Visual search pattern during the line quadrisection task in normal subjects.

A common test for the assessment of hemispatial neglect is the line bisection test. A recent modification of the task has been reported wherein subjects place a mark at either 25% or 75% of line length ('quadrisection'). In a previous study, we suggested that line quadrisection might entail iteration of two steps involved in standard line bisection: (1) finding the midpoint of the entire line, and then (2) bisecting the half-line segment between the adjudged midpoint and the line end ipsilateral to the target quadrisection point. The present study investigated eye movement patterns during line quadrisection in order to test this hypothesis. Twenty normal volunteers determined the left or right quadrisection point of a horizontal line while their eye movements were recorded using two-dimensional video-oculography. Analysis revealed that, like the results of previous studies on bisection, most normal subjects fixate the central portion of the line segment before reaching the target area during the quadrisection task. Normal subjects' eye movements during quadrisection performance share a number of features with line bisection and provide support for the two-step processing hypothesis.