Deirdre Birtles

Motion- and orientation-specific cortical responses in infancy.

During the first 3 months, infants develop visual evoked potential (VEP) responses that are signatures of cortical orientation-selectivity and directional motion selectivity. Orientation-specific cortical responses develop in early infancy. This study compared these responses directly in the same infants, to investigate whether the later appearance of direction selectivity was intrinsic, or a function of the spatio-temporal characteristics of the stimuli used. Steady-state orientation-reversal (OR-) VEPs and direction-reversal (DR-) VEPs were recorded in infants aged 4-18 weeks. DR-VEPs were elicited with random pixel patterns and with gratings spatially similar to those used for OR-VEPs, at velocities of 5.5 and 11 deg/s, and reversal rates of 2 and 4 reversals/s. Infants throughout the age range showed significant responses to orientation-reversal. Direction-reversal responses appeared in less than 25% of infants under 7 weeks of age, rising to 80% or more at 11-13 weeks, whether tested with dots or gratings and for both speeds and reversal rates. However, 2 reversals/s elicits the DR-VEP on average about 2 weeks earlier than 4 reversal/s stimulation. We conclude that human cortical direction selectivity develops separately from orientation-selectivity and emerges at a later age, even with tests that are designed to optimise the former.

Cortical vision, MRI and developmental outcome in preterm infants

Objectives: To test two measures of visual cortical function in the first year of life as early markers of functionally significant brain damage in infants born preterm: orientation-reversal visual event-related potentials (OR-VERP) and a behavioural test of cortically controlled visual attention-fixation shifts under competition (FS). Also to examine how these measures relate to (1) perinatal brain insults identified by MRI, and (2) later neurodevelopmental status. Patients and methods: After neonatal and term-age-equivalent MRI, 26 preterm infants (<32 weeks of gestational age, mean 28.1 weeks) were given the OR-VERP and FS tests before 12 months post-term age and a neurodevelopmental assessment (Griffiths Scales) at 2 years. MRI scans examined for parenchymal lesions, intraventricular haemorrhage, ventricular dilatation and diffuse excessive high signal intensity were classified into three categories of severity. Cortical visual test results were compared across these categories and examined as predictors of developmental status at 2 years. Results: 26 infants were studied. 13/25 infants showed significant OR-VERP responses. 12/26 showed normal FS performance. On both tests, the proportion of infants meeting these criteria decreased significantly with MRI severity. As predictors of Griffiths developmental quotient ⩽80, the FS test had a sensitivity of 100%, a specificity of 61%, and positive and negative predictive values of 50% and 100%, respectively; corresponding values for OR-VERP were 86%, 65%, 50% and 92% . Conclusions: Visual cortical tests can provide early indicators of the functional impact of perinatal brain damage in the preterm infant.

Orientation and motion-specific visual cortex responses in infants born preterm.

Orientation-specific cortical responses develop earlier in infancy than motion-specific responses. The maturation of orientation-reversal and direction-reversal visual evoked potentials was evaluated in 17 healthy, low risk, preterm infants (born <32 weeks gestation), compared with a group of 26 infants born at term. Both groups were studied at a corrected age of 2–4 months. The age function and magnitude of the orientation-reversal responses was similar in the two groups. Direction-reversal responses across the age range, however, were smaller in the preterm infants, suggesting a delayed maturation of motion processing. Reasons for the vulnerability of motion processing are discussed; the results may reflect anomalies of white matter development in preterm infants that are undetected by ultrasonography.

Chronic exertional compartment syndrome : muscle changes with isometric exercise

UNLABELLED Chronic exertional compartment syndrome (CECS) is a well-documented cause of lower leg pain in active individuals. The pathophysiology is unclear, although it is generally believed to be associated with increased intramuscular pressure, but there is very little information about muscle function in relation to the onset of pain. PURPOSE To investigate strength, fatigue, and recovery of the anterior tibial muscles in CECS patients and healthy subjects during an isometric exercise protocol. METHODS Twenty patients and 22 control subjects (mean age 27.6 yr and 33.0 yr, respectively) performed a 20-min isometric exercise protocol consisting of intermittent maximal voluntary contractions (MVC). Central fatigue was evaluated by comparing changes in electrically stimulated (2 s at 50 Hz) and voluntary contraction force before and during the exercise, and then throughout 10 min of recovery. Muscle size was measured by ultrasonography. Pain and cardiovascular parameters were also examined. RESULTS The absolute MVC forces were similar, but MVC:body mass of the patients was lower (P < 0.05) as was the ratio of MVC to muscle cross-sectional area (P < 0.01). The extent of central and peripheral fatigue was similar in the two groups. The patients reported significantly higher levels of pain during exercise (P < 0.05 at 4 min) and after the first minute of recovery (P < 0.001). An 8% increase in muscle size after exercise was observed for both groups. There were no differences in the cardiovascular responses of the two groups. CONCLUSIONS CECS patients were somewhat weaker than normal but fatigued at a similar rate during isometric exercise. Patients reported higher pain than controls despite comparable changes in muscle size, suggesting that abnormally tight fascia are not the main cause of CECS symptoms.

