The Visual Psychophysics Engine is a versatile software tool for ViSaGe that was developed by Dr Neil Parry (Clinical Vision Scientist, The University of Manchester). It is supplied with a suite of pre programmed experimental paradigms which allow you to measure luminance and chromatic Contrast Sensitivity Functions, increment thresholds and De Lange curves. You can also create your own bespoke protocols and present standard visual stimuli like sinewave gratings and Gabor patches using the easy-to-use, menu-driven interface.
Guide Price: £1500
Ready to use protocols for standard experiments
Four pre-programmed experiments are provided with VPE. You can use these tests as they stand, or as a basis to configure your own experimental protocols.
This experiment is in four parts.
a) Increment threshold - method of adjustment.
A 1 degree circle is modulated at 1Hz between a 2 cd.m-2 background and a luminance increment of up to 100cd.m-2. A white fixation cross is displayed. The dependent variable is log luminance increment (LogDeltaL) and there are 5 repeats.
b) Finding the range for the psychometric curve
Once a mean value had been established for the first test spot, the next experiment is selected.
Before running the psychometric curve experiment, the user needs to find the best range of luminances to use. In this preliminary experiment, there are 20 levels of luminance (LogDeltaL) from -2 to 0. This is a yes-no task. Each luminance is presented once, in random order, and the observer simply states whether it is seen or not. At the end of the experiment the results will show a block of seen and a block of unseen luminances. The final range in the psychometric curve experiment should straddle the border.
c) Psychometric curve
In this experiment there are 9 levels of the independent variable, which is spot luminance (LogDeltaL). They depend on the results of experiment 1b. This is a "yes-no‟ experiment and 10 reps are used. The experimenter manually selects the test (e.g. rep 1, middle contrast), and the graph is displayed so that the POS function can be seen developing as the subject responds. Once the whole table has been filled, any repeats can be run and then the data saved.
d) Weber’s law
In this experiment 3 thresholds are set for each of 7 background intensities, from 1 to 64 cd.m-2. The dependent variable is spot luminance (LogDeltaL) as in experiment 1a.
a) De Lange Central
This measures temporal sensitivity for a spot sinusoidally modulated above and below mean luminance, on a 100cd.m-2>/sup> background. A profiled grating of very low spatial frequency is used to draw the spot. The plateau is 1deg wide and the skirt is 0.5deg wide. The independent variable is temporal frequency. The dependent variable is contrast (dB). 3 repeats are run for each temporal frequency.
b) 10Hz sensitivity – effect of eccentricity
Here the modulation frequency is fixed at 10Hz and the independent variable is eccentricity, from -10 to +10 degrees. Each eccentricity is tested 3 times while the subject adjusts contrast in dB. A lower background luminance (5cd.m-2 ) is used to encourage the rods to exert an effect.
This presents a 16Hz reversing red-green grating in RGBLC mode. (Frequency can altered if required.) Red and green luminances is adjusted by the subject to determine the Red:Green ratio (RGR). 3 reps are provided.
b) chromatic CSF
The mean RGR from experiment 3a is used as the control variable for the luminance ratio. The dependent variable is now contrast in dB and the independent is fx. After the CSF has been measured at isoluminance it can be repeated at different luminance ratios.
a) sine-wave, 114cm
7 spatial frequencies from 0.3 to 8c.deg-1 are tested whilst contrast is adjusted in dB. There are 3 reps. Spatial envelope is sinusoidal.
b) square-wave, 114cm
4a is repeated with a square-wave temporal profile.
c) sine-wave, 342cm
6 further spatial frequencies between 8 and 24 c.deg-1 are tested with a sinusoidal profile.
d) square-wave, 342cm.
4c is repeated with a square-wave temporal profile.