptiles, insects, and certain fish possess sheer visual acuity, underwater vision, or night vision vastly greater than our own. A number of animals possess a sense of hearing that is far more acute than ours. So human superiority does not come from our ability to sense energies and chemicals. It lies in our ability to interpret these stimuli and act on them. The ability of animals to vary their behavior in reaction to their perception of stimuli lies on a continuum from no ability whatsoever to very broad and unpredictable abilities. The human brain puts us at the far end of that continuum, and makes us the most behaviorally flexible and least predictable of all living beings.