Visual scanning and cognitive performance in prediagnostic and early-stage Huntington's disease

Abstract

The objective of this study was to evaluate visual scanning strategies in carriers of the Huntington disease (HD) gene expansion and to test whether there is an association between measures of visual scanning and cognitive performance. The study sample included control (NC, n = 23), prediagnostic (PDHD, n = 21), and subjects recently diagnosed with HD (HD, n = 19). All participants completed a uniform clinical evaluation that included examination by neurologist and molecular testing. Eye movements were recorded during completion of the Digit Symbol Subscale (DS) test. Quantitative measures of the subject's visual scanning were evaluated using joint analysis of eye movements and performance on the DS test. All participants employed a simple visual scanning strategy when completing the DS test. There was a significant group effect and a linear trend of decreasing frequency and regularity of visual scanning from NC to PDHD to HD. The performance of all groups improved slightly and in a parallel fashion across the duration of the DS test. There was a strong correlation between visual scanning measures and the DS cognitive scores. While all individuals employed a similar visual scanning strategy, the visual scanning measures grew progressively worse from NC to PDHD to HD. The deficits in visual scanning accounted, at least in part, for the decrease in the DS score.

FIG. 1

Eye movement of a NC subject. A: Eye path on DS page illustration; B: Vertical coordinates of the eye position as a function of time during a 15-second interval (from 25th to 40th seconds of DS testing). The participant was writing symbols at the beginning of the second row. The participant's eye moved back and forth from the writing area to the key area. I, II, III, IV shows a typical periodic maneuver that included: (I) shift to the key area, (II) fixation in key area, (III) shift back to the second row, and (IV) fixation on the second row of digits. During step IV, the participant wrote a symbol in the writing area. The maneuver I–IV was repeated 11 times in 15 seconds time interval and 11 symbols were written in the second row of the DS page.

FIG. 2

Eye movement of a PDHD subject. A: Eye path on DS page illustration; B: Vertical coordinates of the eye position as a function of time during a 15-second interval. The participant was writing symbols at the beginning of the second row.

FIG. 3

Power spectrum of the Fourier transform of vertical eye coordinates (32-second interval at the beginning of DS testing, from 10th to 42nd second). For all participants, more then 95% of power spectrum was allocated between 0 and 2 Hz frequencies. A: NC subject; B: PDHD subject. ◆ shows mean frequency.

FIG. 4

DS scores and number of maneuvers.