Title: Psychophysics Methods & Contrast Sensitivity Function

Reading Assignment

  • Lecture Notes Only

  • General Objectives for Lecture 03

  • Define Psychophysics.
  • Know psychophysics methods used to obtain sensory thresholds.
  • See example psychometric function
  • Explain and define Absolute and 50% or 70% Thresholds.
  • Know how to determine sensitivity from thresholds.
  • Understand Relationship between Contrast and Acuity
  • How do shifts in the CSF relate to visual acuity of owl versus human.
  • Understand how acuity and luminance sensitivy trade-off
  • Define contrast and mean luminance and how these parameters are modified.
  • Compare adult and infant CSFs
  • Use Psychophysics to Generate Contrast-Sensitivity Function (CSF).

  • What is Psychophysics?

    "Psychophysics is commonly defined as the quantitative branch of the study of perception, examining the relations between observed stimuli and responses and the reasons for those relations. This is, however, a very narrow view of the influence it has had on much of psychology. Since its inception, psychophysics has been based on the assumption that the human perceptual system is a measureing instrument yielding results (experiences, judgments, responses) that may be systematically analyzed. Because of its long history (over 140 years), its experimental methods, data analyses, and models of underlying perceptual and cognitive processes have reached a high level of refinement. For this reason, many techniques originally developed in psychophysics have been used to unravel problems in learning, memory, attitude measurement, and social psychology. In addition, scaling and measurement theory have adapted these methods and models to analyze decision making in contexts entirely divorced from perception." Psychophysic tests use independent variables (e.g., light, sound, or mechanical pressure) to probe detection, discrimination and behavioral response to dependent variables (e.g., I saw it or button presses responses). (Source of Quote: International Society of Psychophysics)

    Method of Adjustment is a simple, quick way to identify an absolute threshold for sensation. For example, simply adjust the volume control on an ipod or computer until you can just hear it and the absolute threshold is the corresponding volume level. This methods does not allow you to quantify statistical differences in sensory sensitivity or thresholds

    Method of Limits used to find sensory thresholds involves presenting a series of stimuli with ascending or descending levels of difficulty and asking individuals to report "yes" (or "no") for detection. Outside the laboratory setting we talk about our "visual sensitivity" not our "visual threshold". Sensitivity is the inverse of threshold (S=1/T) and high sensitivity means low threshold. Subjects identify when they detect a stimulus change and this stimulus level is called the Absolute Threshold. Joe Wolfe at University of South Whales demonstrations of method of limits for detecting a change in sound loundess (JAVA Demo).

    Method of Constant Stimuli and Contrast Sensitivity Testing Setup. First subjects have to be placed in a setting where they can see, hear, touch or smell the sensory stimulus and the stimulus parameters can be precisely varied. In this example setup the owl sits in front of a computer monitor. The table in front has response boxes for him to press on his left (L) and right (R) side and there is a food cup (F) where he gets his preferred reward (meat) indicating a correct choice. The Two-Alaternative Forced-Choice Procedure (2-AFC)is a variation of the Method of Constant Stimuli that has subjects make a forced choice between two alternatives. 2-AFC method is a statistically robust procedure and avoids ambiguities associated with language. To generate contrast sensitivity curves (below) owls are placed a fixed distance (85 cm) in front of the stimulus and trained to press the right and left box to indicate horizontal or vertical stimulus orientation, respectively (Figure and Text Source: Harmening, 2009) .

  • Gender perception illusion ascribed to image contrast (Scroll down to Third Place Illusion)
  • (Audio Video Interleave, AVI file, Harmening, 2009)

    Method of Constant Stimuli: Visual Acuity Stimuli
  • Square wave grating: square wave modulation of light intensity (a).
  • Sinusoid wave: sinusoid modulation of light intensity (b).
  • Gabor function: gabor function modulation of light intensity (c).
  • Gabor functions allow for subtle modulation of light contrast and spatial frequency.

  • Method of Constant Stimuli: Variation of Stimulus Spatial Frequency
  • visual acuity is measured with spatial frequency test stimuli.
  • Spatial frequencies varied similarly for sinusoid and gabor functions.
  • Spatial frequency: measured in cycles/degree visual angle.
  • Lower spatial frequency Example (a) 1 cycle/degree.
  • Higher spatial frequency Example (b) 2 cycles/degree.
  • (Figure source & optional reading at webvision.com scroll down to correct figures)

    Method of Constant Stimuli: Psychometric Function. The psychometric function above is generated by graphing the behavioral response (percent correct orientation discrimination) as a function of stimulus variable (i.e. dependent, y-axis versus independent, x-axis variables). When the spatial frequency was too high (5 cycles/degree) the owl randomly chose horizontal or vertical orientation hence his percent correct was 50% (black dotted line). When the spatial frequency was low 4 cycles/degree the owl correctly discriminated the correct orientation 70% of the time (blue dotted line). The spatial frequencies with 50% or 70% discrimination are called the 50% and 70% threshold, respectively. You can tell the differences between the visual acuity of subjects by comparing these thresholds (Figure and Text Source: Harmening, 2009).

