"Each of us believes himself to live directly within the world that surrounds him, to sense its objects and events precisely, to live in real and current time. I assert that these are perceptual illusions, for each of us confronts the world from a brain linked to what is 'out there' by a few million fragile sensory nerve fibres. These are our only information channels, our lifelines to reality. These sensory nerve fibres are not high-fidelity recorders, for they accentuate certain stimulus features, neglect others. The central neuron is a story-teller with regard to the afferent nerve fibres ; and he is never completely trustworthy, allowing distortions of quality and measure, within a stained but isomorphic spatial relation between 'outside' and 'inside'. Sensation is an abstraction, not a replication, of the real world."
Mountcastle, 1975, cited in Popper & Eccles, 1981, p.253.

the preliminary codification

The neurophilosophy of the transport of information from the PNS (Peripheral Nervous System) to the CNS (Central Nervous System), studies the afferent, sensoric, incoming impulses from the five senses, crucial to distinguish perception from sensation.

The efficient neurological cause of perception is called "transduction" ("to lead across"). This is the logic by which a receptor cell, exposed to an environmental stimulus, causes an electrical response. On a deeper level, the overall functioning of the brain is thus underpinned by complex flows of electric charge (from the Greek "electron" or "amber), which, together with magnetism, shapes the fundamental interaction known as electromagnetism (next to universal gravity and the subatomic strong & weak forces). Electromagnetism implies the simultaneity of electrical & magnetic forces. The magnetic field is caused by the electric current or motion of electric charges. The electromagnetic field is the space which exerts a force on particles possessing electric charge, in turn affected by these particles and their motion.

Sensation is defined as the faculty through which the external world is perceived. Hence, the sensory system is two-tiered : on the one hand perception, the raw, naked immediacy of the receptor organs for smell, taste, touch, audition & sight, the "doors of perception" at the periphery of the olfactory, gustative, soma-esthetic (or somato-sensory), visual & auditory systems of the CNS. On the other hand sensation, the end result of an array of central neural systems committed to process the coded form of the impulse perceived by the receptors, like the secondary & tertiary sensory areas, the spatial association area situated in the posterior parietal cortex (of both hemispheres), the angular gyrus in the inferior parietal lobe, sitting at the juncture of the tactile, visual & auditory areas, the limbic system (for emotional coloring) and the Ascending Reticular Activation System in the brainstem for the general arousal-level of the CNS.

The transmission of afferent impulses is never direct but by synaptic relays, changing the massage into a "code". In every neuronal relay station, this coded impulse is modified. Although each sense has its primary receiving area laid out as a cortical "map" (cf. the Brodmann areas), the neuronal relays from the PNS to the CNS cause the preliminary "codification" of the raw impulse hitting the reception surface of nose, tongue, skin, ears and/or eyes. So when the impulses in some sensory pathway reach the primary sensory areas in the CNS, preliminary codification has already taken place (cf. Kant's distinction between experience, "Empfindung" versus appearance, or "Erscheinung").

"In general it can be stated that the intensity of the stimulus is encoded as frequency of discharge of impulses."
Popper & Eccles, 1981, p.252.

from perception to sensation

Mental states are either based on sensation or are non-sensational.

Sensations have a clear bodily location and possess "raw feels" or qualia, defined by the five-tiered sensory input of the five physical organs of sense (smell, taste, touch, audition and sight). More or less spatially defined, sensations are always the experience of a conscious subject. Without this conscious experience, sensations are not.

The distinction between sensation and perception is important. Sensations occur to a subject of experience, and manifest as nose-consciousness (smelling), tongue-consciousness (tasting), skin-consciousness (touching), ear-consciousness (hearing), eye-consciousness (seeing) & the concert of these. They represent the final, "constructive" result of a process starting with naked, "unconstructed" perception. Sensations happen to an empirical ego (largely processed by the prefrontal lobes of the neocortex) with a unique perspective on the ongoing, sensational & non-sensational stream of functional differences or "energies" within consciousness.

Perception is three-fold. The root of perception is the impulse affecting the receptor. Next, the afferent relay to the CNS is coded, finally projecting the coded impulse in the primary sensory area. Because these perceptional data are introduced through sensory pathways to which consciousness has no direct access, perception is, paradoxically, non-sensational. To clarify this idea, the neurophilosophy of the primary, secondary & ternary sensory areas will be helpful.

Non-sensational mental states have no distinct, outer events associated with them. These mental states, also emerging without one being conscious of them, may be classified as :

The earliest organism abided in chemical substances signaling food, poison or sex. In humans, externally located neuronal cell bodies are concentrated within the nose. These nasal-located neurons, like those of other, more ancient creatures, analyze the pheromonal, olfactory and chemical nature of the environment for data concerning food, sex, the weather and the like. Over the course of evolution, only two groups of primal sensory cells formed like-minded cells : the olfactory lobe and the optic lobe. With the expansion and axonal-dendritic interconnection of these lobes the modern brain emerged.

the olfactory bulbs & optic vesicles of the neural tube
from Bear, Connors & Paradiso, 2001, figure 7.10 p.182.

