Las tendencias de consumidores mundiales de moda de lujo

Memory, inseparable in practice from perception, imports past into the present, contracts into a single intuition many moments of duration, and thus by a twofold operation compels us, de facto, to perceive matter in ourselves, whereas we, de jure, perceive matter within matter. – Henri Bergson, in Matter and Memory.

A woman in her 40’s lay on the surgical table of neurosurgical ward in the Christian Medical College (CMC), Vellore, a small, unremarkable town in the southern part of India. The woman, most probably from one of the north-eastern states had travelled a long way to Vellore, obviously to access the superior medical services offered by CMC. A member of the team of surgeons that surrounded the woman was asking her to count numbers from 1 through 10. She began to count aloud with her strong north-eastern accent but stopped suddenly midway as though she was interrupted by an invisible force. That force was the electrical stimulation – mild shocks – delivered by another member of the team to Broca’s area, a brain region responsible for control of our speech, typically located in the left hemisphere in right-handed people. Shocks delivered to this area by the surgeon interfered with ongoing counting. The surgeon placed a tiny piece of paper with a number printed on it, at the spot where they found a moment ago where stimulation stopped speech. The piece of paper is the surgeon’s landmark for Broca’s area.

The very sight of a person talking, or for that matter doing anything normally, with a part of the scalp pinned to an external support, with a piece of the cranium sawed and taken out, and half of the brain exposed, might seem surreal to the uninitiated, but not an uncommon scene in neurosurgery operation theaters all over the world. There are occasions when neurosurgeons choose to operate on the brain while the patient remains conscious, “cooperating” with the surgeon, meekly doing his/her bidding, - be it counting digits or twiddling fingers, - not out of a morbid intent to impose unearthly torture on an already suffering individual. The choice is made out of a simple surgical exigency. While marking the brain tissue that has to be removed, the surgeon has to ascertain that intact tissue is maximally spared. Unfortunately often the affected tissue appears quite similar to intact tissue, leaving only one option to the surgeon: to distinguish between normalcy and dysfunction by electrical stimulation. When the probing electrode of the surgeon hit a spot that was able to halt the ongoing counting, the surgeon discovered an intact Broca’s area.

Surgeries of this kind, which experts fondly call ‘brain mapping by electrical stimulation,’ were pioneered by Wilder Penfield, a Canadian-American neurosurgeon who worked in the earlier part of the last century. Penfield sought to find a surgical cure to epilepsy, a devastating disease that resisted pharmacological treatment. Epilepsy is a condition in which electrical signals in the brain go awry. The regulated background chatter in a normal brain is displaced by large synchronized waves of neural forest fire that often originates from certain vulnerable spots – the “epilectic foci” – and spread to brain areas far and near. These bouts of uncontrolled neural activity, termed seizures, may last only a few seconds or, in extreme cases, may continue unabated for hours or days until subdued by strong pharmacological intervention.

A mild seizure may be experienced as a brief spell of unconsciousness or it may precipitate into a full-blow convulsion.

Penfield explored surgical options to control intractable epilepsy. His strategy was to locate epilectic foci, through systematic electrical stimulation, and lesion the focal area, thereby cutting off the problem at its roots. These explorations led to him map various regions of brain’s surface, particularly temporal lobe where epileptic foci are often located. Very often patients reported vivid recall of past auditory experiences, most probably because superior temporal lobe is the site of auditory area, a part of the brain that processes sounds. For example, the patient might hear the voice of his cousin in Africa, or recall the well-known march of Aida. The experience of recall is so vivid and living, that the sounds seemed to originate from the immediate vicinity of the patient. Yet the patients were not overwhelmed by the hallucination and were aware of their real, immediate surroundings – the operating room, the surgeon, and the intimidating instrumentation. It was as though the patients had a double stream of consciousness at the time of stimulation, one corresponding to their real surroundings, and the other pertaining to the hallucinatory auditory experience.

Similarly when borderlands of parietal and temporal lobes were stimulated, the patients reported complex visual experiences. More of such spots were discovered in the right hemisphere than in the left hemisphere. When these spots were stimulated, patients re-experienced vivid scenes from their past. On subsequent enquiry, the patients were able to clearly link their experiences on the surgical table, with actual past events in their life.

Figure 5.1: Wilder Penfield

These findings led Penfield to believe that memories of one’s past are stored, as in a tape recorder, in specific brain sites. It is when these sites are electrically stimulated that the patients recall the corresponding memories. Thus Penfield’s research supported a “localized” view of organization of memory. Memories are stored at specific sites in the brain, and can be recalled by stimulating those sites.

But quite a contrary view on the subject of memories was arrived at by Karl Lashley who sought to find the sites of memories, by performing on rats, experiments that are far bolder than those performed by Penfield on humans. A big goal of Lashley’s research was to search for the engram, a word that denotes the biochemical and biophysical machinery for storing memories in the brain. Penfield’s work with humans revealed that brain stimulation at specific sites activated these engrams and retrieved memories. Lashley sought to accomplish the same in experimental animals. Early in his career Lashley came under the influence of John Watson, a pioneer in behaviorism. At a time when the techniques of brain research were not sophisticated enough to provide detailed structural and functional information about the brain, neuroscientists attempted to fill the vacuum by subjective, speculative accounts of how brain performs its functions.

