Biomedical Engineering: A Path Toward Artificial Intelligence

Part I

From the Photon to the Mental Image: The Process of Constructing Visual Reality

Author: Prof. Eng. Carlos Serna II, PE MSc.

The photon is an energy particle with no rest mass. It is the quantum of the electromagnetic field and the mediator of the electromagnetic force in quantum field theory. Although the photon has no rest mass, it carries energy and momentum, allowing it to play a fundamental role in electromagnetic interactions and the transmission of information through light.

Visual perception is a highly complex process that transforms objective electromagnetic waves into subjective experiences of color, shape, and motion. Although the photons that reach our eyes are a physical reality, the image we perceive is a construction of the brain, influenced by biological and cognitive factors. This process involves multiple stages, from light capture to the conscious interpretation of the visual scene.

1. Capturing Visual Frequency Signals

1.1. The Nature of Light and Its Interaction with the Eye

Light is an electromagnetic wave that varies in frequency and wavelength. The visible spectrum for humans ranges from approximately 380 to 750 nanometers (nm), with different wavelengths perceived as distinct colors (e.g., 450 nm as blue, 550 nm as green, and 700 nm as red). When light is reflected or emitted by objects and enters the eye, it passes through several structures before reaching the retina:

• Cornea: The first structure that refracts light, directing it toward the lens.

• Lens: Adjusts its shape to focus light on the retina through the process of accommodation.

• Vitreous humor: A gelatinous substance that stabilizes the shape of the eye and allows light to pass through.

1.2. Conversion of Light into Electrical Signals: The Retina and Photoreceptors

The retina is a tissue layer at the back of the eye containing photoreceptor cells:

• Cones: Responsible for color vision, sensitive to specific wavelengths:

• L cones (long wavelength, red, ~560–580 nm).

• M cones (medium wavelength, green, ~530–550 nm).

• S cones (short wavelength, blue, ~420–440 nm).

• Rods: Responsible for low-light vision, sensitive to light intensity but not color.

When photons strike the photoreceptors, they trigger a biochemical cascade in which rhodopsin (in rods) and photopsins (in cones) change their molecular structure, generating electrical impulses.

1.3. Transmission of Signals to the Brain. Photoreceptors send signals to bipolar cells and ganglion cells in the retina. The ganglion neurons’ axons form the optic nerve, which transmits visual information to the brain. At this stage, the signal is merely a series of electrical impulses without perceptual meaning.

2. Processing in the Brain: Constructing the Subjective Image

2.1. Information Crossing at the Optic Chiasm. Each eye captures information from both sides of the visual field. At the optic chiasm:

• Nasal fibers from each retina cross to the opposite hemisphere.

• Temporal fibers remain on the same side.

This organization ensures that:

• The right visual field is processed in the left hemisphere.

• The left visual field is processed in the right hemisphere.

2.2. Processing in the Thalamus (Lateral Geniculate Nucleus, LGN). Visual signals arrive at the lateral geniculate nucleus (LGN) of the thalamus, where information is organized and filtered before being sent to the primary visual cortex (V1) in the occipital lobe.

2.3. Image Construction in the Visual Cortex. The occipital lobe contains the primary visual cortex (V1) and several associated areas (V2, V3, V4, V5) responsible for analyzing different aspects of the image:

• V1: Detects edges, contrast, and line orientation.

• V2-V3: Process depth and motion.

• V4: Specializes in color perception.

• V5 (MT): Processes object motion.

From this segmentation, the information is transmitted via two main pathways:

• Dorsal pathway (“where”): Leads to the parietal lobe, processing the location and movement of objects in space.

• Ventral pathway (“what”): Leads to the temporal lobe, where shapes, faces, and objects are recognized.

2.4. Constructing Subjective Perception. At this stage, the image remains a collection of electrical signals distributed across various brain regions. Subjective perception arises when these signals are integrated with memory, attention, and prior experiences.