We do not smell with the nose, but with a small, thin sheet of cells high up in the nasal cavity. This olfactory epithelium, about 10 cm², has three main cell types : (a) olfactory receptor cells, or neurons with axons of their own penetrating into the CNS, (b) supporting cells, similar to glia, helping to produce mucus and (c) basal cells which are the source of new receptor cells. Indeed, the receptor cells continually grow, die and regenerate in a cycle lasting ca. 4 to 8 weeks. The olfactory epithelium and the retina of the eye are both literal extensions of the brain. The olfactory system gave rise to the evolution of the primitive amygdala and rudimentary hippocampus ca. 500 million years ago. Because odors are inherently slow stimuli, rapid timing of action potentials is unnecessary to encode the timing of odors. Rather, temporal coding, based on the timing of spikes, is supposed to encode the quality of odors, and transduce the chemical stimulus into an electric charge. Temporal patterns of spiking would then be the logic behind the olfactory coding.

In lower mammals olfaction is the dominant sensory input, but in humans it became subordinate to sight, hearing and somaesthesis. Humans are relatively weak smellers. A smell is detected when about 10 trillion molecules of one of the ca. 30.000 or so available odor molecules enter the nose and stimulate receptor cells. Humans are able to detect and remember ca. 10.000 odors. Each of these cells express only one of the 1000 types of odorant receptor genes. Dogs have more than 100 times more receptors in each square centimeter of the olfactory epithelium, which may be over 170 cm².

Olfactory receptor neurons send axons into two olfactory bulbs, full of neural circuits with complex dendritic arrangements, reciprocal synapses and high levels of various neurotransmitters. Olfactory information is modified by inhibitory & excitatory interactions within the structures of the bulbs and between them. Neurons in the bulbs are also subject to modulation from systems of axons descending from higher areas in the brain. The output axons of the olfactory bulbs run through the olfactory tracts and have a complex distribution, the principal termination being in the piriform cortex (or olfactory cortex), the primary sensory area of olfaction, thus making various direct connections to many structures of the limbic system, the "nose brain". From there, the axons go to the thalamus on to the neocortex, were conscious recognition of smell occurs in the orbito-frontal cortex right behind the eyes.

This anatomical feature makes olfaction unique, for all other sensory systems first pass through the thalamus before projecting into the neocortex. Only olfactory connects with a primary sensory area directly related to temporal lobe structures & the limbic system.

The limbic system has been referred to as the "nose brain". The afferent axons stimulate this system directly, without the thalamus or "universal gateway" of the CNS. Indeed, the spinothalamic pathway is the major route by which afferents (registering for example pain or temperature) ascent to the neocortex. The thalamus is thus the gate through which information carried by sensoric axons enters the CNS. Here, these afferents are pre-processed to branch out to the relevant cortical areas & the limbic system.

The unicity of olfaction is clear. Like all sensory systems, it makes use of a method of preliminary codation to relay information to the CNS, but unlike any other system, it branches out in the limbic system before being pre-processed in the thalamus and relayed to the neocortex and its primary sensory area in the forebrain.

Olfaction directly connects with emotion and the latter evolved from feeding, fighting, fleeing and sex.

The olfactory system is related to eating and assists the gustatory system. Flavor can only be detected if both nose & tongue are used together. Although completely independent of the taste buds localized along the tongue, both system may have started out as one chemoreceptive system becoming distinct over the course of evolution. In reptiles and many other animals, an auxiliary olfactory organ is located within the roof of the mouth. But, in this case too, the two systems are separate. Indeed, some food, although smelling good, may taste terrible and have no nutritive value. The taste test allowed for additional differentiation, although some stuff smell & taste great whole still being poisonous.

Both smell & taste are chemical senses using a variety of transduction mechanisms to recognize the large amount of chemicals encountered. For an omnivore, a sensitive and versatile system of taste was essential to survive. Some taste preferences, like sweetness, is innate, but experience strongly modifies these instincts. The body has the capacity to recognize a deficiency and adjust this by causing cravings for particular food. We recognize four basic tastes : sweetness, saltiness, sourness & bitterness.

Although we mainly taste with our tongue, the palate, pharynx and epiglottis also participate, as well as the olfactory system. Scattered about the surface of the tongue are small projections called papillae (or bumps), shaped like ridges, pimples or mushrooms. Each papilla has hundreds of taste buds, composed of ca. 50 - 150 taste receptor cells, only about 1% of the tongue epithelium. Taste buds have also basal cells surrounding the receptors and a set of gustatory afferent axons. A person has ca. 2000 - 5000 taste buds, while exceptional people have as few as 500 or as many as 20.000.

