Color Atlas of Physiology 2003 thieme

Plate 12.25 Visual Field, Visual Pathway

Color, high-resolution stationary shapes, movement, and stereoscopic depth are processed in some subcortical visual pathways, and from V1 onward in separate information channels. These individual aspects must be integrated to achieve visual perception. In diurnally active primates like humans, over half of the cortex is involved in processing visual information. On a simplified scale, the parietal cortex analyzes the “where” and involves motor systems, and the temporal cortex takes care of the “what” of visual input comparing it with memory.

Axons of the optic tract (especially those of M and γ cells) also project to subcortical regions of the brain such as the pretectal region, which regulates the diameter of the pupils (see below); the superior colliculi (!B), which are involved in oculomotor function (!p. 360);

the hypothalamus, which is responsible for circadian rhythms (!p. 334).

The pupillary reflex is induced by sudden exposure of the retina to light (!p. 350). The signal is relayed to the pretectal region; from here, a parasympathetic signal flows via the Edinger–Westphal nucleus, the ciliary ganglion and the oculomotor nerve, and induces narrowing of the pupils (miosis) within less than 1 s. Since both pupils respond simultaneously even if the light stimulus is unilateral, this is called a consensual light response. Meiosis also occurs when the eyes adjust for near vision (near-vision response !p. 360).

The corneal reflex protects the eye. An object touching the cornea (afferent: trigeminal

nerve) or approaching the eye (afferent: optic 359 nerve) results in reflex closure of the eyelids.

Eye Movements, Stereoscopic Vision,

Depth Perception

cade), a new object can be brought into focus. Damage to the cerebellum or organ of balance (!p. 342) can result in pathological nystagmus.

The brain stem is the main center responsible for programming of eye movements. Rapid horizontal (conjugated) movements such as saccades and rapid nystagmus movement are programmed in the pons, whereas vertical and torsion movements are programmed in the mesencephalon. The cerebellum provides the necessary fine tuning (!p. 326). Neurons in the region of the Edinger–Westphal nucleus are responsible for vergence movements.

In near vision, depth vision and three-di- mensional vision are primarily achieved through the coordinated efforts of both eyes and are therefore limited to the binocular field of vision (!A). If both eyes focus on point A (!B), an image of the fixation point is projected on both foveae (AL, AR), i.e., on the corresponding areas of the retina. The same applies for points B and C (!B) since they both lie on a circle that intersects fixation point A and nodal points N (!p. 347 B) of the two eyes (Vieth– Müller horopter). If there were an imaginary middle eye in which the two retinal regions (in the cortex) precisely overlapped, the retinal sites would correspond to a central point AC ! AL + AR (!C). Assuming there is a point D outside the horopter (!C, left), the middle eye would see a double image (D!, D ) instead of point D, where D! is from the left eye (DL). If D and A are not too far apart, central processing of the double image creates the perception that D is located behind D, i.e., depth perception occurs. A similar effect occurs when a point E (!C, right) is closer than A; in this case, the E! image will arise in the right eye (E!R) and E will be perceived as being closer.

Depth perception from a distance. When viewing objects from great distances or with only one eye, contour overlap, haze, shadows, size differences, etc. are cues for depth perception (!D). A nearby object moves across the field of vision more quickly than a distant object, e.g., in the case of the sign compared to the wall in plate D). In addition, the moon appears to migrate with the moving car, while the mountains disappear from sight.

A. Binocular visual field

C. Three-dimensional vision (binocular vision)

D’ D D’’

More distant

A A

Closer

Plate 12.26

D. Cues for depth vision

Photo: A. Rothenburger

Direction of train movement

Physical Principles of Sound—Sound

Stimulus and Perception

damental (lowest) tone in the complex determines the pitch of the sound, and the higher ones determine its timbre (overtones). An a1 (440 Hz) sung by a tenor or played on a harp has a different sound than one produced on an organ or piano. The overlap of two very similar tones produces a distinct effect characterized by a beat tone of a much lower frequency (!A3, blue/red).

Audibility limits. Healthy young persons can hear sounds ranging in frequency from 16 to 20 000 Hz. The upper limit of audibility can drop to 5000 Hz due to aging (presbycusis). At 1000 Hz, the absolute auditory threshold or lowest sound pressure perceived as sound is 3 · 10– 5 Pa. The threshold of sound is frequency-dependent (!B, green curve). The threshold of hearing for a tone rises tremendously when other tones are heard simultaneously. This phenomenon called masking is the reason why it is so difficult to carry on a conversation against loud background noise.

362The ear is overwhelmed by sound pressures over 60 Pa, which corresponds to 2 · 106 times

the sound pressure of the limit of audibility at

1000 Hz. Sounds above this level induce the sensation of pain (!B, red curve).

For practical reasons, the decibel (dB) is used as a logarithmic measure of the sound pressure level (SPL). Given an arbitrary reference sound pressure of po = 2 · 10– 5 Pa, the sound pressure level (SPL) can be calculated as follows:

where px is the actual sound pressure. A tenfold increase in the sound pressure therefore means that the SPL rises by 20 dB.

