Occlusion and the interpretation of visual motion: perceptual and neuronal effects of context.

Visual motion can be represented in terms of the dynamic visual features in the retinal image or in terms of the moving surfaces in the environment that give rise to these features. For natural images, the two types of representation are necessarily quite different because many moving features are only spuriously related to the motion of surfaces in the visual scene. Such "extrinsic" features arise at occlusion boundaries and may be detected by virtue of the depth-ordering cues that exist at those boundaries. Although a number of studies have provided evidence of the impact of depth ordering on the perception of visual motion, few attempts have been made to identify the neuronal substrate of this interaction. To address this issue, we devised a simple contextual manipulation that decouples surface motion from the motions of visual image features. By altering the depth ordering between a moving pattern and abutting static regions, the perceived direction of motion changes dramatically while image motion remains constant. When stimulated with these displays, many neurons in the primate middle temporal visual area (area MT) represent the implied surface motion rather than the motion of retinal image features. These neurons thus use contextual depth-ordering information to achieve a representation of the visual scene consistent with perceptual experience.

The value of serum human placental lactogen and Schwangerschaftsprotein 1 to determine gestation in an ante-natal population.

Single assays of Schwangerschaftsprotein 1 (SP1) and serum human placental lactogen (HPL) were performed in 500 consecutive patients attending a routine ante-natal clinic for their first visit. All samples were taken before 112 days gestation. Of these 500 patients, 233 had a regular menstrual cycle, were certain of last menstrual period (LMP) and delivered a normal infant after spontaneous labour. Regression equations were calculated from these results to assess their value for correcting gestation. Regression equations from previous publications were also applied to our data. The gestation and estimated date of delivery were calculated using SP1 and HPL and were compared with the actual gestation and date of delivery. The correlation was poor and did not indicate that SP1 or HPL could be used to calculate these values in the normal ante-natal population.

Trisomy 1 in an eight cell human pre-embryo.

The high incidence of chromosome abnormalities in clinically recognised pregnancies is well documented, but experience of these problems at the time of conception is extremely limited. Using donated oocytes from women seeking surgical sterilisation, we have established reliable cytogenetic techniques for chromosome analysis of human pre-embryos. These have resulted in the first report of trisomy 1. The pre-embryo showed no other obvious abnormality in relation to follicular characteristics, embryo morphology, and cleavage kinetics. The usefulness of such data in explaining the high incidence of occult human pregnancy loss and the current poor success following embryo replacement is emphasised.