As is the case for the other sensory receptors, papillae tend to be sensitive to only one basic taste and only at some critical concentrations just above threshold is a stimulus evoked. This does not mean sweetness is only tasted with the tip of the tongue. The tongue map implies certain areas of the tongue are more sensitive to the basic tastes than are other regions, while most of the tongue is sensitive to all basic tastes. Single receptors show small differences in response, and subtle distinctions are made in the brain. When a taste receptor cell is stimulated by an appropriate chemical, its membrane potential changes, either depolarizing or hyperpolarizing. This voltage shift, or receptor potential, causes the cell to fire action potentials.

The neuronal coding of taste is not based on specific receptor types, axons and neurons. Taste buds are broadly tuned to stimuli and this is the case all the way into the CNS. Receptor cell inputs converge onto afferent axons, and each receptor synapses into a primary taste axon also receiving input from several other receptors. One axon may combine taste data from several papillae. This is called population coding, used throughout the sensory and motor systems of the brain. This seems to be an architecture already at work at the level of the action potential of the neuron, making a combined decision based on all stimulating (yes) and inhibiting (no) nerve impulses influencing it (cf. the "democratic neuron").

The main flow of taste data is from the taste buds to the afferent gustatory axons, into the brain stem (medulla), up to the thalamus and finally to the neocortex. In the brain stem, the axons synapse with the gustatory nucleus and diverge from there. The thalamus sends axons to the primary gustatory area, the cortical area in the anterior insula of the cortex (in the parietal lobe).

The experience of touch starts at the skin. Most sensory receptors in the somatic sensory system, are mechanoreceptors, sensitive to physical distortions such as bending or stretching, enabling the body to feel, to ache & to chill (in the context of this paper, the term "somatic sensation" is avoided). Present throughout the body, they monitor all contact with the skin as well as pressure in the heart & blood vessels, stretching of the digestive system, urinary bladder and force against the teeth. The axons branches characterizing each mechanoreceptor have mechano-sensitive ion channels, not well understood.

As the largest organ of the body, the skin is richly innervated by axons part of the vast network of peripheral nerves. In the visceral system, primary afferent axons bring information from the somatic sensory receptors up the spinal cord, only synapsing in the dorsal root ganglia (or cuneate nucleus at the base of the head). Information about touch or vibration of the skin takes a route to the CNS entirely distinct from pain and temperature stimuli. Indeed, some of the axons terminating in the root ganglia start at the skin of the big toe. At this point, the information is still represented ipsilaterally (right side body, right dorsal nuclei). But axons from these cells arch and decussate. From this point onwards, the somatic system of one side is concerned with sensoric data deriving from the other side of the body.

After only one synapse, the afferent impulse travels to the thalamus & the cerebral cortex. At each of two relays (root ganglia & thalamus), the opportunity for an inhibitory action is given, sharpening the neuronal signals by eliminating the weaker excitatory stimuli. By this inhibition, a precise localization of touch stimuli becomes possible.

Somaesthetic or somatosensory processing occurs in the cerebral cortex, namely the parietal lobe. The primary somatosensory cortex occupies an exposed cortical strip, the postcentral gyrus. The somatotopy of this gyrus has been called a homunculus (or "little man"), a mapping of the body's surface sensations.

Sounds are audible variation in air pressure caused by moving air molecules. When an object moves away, air is made less dense (rarefied). Many sounds produce periodic variations in air pressure. The frequency of sound is the number of compressed patches of air passing by our ears each second. One cycle is the distance between two successive patches. Sound frequency is expressed in hertz, or the number of cycles per second. The auditory system responds to pressure waves over the range of 20 - 20.000 Hz, decreasing with age & exposure to noise of the high-frequency end (a low organ tone is about 20 Hz, while a high note on a piccolo is about 10.000 Hz). Intensity of sound is difference in pressure between compressed patches of air, and determines the loudness we perceive. The higher the intensity, the louder the sound. The intensity range is remarkable, for the loudest sound leaving our ears undamaged is about a trillion times greater than the intensity of the faintest sound heard.

The ear has three main divisions. The structures from the funnel (pinna) to the eardrum is called the "outer ear". The tympanic membrane and the ossicles constitute the middle ear and what lies behind the oval window is the inner ear. These structures of the auditory pathway play the following roles :

• pinna (or funnel) : helps collecting and localizing sounds ;

• auditory canal : extending ca.2.5cm inside the skull ending at the tympanic membrane ;

• tympanic membrane : the eardrum ;

• ossicles ("little bones") : a series of bones or transferring movements of the tympanic membrane into movements of a second membrane covering a whole in the bone of the skull called the oval window ;

• cochlea ("snail") : behind the oval window, this is a fluid-filled space containing the apparatus transforming physical motion of the oval window membrane into neuronal responses. This involves a frequency analysis of the patterns of sound waves and their conversion into the discharges of neurons. The auditory receptors converge mechanical energy into a change in membrane polarization (cf. the organ of Corti, consisting of hair cells, the rods of Corti and various supporting cells).

