>Denis,dear vince
>
>Thanks for the feedback. I have now purchased and installed an ISR video
>attenuator on our Mac G4 with a Sony GDM F500 monitor. However, I am unclear
>on how to determine if I am getting the increased resolution, or perhaps
>more precisely, if the matlab routines I am using are able to access the
>full resolution provided by the attenuator. Any advice would be greatly
>appreciated.
>
>1. What exactly is the resolution provided by the attenuator? Is it
>12-bits? Are there test routines that will confirm the exact resolution?
>
>2. I have modified the existing ContrastThresholdDemo routines to present
>multiple moving gratings at different locations on the screen and to
>estimate contrast thresholds for the detection of movement at each
>location. However, I have not tinkered with the code that determines
>contrast steps, CLUTs, or QUEST parameters. It appears that these routines
>assume that only 256 luminance levels are available (8 bits). Do I need to
>alter the statements that apparently refer to 256 levels in various matlab
>routines to reflect the higher resolution provided by the attenuator in
>order to get the QUEST routines to modulate stimulus contrast with finer
>resolution? I've tried doing this with the statements I thought were
>relevant, but I typically break the programs when I do so. Generally, if one
>has a contrast threshold estimation program based on the QUEST routines set
>up to expect 8-bit resolution, how does one alter the program to expect a
>resolution of 12-bits (or whatever the resolution of the video attenuator
>actually is)?
>
>Thanks for your advice,
>
>Vince Samar
1. you'll get an effective resolution of around 12 bits. no, we don't
have any MATLAB test routines to show that. i would be good to have such.
2. even though we use the Pelli-Zhang attenuator in nearly all our work,
we're still using the original software that I wrote for it to do that,
which has only minimal documentation (usage help) and is needlessly
restricted to the Mac. We use a free-standing application called
CalibrateLuminance and Attenuator.mex (formerly called "CLUT.mex"). (Both
are available, see below; you seem to have them already.) Attenuator.mex
reads the luminance calibration file produced by CalibrateLuminance and
computes appropriate color lookup table (i.e. clut) values to linearly
map a range of pixel values (entry numbers) to luminances. It takes
advantage of the video attenuator to achieve very fine steps in that
mapping.
The normal way to use the video attenuator is to set the video card to
use 8-bit pixels. In this case the pixel value (0 to 255) is used to
select one of the entries (0 to 255) in the hardward color lookup table
(clut). Each of those entries is a triplet of numbers, which drive the
three DACs. The video attenuator combines those three voltages, with
unequal gains, to produce one voltage, which drives your monitor. Thus,
for a fixed clut, you can produce only 256 luminances. If you are
simultaneously displaying several gratings you should optimize the clut
for the highest contrast grating. Of course, the clut can be changed very
quickly between frames, allowing you to have a completely different clut
on each frame if you like. in practice, i rarely do that, but we do
always compute a new clut for each stimulus presentation (i.e. each new
trial at a new contrast).
we have thought for some time that it would be very nice to replace the C
programs CalibrateLuminance (standalone application) and Attenuator.mex
by plain MATLAB code, but we don't have a schedule for that. the Pelli
and Zhang article contains all the necessary information.
Attenuator.mex will be included in the next release of the PsychToolbox,
in a new category called "PsychBeta" of unsupported software that
nevertheless seems to be of wide interest. I'm also enclosing it in this
message.
good luck
best
denis
p.s.
announcement for Attenuator.mex
http://psychtoolbox.org/mac.html#changes
attenuator web page
http://vision.nyu.edu/Tips/Attenuator.html
CalibrateLuminance (application and sources included in VideoToolbox)
http://vision.nyu.edu/VideoToolbox/Contents.html
http://vision.nyu.edu/VideoToolbox/Download.html
NOTE TO ALL: denis.pelli@... is my preferred email address. Please
use that address. My "verizon.net" address will go away soon, partly
because Verizon forces me to use that as my "From:" address. My current
email program won't allow me to designate a separate "Reply-to:" address.