Behaviorism emerged as a reaction to this unhealthy trend and insisted on casting ones theories

of mind/brain strictly in terms of observables and quantifiable aspects of behavior. For example, thou shalt not say that an animal pressed a bar vigorously; a behaviorist would say that the animal pressed the bar so many times a second. Trained in such a tradition Lashley set out to study the link between brain damage, which he tried to quantify, and behavior.

Figure 5.2: Karl Lashley

Although Lashley worked with a variety of animals, his most eminent work was on maze learning in rats. Rats are known for their remarkable ability for finding their way out of complex mazes. Thanks to this special gift, rats have been used for over a century for studying how brain represents and navigates through space. Lashley exposed rats’ brains and made long straight cuts through the surface in various regions. The total length of the cuts is treated as a control parameter. As this ‘parameter’ is varied the efficiency with which rats escaped from the maze is noted. What Lashley observed, to his astonishment, is that the escape efficiency depended more or less strictly on the total amount of damage, not much on the location of the damage. This result flies in the face of earlier work of Penfield and others who concluded that brain function, and memories, are localized. Thus Lashley’s search for a precise location of the engram ended in a failure.

Lashley summarized his experimental findings into two “laws” of brain function.

The first law, the law of mass action, states that the cerebral cortex works as a whole and not as a patchwork of modules each working independently. The second law, termed the principle of equipotentiality, states that if a certain part of the brain is damaged, other parts may reorganize themselves so as to take up the function of the damaged region.

Considering the strong reductionist tendencies of twentieth century science in general, and the ‘atomistic’ thinking (one gene
one protein
one phenotype or one germ
one disease
one drug) that was strong in biology, it would have been easier to accept localization in brain, than to appreciate a theory of brain function, or of engram, that is delocalized. Paradoxically, localization and delocalization are equally true and important facets of the brain. A single neuron in visual cortex responds only when a bar of a certain length and orientation is placed precisely at a certain spot in the visual space, - a case of extreme localization. There have been cases when a whole hemisphere has been removed in childhood, and the individual grew up almost normally, went to college and led a successful life – a case of extreme delocalization. two-dimensional photographs. An ordinary photograph, with only length and breadth dimensions, looks the same whichever angle you see it from. Contrarily, a real three-dimensional object, with the added depth dimension, reveals hidden features as you move around it seeing it from

different angles. Viewing a hologram is similar, in a limited sense, to viewing a real, 3D object.

It reveals hidden features as you change your vantage point.

Another property of a hologram is the manner in which it responds to damage. When a 2D photo is damaged locally – say a local burn, or an ink spill – the specific part of the image is destroyed forever. Information in a photo is “localized.” But in a hologram, local damage degrades the entire image to a small extent – thus information in a hologram is delocalized. This aspect of a hologram intrigued Pribram since he saw a possibly analogy to the engram that he was just searching for. He decided to learn more about holograms from his son, who was a physicist.

Mechanisms that underlie creation of a normal photographic image can be explained using geometric or ray optics, the earliest version of optics which every highschooler learns. In geometric optics, light is described as a bundle of vanishingly thin “light rays.” These rays travel in straight lines until they bounce off reflecting surfaces or bend at the interfaces between unlike media. When we see an object, rays from a light source fall on the object, bounce off the object’s surface, and pass through the pupil of our eye to form an image at the back of the eye, on the retina. If we replace the eye with a camera, we can understand the formation of an image on the photographic film. This ‘ray’ story is adequate to explain the formation of the image on the retina, or on a photographic film, but insufficient to explain the physics of the hologram.

It turns out that the ray description of light is only an approximation valid at sufficiently large length scales. A more accurate description of light is as a wave – not a material wave that we observe on the surface of a lake, but a wave, or a propagating disturbance, in an electromagnetic field. In the world of ‘ray’ optics, the ray coming from an object conveys several

properties about the spot on the object from which it originates - intensity (is the spot bright or dim?), direction (which way is the spot located?). In the world of wave optics, the wave not only carries information about intensity and direction, but in addition, carries a new property known as phase, which has no counterpart in the ‘ray’ world.

Simple domestic experiments with a large tub of water can reveal a few things about waves. If you stick your finger suddenly in a large tranquil tub of water, you will notice that circular waves originating from the spot where you dunk you finger, and expanding in all directions. If you just looked at a single spot on the surface of the water, you could notice water going up and down. There is no real movement of water, as you can test by dropping a tiny piece of paper, and watch it bob up and down at a single spot. If it were a mere gentle dip you would see that the paper makes only a small oscillation. If you struck harder, making something of a splash, you will notice that the paper swings up and down by a greater extent. This extent of the wave is known as its amplitude, a property that is related to the ‘energy’ of the wave. Another property of the wave is the rate at which a point on the wave bobs up and down – its frequency.