The sound intensity (I) is the amount of sound energy passing through a given unit of area per unit of time (W · m2). The sound intensity is proportional to the square of px. Therefore, dB values cannot be calculated on a simple linear basis. If, for example, two loudspeakers produce 70 dB each (px = 6.3 · 10-2 Pa), they do not produce 140 dB together, but a mere 73 dB because px only increases by a factor of !"2 when the intensity level doubles. Thus, !"2 · 6.3 · 10–2 Pa has to be inserted for px into Eq. 12.2.

Sound waves with different frequencies but equal sound pressures are not subjectively perceived as equally loud. A 63 Hz tone is only perceived to be as loud as a 20 dB/1000 Hz reference tone if the sound pressure of the 63 Hz tone is 30-fold higher (+ 29 dB). In this case, the sound pressure level of the reference tone (20 dB/1000 Hz) gives the loudness level of the 63 Hz tone in phon (20 phon) as at a frequency of 1000 Hz, the phon scale is numerically equals the dB SPL scale (!B). Equal loudness contours or isophones can be obtained by plotting the subjective values of equal loudness for test frequencies over the whole audible range (!B, blue curves). The absolute auditory threshold is also an isophone (4 phons; !B, green curve). Human hearing is most sensitive in the 2000–5000 Hz range (!B).

Note: Another unit is used to describe how a tone of constant frequency is subjectively perceived as louder or less loud. Sone is the unit of this type of loudness, where 1 sone = 40 phons at 1000 Hz. 2 sones equal twice the reference loudness, and 0.5 sone is 1/2 the reference loudness.

The auditory area in diagram B is limited by the highest and lowest audible frequencies on the one side, and by isophones of the thresholds of hearing and pain on the other. The green area in plate B represents the range of frequencies and intensities required for comprehension of ordinary speech (!B).

Conduction of Sound, Sound Sensors

tines of the tuning fork are placed in front of the ear (air conduction). Individuals with normal hearing or sensorineural deafness can hear the turning fork in the latter position anew (positive test result), whereas those with conduction deafness cannot (test negative).

The internal ear consists of the equilibrium organ (!p. 342) and the cochlea, a spiraling bony tube that is 3–4 cm in length. Inside the cochlea is an en- dolymph-filled duct called the scala media (cochlear duct); the ductus reuniens connects the base of the cochlear duct to the endolymph-filled part of the equilibrium organ. The scala media is accompanied on either side by two perilymph-filled cavities: the scala vestibuli and scala tympani. These cavities merge at the apex of the cochlea to form the helicotrema. The scala vestibuli arises from the oval window, and the scala tympani terminates on the membrane of the round window (!A2). The composition of perilymph is similar to that of plasma water (!p. 93 C), and the composition of endolymph is similar to that of the cytosol (see below). Perilymph circulates in Corti’s tunnel and Nuel’s spaces (!A4).

Organ of Corti. The (secondary) sensory cells of the hearing organ consist of approximately 10 000–12 000 external hair cells (HCs) and 3500 internal hair cells that sit upon the basilar membrane ( !A4). Their structure is very similar to that of the vestibular organ (!p. 342) with the main difference being that the kinocilia are absent or rudimentary.

There are three rows of slender, cylindrical outer hair cells, each of which contains approximately 100 cilia (actually microvilli) which touch the tectorial membrane. The bases of the hair cells are firmly attached to the basilar membrane by supporting cells, and their cell bodies float in perilymph of Nuel’s spaces (!A4). The outer hair cells are principally innervated by efferent, mostly cholinergic neurons from the spiral ganglion (NM-cholinoceptors; !p. 82). The inner hair cells are pear-shaped and completely surrounded by supporting cells. Their cilia project freely into the endolymph. The inner hair cells are arranged in a single row and synapse with over 90% of the afferent fibers of the spiral ganglion. Efferent axons from the nucleus olivaris superior lateralis synapse with the afferent endings.

Sound conduction in the inner ear. The stapes moves against the membrane of the oval window membrane, causing it to vibrate. These are transmitted via the perilymph to the membrane of the round window (!A2). The walls of the endolymph-filled cochlear duct, i.e. Reissner’s membrane and the basilar mem-

!

cell cilia at the site of maximum reaction to the sound frequency (!D4), resulting in opening of transduction channels and depolarization of the cells (sensor potential). This leads to transmitter release (glutamate coupling to AMPA receptors; !p. 55 F) by internal hair cells and the subsequent conduction of impulses to the CNS.

Vibrations in the internal ear set off an outward emission of sound. These evoked otoacoustic emissions can be measured by placing a microphone in front of the tympanic membrane, e.g., to test internal ear function in infants and other individuals incapable of reporting their hearing sensations.