Factors influencing the subjective construction of visual reality:

• Optical illusions: Demonstrate how the brain can interpret the same visual information in different ways.

• Experience and learning: Color and shape perception can be modified by culture and education.

• Attention: Not everything we see is consciously processed; the brain filters irrelevant information.

A notable phenomenon is perceptual constancy, where the brain maintains color and shape stability despite changes in lighting and perspective.

3. Comparison with Visual Perception in Other Species. Different species have visual systems adapted to their evolutionary needs:

• Dogs and cats: See a limited color range (primarily blue and yellow) but have superior night vision due to a higher number of rods.

• Bees: Can see ultraviolet light, allowing them to detect flower patterns invisible to humans.

• Mantis shrimp: Has 12 types of photoreceptors, perceiving a much broader color spectrum than humans.

• Snakes: Possess infrared receptors, enabling them to detect heat emitted by prey in the dark.

This comparison highlights that visual perception is not an absolute representation of reality, but rather an adaptive construction unique to each species.

Conclusion. Vision is a process that transforms electromagnetic signals into a highly elaborated subjective experience. From the reception of photons in the retina to their integration in the cerebral cortex, each stage converts physical information into an internal representation of the world. However, this image is not an exact reflection of reality but an adaptive interface designed to maximize survival and perceptual efficiency.

Comparing human vision with that of other species reinforces the idea that each organism constructs its own version of reality, tailored to its evolutionary needs. This analysis leads to deeper questions about the nature of perception, reminding us that reality, as we experience it, is not a faithful representation of the objective universe but rather a construct of our brain.

From Stimulus to Feeling: The Process of Constructing Tactile Perception

The sense of touch is one of the most fundamental ways of interacting with the world, allowing us to detect temperature, pressure, vibration, texture, and pain. Although tactile stimuli are objective physical signals, the perception of touch is a subjective construction of the brain, influenced by experience, memory, and context. This process involves multiple stages, from the reception of mechanical signals in the skin to their conscious interpretation in the cerebral cortex.

1. Reception of Tactile Signals: Receptors in the Skin. The skin is the largest sensory organ of the human body and contains a variety of specialized mechanoreceptors that respond to different types of tactile stimuli. These receptors are distributed in the epidermis and dermis and can be classified into four main categories:

1.1. Skin Mechanoreceptors

a. Meissner’s corpuscles (sensitive to light touch and low-frequency vibrations). Located in the dermis of fingertips, lips, and sensitive areas. They detect rapid changes in texture (such as sliding fingers over a rough surface).

b. Pacinian corpuscles (sensitive to vibration and deep pressure). Found in the deep dermis. They detect high-frequency vibrations and rapid pressure changes.

c. Merkel discs (sensitive to sustained pressure and texture). Located in the epidermis, they are responsible for detecting edges and fine patterns. They play a crucial role in reading Braille and perceiving detailed object shapes.

d. Ruffini corpuscles (sensitive to skin tension and sustained pressure). Located in the deep dermis, they detect skin stretching and are fundamental for perceiving finger position and object manipulation.

1.2. Pain and Temperature Receptors. In addition to mechanoreceptors, the skin contains nociceptors (pain receptors) and thermoreceptors (temperature-sensitive receptors):

• Nociceptors: Detect potentially harmful stimuli, such as extreme heat, excessive pressure, or irritating chemicals.

• Thermoreceptors: Divided into cold and heat receptors, they help maintain thermal homeostasis.

2. Transmission of Tactile Signals to the Central Nervous System. When a tactile stimulus activates a receptor in the skin, it generates an action potential, an electrical signal that is transmitted through nerve fibers to the brain.

2.1. Types of Nerve Fibers. Tactile signals travel through different types of nerve fibers:

• Aβ fibers (fast, myelinated): Transmit information about fine touch and vibration at high speed.

• Aδ fibers (moderately fast, myelinated): Transmit sharp pain sensations and cold temperature.

• C fibers (slow, unmyelinated): Responsible for dull pain and prolonged heat sensation.

