Allison M. McKendrick

Motion processing deficits in migraine

This study was designed to determine whether cortical motion processing abnormalities are present in individuals with migraine. Performance was measured using a visual motion coherence task (motion coherence perimetry, MCP) thought to depend on the operation of cortical area V5. Motion coherence thresholds were measured using stimuli composed of moving dots at 17 locations in the central ± 20° of visual field. Pre-cortical visual function was also measured using frequency doubling perimetry (FDP) at the same 17 locations. Several migraine subjects demonstrated significant pre-cortical visual functional abnormalities, however, most subjects had normal visual fields measured with FDP. Abnormal MCP performance was measured in 15 of 19 migraine-with-aura subjects, and 11 of 17 migraine-without-aura subjects. A decreased ability to detect coherent motion may possibly be explained by an increase in baseline neuronal noise, such as would be consistent with the concept of cortical hyperexcitability in migraine.

The effect of normal aging on closed contour shape discrimination.

Our experiments explore whether contour processing of closed shapes is altered by healthy aging. Contour processing was measured using a closed contour (circle or ellipse) constructed of Gabor elements. The contour was presented either on a blank background or embedded in noise (identical Gabor elements of random orientation). Twenty-one older (age range: 61-80 years) and 21 younger (age range: 22-38 years) adults participated in three experiments: 1) the number of Gabors comprising the contour was fixed (10, 12 or 15) and the threshold aspect ratio required to discriminate the shape (circle versus ellipse) was measured; 2) orientation jitter was added to the Gabor elements comprising the contour and shape aspect ratio discrimination thresholds were measured; and 3) the aspect ratio was fixed (three times the individual threshold aspect ratios) and the threshold number of elements required to determine the shape was measured. Older adults required a larger number of elements to discriminate the global contour shape (F(1, 41) = 15, p < 0.001), even when stimulus saliency was matched for contrast sensitivity and aspect ratio threshold. This finding is consistent with other recent work showing deteriorations in cortically mediated visual processing with age.

Enhanced Structure–Function Relationship in Glaucoma With an Anatomically and Geometrically Accurate Neuroretinal Rim Measurement

PURPOSE To evaluate the structure-function relationship between disc margin-based rim area (DM-RA) obtained with confocal scanning laser tomography (CSLT), Bruchs membrane opening-based horizontal rim width (BMO-HRW), minimum rim width (BMO-MRW), peripapillary retinal nerve fiber layer thickness (RNFLT) obtained with spectral-domain optical coherence tomography (SD-OCT), and visual field sensitivity. METHODS We examined 151 glaucoma patients with CSLT, SD-OCT, and standard automated perimetry on the same day. Optic nerve head (ONH) and RNFL with SD-OCT were acquired relative to a fixed coordinate system (acquired image frame [AIF]) and to the eye-specific fovea-BMO center (FoBMO) axis. Visual field locations were mapped to ONH and RNFL sectors with fixed Garway-Heath (VF(GH)) and patient-specific (VF(PS)) maps customized for various biometric parameters. RESULTS Globally and sectorally, the structure-function relationships between DM-RA and VF(GH), BMO-HRW(AIF) and VF(GH), and BMO-HRW(FoBMO) and VF(PS) were equally weak. The R(2) for the relationship between DM-RA and VF(GH) ranged from 0.1% (inferonasal) to 11% (superotemporal) whereas that between BMO-HRW(AIF) and VF(GH) ranged from 0.1% (nasal) to 10% (superotemporal). Relatively stronger global and sectoral structure-function relationships with BMO-MRW(AIF) and with BMO-MRW(FoBMO) were obtained. The R(2) between BMO-MRW(AIF) and VF(GH) ranged from 5% (nasal) to 30% (superotemporal), whereas that between BMO-MRW(FoBMO) and VF(PS) ranged from 5% (nasal) to 25% (inferotemporal). The structure-function relationship with RNFLT was not significantly different from that with BMO-MRW, regardless of image acquisition method. CONCLUSIONS The structure-function relationship was enhanced with BMO-MRW compared with the other neuroretinal rim measurements, due mainly to its geometrically accurate properties.