Once the inner ear generates the neural response to sound, the signal is transferred to and processed in nuclei in the brainstem, and sent to a relay in the thalamus, finally projecting to the primary auditory cortex in the temporal lobe (Heschl's gyrus). Both audition & sight start with sensory receptors connecting to early integration stages (retina for sight and brain stem for audition), relay to the thalamus and then to the sensory cortex. Nevertheless, given there are more synapses at nuclei intermediate between the sensory organ and the cortex, the auditory pathway appears more complex than the visual pathway. However, the cells and synapses of the auditory system in the brain stem are analogous to the interactions in the layers of the retina. All ascending (afferent) auditory pathways converge onto the inferior colliculus of the midbrain. The right cochlea projects mostly to the left primary auditory area, and vice versa for the left cochlea.

Neurons processing sound information are timing machines. They are designed to preserve & analyze very rapid neural signals encoding small but meaningful variation in sound signals. A trained pianist can distinguish between two tones of 1000 Hz and 1001 Hz, or the detection of a difference of only 1 μsec in the wavelengths ! A single action potential lasts about 1000 times longer. The detection of a sound source in the horizontal plane with a precision of 2° is possible, demanding the discrimination of 11 μsec difference between the time it takes a sound to reach their two ears. Many auditory neurons in the brain stem have an architecture & physiology optimized for speed and electrical conduction. This precise localization is also necessary to report position and movement of the head (the vestibular system). Indeed, both the auditory and vestibular systems use hair cells to transduce movements.

light

The majority of the light hitting the surface of the Earth comes from the Sun, and this is only a fraction of what our star disperses into outer space in all directions. Light, electricity & magnetism are all electromagnetic radiation. The electric and magnetic fields oscillate at right angles to each other, while the combined wave moves in a direction perpendicular to both of these two field oscillations.

Light, of constant speed in empty space, may be conceived as moving packages of energy called photons. Paradoxically, a photon is a particle of electromagnetic radiation. Light is both particle-like & wave-like. Whether light moves like a wave or as a particle depends on how it is observed (cf. the importance of the experimental setup in the two-slit experiment).

In a digital camera, both aspects of light are addressed. The lens of the camera refracts (bends & focuses) incoming waves of light. These waves are made to hit a charge-coupled device (CCD). This is a light-sensitive integrated circuit storing & displaying image-data. Each picture element (pixel) is converted into an electrical charge related, by intensity, to a color in the color spectrum. Subatomically, this intensity is measured by light photons kicking electrons out of the silicon contained in the bombarded surface. These electrons are finally detected by electronics interpreting the number of electrons released and their position of release from the silicon to create an image.

The length of a light wave λ is the distance between successive peaks or troughs, its frequency ν is the number of waves per second, and its amplitude is the difference between peak and trough, related to the intensity or brightness of a wave relative to other light waves of the same wavelength. Light features a simple relation between its speed (c), wavelength (λ) and frequency (ν), namely (1) ν = c/λ. Since λ, the wavelength in Ångstroms (1 Ångstrom = 10^-10 meter), bottoms the fraction, frequency is inversely proportional to the wavelength. Light with a smaller wavelength has a higher (larger) frequency and vice versa.

White light is made of different colors or wavelengths. When passed through a prism, it spreads out in different colors (cf. the rainbow of the visible spectrum). This phenomenon shows how all possible wavelengths become manifest. "Hot" colors such as red or orange consist of light with a longer wavelength, and these have less energy than "cool" colors such as blue or violet.

In physics, a quantum (plural : quanta) is an indivisible entity of energy. For instance, the photon, being the unit of light, is a "light quantum". In empty space, a photon moves at a constant speed, has no rest mass and no charge. Einstein (1879 - 1955) found the relationship between the energy of light E and its frequency ν to be : (2) E = h × ν, h being Planck's constant, or 6.626 × 10^-34 J·sec, used in the quantization of energy. The energy of electromagnetic radiation is proportional to its frequency. Emitted at high frequency (or short wavelengths) it has the highest energy. (1) & (2) give E = h.c/λ, with c = 299.800 km/s (the speed of light in empty space).

Light rays travel in straight lines until interacting with the atoms & molecules of the atmosphere and objects. These interactions include reflection, absorption & refraction.

Reflection is the bouncing of light rays off a surface. Most light we see is reflected off objects. Striking a mirror perpendicularly will reflect light 180° back upon itself, at 45° a reflection of 90° occurs, etc.

