The Effects of Duration

1
The Effects of Duration Masking on Orientation
Discrimination
Alana Rojewski Nestor Matthews
Department of Psychology, Denison University,
Granville OH 43023 USA
Purpose To explore factors affecting the time
course of humans ability to identify subtle
angular differences.
Discussion
Results
Data from the two groups (blue red) overlapped
during threshold estimation, showing initial
similarity. During the main experiment, however,
performance was significantly worse in the masked
group.

• Our data indicate that OD is excellent and flat
  between 16 and 80 msec durations when there is no
  mask, i.e., when oriented stimulation is allowed
  to persist in the CNS.
• However, OD increases significantly with stimulus
  duration when masks prohibit the persistence of
  oriented stimulation in the CNS. Recent
  recordings from primate brain region V1 indicate
  that masking attenuates the response of
  orientation tuned neurons5,6. As a speculation
  then, our masks may have suppresedV1 activity.
• Comparable trends were observed here for cardinal
  and oblique orientations, probably because we
  used gratings that were smaller and more
  peripheral than those1,2,3,4 that had previously
  generated an oblique effect in OD. Our future
  directions include using larger and less
  peripheral stimuli to explore whether the oblique
  effect in OD grows with time, or is a static
  characteristic of the visual system.

Background Orientation discrimination (OD) is
known to depend on size1, retinal position2, and
whether the stimuli are oriented around cardinal
(horizontal vertical) or oblique (diagonal)
axes3,4. While such spatial aspects of OD are
well documented, little is known about the
temporal aspects of OD.
Threshold Estimation
Oblique
Cardinal
Method During threshold-estimation, two gratings
were presented simultaneously for 80 msec, and
were preceded and followed by bulls-eye masks
(see below). Participants (n16) identified the
grating on the right as being oriented
clockwise or anti-clockwise to the grating on
the left. We computed each participants 84
discrimination threshold and presented that
angular difference to the participant during the
main experiment, wherein half the participants
viewed masked gratings and half viewed unmasked
gratings. Grating durations in the main
experiment varied from 16 to 80 msec in 8 msec
steps, randomly.
The Bottom Line
Performance in the no-mask group significantly
exceeded that of the mask group, even though the
grating durations were identical in the two
groups. This differences suggests that neural
persistence significantly enhances OD.
Main Experiment
Cardinal
Oblique
Proficiency (d'/RT)
Precision (d')
Reaction Time
Acknowledgment
This project was supported by a Hughes Early
Research Experience Scholarship to Alana
Rojewski.
Time
Time
References
1. Vision Res. (1984). 24(2)121-8. PMID
6710874 2. Invest Ophthalmol Vis Sci. (1986).
27(2)237-45. PMID 3943947 3. Vision Res.
(1985). 25(11)1679-87. PMID 3832592 4.
Percept Psychophys. (1997). 59(1), 60-72. PMID
9038408 5. Nat Neurosci. (1998). 1(2)144-9.
PMID 10195130 6. J Neurophysiol. (2000).
83(2)941-54. PMID 10669506 This poster can be
viewed and downloaded at http//denison.edu/matth
ewsn/hughesorientation.html
Clockwise Or Anti-Clockwise?
Clockwise Or Anti-Clockwise?