2.2. Transmission Pathways in the Spinal Cord. Tactile information enters the central nervous system through peripheral nerves and follows two main pathways in the spinal cord:

A. Medial lemniscal pathway (dorsal column): Processes fine touch, vibration, and proprioception (body position). Signals ascend the spinal cord to the gracile and cuneate nuclei in the medulla oblongata. They then cross to the opposite side of the brain and reach the thalamus before being sent to the primary somatosensory cortex.

B. Spinothalamic pathway: Processes pain, temperature, and crude touch. Signals cross to the opposite side of the spinal cord immediately after entering and ascend directly to the thalamus.

3. Processing in the Brain: Construction of Tactile Perception

3.1. The Thalamus: Sensory Relay Center. The thalamus acts as a processing station that filters and organizes tactile information before sending it to the primary somatosensory cortex (S1) in the parietal lobe.

3.2. The Somatosensory Cortex: Mapping the Body in the Brain. The somatosensory cortex is organized as a sensory homunculus, a brain map where areas with higher sensitivity (such as hands and lips) occupy more cortical space.

Key functions include:

• Texture and shape differentiation: Mechanoreceptor information is combined to generate a three-dimensional representation of touched objects.

• Stimulus localization: The brain determines the exact location of skin contact.

• Integration with proprioception: Tactile perception combines with information from muscles and joints to generate a sense of body position in space.

3.3. Subjective Construction of Touch. Although tactile stimuli are objective physical signals, the perception of touch is influenced by several factors:

• Expectation and context: The same stimulus may feel different depending on context (e.g., warm water may feel cold after touching hot water).

• Attention: Distracted individuals may not perceive certain tactile stimuli.

• Previous experience: Familiarity with textures or temperatures affects how we interpret them.

• Emotional modulation: The perception of pain and tactile pleasure can be influenced by an individual’s emotional state.

4. Comparison with Tactile Perception in Other Species. Tactile perception varies significantly among species due to differences in the distribution and specialization of sensory receptors:

• Cats and rodents: Have whiskers (vibrissae) with highly sensitive mechanoreceptors that allow them to detect vibrations and textures with precision.

• Snakes: Some species have heat-sensing pits that enable them to detect infrared radiation from their prey, providing a form of “thermal vision.”

• Elephants: Their trunks contain an extensive number of tactile receptors, allowing them to perceive textures with extreme precision.

• Octopuses: Their tentacles possess chemical and tactile receptors, enabling them to “taste” objects through touch.

These differences show that tactile perception is an evolutionary adaptation specific to each environment and biological need.

Conclusion. The sense of touch converts mechanical and thermal stimuli into a complex subjective experience. From the activation of mechanoreceptors in the skin to interpretation in the somatosensory cortex, the tactile process is a neural construction influenced by context, emotion, and memory. Just as in vision, tactile perception is not a direct reflection of external reality but rather an interpretation designed to optimize interaction with the environment. The comparison with other species reinforces the idea that each organism constructs its own tactile version of the world, tailored to its evolutionary needs.

From Chemical Stimulus to Olfactory Experience: The Subjective Construction of Smell

The sense of smell is one of the most primitive and fundamental for survival. It allows us to detect dangers (such as spoiled food or smoke), identify edible substances, and recognize individuals through pheromones. Although olfactory stimuli are objective chemical signals, the perception of odors is a subjective process that depends on brain interpretation, memory, and emotion.

1. Capturing Olfactory Signals: Receptors in the Nasal Mucosa. Unlike touch or vision, where stimuli are physical (mechanical pressure or light), smell relies on detecting airborne chemical molecules. These molecules, known as odorants, enter the nasal cavity with each inhalation.

1.1. Olfactory Epithelium: The First Line of Detection. Located in the upper part of the nasal cavity, the olfactory epithelium contains millions of olfactory receptor neurons (ORNs). These neurons have cilia covered by a mucus layer, where odorants dissolve before interacting with specific receptors.

