Aristotelian and medieval concept of five senses projecting to the heart and the sensorium commune, made around 1500. An unknown English scholar added this pen drawing to the sensory chapter of G. de Hardewyck's book of 1496 on Aristotle. Lower part: stimuli of five senses (acoustic, visual, olfactory, gustatory, and somatosensory for heat and pain). Sense organs project by lines to heart as seat of the soul (according to Aristotle's scheme), either directly or after coordination in “sensus communis” in the anterior part of head. Upper part: two heads with a fancy cerebral location of labeled division of four and five brain compartments; left, Galen's and Avicenna's “sensus communis,” “phantasia,” “cogitativa,” and “memorativa”; right: Albertus Magnus's similar concept of five faculties: “sensus communis,” “imaginativa,” “estimativa,” “phantasia,” and “memorativa.”

Emperor Frederick II's method of visual learning conditioning of falcons after visual deprivation, written ca. A.D. 1240. During a period of visual exclusion by lid suture the falcon is accustomed to the voice and touch of the falconer. After this period of visual deprivation the falconer trains the seeing falcon for hunting in a series of stages. A: some weeks after opening the lids the falcon is tied to a bar by a long line and quieted by feeding, the attendant keeping his face averted. Then the falcon is accustomed to the man wearing a hat. B: after this imprinting to a human being the falcon learns to sit on a leather glove and is trained to remain there with and without a hood covering the eyes. C: unrest is quieted by feeding with averted face and by caressing. This procedure is repeated until the falcon remains on the gloved left fist of the falconer. D: conditioned falcon keeps sitting on the falconer's glove, first while walking, then while riding. Frederick's original illustrated manuscript was lost during the siege of Parma in 1248, but several copies were preserved, the best being that of his son King Manfred. These figures are taken from a good facsimile of Manfred's manuscript.

Experiments on light refraction, A, and theory of the rainbow, B, drawn by Dietrich (Theodoricus) of Freiberg around 1300. To explain the rainbow colors Dietrich studied the spectral refraction of light in glass models of raindrops. A: refraction study with hexagonal crystals and vessels filled with water showed spectral colors: he found that red is nearest, blue farthest, from original angle of the light rays, and yellow and green lie between them. White light beam enters vessel, refracting at k, is partly reflected at inner wall, passing out at left; partly refracted again, it leaves vessel downward in q. B: Scheme of the rainbow originating from sun, a, by double spectral refraction and reflection in 4 raindrops (right). Observer, c, sees red light from upper drop and yellow, green, and blue from other drops in order of the rainbow. Divergence of doubly refracted colored light is drawn incorrectly in the parallel lines for each color.

Alberti's illustrations of visual pyramid, perspective convergence of parallel lines, and foreshortening of horizontal and vertical patterns. Top: visual angle of eye looking to a quadrangle forms a pyramid; length and width of base increase linearly with distance. Middle: parallel lines on a plane converge to a central point (punto centrico) at horizon. Bottom: quadratic pattern on floor and two columns (A and B) show the same perspective convergence. Text says, “linea giacente, 9 bracchia. A, B pilastri o muri di 10 bracchia” (Floor lines of 9 ells and columns of 10 ells).

Dürer's sketch and woodcut of perspective reproduction with help of squared glass window grid. (made around 1524). Top: draftsman marks main object contours on glass window, where they are seen with perspective foreshortening. In addition he uses a string for measuring distances and for demonstrating direction of visual pyramid from eye to objects seen. Bottom: draftsman copies foreshortened contours of a woman on paper from projection of glass window that has same squared network as the paper. Pointed bar marks eye position and window distance.

Leonardo's drawing of dependence of illumination on angle of reflecting surface. The light rays b‐m from source a meet different parts of face, with maximal luminance in rectangular projection (g, h, “equal angles”). Leonardo's mirror script reads in MacCurdy's translation: “Since it is proved that every light with fixed boundaries emanates or appears to emanate from a single point, that part illuminated by it will have those portions in highest light upon which the line of radiance falls, between two equal angles, as is shown above in the lines a‐g, also in a‐h, and similarly in a‐l; and that portion of the illuminated part will be less luminous upon which the line of incidence strikes at two more unequal angles, as may be seen in b, c and d; and in this way you will also be able to discern the parts deprived of light, as may be seen at m and k.”

Leonardo da Vinci's note and sketch of his experiments with frog spinal cord (about 1500). Leonardo drew the spinal cord in the vertebral canal with vertebrate bodies. He wrote in mirror script on the cord “generative power” and described on left and right sides the experiment of cord destruction. English translation of the essential result of abolishing reflex action: “The frog instantly dies when its spinal cord is perforated. Although it lived before without head, without heart, or any entrails of intestines or skin. It thus seems that here lies the fundamental of motion and of life.”

