SENSATION AND PERCEPTION
Sensory Systems
Sensory systems are crucial for gathering information about our surroundings, enabling successful navigation and interaction with environments.
Sensation
Sensory receptors: Specialized neurons designed to respond to specific types of stimuli.
Sensation: The process that occurs when sensory receptors merely detect sensory stimuli.
Transduction: The fundamental process where sensory receptors convert sensory energy (e.g., light, sound, chemicals) into an electrical signal, specifically an action potential, which the brain can interpret.
Primary Sensory Systems:
Vision
Hearing (audition)
Smell (olfaction)
Taste (gustation)
Touch (somatosensation)
Balance (vestibular sense)
Body position (proprioception)
Movement (kinesthesia)
Pain (nociception)
Temperature (thermoception)
Sensory Thresholds
For a stimulus to generate an action potential (AP) that reaches the brain, it must first possess sufficient strength.
Absolute threshold: Defined as the minimum amount of stimulus energy required for the stimulus to be detected at least 50% of the time.
Example: On a clear night, the most sensitive photoreceptors in the human eye can detect a candle flame from up to 30 miles away.
The emphasis of the absolute threshold is on whether a stimulus can be detected at all.
Subliminal messages: These refer to sensory inputs that are present and cause an action potential, but are below the threshold of conscious awareness.
Laboratory experiments have demonstrated that individuals can unconsciously respond to subliminal messages, even without explicit awareness of their detection.
Just Noticeable Difference (JND)
Just noticeable difference (JND): Also known as the difference threshold, this is the minimum difference in the intensity of 2 stimuli that is required for a person to detect a change or a difference between them.
The JND is not static; it can vary depending on the initial intensity of the stimulus.
Example 1: Car radio volume:
Changing the volume from 5 to 15 is a much easier difference to detect.
Changing the volume from 5 to 6 is significantly harder to detect.
In this context, the JND would be the minimum increase in volume needed for that change to be perceived.
Distinction between Absolute Threshold and JND:
Absolute threshold: Focuses on the ability to detect a stimulus at all.
JND: Focuses on the ability to detect changes in the intensity of a stimulus.
Weber's Law: This law posits that the JND is not a fixed amount but is consistently proportional to the initial intensity of the stimulus; it's about a proportion or percentage of change.
Perception
While sensation involves collecting information about the environment via our senses, perception is the subsequent process.
Perception: The intricate way that sensory information is interpreted, organized, and consciously experienced by the brain.
Our perception profoundly influences how we interact with and navigate the world.
Perceptual Processing
Perception involves an interplay of 2 primary forms of processing:
Bottom-up processing: A system where perceptions are constructed directly from individual sensory inputs. It starts with specific sensory details and builds up to a complete perception.
Top-down processing: A system where the interpretation of sensations is heavily influenced by existing knowledge, past experiences, expectations, and thoughts. It uses broader context to interpret sensory data.
Example: When viewing an ambiguous figure that could be interpreted as the letter "B" or the number "13":
Bottom-up processing: The sensory information alone reveals 3 horizontal lines and 2 vertical lines.
Top-down processing: Prior knowledge, such as the surrounding context of letters or numbers, suggests whether it's a "B" or a "13".
Factors Impacting Perception
Sensory adaptation: The phenomenon of not consciously perceiving stimuli that remain relatively constant over prolonged periods of time (e.g., getting used to the smell of your own house).
Attention: Our selective focus plays a critical role.
Inattentional blindness: A notable failure to notice something that is completely visible directly in front of us due to a lack of attention directed towards it.
Motivation: Our desires and needs can influence perception, sometimes leading us to perceive something even when the stimulus is absent simply because we want to perceive it.
Beliefs, values, prejudices, expectations: These cognitive frameworks strongly color our interpretations.
Example: An individual's attitudes or expectations about a certain food can significantly alter their perception of its taste.
Life and cultural experiences: Our unique backgrounds and cultural contexts shape our perceptual biases.
Example: The Muller-Lyer illusion, where cultural exposure to certain architectural features can influence how lines are perceived.
Psychophysics: Waves & Wavelengths
Both visual and auditory stimuli are transmitted in the form of waves, possessing 2 fundamental physical properties:
Amplitude: The height of a wave, measured from its peak (highest point) to its trough (lowest point).
Wavelength: The horizontal distance of a wave, measured from one peak to the next adjacent peak.
Frequency
Frequency: Refers to the number of waves that pass a given point in a specific time period.
It is typically expressed in hertz (Hz).
Relationship between Wavelength and Frequency: Wavelength is inversely related to frequency.
Longer wavelengths correspond to lower frequencies.