Processing of global form and motion in migraineurs

Previous studies have identified anomalies of cortical visual processing in migraineurs that appear to extend beyond V1. Migraineurs respond differently than controls to transcranial magnetic stimulation of V5, and can demonstrate impairments of global motion processing. This study was designed to assess the integrity of intermediate stages of both motion and form processing in people with migraine. We measured the ability to integrate local orientation information into a global form percept, and to integrate local motion information into a global motion percept. Control subjects performed significantly better than migraineurs on both tasks, suggesting a diffuse visual cortical processing anomaly in migraine.

Aging alters surround modulation of perceived contrast.

It is well established that many visual functions deteriorate with age. Perhaps counter-intuitively, a recent study revealed that older people actually require less time to discriminate the direction of motion of large, high contrast moving stimuli than young adults (L. R. Betts, C. P. Taylor, A. B. Sekuler, & P. J. Bennett, 2005). L. R. Betts et al. (2005) proposed their finding as evidence for a reduction of cortical inhibitory function within the aging visual system. There is some neurophysiological support for this suggestion, as broadening of visual cortical neural tuning consistent with reduced inhibitory function has been observed in older animals. Here we explore the perceptual consequences of center-surround suppression within the healthy aging human visual system and report data from a center-surround contrast discrimination task (the Chubb contrast illusion). We predicted that older observers would demonstrate less center-surround suppression than younger subjects (consistent with reduced inhibition). Our data does not support this prediction as perceived contrast was altered more by surround modulation in the older than younger group (t(33) = 2.53, p = 0.02). A possible explanation for our findings is a decrease in perceptual brightness induction in the elderly group. Brightness induction relies on neural synchronization which might be disrupted by aging.

Recent developments in perimetry: test stimuli and procedures.

Automated perimetry has evolved substantially in recent years, in part due to modern computer technology that enables more complex visual stimuli and test procedures to be realised than those incorporated in traditional white‐on‐white luminance increment perimetry. This paper reviews briefly a number of advances in automated perimetry. The review includes discussion of new test types: frequency doubling technology perimetry, short wavelength automated perimetry, flicker perimetry, high‐pass resolution perimetry and rarebit perimetry. Test algorithms applied to perimetry such as zippy estimation of sequential thresholds (ZEST), Swedish interactive thresholding algorithm (SITA), tendency‐oriented perimetry (TOP) and multi‐sampling supra‐threshold perimetry are also discussed.

An anatomically customizable computational model relating the visual field to the optic nerve head in individual eyes.

PURPOSE To present a computational model mapping visual field (VF) locations to optic nerve head (ONH) sectors accounting for individual ocular anatomy, and to describe the effects of anatomical variability on maps produced. METHODS A previous model that related retinal locations to ONH sectors was adapted to model eyes with varying axial length, ONH position and ONH dimensions. Maps (n = 11,550) relating VF locations (24-2 pattern, n = 52 non-blind-spot locations) to 1° ONH sectors were generated for a range of clinically plausible anatomical parameters. Infrequently mapped ONH sectors (5%) were discarded for all locations. The influence of anatomical variables on the maps was explored by multiple linear regression. RESULTS Across all anatomical variants, for individual VF locations (24-2), total number of mapped 1° ONH sectors ranged from 12 to 90. Forty-one locations varied more than 30°. In five nasal-step locations, mapped ONH sectors were bimodally distributed, mapping to vertically opposite ONH sectors depending on vertical ONH position. Mapped ONH sectors were significantly influenced (P < 0.0002) by axial length, ONH position, and ONH dimensions for 39, 52, and 30 VF locations, respectively. On average across all VF locations, vertical ONH position explained the most variance in mapped ONH sector, followed by horizontal ONH position, axial length, and ONH dimensions. CONCLUSIONS Relations between ONH sectors and many VF locations are strongly anatomy-dependent. Our model may be used to produce customized maps from VF locations to the ONH in individual eyes where some simple biometric parameters are known.