Inner ear potentials (!p. 369 C). On the cilia side, the hair cells border with the endolymphfilled space, which has a potential difference (endocochlear potential) of ca. + 80 to + 110 mV relative to perilymph (!p. 369 C). This potential difference is maintained by an active transport mechanism in the stria vascularis. Since the cell potential of outer (inner) hair cells is

– 70 mV (– 40 mV), a potential difference of roughly 150–180 mV (120–150 mV) prevails across the cell membrane occupied by cilia (cell interior negative). Since the K+ conc. in the endolymph and hair cells are roughly equal (!140 mmol/L), the prevailing K+ equilibrium potential is ca. 0 mV (!p. 32). These high potentials provide the driving forces for the influx not only of Ca2+ and Na+, but also of K+, prerequisites for provoking the sensor potential.

Hearing tests are performed using an audiometer. The patient is presented sounds of various frequencies and routes of conduction (bone, air). The sound pressure is initially set at a level under the threshold of hearing and is raised in increments until the patient is able to hear the presented sound (threshold audiogram). If the patient is unable to hear the sounds at normal levels, he or she has an hearing loss, which is quantitated in decibels (dB). In audiometry, all frequencies at the normal threshold of hearing are assigned the value of 0 dB (unlike the diagram on p. 363 B, green curve). Hearing loss can be caused by presbycusis (!p. 362), middle ear infection (impaired air conduction), and damage to the internal ear (impaired air and bone conduction) caused, for example, by prolonged exposure to excessive sound pressure (!90 dB, e.g. disco music, pneumatic drill etc.).

12 Central Nervous System and Senses

368

Central Processing of Acoustic

Information

Various qualities of sound must be coded for signal transmission in the acoustic pathway. These include the frequency, intensity and direction of sound waves as well as the distance of the sound source from the listener.

Frequency imaging. Tones of various frequencies are “imaged” along the cochlea, conducted in separate fibers of the auditory pathway and centrally identified. Assuming that a tone of 1000 Hz can just be distinguished from one of 1003 Hz (resembling true conditions), the frequency difference of 3 Hz corresponds to a relative frequency differential threshold of 0.003 (!p. 352). This fine differential capacity is mainly due to frequency imaging in the cochlea, amplification by its outer hair cells (!p. 366), and neuronal contrast along the auditory pathway (!p. 313 D). This fine tuning ensures that a certain frequency has a particularly low threshold at its “imaging” site. Adjacent fibers are not recruited until higher sound pressures are encountered.

Intensity. Higher intensity levels result in higher action potential frequencies in afferent nerve fibers and recruitment of neighboring nerve fibers (!A). The relative intensity differential threshold is 0.1 (!p. 352), which is very crude compared to the frequency differential threshold. Hence, differences in loudness of sound are not perceived by the human ear until the intensity level changes by a factor of over 1.1, that is, until the sound pressure changes by a factor of over !1,1 = 1,05.

Direction. Binaural hearing is needed to identify the direction of sound waves and is based on the following two effects. (1) Sound waves that strike the ear obliquely reach the averted ear later than the other, resulting in a lag time. The change in direction that a normal human subject can just barely detect (direction threshold) is roughly 3 degrees. This angle delays the arrival of the sound waves in the averted ear by about 3 · 10-5 s (!B, left). (2) Sound reaching the averted ear is also perceived as being quieter; differences as small as 1 dB can be distinguished. A lower sound pressure results in delayed firing of actions potentials, i.e., in increased latency (!B, right). Thus,

the impulses from the averted ear reach the CNS later (nucleus accessorius, !D5). Effects

(1) and (2) are additive effects (!B). The external ear helps to decide whether the sound is coming from front or back, above or below. Binaural hearing also helps to distinguish a certain voice against high background noise, e.g., at a party. Visibility of the speaker’s mouth also facilitates comprehension.

Distance to the sound source can be determined because high frequencies are attenuated more strongly than low frequencies during sound wave conduction. The longer the sound wave travels, the lower the proportion of high frequencies when it reaches the listener. This helps, for instance, to determine whether a thunderstorm is nearby or far away.

Auditory pathway (!D). The auditory nerve fibers with somata positioned in the spiral ganglion of the cochlea project from the cochlea (!D1) to the anterolateral ( !D2), posteroventral and dorsal cochlear nuclei (!D3). Afferents in these three nuclei exhibit tonotopicity, i.e., they are arranged according to tone frequency at different levels of complexity. In these areas, lateral inhibition (!p. 313 D) enhances contrast, i.e., suppresses noise. Binaural comparison of intensity and transit time of sound waves (direction of sound) takes place at the next-higher station of the auditory pathway, i.e. in the superior olive (!D4) and accessory nucleus (!D5). The next stations are in the nucleus of lateral lemniscus (!D6) and, after most fibers cross over to the opposite side, the inferior quadrigeminal bodies (!D7). They synapse with numerous afferents and serve as a reflex station (e.g., muscles of the middle ear; !p. 366). Here, sensory information from the cochlear nuclei is compared with spatial information from the superior olive. Via connections to the superior quadrigeminal bodies (!D8), they also ensure coordination of the auditory and visual space. By way of the thalamus (medial geniculate body, MGB; !D9), the afferents ultimately reach the primary auditory cortex (!D10) and the surrounding secondary auditory areas (!p. 311 E, areas 41 and 22). Analysis of complex sounds, short-term memory for comparison of tones, and tasks required for “eavesdropping” are some of their functions.