Exam 1 Flashcards
Variables required to determine the visual angle a stimulus subtends on the retina?
1) Dimensions of the object
2) Distance between observer and object
equation used to calculate the visual angle an object subtends on the retina
tan(theta)=opposite/adjacent
where opposite is the object size and adjacent is the distance from the observer
Subject is instructed to look at fixation cross. Test flash (of diameter=15cm) is presented 60 cm to the right of the fixation cross. Assume subject is 2 m from the stimulus display. Test flash is projected to retinal region ______ degrees from the fovea.
Ɵ= tan-1 (60/200) = 16.70°
On a monitor, 2 stimuli are 25 cm apart. The subject is 1 m from the monitor. How far apart are the stimuli?
Ɵ= tan-1(25/100) =14.04°
What is a sinewave grating pattern
•A grating pattern is a series of light and dark bars.
-a pattern in which bars have fuzzy edges. Their luminance changes very gradually from dark to light, then from light back to dark.
What is a square-wave grating pattern?
a pattern in which bars have crisp sharp edges. Their luminance changes abruptly from dark to light and from light to dark.
What is luminance profile?
•Luminance profile is a graph showing how luminance/ intensity changes over space.
◦Y-axis= luminance and color appearance
◦X-axis= position
Luminance profile for the square-wave pattern
looks like blocks
if in the dark bar the luminance is lower on the graph
If in the light bar the luminance is higher on the graph
if right on the border the line is vertical
Luminance profile for the sinewave pattern
the graph is a sine wave
the middle of the bars are the darkest/lightest points
What are the four characteristics that are needed to fully describe/define a sinewave/squarewave pattern?
a. Spatial frequency
b. Contrast
c. Orientation
d. Phase
Spatial frequency
a. width of bars (bars are of equal width in a grating pattern)
i. High when narrow bars; Low when wide bars
ii. Recorded as cycles per unit area (i.e. cpd)
Contrast
a. difference in lightness between the dark and light bars.
i. High when the difference in brightness is great
ii. Recorded as percentage calculated using Michelson Contrast Equation
orientation
a. tilt of the grating pattern
i. Horizontal, vertical or oblique
Phase
a. position relative to some landmark; alignment of the grating pattern bars
i. In phase= light bars line up with light bars and dark bars with dark bars
ii. Out of phase= bars do not line up nicely
What is a cycle?
a pair of bars
one dark and one light
Bar width = 2 cm. Observer’s distance = 40 cm
Calculate the cycles per degree of visual angle a grating pattern subtends
Ɵ= tan -1 (2/40)=2.86° per bar and 5.71° per cycle so 0.17 cpd
•Remember that if single bar subtends 15’ than 1 cycle subtends 30’and so 2 cycles per degree because 60’ per degree
Michelson contrast equation
Contrast= (Luminance max - Luminance min)/(Luminance max + Luminance min)
Lmax=50cd/m2 and Lmin=20 cd/m2
calculate Michelson contrast
Contrast = (50-20)/(50+20)=30/70= 42.86
Psychophysics
scientific study of the relationship between physical stimuli and their resulting sensations.
Detection task
measurement of minimum amount of energy needed to just detect the presence of a stimulus. Absolute threshold= intensity which produced detection of the stimulus 50% of the time.
Discrimination task
measurement of minimum amount of a stimulus that must be added or taken away from one stimulus so that it is just noticeably different from a second stimulus. Difference threshold is the difference between two stimuli which is detected 50 % of the time.
Recognition Task
measurement of minimum amount of stimulus needed to be present to correctly name or categorize the visual stimulus.
Example of detection threshold
AKA absolute threshold
- Is stimulus detectable?
- at what luminance can target just be detected?
Example of discrimination threshold
AKA difference threshold
Is one suprathreshold stimulus different from another?
-how bright must light be to be just noticeably different from another?
-how far apart must 2 point sources be in order to appear separate?
-when are 2 lines just noticeably different in length?
-select visual match for target from group of objects.
-which object is different?