Descartes's schemes of multisensory perception. A: visual and somatosensory control. Under visual control two bars N and O are directed by hands to a target B. This movement elicits afferent “animal spirits” of nerves to brain and pineal body H. The pineal is supposed to bend for receiving and sending impulses from and to eyes, 7, and arms, 8. B: bisensory interaction of vision and olfaction, and visual attention. Nerve impulses from both eyes via optic nerves converge in brain from 2, 4, and 6 to the pineal. There visual attention is aroused and causes an efferent flow (abc) facilitating visual input from the arrow (ABC). This suppresses olfactory afference from flower D via olfactory nerves 8 having less access to the pineal (dotted lines).

Descartes's schemes of sensorimotor coordination in the brain show cerebral mechanisms of reflex movements (A) and voluntary and emotional action (B) elicited by sensation of warmth. A: cerebral mechanisms of reflex movements. Heat of fire A stimulates reflex withdrawal of foot B and defense movements of hand. This is effected by afferent and efferent impulses (“animal spirits”) of nerves, conducted along filament CC to and from brain after convergence at pineal gland F. Descartes's text reads in English: “If the fire A is close to the foot B, the small parts of this fire, which move, as you know, very rapidly, have the power to move along with them the part of the skin of this foot which they touch; and by this means, pulling the small filament CC, which you see attached to it, they open at the same time the entrance of the pore de, where this small filament ends, just as, by pulling one of the ends of a cord, you cause a bell attached to the other end to ring at the same time.” B: voluntary and emotional action elicited by sensation of warmth. Burning heat of a fire causes different cerebral responses of attraction or avoidance, sent by pineal body into different nerve channels. When hand B is cold it is extended voluntarily toward fire A for warmth. If fire burns hand the strong sensation of pain, conducted by more animal spirits in nerves to the brain, causes efferent impulses in cranial nerves oprs and an outburst of crying and tears. Descartes's general conclusion for sensory projections to the brain is this: “There are many small filaments similar to CC; they all begin to separate from one another at the inner surface of the brain, from whence they originate, and going from there to disperse throughout the rest of the body they serve as the organs for the sense of touch.”

Descartes's concept of reciprocal innervation of two antagonistic eye muscles. Tube CB represents a nerve branching at CB to innervate the two antagonistic ocular muscles E and AD. Muscle AD is contracting after receiving more impulses (esprits animaux) than relaxing muscle E and moves eye to side F. In addition to centrifugal nerve impulses Descartes discussed a nonexisting peripheral interaction between the two antagonists by valves in sclera; these allow impulses to flow from e to i, but not in opposite direction, because valve g is closed.

Descartes's picture of inverted retinal image. It is observed after removal of posterior part of sclera as in Scheiner's experiment on ox's eye. The observer at P sees inverted image TSR of the outer object VXY projected through cornea BCD and lens 1–6 to retina.

Newton's sketch of his prismatic experiments made around 1700. This original design, preserved among other “non‐optic” writings of Newton, shows how rays of white sunlight from the window hole are refracted by a prism into their spectral colors and are projected to a perforated screen at left. Lowest spectral color is projected further, beyond a screen hole and through a second prism to left side of wall, excluding the other spectral components that were projected above. Newton's Latin script says twice: “Nec variat lux fracta colorem” (the refracted light does not change its color).

Newton's postulate of partial decussation of optic nerves in the chiasma as illustrated in the eighteenth century by Taylor (1738) and Harris (1775). A: Taylor's figure of optic chiasma depicts crossing over of nasal parts of the retina to explain single vision in two half‐fields of both eyes without reference to Newton; images of the object (arrows BA) are projected to retinas of both eyes (a‐b in left eye and α‐β in right eye). From chiasma onward the right parts of both retinas (continuous lines) project to right (XWS), and left parts (dashed lines) to left optic tract (rtu). B: Harris's figure of binocular convergence to illustrate Newton's concept of central projection of corresponding parts of retina when eyes converge on line PS. After crossing of nasal parts of the retina the corresponding retinal areas T and X and V and Y are drawn to unite in FGH so that binocular convergence occurs in brain and is projected to a hypothetical central area abcd.

G. W. Leibniz's manuscript of the binary number theory, written on March 15, 1679. Latin text explains the principle to use the two signs 1 and 0 for all numbers.

Helmholtz's depictions of Thomas Young's three‐color concept compared with his own spectral sensitivity curves. A: for Young's theory of three receptor elements Helmholtz postulated in 1850 relatively broad spectral absorption curves showing 3 maxima—in orange, in green, and in blue‐violet, respectively. B: Helmholtz used König's and Dieterici's psychophysical data of their normal trichromatic eyes to obtain sensitivity curves of three elementary color sensations. Red curve R is shifted toward short‐wave side with a maximum at 560 nm, green curve G has a maximum at 550 nm, and violet V at 450 nm. This corresponds approximately to modern objective measurements of receptor absorptions (570, 540, 450 nm). Some individual differences are marked K for König and D for Dieterici.