    Method of Constant Stimuli: Vary Stimulus Contrast. The Contrast is the amplitude of light intensity modulation. Two sinusoid frequency gratings are illustrated with high and low contrast corresponding to luminance of 1.0 and 0.5, respectively.

    Owl Contrast Sensitivity Function (CSF). At one point psychophysicists lowered the contrast of their stimuli and found that it was harder to discriminate the orientation even with low spatial frequency stimuli as evident in the Contrast Sensitivity Function (CSF) generated by varying contrast and spatial frequency (independent variables) and measuring percent correct orientation discrimination (dependent variable, metric of threshold and sensitivity). For all visual animals tested, CSFs are peaked reflecting a limited range of maximal sensitivity and low threshold. In the same way that psychophysicist compare 70% thresholds of the single variable psychometric function (see above) you can compare the peak and drop off of the CSF. The barn owl CSF hits its maximum at around 1 cyc/deg and drops off to chance levels at around 3 cyc/deg (Figure and Text Source: Harmening, 2009).

    Species Variation in Acuity. Visual acuity as measured from the peak of the CSF varies logarithmically across animals. The hooded rat CSF peaks at 0.1 cycles/degrees. In owl monkeys (and many New World monkeys) the peak of the CSF is below (2 c/deg) that of Old world macaque monkeys and humans (10 c/deg). Lizards have relatively low visual acuity but high motion detection abilities

    Human Contrast Sensitivity Function. The human CSF peaks at higher spatial frequencies than the barn owl. Thus humans have a higher visual spatial acuity than barn owls. Adult humans have higher acuity spatial frequency) sensitivity than infants (Non-required reading: Effects of Multiple Sclerosis (MS) and cataracts on CSF).

  • Adult CSF peaks at about 3 cycles/degree.
  • Infant CSF peaks at about 1 cycle/degree.
  • Infant CSF upper frequency limit is <10 cycles/degree whereas this limit in adults closer to 80 cycles/deg.
  • Retinal rods develop before retinal cones and could contribute to the reduced visual acuity observed in infants; however, underdeveloped central neural sensory pathways also likely contribute.

  • Psychophysical Test of Photoreceptor Function: Central vs Peripheral Dark Adaptation. Dark adaptation was one of the first sensory phenomenon studied in the field of psychophysics. If you step outside at night you'll notice it takes time for your eyes to adjust to the dark (low luminance levels). Likewise when there is a bright flash of light initially it is hard to see and it takes time for your eyes to adjust. Psychophysicists used this phenomenon to study photoreceptor adaptation to light and dark and called it "dark adaptation". They exposed subjects to a large flash of light then tests their thresholds (luminance level) for detection of a small (2 degree diameter) spot of light positioned 5 degrees visual angle away from fixation (see Setup). Initially thresholds were high as shown in the Dark Adaptation Curve above at time 0. With time thresholds dropped. Surprisingly they found thresholds dropped in two stages. There was a fast inital adaptation that leveled off at 10 minutes and a second long adaptation that leveled off by about 30 minutes. They hypothesized that this was due to different adaptation capabilities of cone versus rod photoreceptors. Knowing what you know about cone and rod photoreceptor distribution on the retina, what experiments could you do to test this hypothesis? Click on the figure above to see the psychophysics curves tested at 0, 5, 10 and 25 degrees visual angle to support this theory. Note that moving there is only one slope to the dark adaptation curve if the light is positioned at 0 deg (activating fovea) or at 25 deg (activating primarily rods). (Supplemental Extra Reading at Webvision) and (Figure Source, Hecht et al., J. Gen. Physiol., 1935).

    We now know that dark adaptation of photoreceptors requires rapid enzymatic regeneration of 11 cis retinal. If people lack this ability due to genetic mutation they can not fully adapt to the dark and thresholds will remain high when tested for dark adaptation.

  • (Extra Reading tactile acuity in blind people)
  • (Source and Extra Reading CSF of albino rat)
  • (Figure Source: Keller J Neurosci Methods 2000)
  • (Advanced Reading: Arrese Numbat CSF Brain, Behav Evolution 2000)
  • (Advanced Reading: Effects of gestational length CSF, J Scienti Res Bio Vis, Dobkins, 2009)
  • (video: pigeon playing Tap Tap Revenge on iphone)
  • Topic cZar: Heather Read