Absorption is the transfer of light energy to a particle or a surface of molecules. Black surfaces absorb the energy of all visible wavelengths, while some compounds absorb a limited range of them and reflect the remaining. "Violet" absorbs long wavelengths but reflect a range of short ones centered on 430 nm (4300 Å), perceived as "violet".

Refraction is the bending of light rays traveling from one transparant medium to another. Striking a surface at an angle will bend the light toward a line perpendicular to it. This bending occurs because the speed of light differs in the two media, passing through air more rapidly than through water. The greater the difference between the speed of light in the two media, the greater the angle of refraction.

the visual system

A large part of the cerebral cortex is involved with analyzing the visual world captured as the electromagnetic radiation visible to our eyes. In the electromagnetic spectrum, ranging from Gamma rays to AC circuits, the visible spectrum of rainbow colors lies between wavelengths of 400 & 800 nm (1 nanometer = 10^-9 meter = 10 Ångstroms), in other words, light visible to our eyes has wavelengths between 4000 - 8000 Ångstroms.

The structures involved in all steps of the visual pathways are complex. The eye is an organ specialized for the detection, localization and analysis of light. The gross anatomy of the eye is as follows :

• pupil : the opening allowing light to enter the eye and reach the retina ;

• iris : surrounds the pupil, and is pigmented to provide the color of the eyes. It contains two muscles varying the size of the pupil (one makes it smaller when it contracts, the other larger) ;

• cornea : pupil & iris are covered by a glassy transparant external surface, lacking blood vessels and nourished by the fluid behind it, the aqueous humor. It is continuous with the sclera, the "white of the eye", forming the tough wall of the eyeballs ;

• extraocular muscles : inserted into the sclera, they move the eyeball in the bony orbits of the skull ;

• conjunctiva : membrane folding back from the inside of the eyelids and attached to the sclera ;

• retina : a sheet of closely packed visual receptors (ca. 10⁷ cones and 10⁸ rods) at the back of the eyeball, on which an image is formed ;

• optic nerve : carries axons from the retina and exits the eye at the back, passes through the orbit and reaches the brain at its base, near the pituitary gland.

The conversion of light energy into neuronal activity happens in the retina. The basic flow of light in the retina is from the photoreceptors to bipolar cells to ganglion cells. The only light-sensitive cells in the retina are the photoreceptors, while all other cells are influenced by light via direct or indirect synaptic interactions with these. The ganglion cells are the only source of output from the retina. They alone form action potentials. The actual conversion of electromagnetic radiation into neural signals occurs in the 125 million photoreceptors at the back of the retina. They convert light energy into changes in membrane potential, using biochemical cascade.

Each cell has four regions : an outer segment, an inner segment, a cell body and a synaptic terminal. Light-sensitive photopigments absorb light and trigger changes in the membrane potential of the photoreceptor. Rod photoreceptors have a long, cylindrical outer segment, while cone photoreceptors have a shorter segment. Rods are 1000 times more sensitive to light, while there are three types of cones, each containing a different pigment, making them sensitive to different wavelenghts of light. Only the cones are responsible for our ability to see color.

The axons of the ca. million ganglion cells travel in the optic nerve. About 10% of this retinofugal projection courses from each eye to the midbrain (the superior colliculus), while most of them innervate the thalamus, and from there go to the primary visual cortex or striate cortex in the occipital lobe (the nonthalamic targets of the optic tract involves about 150.000 neurons). The optic nerves exit the left and right eyes and travel through the fatty tissues behind the eyes and pass through holes in the floor of the skull. These nerves from both eyes form the optic chiasm, which lies at the base of the brain, anterior to where the pituitary gland dangles down.  In this chiasm, optic nerve fibers cross from one side to the other (decussation). Hence, the left visual field is viewed by the right hemisphere and the right visual field is viewed by the left hemisphere. The actual viewing happens in the primary visual cortex.

From the striate cortex, a ventral stream of information projects into the inferior temporal cortex, where the highest integration of visual function & analysis occurs. This is the end station of a system of recognition of specific and particular shapes and objects of interest, both cognitively as well as emotionally, for interconnected with the amygdala, hippocampus, limbic system and the autonomous nervous system.

"If the doors of perception were cleansed,
every thing will appear to man as it is, infinite.
For man has closed himself up,
till he sees all things thru' narrow chinks of his cavern."
Blake, 1790/2, The Marriage of Heaven and Hell.

The receptor organs of the sensory system are fed by impulses based on chemical substances, collisions & frictions, air pressures and electromagnetic radiation. These impulses are the first cause of perception, nothing else. Stimuli are the direct, external changes caused by a narrow band of material objects on the surface of the receptor organs of the sensory system.

Molecules alter the chemistry of nose & tongue. The mechanics of stretching & bending triggers somatosensoric responses. Each second, compressed patches of air pass by our ears. Variations in electromagnetic energy stimulates the retina. Take away these stimuli or disable the receptor organs, and perception is either absent, partial or impossible. The receptor organs are the "doors of perception" ...

