Effects of time-of-day on neuropsychological testing as measured by dichotic listening.

In a study of 30 reading-disabled and 30 normal-achieving children tested on a dichotic listening task using digits in either morning or afternoon settings neither laterality nor capacity differences were evident between groups in the afternoon. In the morning, the normal-achieving children compared to the disabled readers were more strongly lateralized when attending selectively to the right channel and less lateralized when attending to the left channel, suggesting disabled readers do not respond normally to circadian influences and may suffer from a time-locked dysfunction which reduces their control over attentional resources in the morning. To examine the normal circadian effect further a second study (using 40 normal-achieving, right-handed male children) was conducted using the same dichotic listening stimuli with an order variable included. The increased report in the morning was seen to be linked to a right ear (left hemisphere) priming effect which served to increase the overall report. The effect was evident only for subjects directed to report the right ear first. The results are consistent with a prepotent left hemisphere in the morning which appears to facilitate attending in the morning for subjects directed to attend to their right ear (left hemisphere) first. Potential research and educational implications are discussed.