Sensation and Perception, Exam 1

Physical stimulus energy 

• Your sensory receptors pick it up

  • Eye, retina 

  • Ear

  • Touch 

    • Convert physical energy to impulses to neurons

      • Leads to awareness 

Why bother studying perception?

• Understand and reduce instances of dangerous perceptions 

  • Your mirror at the side of your car is there to help compensate for your limited perception

  • Blind spot warning light

• Make it easier for us to perceive critical information

• Simulate the world

  • Create perception (i.e. VR)

  • Create machines to do perception (i.e. Waymo)

  • Machines used to not be good at angle invariance but now they are

    • Also not perceptual completion

    • They are decent at it now

  • Teslas are not good at recognizing vehicles with flashing lights?

    • Tesla has low rate of accidents and are a lot safe 

  • Alexa and Siri can also sense sound and perception

• Better diagnosis and treatment of disorders related to perception

Single-cell imaging 

• Look at an individual neuron 

• What are they responding to? (Increasing/decreasing activity)

David Hubel and Torsten Wiesel experiments

Hubel and Wiesel discovered that individual neurons in the primary visual cortex (V1) do not just respond to random light. Instead, they act as feature detectors.

• Orientation Selectivity: A specific neuron fires rapidly only when a bar of light is oriented at a precise angle (e.g., horizontal, vertical, or diagonal).

(Hubel and Wiesel inserted tiny glass electrodes into the primary visual cortex of anesthetized cats. The visual cortex is the part of the brain that processes what we see. [1]

• The Accidental Discovery: While projecting glass slides onto a screen, a dark line appeared when the edge of a slide slipped into the projector. A cluster of brain cells suddenly fired electrical signals. [1, 2]

• Finding Simple and Complex Cells: They realized that individual neurons do not just respond to dots of light. Instead, they respond to lines, bars, or edges at specific angles (orientations).)

Positron Emmision Tomography (PET)

• Individual would be injected with a substance that had some radioactive markers. The radioactive marker would travel to the brain and they looked at the brain and scanned where it was in different tasks 

• limitations of PET scans

  • low spatial resolutions, low temporal resolutions, radioactivtiy

first human Blood Oxygen Level-Dependent (BOLD) functional Magnetic Resonance Imaging (fMRI) experiments

• The Baseline (Off Condition): The subject looks at a blank, dark screen. [1, 2]

• The Stimulus (On Condition): A high-contrast, black-and-white checkerboard pattern flickers or reverses its contrast rapidly (typically at 8 Hz). [1, 2]

• The Result: The flickering pattern demands heavy sensory processing, causing localized neurons in the primary visual cortex (V1) within the occipital lobe to fire intensely. [1]

Functional magnetic resonance imaging 

• Measures oxygen level in blood in the brain.

• More activity leads to more oxygen-rich (as opposed to oxygen-depleted) blood in the area

• Better spatial resolution: 3-dimensional pixel = “VOXEL” = about 100,000 neurons

• Percent of brain activity from an MRI on a graph you can see the activity in the brain changes almost immediately, good temporal resolution 

fMRI graph

• Wherever this part of the brain is involved in seeing things but doesn’t care about whether that is moving or not

Diffuse Optical Tomography (DOT)

• Measures oxygen level in blood in brain by sending changes in the light reflected by the brain, and different oxygenated blood levels absorb different wavelengths 

  • Temporal resolution as good as fMRI, but spatial resolution a little poorer. Portable equipment.

Psychophysics

• The study of the relationship between the physical energy in a stimulus and the perceptual experience that it produces

• To learn about what’s going on in the perceptual system without directly observing brain activity.

  • With airplanes, can you change some aspect of the jet engine sound to make it less annoying to people? 

Gustav Fechner 

• The father of psychophysics 

  • Is it possible to systematically answer how we perceive things? 

Absolute threshold 

• The minimum intensity of stimulation required to produce a sensation (the lowest detectable intensity of a stimulus).

  • Lowest concentration of sugar to be tasted? Lowest light to be seen?

Difference threshold

The minimum difference in intensity between two stimuli that can be detected (smallest detectable amount of change in a stimulus). This is also known as the JUST NOTICEABLE DIFFERENCE (JND).

Method of Limits

• 1. Present an orderly series of stimulus intensities.

• 2. At each presentation: Did the subject detect the stimulus?

• 3. Threshold = midpoint between intensities where the subject’s response changed.

  • In the image below, people failed at 48, so the threshold is between 49 to 48, so 48.5

    • Potential problems with this method 

      • Problem: The subject tends to keep reporting that they detected the stimulus. (Error of habituation/perseveration)

      • Solution: Present both descending and ascending series (little louder over time from extremely quiet) and (quieter to louder)

      • Problem: The subject produces a consistent pattern of responses. They remember how many stimuli they detected (or didn’t detect) at the beginning of each type of series. (Error of anticipation)

      • Solution: Stagger the starting point of successive series:

Method of Adjustment 

• This method is the SAME as the method of limits except that the subject (or the experimenter) CONTINUOUSLY ADJUSTS the stimulus

  • Similar to a descending series “turn the volume down until you can no longer hear it (or opposite)”

Method of Constant Stimuli

• 1. Use a fixed set of stimulus intensities selected in advance (there should be intensities that are clearly above and below threshold).

• 2. Present stimulus intensities in a RANDOM ORDER (NOT an orderly series).

• 3. Present each intensity several times, each time asking the subject for a “yes/no” judgment—did they detect the stimulus?

  • Plot it for % of trials you detect it vs physical stimulus intensity

    • Theoretically, this graph would show a range of intensities the subject cannot detect, followed by a spontaneous increase. This is not how it actually is. Real results have a gradual transition. This is due to random noise and variation in all processes involving humans.

  • Absolute threshold is the percentage of the trials you detect 50% of the time

Staircase Method 

• 1. Begin with an intensity far below (or above) the threshold.

• 2. Increase the intensity one step if not detected; decrease one step if detected.

• 3. Stop after a predetermined number of response reversals.

• 4. Threshold = average intensity of the reversals.

• 
  

  • More efficient than constant stimuli because it adapts to your response

Method of Constant Stimuli for Difference Threshold

• 1. Use pre-selected set of stimulus intensities presented in a random order.

• 2. On each trial present TWO stimuli: One is the standard, used as a reference stimulus. The other is the comparison: intensity varies from trial to trial.

• 3. Possible responses: “The comparison is bigger/brighter/heavier than the standard” or “The comparison is smaller/dimmer/lighter than the standard”.

Point of Subjective Equality

• the specific stimulus value where a person perceives two different inputs as identical

Point of Objective Equality (POE)

is the physical point where the two stimuli are actually identical

• THE DIFFERENCE THRESHOLD IS THE SLOPE OF THOSE TWO POINTS (50% of the time and 25% of the time, is 2 different thresholds. The actual difference threshold is 1 half of that interval)

  • Start at the left, find the 75% line, then the 25%, and the difference half of that is the DT

  • In this image the difference threshold is 1 mm

Size-Weight Illusion 

• The larger of two equal-weight objects feels lighter. (This is an example of how context can influence perception)

• Perception depends on the context in which we percieve things