Visual Half Field Tasks

In these tasks Visual Half field (VHF) asymmetries are employed as a determinant of cerebral dominance.

The subject is asked to repeat aloud the numbers that appear in the centre of the screen. These numbers mark a fixation point. After a random sequence of numbers (between 3 and 5) the target stimulus appears either in the left or right Visual Field (LVF or RVF) for a period of 60 msec. The various stimuli are described below. After the patient's response the assistant (sitting on the right of the subject) has to type the last named "fixation number" on the numeric keyboard. If the last number the subject named was correct, the computer accepts this trial, on the assumption that the subject was indeed fixating the centre of the screen.

A faster reaction time for RVF verbal items gives support for LH dominance. A faster reaction time for LVF nonverbal items gives support for RH non-dominance. However the latter effect is usually much smaller than is the case for the relation verbal-LH.

For proper projection onto the LVF or RVF the subject has to sit about 70 cm from the screen.

The order of left/right projections is balanced in real time. No more than 4 succes- sive projections will take place on the same side. At least 15 projections (which have passed the fixation test) to either side will be presented, with a minimum of 30 trials in each mode.

There are 3 different forms of presentation of the VHF, each in two modes ( words and figures):

  • A word or figure (target item) either on the left or right side of the screen appears. Then 4 words are presented, one of these matching the target item. The testee has to indicate which of the 4 words matches the target item.
  • A word or figure appears in the centre and, simultaneously, either on the left or right side of the screen a second word. The purpose is to indicate whether the two words are same or different.
  • Two words or figures either on the left or right side of the screen appear above each other. The purpose is to indicate whether the two words are same or different.