In case of a light wave, this frequency refers to the color of light. Red, for example, has a lower frequency than blue. A third property of the wave, the most relevant one for our present purpose, is phase. It refers to the state of oscillation of a wave in its cycle. By analogy, we may consider a season to represent the phase of a year.

A phase is a property of a single wave. Now it is possible to compare the phases of two waves, and talk of phase difference. Imagine two joggers going around a circular track running at equal speeds. Jogger A, however, had a head start over jogger B, and continues to lead B for ever since their speeds are the same. Thus we say that A’s phase is greater than that of B, or A’s phase leads B’s phase. If B had a head start then we say that A’s phase lags that of B. Phase difference

is the basis of another important wave-related phenomenon known as interference, a phenomenon that is closely related to holograms.

It is straightforward to observe interference in our familiar tub of water. Instead of sticking a single finger, stick two at different ends of the tub, simultaneously. You will watch two sets of expanding, circular waves originating from two different spots. These expanding waves meet somewhere in the middle of the tub, like clashing armies, and form interesting grid-like patterns of troughs and crests on the water surface. This meeting of waves is known as interference, and the patterns such trysting waves form are called interference patterns (fig. 5.3).

Figure 5.3: Interference pattern produced when two sets of circular waves originating from two points meet and interfere.

If the two waves meet such that the crests of either wave coincide, (or the phase difference is zero), then the two waves add up, leading to what is known as constructive interference (fig. 5.4a). When the two waves meet such that the crest of one, meets the trough of the other, their phases are opposite, resulting in what is known as destructive interference, since the two waves cancel each other at such points (fig. 5.4b). Thus an interference pattern may be

regarded as a diagram of phase differences among waves. It is this interference pattern that forms the basis of holography, that adds depth to otherwise flat images,

Figure 5.4: Constructive and destructive interference

The fact that light is a wave and not a stream of “corpuscles” was established in a classic experiment by Thomas Young in early 1800’s. Young’s experiment, famously known

double-slit experiment, consists of a light source whose light falls on a screen with two slits. The two slits now act as point sources of light, from which light expands outwards in circular waves that interfere, constructively and destructively,

falling on another screen downstream shows a band

dark bands representing constructive and destructive interference respectively. Thus the interference pattern may be thought of as a diagram of the phase of the wavefront.

regarded as a diagram of phase differences among waves. It is this interference pattern that forms the basis of holography, that adds depth to otherwise flat images, and make them spring to life.

Figure 5.4: Constructive and destructive interference

The fact that light is a wave and not a stream of “corpuscles” was established in a classic experiment by Thomas Young in early 1800’s. Young’s experiment, famously known

slit experiment, consists of a light source whose light falls on a screen with two slits. The two slits now act as point sources of light, from which light expands outwards in circular waves that interfere, constructively and destructively, forming intricate interference patterns. Light falling on another screen downstream shows a band-like pattern of intensity, with the bright and dark bands representing constructive and destructive interference respectively. Thus the

y be thought of as a diagram of the phase of the wavefront.

regarded as a diagram of phase differences among waves. It is this interference pattern that forms and make them spring to life.

The fact that light is a wave and not a stream of “corpuscles” was established in a classic experiment by Thomas Young in early 1800’s. Young’s experiment, famously known as the slit experiment, consists of a light source whose light falls on a screen with two slits. The two slits now act as point sources of light, from which light expands outwards in circular waves forming intricate interference patterns. Light like pattern of intensity, with the bright and dark bands representing constructive and destructive interference respectively. Thus the

y be thought of as a diagram of the phase of the wavefront.

Figure 5.5: A sketch of interference pattern obtained in Young’s double slit experiment.

Phase differences among waves have a lot of significance to our perception of three-dimensional objects and to the principle of hologram. Phase difference has also a key role in our ability to localize a sound source, an auditory equivalent of perceiving depth and direction visually. Consider a sound that originated from your right. You can identify where it came from, even with your eyes closed, since the signal reaches your right ear first, before it reaches the left one. Or, in the language of phases, the phase of the sound vibrations near your right ear, leads that of the vibrations at your left ear. This is the principle used in stereo sound. By gradually varying the phase difference between the sounds that are played to the two ears, it is possible to control the perceived location of the sound source. Stretching the analogy to optics, when light that bounces off a real object meets our eye, what it brings is not just a pattern of intensity (or amplitude) but also the phase. This phase is critical to our perception of a three-dimensional world of depth and shape. Note that phase is only one factor that defines our depth, or three-dimensional perception. Though there are others - use of two eyes to find depth, use of motion cues, or use of shading that arises from play of light and shadow, each providing its supply of depth-related information to the viewer – phase is primary.

The photofilm in a camera only records the intensity pattern, - the 2D photo, - of the light

The photofilm in a camera only records the intensity pattern, - the 2D photo, - of the light