E. D. Adrian's drawings of sensory nerve messages and their recording. A: cutaneous sensation. , Stimulus touching skin elicits rhythmic action potentials in nerve fiber. These messages are picked up from electrodes and magnified by an amplifier for recording instrument, . B: stretch receptors from muscle, lung, and carotid sinus are excited by muscle‐lengthening lung extension and carotid sinus dilatation, respectively. C: afferent neurons signaling touch, light, heat, and smell, and their processing of the receptor excitation: afferent fiber is connected with a hair touch receptor; optic nerve fiber is activated from a cone over two synapses; thermoreceptor activating a posterior root fiber and olfactory receptor with its primary fiber send their messages to central nervous system directly or after various synaptic transformations.

Hans Berger's early 1929 and 1930 recordings of desynchronization of electroencephalogram (EEG) in man by sensory stimuli and attention. A, B: scalp recordings show blocking of α‐waves and EEG flattening caused by sensory stimuli, A, and during periodic alterations of attention, B, with EEG (middle records) and time 50/s (lowest records). Subjects: healthy men aged 30 (A) and 34 years (B). C, D, E: Berger's unpublished first direct recordings from human brain in 1930: C: Berger's drawing shows how the coated silver needle pairs were inserted in the parietal cortex and 4 cm below in the white matter during a diagnostic cerebral puncture in 20‐yr‐old man with brain tumor. D, E: simultaneous records demonstrate origin of EEG waves in cortex, the white matter having less electrical activity. On D Berger wrote “Cortex, Centr. semiovale,” and after name and date, “Nadelabl.”, in shorthand: “Augen geschlossen; eingeführte El.” (needle recording; eyes shut, electrodes inserted). E: during expectation of a second sensory stimulus the EEG flattened.

Diagrams of intentional perception, aimed action, and directive attention. Schemes demonstrate anticipating preparation of perception and action in inner world and sensorimotor processes of behavior in outer world. A: Time relations of anticipating purpose, intention, and action as conceived by Aristotle. Aristotle described in the “Metaphysics” the cooperation of human knowledge and action by anticipation (noesis) of a foreseen aim (eidos) which acts in reverse in determining the action (poiesis) toward a goal. B: time relations of anticipating purpose, intention, and action, by scheme of N. Hartmann. Hartmann uses arrows to depict the peculiar time relations in the anticipating preparation of goal‐directed action. Willed intention must precede selection of means for the ends and this selection also precedes realization. Time scale of purpose (Zwecksetzung) and selection (Mittelwahl) shows opposite directions: purpose projects to future, and selection paradoxically acts against time flow as shown by arrows and time abscissa. This seemingly paradoxical inversion of time by preconceived future is present also in perceptual attention using internal world models given by sensory experience to search for and perceive the intended objects. C: psychophysical scheme of attentive perception, anticipation, and action. Interactions between perceiving subject and intended object are schematized for three active processes of expectation, attention, and orientation, and for passive afferent messages from sense organs to brain. Above the middle horizontal line are shown anticipatory processes of directive attention in the mind; below are shown physiological correlates of attentive orientation and perception in body and brain. Upper processes are psychic; lower are neurophysiological and behavioral functions. For vision the attentive and orientating processes include eye movements.

Slow negative cortical potentials in man, correlated with intention to act (A, B) and to reach a target (C). Potential shifts (▭ before action, ▪ during action) are maximal at vertex and show higher amplitudes over motor cortex contralateral to moving limb. Motion is marked by arrows (↕ for brief movements; for longer movements). A: expectation wave or contingent negative variation (CNV) of Walter and co‐workers is dependent on sensory stimuli during conditioning. It is elicited by a first stimulus (here auditory and conditional), increases during expectancy of second stimulus (visual and imperative), and is terminated by a positive shift when conditioned action (key‐pressing) starts. B: readiness potential (Bereitschaftspotential ▭ of Kornhuber and co‐workers) precedes voluntary movements during intention to act and ends with a positive shift when brief movement (left‐hand flexion) begins. C: goal‐directed movement potential. increases premotion negativity (▭) during monitored movement for seconds until target is reached and a positive shift occurs. An intentional act with directed attention is common condition of these negative potential shifts. Maximal negativity appears in A B before brief movements and in C during goal‐directed movements.

Diagram of Nicolai Hartmann's level concept of world and its relation to sensory sciences demonstrates structural stratification in four levels (inorganic, biological, psychic, and cultural) on left and shows the related hierarchical order of four classes of science. Sensory research concerns mainly the three lower levels, specialist studies being limited to one level and correlative sciences connecting several levels. Psychophysical research compares physical stimuli at inorganic level with mental experience at psychic level. Recordings of sensory messages belong to biological level. Psychophysiology relates afferent information processing by the brain to percepts. Gulf between organic and psychic strata is bridged by correlating perception at psychic level with sensory messages at biological, and physical stimuli at inorganic level. Each stratum depends on basic functions of all subjacent levels but has its specific new laws. These special laws increase in complexity and differentiation for higher levels, although natural laws of lower are also valid in all upper levels (as indicated by arrows).