Without perception, no interpretation of perception and no sensation. To be physically in touch with our environment, evolution provided five doors. Although more may be available (cf. imagination & mind), sense perception is the primary fact of physical experience shared by all humans at birth. It is nominal and the cause of sensate objects.

"Literary or scientific, liberal or specialist, all our education is predominantly verbal and therefore fails to accomplish what it is supposed to do. Instead of transforming children into fully developed adults, it turns out students of the natural sciences who are completely unaware of Nature as the primary fact of experience, it inflicts upon the world students of the Humanities who know nothing of humanity, their own or anyone else's."
Huxley, 1957, p.59, my italics.

We first smell, taste, touch, hear and/or see (perceive) and then consciously experience odor, taste, feels, sound & light (sense). Throughout the sensory system population coding is used, implementing a threshold for combined action-potentials. This procedure enables broad responses.

Between the moment the receptor organ changes (stimulus) and the actual conscious sensation (response), two levels of interpretation exist : automatic & processed.

• automatic interpretation from receptor organ to thalamus : evolutionary, biological software integrated in the hardware of the brain, involving transduction, coded relays & the reception by thalamus ;

• processed interpretation from thalamus, primary sensory cortex to prefrontal cortex  : evolutionary software plus userware (volitional & processed), able to change software & influence hardware, calling for the secondary sensory cortex, the association areas, the angular gyrus & the prefrontal cortex.

In each receptor organ, a particular transduction is operational  from, on the one hand, chemical (smell, taste, touch), mechanical (touch, audition) or electromagnetic energy (sight) to, on the other hand, encoded sequences of electric voltages running through neurons and their axons and dendrites.

• smell : transduction of chemical stimuli (odorants) by temporal coding (the timing of spikes) ;

• taste  : transduction of chemical stimuli by membrane potential changes, either depolarizing or hyperpolarizing (voltage shift) ;

• touch : transduction of mechanical and chemical stimuli by membrane potential changes & mechanoreceptors (with mechano-sensitive ion channels ?) ;

• audition : transduction of mechanical energy by a change in membrane polarization ;

• sight : transduction of electromagnetic radiation by a change in membrane polarization.

The axons of the olfactory bulbs run through the olfactory tracts and project directly into the olfactory cortex. This happens without passing through the thalamus first, as is the case for taste (gustatory afferent axons), touch (somatosensoric axons), audition (auditory nerve) & sight (optic nerves), projecting into the neocortex by thalamic relay.

Smell is an exceptional sense, able to swiftly trigger massive limbic responses. Indeed, its primary sensory cortex belongs to the primitive cortex, which is part of the limbic brain, the nose brain. Olfactory afferent input and its projection into the primitive regions of the cortex (piriform cortex) is nonthalamic, making smell unique among the senses. This cortex has three layers, the neocortex six. From this old piriform cortex, many connections to many structures in the limbic brain are made.  Many parallel pathways mediate the olfactory functions, such as odor discrimination, emotions, motivation & behaviours from reproduction, feeding to imprinting and memorizing.

The role between odorants and emotional memory (hippocampus) is pertinent. The olfactory system is the outer organ of the play of emotional tensions between inhibiting and exciting. It heralds danger, sexual activity & a feeling of well-being (cf. the role of pheromonal communication between animals). Conscious smelling is mediated by pathways between the medial dorsal nucleus of the thalamus and the prefrontal cortex.

brain stem and cerebellum removed
adapted from Bear, Connors & Paradiso, 2001, p.211.

Both transduction & the axonal relay (by way of synapses) as well as the thalamic relay are important automatic interpretations, each altering the code, upgrading it from (1) receptor (from receptor neurons to thalamus) to (2) integrator (thalamus).

It took millions of years for receptor neurons to be able to receive & transduce. This "automatic" level of perception is called "naked" because it touches, so must we think, the absolute (or "Ding-an-sich"). Reality-as-such & ideality-as-such, the Real-Ideal, is onefold and crucial in logic, epistemology, ethics, esthetics & ontology. Naked perception is the truth-core of realism. The latter is methodological ("as if"), not ontological. This means it does not operate as ground, foundation or hypokeimenon of thought, knowledge, goodness & beauty (cf. Clearings, 2006). By way of method, we accept certain physical stimuli out there cause changes in the receptor organs, effectuating a chain of events relayed, in a coded format, to the thalamus.

Insofar as stimuli cause material changes, transduction causes neuronal information to be relayed to the thalamus.

To reach the neocortex and become conscious sensation, all afferent sensory inputs directly (taste, touch, audition, sight) or indirectly (smell) enter the thalamus. The thalamus is the gate, integrator and translator of various inputs processed into a form readable by the neocortex. As a projector, the thalamus relays selectively to various parts of the neocortex, and one thalamic point may reach more than one area of the cerebral cortex.