Shorter wavelengths correspond to higher frequencies.
Perception of Color
Wavelength and Color: Different wavelengths of light are directly associated with our perception of various colors.
Longer wavelengths (and thus lower frequencies) are perceived as reds.
Intermediate wavelengths are perceived as greens.
Shorter wavelengths (and thus higher frequencies) are perceived as blues and violets.
Amplitude and Color: The amplitude of light waves is associated with the brightness or intensity of a color.
Larger amplitudes result in colors appearing brighter.
Smaller amplitudes result in colors appearing dimmer.
Soundwaves
Frequency of soundwaves: Directly perceived as pitch.
High frequency corresponds to a high-pitched sound.
Low frequency corresponds to a low-pitched sound.
Amplitude of soundwaves: Directly perceived as loudness.
Higher amplitude results in louder sounds.
Lower amplitude results in quieter sounds.
Loudness is measured in decibels (dB).
The audible range of sound frequencies for humans is between 20–2000 Hz.
Vision
Anatomy of the Visual System
Light waves are first transmitted across the transparent cornea and then enter the eye through the pupil.
The iris, the colored part of the eye, contains muscles that control the size of the pupil, regulating the amount of light entering.
The light then crosses the lens, which focuses it onto the fovea, a small depression in the retina.
The fovea is a crucial part of the retina and contains a high concentration of photoreceptors.
Photoreceptors (rods and cones) within the retina connect to retinal ganglion cells.
The axons from these ganglion cells converge and exit the back of the eye, collectively forming the optic nerve.
The optic nerve then transmits the visual information from the eye to the brain for further processing.
Blind spot: This is a specific point on the retina where the optic nerve exits the eye. Because there are no photoreceptors here, it's a region where we literally cannot respond to visual information, creating a "blind" area in our visual field.
Photoreceptors
Cones:
Primarily responsible for phototopic (daytime) vision.
Function optimally in bright light conditions.
Provide high-acuity color information (mnemonic: "c" for cones, "c" for color).
Are highly concentrated in the fovea.
Rods:
Primarily responsible for scotopic (nighttime) vision.
Function best in low light conditions.
Exhibit high sensitivity to light, allowing for vision in dim environments.
Provide low-acuity vision.
Are significantly involved in the perception of movement in our peripheral vision.
Are located predominantly in the periphery of the retina.
Optic Chiasm and Visual Pathways
The optic nerve from each eye merges at the optic chiasm, an X-shaped structure situated just below the cerebral cortex at the front of the brain.
At the optic chiasm, visual information is split:
Information originating from the right visual field (from both eyes) is directed to the left hemisphere of the brain.
Information originating from the left visual field (from both eyes) is directed to the right hemisphere of the brain.
After the optic chiasm, this processed information continues its journey, first through the thalamus, and then is ultimately sent to the occipital lobe at the back of the brain for final visual processing and perception.
Color Vision Theories
2 prevalent theories explain color vision, each applying to different stages of the visual system:
Trichromatic Theory of Color Vision:
Proposes that all colors we perceive can be produced by combining just 3 primary colors: red, green, and blue.
This theory applies specifically to the retina, where color vision is mediated by 3 distinct types of cones, each maximally sensitive to a different wavelength of light (short, medium, and long).
Opponent-Process Theory:
Suggests that color is coded in opponent pairs: black-white, yellow-blue, and green-red.
According to this theory, some cells in the visual system are excited by one of an opponent color pair (e.g., green) and simultaneously inhibited by the other color in that pair (e.g., red).
This theory applies to cells after the retina, such as in the retinal ganglion cells and neurons in the thalamus.
Conclusion: Research has definitively shown that both the Trichromatic Theory and the Opponent-Process Theory are true and essential for a complete understanding of color vision, with each explaining processes occurring in different parts of the visual system.
Audition (Hearing)
Anatomy of the Auditory System
The human ear is conventionally divided into 3 main divisions:
Outer Ear: Consists of the pinna (the visible part of the ear) and the tympanic membrane (eardrum).
Middle Ear: Contains the 3 smallest bones in the body, collectively known as the ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup).
Inner Ear: Houses critical structures including the snail-shaped cochlea and the basilar membrane within it.
Auditory Pathway Summary:
Sound waves enter the ear canal and cause the tympanic membrane (eardrum) to vibrate.
These vibrations are then transmitted to the ossicles (malleus, incus, stapes), causing them to vibrate.
The stapes vibrate against the oval window, setting the cochlear fluid in motion.
The movement of this fluid stimulates hair cells on the basilar membrane within the cochlea, which in turn generate nerve impulses.