Combining Ganglion Cell Topology and Data of Patients with Glaucoma to Determine a Structure―Function Map

PURPOSE To introduce techniques for deriving a map that relates visual field locations to optic nerve head (ONH) sectors and to use the techniques to derive a map relating Medmont perimetric data to data from the Heidelberg Retinal Tomograph. METHODS Spearman correlation coefficients were calculated relating each visual field location (Medmont M700) to rim area and volume measures for 10 degrees ONH sectors (HRT III software) for 57 participants: 34 with glaucoma, 18 with suspected glaucoma, and 5 with ocular hypertension. Correlations were constrained to be anatomically plausible with a computational model of the axon growth of retinal ganglion cells (Algorithm GROW). GROW generated a map relating field locations to sectors of the ONH. The sector with the maximum statistically significant (P < 0.05) correlation coefficient within 40 degrees of the angle predicted by GROW for each location was computed. Before correlation, both functional and structural data were normalized by either normative data or the fellow eye in each participant. RESULTS The model of axon growth produced a 24-2 map that is qualitatively similar to existing maps derived from empiric data. When GROW was used in conjunction with normative data, 31% of field locations exhibited a statistically significant relationship. This significance increased to 67% (z-test, z = 4.84; P < 0.001) when both field and rim area data were normalized with the fellow eye. CONCLUSIONS A computational model of axon growth and normalizing data by the fellow eye can assist in constructing an anatomically plausible map connecting visual field data and sectoral ONH data.

Migraine Increases Centre-Surround Suppression for Drifting Visual Stimuli

Background The pathophysiology of migraine is incompletely understood, but evidence points to hyper-responsivity of cortical neurons being a key feature. The basis of hyper-responsiveness is not clear, with an excitability imbalance potentially arising from either reduced inhibition or increased excitation. In this study, we measure centre-surround contrast suppression in people with migraine as a perceptual analogue of the interplay between inhibition and excitation in cortical areas responsible for vision. We predicted that reduced inhibitory function in migraine would reduce perceptual surround suppression. Recent models of neuronal surround suppression incorporate excitatory feedback that drives surround inhibition. Consequently, an increase in excitation predicts an increase in perceptual surround suppression. Methods and Findings Twenty-six people with migraine and twenty approximately age- and gender-matched non-headache controls participated. The perceived contrast of a central sinusoidal grating patch (4 c/deg stationary grating, or 2 c/deg drifting at 2 deg/sec, 40% contrast) was measured in the presence and absence of a 95% contrast annular grating (same orientation, spatial frequency, and drift rate). For the static grating, similar surround suppression strength was present in control and migraine groups with the presence of the surround resulting in the central patch appearing to be 72% and 65% of its true contrast for control and migraine groups respectively (t(44) = 0.81, p = 0.42). For the drifting stimulus, the migraine group showed significantly increased surround suppression (t(44) = 2.86, p<0.01), with perceived contrast being on average 53% of actual contrast for the migraine group and 68% for non-headache controls. Conclusions In between migraines, when asymptomatic, visual surround suppression for drifting stimuli is greater in individuals with migraine than in controls. The data provides evidence for a behaviourally measurable imbalance in inhibitory and excitatory visual processes in migraine and is incompatible with a simple model of reduced cortical inhibitory function within the visual system.

An analysis of the factors associated with visual field deficits measured with flickering stimuli in-between migraine.

We have previously demonstrated that perimetric performance measured with flickering stimuli is not normal in some individuals who experience migraine with aura in the period between their attacks. In this study, flicker perimetric performance is measured in a broad group of migraineurs to determine whether the existence of such visual field deficits is dependent on the presence of visual aura, is correlated with the duration of migraine history, or frequency of attacks. Twenty-eight migraine with aura, 25 migraine without aura, and 24 non-headache control subjects participated. The performance of the migraine groups was not significantly different from each other. The migraine groups showed significantly lower general sensitivity across the visual field and higher incidence of localized visual field deficits relative to controls. Both length of migraine history and frequency of migraine occurrence over the past 12 months were significantly correlated with lower general sensitivity to flickering visual stimuli.