At the level of the thalamus, reptilian & mammalian software takes over. Before entry into the neocortex, this "inner room" or "storeroom" (of a Greek or Roman house) receives the neuronal messages of the five senses. This sensory information is spatio-temporalized, integrated and finally projected into the primary sensory cortex, while the intensity of the flow to and fro the neocortex is monitored and if necessary inhibited.

Through this inhibition, the thalamus rules the flow of sensory (and other neuronal) information to the cerebral (neo)cortex and acts as a highly state-dependent "reducing valve" or central sensory gate. This is done by the reticular nucleus, a sheet of acetylcholine inhibiting neurons, covering the whole of the dorsal thalamus. This sheet contains nerve cells gathering information from dendrites draping the outer surface of the thalamus, sampling the activity between thalamus and neocortex. Cortical excitatory states descend and excite the reticular nucleus, blocking perception. Brain stem excitatory states ascend and inhibit the nucleus, allowing more sensate messages to flow to through the thalamus into the neocortex.

Higher mammals have a larger pulvinar ("cushion"), the back of the thalamus. In humans, it occupies one quarter of the thalamus and is the essential thalamic counterpart of the sensate association cortex covering the back of the neocortex. Functionally, it seems to make the initial contribution to the process of automatically grasping & holding items in our visual & auditory space or salience. This allows them to become of meaningful interest. So the thalamus also computes our initial level of attention. And there is more : the medial dorsal nucleus assists frontal scenario's, the anterior nucleus brings in sensual gratification, the intralaminar nuclei stimulate, the lateral genicular nucleus "sees", and the reticular nucleus is a shield (Austin, 1998, pp.263 - 274).

This "automatic" level of perception is called "natural" because our brain shares it with all higher mammals. In humans, the thalamus acts not only as a receptor and an integrator-projector, but also as the initiator of a series of higher cortical functions.

The neocortex is never directly informed about the afferent data provided by both naked & natural perception. Conscious sensation is a posthalamic process.

II : The Sensuous Cortex.

So to consciously observe an object, is to grasp it and hold it before a subject of experience. Sensations are always conscious and they are because resulting from a complex inner process, involving all association areas.

"All our knowledge begins with the senses, proceeds thence to the understanding, and ends with reason. There is nothing higher than reason for working up the material of intuition & comprehending it under the highest unity of thought."
Kant, I. : Critique of Pure Reason, B355.

In Kantian epistemology, the process of acquiring knowledge runs as follows :

1. transcendental aesthetic : empirical knowledge : a variety of direct, multiple, unordered, nameless impressions (Hume), called "Empfindungen" (or perceptions) are synthesized by the forms of representation "space" (related to geometry) and "time" (related to arithmetics) and turned into "Erscheinungen" (or phenomena). These representations reflect the structure of our receptive apparatus, and are meant to structure sensations into phenomena ;

2. transcendental analytic : scientific knowledge : phenomena are only objectified by thought, but do not constitute an object of knowledge, for this is realized in propositions. The phenomena need to be structured by the 12 categories of understanding, corresponding to 12 different types of propositions (quantity, quality, relation and modality, each viewed from three angels). This categorization of phenomena leads to object-knowledge (synthetic propositions a priori). The categories are meant to structure phenomena into object-knowledge ;

3. transcendental dialectic : metaphysical knowledge : the variety of objects known is brought to a higher unity. A last, sufficient ground is sought and found in the ideas of reason : "ego", "world" and "God" (derived from the category of relation). These words are not things and only serve understanding, nothing more. While stimulating the mind's continuous expansion, these ideas regulate understanding and bring it to a more comprehensive, reasonable unity. They are meant to structure understanding into an immanent metaphysics.

The 2 forms of representation, 12 categories (brought to unity by 3 ideas) make the object possible, rather than vice versa. The human mind is the active originator of experience, rather than just a passive recipient of perceptions, as Hume (1711 - 1776) thought. The mind can not be a tabula rasa, a "blank tablet", so Descartes (1596 - 1650) is right. The whole transcendental system is innate. Even on the level of the transcendental aesthetics, sensations, the only source of knowledge acknowledged, as Locke (1632 - 1704) claimed, must always be processed to be recognized, or they would just be "less even than a dream" or "nothing to us". Both sensations, representation and categorization are necessary to constitute an object of knowledge.

This theory of knowledge is in tune with the neurophilosophy of sensation. First there are perceptions ("Empfindungen", "sinnliche Anschauung" or "Sinnlichkeit") relayed to the thalamus, integrating & spatiotemporalizing them as phenomena ("Erscheinungen"). The latter are projected in the primary sensory cortex to be recognized by the verbal association area and the attention association area as object-knowledge. Kant's categorial scheme, although it does have general characteristics (the neuronal structures of the areas), does not yield synthetic propositions a priori (cf. Kant's acceptance of foundationalism), but a series of conceptualization (of sensations), or perceptions molded in an individual cognitive framework a posteriori. The higher order organization of the mind by reason, is executed by the prefrontal lobes.

