Visual Perception: Pathways and Processing

• Presents an overview of visual perception, starting from the initial signals to processing in the brain

Visual Pathway

• Signals travel from the visual field to the opposite side of the eye
• Followed by the optic nerve then switch to the contralateral side, then to the Lateral Geniculate Nucleus (LGN) and superior colliculus
• Signals terminate in the primary visual cortex (V1)

Optic Nerve to LGN

• LGN receives different messages from both eyes and organizes them by:
  • Location in the retina
  • Information from rods and cones
• LGN selectively reduces neural firing to area V1
• LGN receives signals from throughout the brain to suppress specific signals before sending them to V1
  • Feedback signals from V1 suppress or amplify signals, differentiating between complex and simple features
  • Integrative V1 communicates bidirectionally with ganglion cells
  • Feedforward signals transmit simple to complex information

Superior Colliculus

• The superior colliculus sends neural impulses to eye muscles to move the fovea
• Partially directs saccades (small, rapid eye movements)
• Amplifies details of light that are of interest in the visual field

Top-Down vs. Bottom-Up Processing

• Top-down processing relies on pre-existing memories, knowledge, expectations, and goals
  • It involves complex perception and basic sensation
  • Forms perception using a big picture approach before focusing on precise details

V1 Area Functions

• Neurons in area V1 respond to bars of light
• Bar orientation cells receive input from dots (ganglion cells) and process them into lines and bars
• V1, specifically the striate cortex in the primary visual cortex (PVC), responds to bars and lines
  • Simple cells detect differences
  • Complex cells detect movement and direction of bars, needed for the "WHERE" pathway
• Cortical magnification occurs
• A larger brain area is dedicated to processing point-of-focus light in V1 compared to LGN, especially for the fovea
• Bar/lined receptive fields are orientation-specific and can differentiate colors
• Neural convergence occurs where 4 LGN neurons communicate with 1 V1 cell
• Mental representation includes perceptual boundaries within objects
• Mental representations are formed

Neural Processing

• Neural firing patterns distinguish between similar vs. different stimuli and enable consciousness of difference
• They are based on the number of objects in points of focus vicinity and foreground vs. background distinctions

Gestalt Grouping Rules

• They are innate mental shortcuts (bottom-up) for grouping and separating light
• Form perception based on similarity, proximity, object continuation, closure, and simplicity

Object Recognition

• V2 responds to full shapes
• The WHAT pathways have less orderly receptive field patterns beyond V2
  • Features (horizontal/vertical edges) are processed into component features (shapes, length)
  • Then, component shapes combine to form a table

V1 vs. V2

• V2 neurons respond to nuanced shapes
• V1 neurons respond to bars of light

Factors in Color Perception

• Illumination available, including spectrum and intensity, such as sunlight
• Light saturation versus light intensity
• Reflective properties of the surface light strikes where distal stimulus is absorbed vs. reflected
  • An orange t-shirt and coffee mug reflect wavelengths perceived as orange, with potentially different reflective distributions
• Photoreceptor response (mostly cones) to light reaching the retina which is comparative based on different photoreceptor types
  • Cone change firing rates are based on proximal stimulus wavelength and amplitude
• Advanced brain regions on the "what pathway" are used in color perception

Additional Color Concepts

• Solve problems with same cone firing, indicating different mental representations
• Metamers are different spectral distributions of light that reach the retina but cause the same cone activity
  • They lead to the same perceived hue, brightness, and saturation
• Proximal stimuli metamers can be perceived as different colors under different illuminants
• Different objects can reflect light to produce the same cone activity but different proximal stimuli
• Perceive same hue, brightness and saturation
• Color naming distributions help separate categories more quickly and easily

Color Processing

• The mind amplifies or reduces categories to enhance edges
• Distal refers to the object in the world, and proximal refers to light hitting photoreceptors
• Color constancy is the ability to perceive color relatively consistently despite changes in illumination conditions
• Without correction, an orange moved from daylight to incandescent light might appear more reddish

Principle of Univariance

• A single cone type has the same response to stimuli
• Can not perceptually differentiate different things, need multiple cones

Cone Response

• A single cone can only tell you how much light it's absorbing, not the wavelength
• Different wavelengths can cause the same response in a single cone if light intensity is adjusted
• Multiple cone types are needed to differentiate between proximal stimuli
• With three cone types (S, M, and L), the brain can compare patterns of activity to different wavelengths
• With three cones and a "good" illuminant, it is still impossible to differentiate all differences between proximal stimuli because there are metamers and the principle of univariance