Asymmetrical cortical processing of radial expansion/contraction in infants and adults

We report asymmetrical cortical responses (steady-state visual evoked potentials) to radial expansion and contraction in human infants and adults. Forty-four infants (22 3-month-olds and 22 4-month-olds) and nine adults viewed dynamic dot patterns which cyclically (2.1 Hz) alternate between radial expansion (or contraction) and random directional motion. The first harmonic (F1) response in the steady-state VEP response must arise from mechanisms sensitive to the global radial motion structure. We compared F1 amplitudes between expansion-random and contraction-random motion alternations. F1 amplitudes for contraction were significantly larger than those for expansion for the older infants and adults but not for the younger infants. These results suggest that the human cortical motion mechanisms have asymmetrical sensitivity for radial expansion vs. contraction, which develops at around 4 months of age. The relation between development of sensitivity to radial motion and cortical motion mechanisms is discussed.

Latency measures of pattern-reversal VEP in adults and infants: different information from transient P1 response and steady-state phase.

PURPOSE Temporal properties such as the peak latency of pattern-reversal (PR) visual evoked potentials (VEPs) have been found to be a sensitive indicator of visual development. Latency can be assessed from the slope of a plot of phase against temporal frequency (TF) for steady state VEP measurements as well as from the transient P1 peak. This study aimed to discover whether the two methods provide different information regarding early visual development. METHODS Developmental changes of the transient peak latency were tracked using low TFs of one to four reversals per second (r/s) and a spatial frequency (SF) of 0.24 cycles per degree (cpd) in comparison with latencies calculated from the phase versus TF gradient in the range of 1 to 19 r/s. PR-VEP responses were recorded from 81 adults and 137 infants (ages 3.6-79 weeks). RESULTS Values of the calculated and transient peak latencies were similar in adults, but the calculated latency was statistically longer than transient peak latency in younger infants. Moreover, while the transient peak latency asymptoted to an adult value of 104 ms at approximately 15 weeks of age, the calculated latency did not asymptote until after 30 weeks. CONCLUSIONS In this study, the effectiveness of the phase-based method to calculate latency was confirmed. In infants, the rapid decrease of P1 latency may be due to the progressive maturation of conduction time in the afferent visual pathways, with the development of adult levels of phase-based calculated latency being due to the maturation of later cortical processing in infants.

Orientation-reversal VEP: Comparison of phase and peak latencies in adults and infants

The peak latency of pattern-reversal (PR)-VEP has been found to develop rapidly, reaching the adult level around 15 weeks of age. However, the development of orientation-reversal (OR)-VEP, reflecting the specific spatial organization of cortical receptive fields, still remains unknown. OR-VEP was tested in 81 adults at 1-12 reversals/sec (r/s) and 94 infants (age 4-79 weeks) at 2-8r/s. OR data at 4r/s from an additional 123 infants (age 4.0-20.3 weeks) studied previously were also analyzed. In addition to peak transient latencies at 1-4r/s, latency values derived from the gradient of phase against temporal frequency in steady-state recording were also calculated. For both adults and infants, no significant latency differences in the initial positive peaks were found among the low reversal rates. The calculated latency was statistically longer than the transient latency in both groups. While the transient latency asymptoted to adult value of 102 ms at around 50 weeks of age, the calculated latency, unlike that for PR-VEP, showed little variation across the age span. The data suggest a dominant effect of transmission delay on the initial peak in infancy, which reduces with age. However, the overall timing of the cortical response to orientation change remains slower than for pattern reversal in the fully developed visual cortex. Upon reaching maturity, the latencies of the initial positive peak in both pattern and orientation VEPs may arise from the same level of cortical processing in V1, but the overall time course reflected in the steady-state phase continues to show a much more prolonged response to orientation change than the transmission delay seen in the transient VEPs.