Transcript Visual Search Deficits in Williams Buren Syndrome

Visual Search Deficits in
Williams Buren Syndrome
Montfoort, I., Frens, M.A., LagersVan Haselen, G.C., & van der
Geest, J.N.
Williams Syndrome

Genetic disorder
Characteristics of WS
 Impaired global visual processing (Bihrle, Bellugi,
Delis & Marks, 1989)
 Deficits in visuospatial memory (Vicari, Bellucci &
Carlesimo, 2005)
 Motor problems (Van der Geest, et al., 2005, Withers,
1996)
Visual Search
What is visual search?
 Attempt to find a ‘target’ in the visual scene
e.g. Where is the orange
square?
Serial search is likely to use visuospatial memory
and working memory
Visual search and eye movements
Definition of serial search
 “Using saccadic eye movements to look for an item of
interest. Searching for one item after another until the
target is found.”
Visual Search and eye movements
 Saccadic eye movements allow the observer to look
for interesting items in the display
 Foveal fixation = information gathering
 Scan path = the path used to search for items
Predictions

Impairments in visual processing and working
memory will lead to less efficient visual search
in Williams participants than in normal controls
Method
Participants
Group
Sample Size
Age Range
WS
38
8-41
Control
21
18-44
Low IQ*
5
16-19
IQ = 66-85
Measured visual acuity (Landolt-C test), no significant
differences were found in the visual acuity of the three
groups (p=0.2)
Apparatus




Subjects had a chin rest to restrain head movements
Monocular vision with dominant eye
Calibrated eye movements
Eye-Link 2.04
 Records monocular gaze positions using infrared
video-oculography
Design

4-11 white items (squares, circles, triangles)
 Target white with a black dot
 Black dot very small so had to foveate on the target
 10 search displays
 Same order for each participant

Red pop-out stimulus
 To attract attention
 To avoid participants looking straight at the target
Example Trial
Pop-out Distracter
KEY
Start Point,
visible
before start
of trial
Target
Measures
Saccades
 Eye movements classed as saccades if over 30º/s
Fixations
 >80m/s (to exclude fixations prior to correction
saccades)
 Target fixation if within 3º of target
 If more than one item within 3º, closest item was
classed as the item fixated on
Measures

Search time
 Fixation duration
 Number of fixations
 Type of fixations (mis- and re-fixations)
 Don’t analyse the QL group
 Analysed young (<18 years) WS cf. to older (>18
years) WS, no differences, so collapsed across the
group
Visual Search Displays
Search Efficiency
Location of Target
 WS found target within 5 seconds on 67% of trials,
control 99%
 WS were slower than control
Median search time
 WS 3.6 (+/- 0.3sec)
 TD 1.7 (+/- 0.1 sec)
Increase in the number of display elements led to
increase in RT
 WS 334 m/s per element
 Control 157 m/s per element
Fixation Duration
Fixation Duration
 37m/s longer in WS
First fixation
 Longer than subsequent fixations
 WS 363 (+/-6) m/s
 Control 337 (+/-5) m/s
 Marginal difference between groups, p=.06
First Saccade
 58% trials (both groups) directed at the red dot
Number of Fixations
Locate target
 WS group needed an average of 2.2 more fixations than
the control group to find the target
 Had more refixations and misfixations than control group
Increase in the number of items in the display
 WS = 1.4 fixations/element
 Control = 0.7 fixations/element
Random search?
 WS were more similar to a search that had no memory
 Even after removal of fixations, search was more similar
to a search without memory
Refixations and Misfixations
WS Group
 The number of misfixations increased for trials
containing more items
 1 in 8 refixations
 1 in 4 misfixations
Control Group
 Few misfixations in the control group
But…
 Subtract total number of refixations and misfixations
from total fixations then WS group do not need more
fixations than control to complete the task
Discussion
Could this be due ocular motor problems?
 Some degree of saccadic dysmetria is found in WS,
including a higher number of correction saccades (e.g.
Van der Geest et al., 2004)
 However…
 Inaccuracy in eye movements for the WS group
were roughly 2.5º whilst misfixations were 3º
 This does not explain the unsystematic search
pattern in the WS group
Discussion
Could this be due to impaired visuospatial processing?
 The WS group had longer fixations, so perhaps this is
linked to local or global processing, but the current
study does not separate the effect of the two
processes
Could it be memory?
 Hooge et al. (1999) propose the first fixation can be
used to plan search path
 WS and TD group had longer first fixations
 First fixation longer than the mean duration of
subsequent fixations for both groups
Search and Memory
Could it be memory…?
 The difference in the first fixation to subsequent
fixations was smaller in the WS group than the control
group, so perhaps a problem with memory for the WS
group?

WS group: Search was poorer than the predicted
random pattern of search (which proposes each
fixation is at a separate point and there are no
misfixations)

For the WS group, this suggests there may be a
problem in memory for locations
Could it be IQ?

Possibly not, as low-IQ individuals looked similar to
normal controls
 But, results of the low IQ group are not explicitly
discussed in the article
Thank you!