Visual Search Deficits in Williams Buren Syndrome

Montfoort, I., Frens, M.A., Lagers-Van 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

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)

Group Sample Size Age Range

WS 38 8-41

Control 21 18-44

Low IQ* 5 16-19

Participants

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

Target

Pop-out Distracter

Start Point, visible before start of trial

KEY

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 toincrease 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!