Top-Down Color Influence

• Memory has a greater impact when placing an example into a category
• Language amplifies and influences recognition
• Influences perceptual enhancement of color differences like edge enhancement for lines
• Expecting to see differences causes you to be perceptually prepared
• Advanced brain regions make complex color processing adjustments to adapt to illuminations
• Bottom-up processing limits the ability to identify and discriminate between colors

Concepts for Cone Response

• Cone types have unique absorption distributions, which determine perceived color based on the relative activity levels
• Non-spectral colors cannot be created by a single wavelength
• Dimensions of hue, brightness, and saturation are the 7 Perceptual Dimensions
  • Intensity corresponds to brightness based on lightwave amplitude
  • Hue is related to the lightwave frequency
  • Saturation represents the purity or richness of the color
    • Reducing saturation increases whiteness and washes out color richness
    • Logically, the distribution of wavelengths across the spectrum becomes more pure

Color Blindness

• The lighting source (illuminant factor) is vital in color perception, supplemented by:
  • Object reflectance distribution
  • Cone absorbance/sensitivity to the proximal stimulus
  • Complex processing adjustments

Limitations in color perception

• Top-down processing is limited when identifying and discriminating between colors
• Yellow can result from different varieties of proximal stimuli metamers
• Metamers are different spectral distributions reaching the retina that cause the same cone activity

How we hear

• Proximal stimuli are air compression-rarefaction oscillations
  • Higher amplitude indicates a louder perception of volume
  • Hearing a higher frequency indicates you hear more tiny bumps

Distal Stimulus

• Distal stimulus is air molecules
• Perceive volume, pitch, and timbre
• Amplify vibration strength to allow vibrations to withstand being suppressed with traveling through liquid
  • Evolutionary, its communication
  • Perceiving high-pitch 360 degrees, can hear thru barriers

Anatomy of the Ear

• The outer ear, including the canal, amplifies sound
• Sound waves move into the eardrum (tympanic membrane)
• The auditory canal channels sound waves to the eardrum
• Intensifies the vibrations as they become air vibrations
• The eardrum transmits vibrations from sound waves to the ossicles (auditory bones)
• The middle ear (ossicles/oval window) amplifies the sound
  • Ossicles vibrate from the tympanic membrane to the cochlea
  • Muscles add tension to ligaments to suppress vibrations for extremely loud sounds
• In the inner ear (cochlea), hair cells are embedded in the basilar membrane and surrounded by fluid
• Vibrations travel up the Scala Vestibuli
• Causes the up or down displacement of hair cells on the basilar membrane which causes tectorial membrane to be displaced
• Bending left or right causes the tectorial membrane to move which then makes the organ of corti move and those tiny hairs in the basilar membrane move
• Hair cell transduction occurs when tip links (on stereocilia) move mechanically
  • It opens ions channels for depolarization

Perceiving Pitch

• Pitch comes from frequency
• The ear uses temporal coding and place coding (place theory) for main mechansims to encode frequency
• Temporal coding is for lower frequencies (up to around 5,000 HZ)
  • Auditory nerve fibers fire bursts of neural impulses that are locked in phase with the wave amplitude peaks of the soundwave
• Place coding is based on higher frequencies
• Different frequencies cause maximal vibration along different locations of the basilar membrane
• High-frequency noises cause the most upwaard undalation at the base of the basilar membrane
• Low frequency noises cause the upward undalation at the apex on the basilar membrane

Sound Frequencies

• High frequency noises cause upward undulation at the base of basilar membrane using place coding measure
• Low frequency noises cause upward undulation at the apex of basilar membrane utilizing temporal coding measuring the noises

Complex Soundwaves

• Most sounds are complex soundwaves with multiple frequencies
• Includes fundamental frequency and additional frequencies which are the harmonics
• Fundamental frequency is the lowest frequency compontent that we hear the sound at
• Harmonics are higher frequency components that are integer mulitples of the fundamental frequencies
• Combines amplitudes of the harmonics that contribute to the timbre of the sound
• Timbre is determined by the overall pattern of the complex sound wave

Phase Locking

• Auditory nerve firing corresponds to wave amplitude peaks
• Neural activity is therefore locked in phase with the wave
• Fibers fire with periods of silent intervals which creates a patter of firing
• Muscles of the inner ear add tension to the ligaments attached to the ossicles, thereby reducing the intensity which ossicles pivot on their axis.