Oliver Braddick

Development of contrast sensitivity over the first 3 months of life in the human infant.

Abstract Contrast sensitivity functions have been obtained for infants aged 5–12 weeks. Sensitivity was derived from fixation preference for static or drifting sinusoidal gratings over a uniform field, assessed by two-alternative forced choice by a “blind” observer. In terms of acuity, extent of the low-frequency cut, and overall sensitivity, there were large differences between the 5-week group and older age groups. This difference in performance may reflect rapid neural development in the visual system over the first 3 months of life.

Orientation-Specific Learning in Stereopsis

Inexperienced observers show a delay before experiencing the stereoscopic percept from a random-dot stereo pair. This perception time is progressively reduced with repeated exposures of the stereogram. We have investigated the specificity of this perceptual learning effect, using stereograms made up of short oblique line elements. Learning with a stereogram consisting of 45° line elements transferred completely to an uncorrelated pattern with the same element orientation, but there was a marked failure of transfer to a pattern whose elements had the opposed oblique orientation. Thus the stereoscopic skill that has been acquired may be specific to those orientation analysers that were stimulated during the training period.

THE TEMPORAL INTEGRATION AND RESOLUTION OF VELOCITY SIGNALS

The temporal properties of human visual motion detection were explored. Experiment 1 measured thresholds for speed discrimination as a function of stimulus duration. Thresholds fell asymptotically to a Weber fraction around 0.06 over a period of approx. 100 msec, with faster speeds asymptoting at slightly shorter stimulus durations. A second experiment required subjects to discriminate a pattern that was modulated between two speeds from one which remained at a constant speed. The minimum depth of the modulation required to make this judgement was found to be equivalent to a Weber fraction of 0.3 at low modulation rates, around five times greater than when the velocities were presented in isolation (expt 1). At some higher modulation rate performance dramatically declined. The modulation rate at which this occurred decreased with stimulus speed, and increased with stimulus size. The results of expt 1 seem consistent with the known properties of primary motion sensors, while the results of the latter experiments may arise from a later stage integrating the output of these primary motion sensors.

Infant astigmatism: Its disappearance with age

Abstract Photorefractive measurements, including longitudinal measures on a group of infants, show that the marked astigmatism that is common during the first year of life declines to adult levels of incidence by about 18 months.

Temporal properties of the short-range process in apparent motion.

A study is reported of the perception of random-dot two-frame apparent motion in which the durations of each exposure and the interstimulus interval between them were varied. The results are largely consistent with the rule that, for optimal motion detection, a portion of each exposure must fall within the same time interval of about 40 ms. In addition, motion perception is separably dependent on the displacement from one exposure to the next and on the time interval between those exposures, rather than on the ‘velocity implied by their ratio.

Changes in Infants' Ability to Switch Visual Attention in the First Three Months of Life:

The abilities of 1-month-old and 3-month-old infants to shift their gaze from a central target to a peripheral target were compared in four experiments. In experiment 1 targets matched in mean luminance to the background were presented to infants in the periphery at varying levels of contrast. The contrast thresholds for target detection were found to be significantly different for 1-month-olds compared with 3-month-olds. With targets set close to these contrast thresholds, correct refixations and the latency for shifting attention were examined in experiment 2. Two conditions were used: a peripheral target was presented against a homogeneous background (noncompetition); and in the second condition, the patterned target appeared at one of two lighter peripheral windows set against a darker background (competition). Although there was no difference between the two age groups in the latency for shifting visual attention, 1-month-olds were found to make more directional errors in the competition condition. The competition effect of two potential targets on latencies was examined in experiment 3. In the competition condition, two identical peripheral patterned targets were presented to the infants. The 3-month-olds refixated more quickly to one of the double targets in the competition condition than to a single peripheral target, whereas 1-month-olds were slowed down by a double target display. Finally, in experiment 4 the ability of the infants to process and disengage from a central stimulus and to refixate towards a similar peripheral target was examined. This type of competition disrupted both the direction of the first eye movement and the latency to shift attention in both age groups. However, the effect was significantly greater for the 1-month-olds. Taken together, the results of these experiments demonstrate the greater disruption of fixation-shift behaviour in 1-month-olds compared with 3-month-olds when competing visual stimuli are used. This developmental change is explained in terms of maturation of executive cortical orienting systems over the first months of life.

A photorefractive study of infant accommodation.

Abstract The technique of “photorefraction” has been used to investigate the refractive state of freely accommodating infants, when presented with targets at distances up to 150 cm. Groups of infants were aged from birth to 1 yr. All infants tested at six months and older, and most 2–3 mth infants, showed the ability to accommodate correctly on targets at 150 cm and closer. Newborn and 1 mth infants could accommodate more accurately at 75 cm and closer than they could at 150 cm. Many infants who did not consistently accommodate for 150 or 75 cm distant targets nonetheless showed fluctuations of accommodation that included the appropriate distance. 62% of our infant sample showed a significant degree of astigmatism. The errors of accommodation observed are insufficient to be the limiting factor on reported values of infant acuity. We conclude that accommodative errors may instead be the consequence of infants limited ability to process the retinal image so as to detect incorrect accommodation, and possibly also of limitations of visual attention for targets further than 1 m.

The basis of area and dot number effects in random dot motion perception

Random dot kinematograms present a patch of random dots, uniformly displaced from one frame to the next. We have re-examined the question of whether the limiting displacement for this short-range apparent motion is determined by the retinal angle or by the number of pixels across which the patch is displaced; our data support the former hypothesis. The number of dots in the patch is not a confounding variable, and in fact has very little effect. Increasing the area of the patch does increase the displacement limit, apparently due to the invasion of greater retinal eccentricities.

Eccentricity-dependent scaling of the limits for short-range apparent motion perception.

The ability to report the direction of apparent motion when an array of random dots is displaced fails when the displacement exceeds a limiting value (dmax). We find that dmax increases rapidly with retinal eccentricity, in a manner different from spatial measures such as acuity which are believed to depend on the magnification factor of projection to area 17. The minimum displacement giving detectable motion (dmin) shows a shallower increase with eccentricity which is more compatible with the variation of cortical magnification. The dependence of apparent motion on the timing variables (exposure duration, inter-stimulus interval) changes negligibly with eccentricity. Consequently the dynamic range and the upper limit of detectable velocities increases greatly with eccentricity. The increase of dmax with eccentricity means that the perception of apparent motion will show an approximate invariance with display scale, even though dmax has a locally fixed value depending on receptive field structure.

The masking of apparent motion in random-dot patterns.

Abstract Two random dot patterns were alternated to give apparent motion of a central square region. The appearance of the moving square could be masked by exposing a uniform light field in the interval between the patterns. This masking effect (i) was not found when the patterns and blank field were exposed to different eyes, (ii) was of similar magnitude when a visual noise field was used, (iii) was greatest when the masking field luminance was slightly greater than that of the patterns, declining for brighter fields, (iv) was not produced by light adaptation if the interval itself was dark, (v) could not be accounted for by the reduction in apparent pattern contrast, (vi) was greatest when the masking field was exposed early in the interval. Three hypotheses are discussed: (i) that this masking of motion is an aspect of a more general masking of the patterns; (ii) that it is due to stimulation of motion detectors by the masking field; (iii) that it is due to erasure of briefly stored information. Hypothesis (i) is rejected.