Visual Search Deficits in Williams Buren Syndrome

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 Participants 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!

Visual Search Deficits in Williams Buren Syndrome