Although the pair perception/sensation plays a fundamental role in epistemology, it is not without importance in ethics & esthetics. In both, the need to distinguish between the input of the senses (perception) and the irreducible interpretation of the conceptual mind (sensation) is crucial and calls for a critical analysis of sensation. We cannot accept our sensate information at face value, but distinguish between the "raw" sense-data we are bound to affirm and the elaborate appearance of sensate objects in simple to complex conceptual frameworks.  Although the conceptual mind is unable to eliminate interpretation to witness the absolute data, it can introduce elaborate comparisons and try to integrate information from as many subjects of experience as possible. Insofar as an intersubjective consensus is at hand and sensations are repeated over and over again, the subjective margin may be reduced, although never completely eliminated (the scientific language game has no privileged access to naked perception).

For Shankara (788 - 820), the main representative of Advaita-Vedânta and an important renewer of Hinduism after the success of Buddhism in India, "mâyâ" (deception, illusion, enchanting display) is a universal principle inseparably united with Brahman, the absolute.

As universal ignorance or cosmic illusion, "mâyâ" draws a veil over Brahman and so confuses our vision, making us witness diversity rather than unity. Because of illusion, we consider sensate objects to be separate entities with definite characteristics. Here we are in error, and witness illusion rather than true reality. The origin of ignorance is the superimposition of unreal objective conditions on what is truly at hand (adhyâsa). Moroever, the transfer of an object (a sensate not-I) along with its accidents to the subject (the I), is deemed false knowledge ("avidyâ"), mixing up reality with unreality, incapable of distinguishing transient from intransient and real from unreal. In the famous simile, we fear a coiled snake, while it is only a rope. In the Vedanta, the key is always to discriminate (viveka) between the real (Brahman, the absolute) and the unreal (mâyâ, the relative). This eliminates ignorance.

To take away "avidyâ" brings enlightenment (samâdhi), destroying past & future "karma" (or operational causes). Only the "karma" already bearing fruit, sustaining this present life has not yet vanished. The "jîvanmukta" ("one liberated while still alive"), witnesses how he experiences activities caused by "prârabdha karma" (which can not be prevented), but continuously without mixing up reality.

Critical thought (cf. Clearings, 2006) also draws a radical distinction between absolute truth and relative truth, between the Real-Ideal (Kant's "Ding-an-sich") and scientific empirico-formal propositions a posteriori. Avoiding dogmatism (the unchangeable yes) & skepticism (the unbreakable no of dogmatic negation), criticism (the open maybe) also steers away from dogmatism or skepticism regarding absolute reality :

Universal illusion cannot be identified, for positing "mâyâ" turns it into something particular, contradicting its universality. Neither can we exclude universal illusion by assuming "being" equals "being known in thought", for then we move ad hoc from what we assume to be the case to the affirmation of being as knowable as such (cf. the critique of foundationalism). We assume the mental coincides (represents) the extra-mental and move from this assumption to the affirmation this must be the case. This move is unlogical. Classical metaphysics makes this category mistake (assumptions are not certainties). Metaphysical realism (mind corresponds with reality) and metaphysical idealism (mind makes reality) are extremes to avoid.

Although we must assume facts are the place where conceptual rationality & the absolute coincide, and must think the ultimate consensual correspondence (or Real-Ideal), we cannot eliminate the possibility conceptual rationality is self-deluded and, as an illusion-machine, superimposes its own dual display upon the world it sensates and experiences, living out, as if on stage, its own projections in the "mirror" of the world "out there". In other words, in order for knowledge to be possible, we must suppose the absolute to be knowable, even if this is not the case or only partially so. Indeed, conceptual knowledge could well be illusionary, i.e. altogether different from the Real-Ideal. How can this not humble the true scientist ?

Neurological executants are skilled cortical performers backing this interesting state of affairs. The neurophilosophy of sensation clarifies the difference between perception and sensation. The objects we sensate appear as they do because of our interpretation and, as long as conceptual rationality is at hand, this cannot be put to rest or eliminated. This "interpretation" is not something "added" to perceptions, and, by some method, subtracted. The association areas process the construction in which the sensate objects appear as entities (cluster of events) with accidents (quantity, quality, relation, modality, etc.) and this by a subject of experience. Before they "enter" these areas, they have not been introduced to the overall modular activity of the neocortex, the concert of interpretations with an attention area mediating the will of the conductor. Once this happens, the end relay of perception transforms into sensation, for there is interpretation (fabrication) and a subject of experience facing a sensate object of experience.

S(ensation) = P(erception) . I(nterpretation), with I ≠ 1.

The argument of illusion can be explained in objective & subjective terms :