Understanding Sensory Perception

Multiple Levels of Analysis

• Sensory systems process stimuli, leading to behavioral outputs
• Perception is understood thru multiple levels of analysis
• Complex systems necessitate multiple levels of analysis due to their many levels
• Different approaches are used for different scales of analysis
• Physiological approaches involve recording from individual neurons to determine their selectivity
• Neuroimaging/neuropsychology studies pinpoint processing locations in the brain and determine brain modularity using fMRI
• Psychophysics examines subjective experiences and behavioral responses which rely on the visual system
• Determines sensitivity and threshold through tasks like reading and color judgment
• Marr (1982) states studying perception requires understanding different scales of analysis
• Studying bird flight by focusing only on feathers is incomplete

Physiological Approaches

• Neurons use action potentials to communicate, involving brief bursts of electrical activity transmitted along axons, triggering chemical signals
• Microelectrodes are small, insulated electrodes (tungsten wire) that detect electrical activity of neurons
• Microelectrodes placed near neurons detect electrical changes during neuron firing, signals are amplified and visualized on screens
• Multiple neurons can be detected by using unique "signature" signals from each neuron
• Invasive single cell recording involves surgically inserting electrodes through the skull to study areas like the visual cortex
• Non-invasive single cell recording allows easier eyeball access through the retina
• Studies are performed on anaesthetized or non-anaesthetized animals by using surgery implants or tubes for repeated electrode insertion
• Single-cell data collected counts action potentials as spikes per second
• Hubel & Weisel (1968) determined orientation selectivity in a V1 simple cell
• Receptive fields describe neurons with specific stimuli within a restricted window

Neuroimaging

• Neuroimaging determines brain areas involved in visual perception and examines parts involved in different aspects of visual scenes

Electroencephalography

• Microelectrodes on the scalp measure brain waves produced by synchronized activity of large neuron populations
• A polygraph characterizes the brain waves by oscillations and categorizes them into frequency bands
• Useful for infants because no verbal responses are needed
• Rossion & Jacques (2008) identified N170, a large negative potential elicited 170 ms after stimulus presentation, stronger for faces
• Visually evoked potentials (VEPs) are recorded in response to visual stimuli and reflect summed electrical activity of numerous neurons
• Requires averaging due to small signals compared to background noise and ability to isolate visual signals as multiple presentations of a stimulus are recorded

Magnetoencephalography

• Superconducting sensors are used to measure magnetic fields from neurons

Magnetic Resonance Imaging

• Strong magnetic fields emit radio waves to exploit varying magnetic properties of grey and white matter
• Provides static structural image of the brain

Functional Magnetic Resonance Imaging

• Brain activity is mapped based on cerebral blood flow to specific regions involving hemoglobin magnetic properties
• Relies on hemodynamic response
• Firing uses energy which increases blood flow to restore brain region metabolites
• Paramagnetic deoxyhaemoglobin and diamagnetic oxyhaemoglobin are factors
• Neurons require more oxygen which leads to greater local blood flow
• Contrasts are performed between conditions, like stimulus present vs absent
• Changes in blood-oxygenated level dependant (BOLD) signal are measured relative to baseline
• Spatial resolution is more precise than temporal resolution
• Regions of interest can be found and BOLD signals can be compared for different stimuli
• Kanwisher, McDermott & Chun (1997) demonstrated the Fusiform Face Area (FFA) compared BOLD signal and demonstrated brain modularity.

Positron Emission Tomography

• A radioactive tracer is injected into the bloodstream, which accumulates in brain areas with high metabolic activity
• A scanner detects gamma rays emitted during decay

Neuropsychological Approach

• Focuses on what happens when specific brain regions are damaged
• Lesions can be experimentally induced in animals
• Relies on naturally occurring lesions in humans caused by stroke or trauma, which are rarely localized
• Zihl, von Cramon & Mai (1983) found Patient LM had "motion blindness" after stroke

Psychophysics

• Gustav Fechner (1860) published the elements of psychophysics
• Investigates the relationship between physical stimuli and psychological quantities
• A lawful relationship exists between the objective physical world and subjective perceptions
• Behavior in response to a stimulus must be measured precisely due to the inability to directly measure the mind
• The linking hypothesis connects subjective phenomena to objective measurements
• Neural detectors perform assumed mental operations that are connected to fMRI through blood flow
• Underlying operations are inferred from properties of stimuli and a person's response

Psychophysical Experiements

• Understand the relationship between brain activity and visual perception
• Infer properties of the visual system from behavioral responses
• Characterized by the task, the outcome, and the method
• Matching measures appearance through the perceived equivalence of two stimuli
• Color Matching measures metamers by matching a yellow patch with superimposed red and green light

Thresholds

• Ebbinghaus illusion measures size with a test circle to measure contextual modulation
• Perceptual threshold (ie finest detail) is the lowest stimulus quantity that can be reliably seen
• Thresholds measure sensitivity, where low threshold equals high sensitivity
• Absolute/detection thresholds defines the minimum intensity at which a stimulus is detectable
• Example is the lowest brightness value that can be seen
• Difference/discrimination thresholds specify the smallest difference in intensity that is detectable
• Hecht, Haig & Chase (1937) determined the lowest brightness/luminance visible
• Detection thresholds were measured after durations in the dark to measure thresholds as a function of time
• Sensitivity increased over time and threshold decreased in two phases
  • Rods and cones adapt to the dark at different rates/different temporal properties

Measurement of Responses

• Directly asking someone if they can see something
• Thresholds measure sensitivity changes
• Detection thresholds are confounded by subjective criterion
• Subjective criterion is defined as an internal rule that translating sensory experiences into behavior
• Those will a liberal criterion eagerly say 'yes' whereas conservative criterion are reluctant
• Forced-choice is more reliable because the subject has to choose
• Prevents guessing based on chance
• The observer might be asked, "Was the brighter patch on the left or right?" using two-alternative forced choice
• Plots as a psychometric function from 50%-100% and the midpoint is taken at 75%
• Can be used to test perception in animals and pre-verbal children
• Hard to create "objective" scoring

Methods

• Fechner formulated three basic experimental approaches
• The method of limits describes the experimenter who varies stimulus magnitude in small increments where subjects are asked if the new stimulus is more or less than the reference stimulus
• Estimated based on the crossover points
• The method of adjustment describes the subject being allowed to adjust stimulus magnitude within a short amount of time
• The subject stops when they report they have equal magnitude
• Errors of anticipation involve participants expecting anticipation and errors of habituation where responses are long
• To minimize errors with adjustment, approach threshold from both directions
• The method of constant stimuli involves random presentation to suppress adaptions
• More difficult due to stimulus intensities needing to be randomized
• Performance varies from 0% to 100%
• Pilot testing is typical

Adaptive Procedures

• Adaptive procedures modernly employ an algorithm to select stimulus intensity based on responses
• Experimental procedures adapt the physical characteristics of stimuli of trails are determined by stimuli and sequences
• This relies on the knowledge of perpection
• Used when a single intensity value is wanted