1.2. Olfactory Receptors: Keys in a Chemical Lock. Each olfactory receptor neuron expresses a single type of receptor, but each receptor can be activated by multiple molecules with similar chemical structures. Humans have approximately 400 types of olfactory receptors but can distinguish over a trillion odors due to the combination of signals.

When an odorant binds to a specific receptor, an intracellular signaling cascade is triggered via G proteins (Golf), leading to the opening of ion channels in the neuron membrane. This generates an action potential, an electrical signal that travels toward the brain.

2. Transmission of the Olfactory Signal to the Central Nervous System. The electrical signals generated by olfactory receptors are transmitted via the axons of olfactory neurons to the olfactory bulb, the first neural processing station.

2.1. The Olfactory Bulb: Initial Odor Processing. The olfactory bulb is organized into structures called glomeruli, where signals from neurons with the same receptor type converge. Here, signals are refined by inhibitory interneurons (periglomerular and granular cells) before being sent to higher brain regions.

This initial processing helps:

• Encode odorant combinations to form a unique “olfactory signature.”

• Regulate odor intensity through lateral inhibition mechanisms.

• Filter out irrelevant signals, allowing focus on specific odors.

From the olfactory bulb, olfactory signals travel directly to the piriform cortex, the amygdala, and the hypothalamus, bypassing the thalamus (unlike other senses such as vision and touch).

3. Construction of Olfactory Perception in the Brain. Smell is a highly subjective sense, as it is closely linked to memory and emotions.

3.1. Piriform Cortex: Odor Identification. The piriform cortex, located in the temporal lobe, is responsible for identifying and categorizing odors. Here, activation patterns of glomeruli in the olfactory bulb are interpreted as distinct smells.

3.2. Amygdala and Hypothalamus: Emotional Influence and Adaptive Response. The amygdala associates odors with emotional experiences. This explains why certain smells can evoke intense memories (the Proust effect). The hypothalamus modulates physiological responses to odors, such as salivation in response to a pleasant aroma or aversion to an unpleasant smell.

3.3. Orbitofrontal Cortex: Integration with Other Senses. Olfactory signals are also integrated with taste perception in the orbitofrontal cortex, allowing the construction of food flavors (which is why food seems tasteless when we have a stuffy nose).

4. Comparison with Olfactory Perception in Other Species. Olfactory perception varies significantly among species, depending on their evolutionary needs:

• Dogs: Possess up to 220 million olfactory receptors, compared to 5–6 million in humans. Their olfactory bulb is proportionally 40 times larger, allowing them to detect odors at extremely low concentrations.

• Sharks: Can detect a drop of blood in millions of liters of water using specialized chemoreception.

• Butterflies: Have olfactory sensors on their legs, enabling them to detect pheromones from mates over long distances.

• Elephants: Have the most sophisticated olfactory system known in terrestrial mammals, with a capacity superior to that of dogs.

These differences demonstrate that olfactory perception is not universal but adapted to each organism’s needs.

5. The Subjective Construction of Smell. Although odorant molecules are objective chemical signals, odor perception is subjective and influenced by multiple factors:

• Previous experience: Odors associated with positive or negative memories can elicit different emotional responses in individuals.

• Culture: Some societies find certain smells pleasant, while others consider them unpleasant (e.g., certain cheeses or spices).

• Olfactory adaptation: Prolonged exposure to an odor reduces neuronal response, making us less aware of its presence (such as our own perfume).

• Odor blending effect: Some odors can be perceived differently when combined, creating a new impression in the brain.

Conclusion. Smell transforms chemical signals into deeply subjective sensory experiences. From the detection of odorant molecules in the nasal mucosa to their interpretation in the cerebral cortex, smell is a process that combines physical, emotional, and cognitive aspects. Its connection to memory and emotions makes it a powerful sense for decision-making and environmental adaptation. However, olfactory perception is not an objective representation of the world but a neural construction designed to enhance survival and interaction with the environment.