These nerve impulses are carried by the cochlear nerve to the brain for auditory processing.
Pitch Perception
The auditory system differentiates among various pitches through 2 main theories, each explaining different aspects of pitch perception:
Temporal Theory (or Frequency Theory):
Proposes that the frequency of a sound is coded by the activity level (rate of action potentials) of a sensory neuron.
Problem: This theory has limitations because neurons have a maximum firing rate; the frequency of action potentials cannot fully account for the entire range of human hearing (e.g., 20 Hz to 20000 Hz), as a single neuron cannot fire fast enough to match very high frequencies.
Place Theory:
Suggests that different locations (places) along the basilar membrane are maximally sensitive to sounds of different frequencies.
Specifically, the base of the basilar membrane responds best to high frequencies.
The tip (apex) of the basilar membrane responds best to low frequencies.
Laurel vs. Yanny Auditory Illusion:
Neuroscientist Tyler Perrachione, PhD, suggests that the ambiguity of this auditory illusion may largely stem from the "distortion" of the audio clip, which includes a significant amount of high-frequency information.
Individuals who are more adept at hearing high frequencies are more likely to perceive "Yanny."
Other Senses
Taste (Gustation)
Taste is considered a chemical sense, as it involves the chemoreception of dissolved substances.
6 Groupings of Taste: Research has identified several fundamental taste qualities:
Sweet
Salty
Sour
Bitter
Umami (a savory taste, often associated with monosodium glutamate (MSG))
Some emerging research also suggests a distinct taste receptor for the fatty content of food.
Taste buds: These are groupings of specialized taste receptor cells.
Each taste bud features hair-like extensions that protrude into a central pore, which is the site of interaction with taste molecules.
Taste buds have a relatively short life cycle, lasting approximately 10 days to 2 weeks.
Transduction Process for Taste:
Taste molecules from food bind to specific receptors on the hair-like extensions of the taste receptor cells.
This binding triggers a series of chemical changes within the sensory cell.
These chemical changes ultimately result in the generation of neural impulses, which are then transmitted to the brain for interpretation.
Smell (Olfaction)
Smell is also a chemical sense, detecting airborne chemical molecules.
Olfactory receptor cells: These cells are located within a mucous membrane at the very top of the nose.
They possess small hair-like extensions, which serve as the primary sites where odor molecules interact and bind with chemical receptors.
Transduction Process for Smell:
Odor molecules, inhaled through the nose, bind to specific receptors on the olfactory receptor cells.
This binding initiates chemical changes, causing signals to be sent to the olfactory bulb (which is where the olfactory nerves originate).
From the olfactory bulb, sensory information is then transmitted to the limbic system (involved in emotion and memory) and the primary olfactory cortex for processing.
Touch (Somatosensation)
The skin contains numerous types of specialized sensory receptors, each finely attuned to different touch-related stimuli:
Merkel's disks: Respond primarily to light pressure and touch
Meissner's corpuscles: Detect pressure and lower-frequency vibrations
Ruffini corpuscles: Specialized for detecting stretch in the skin
Pacinian corpuscles: Detect transient (short-lasting) pressure and higher-frequency vibrations
Thermoception & Nociception
In addition to the specialized receptors in the skin, there are a number of free nerve endings that perform critical sensory functions.
Thermoception: The sensory ability to perceive temperature (both heat and cold).
Nociception: The sensory signal that indicates potential harm or pain to the body.
This particular type of sensory information (temperature and pain) travels up the spinal cord and directly to various brain regions, including the medulla, thalamus, and eventually the somatosensory cortex located in the parietal lobe.
The Vestibular Sense
The vestibular sense is crucial for maintaining our balance and body posture.
The primary sensory organs responsible for the vestibular system are strategically located adjacent to the cochlea in the inner ear.
These organs are fluid-filled and contain specialized hair cells.
Gestalt Principles of Perception
Gestalt psychology: A field of psychology founded on the central idea that "the whole is different from the sum of its parts." This means the brain actively organizes sensory input into meaningful wholes, rather than merely processing individual components.
The brain creates a perception that is inherently more than simply the sum of the available sensory inputs.
Gestalt psychologists identified predictable ways in which the brain organizes sensory information, translating these into a set of principles.
Key Gestalt Principles Include:
Figure-ground relationship
Proximity
Similarity
Continuity
Closure
Figure-Ground Relationship
This principle explains our tendency to segment our visual world into a specific figure and a surrounding ground.
Figure: The element that is the primary focus of the visual field; it stands out.
Ground: The background against which the figure is perceived.
Our perception can dramatically vary depending on what we interpret as the figure and what we interpret as the ground.