Early impairment of foveal magno- and parvocellular pathways in juxta chiasmal tumours.

Foveal pathway visual function was assessed in 11 patients having tumours extending into the suprasellar region but without evidence of visual impairment as assessed by visual acuity and Bjerrum screen campimetry. Psychophysical and routine visual evoked potential (VEP) measurements were obtained from the eye ipsilateral to the maximal suprasellar extension. The sensitivity of luminance and chromatic pathways was assessed psychophysically by measuring increment thresholds for white and red flashes of light presented on a white adapting field. Temporal sensitivity was assessed psychophysically by measuring threshold modulation sensitivity for sinusoidally modulating stimuli (de Lange attenuation characteristic). The patient group showed approximately equal significant psychophysical losses in chromatic, luminance and temporal sensitivities relative to normal controls. Midline VEP P100 latencies of the patient group did not significantly differ from those of the normal control group. It is concluded that tumours extending into the suprasellar region can cause foveal pathway dysfunction affecting both magno- and parvocellular pathways, even in the presence of normal visual acuity and fields suggesting a more widespread and insidious abnormality of the visual pathways in this condition than previously thought.

Integration across directions in dynamic random dot displays: vector summation or winner take all?

Recent studies have clearly demonstrated that the activity of directionally selective neuronal populations in the middle temporal (MT) and medial superior temporal (MST) cortical areas plays a direct role in the judgment of the direction of visual motion. However, the way in which the information is derived from a population of neurons remains unknown. Two principal models have been suggested in the past: the vector summation model suggests that the responses of neurons encoding all directions of motion are weighted and pooled to obtained an accurate estimate of the mean direction of motion; the winner-take-all model is based on a competition between different direction-specific channels, so that decisions are cast in favor of the channel generating the strongest directional signal. To discriminate between these two models we generated random dot stimuli that contained an asymmetric distribution of directions of motion. Human subjects were asked to adjust the global direction of motion to the upward vertical direction. When the directional signals were of similar strength, subjects tended to perceive global motion in the mean direction of motion (corresponding to vector summation), but as one directional signal became more prominent, most subjects' settings diverged from the mean towards the modal direction of motion. Some subjects could either match the mean or the modal direction of motion in the display, depending on the task instructions. These results suggest that the perceptual judgment of direction of motion is not based on any rigid algorithm generating a single valued output. Rather, human observers are able to judge different aspects of the distribution of activity in a cortical area depending on the task requirements.

What is noise for the motion system?

Motion coherence thresholds in random-dot patterns have been widely adopted as a measure of performance in visual motion processing. However, there has been diversity in the type of "noise" in which a coherent motion signal has to be detected. Here we compare coherence thresholds for three ways of creating motion noise: dots replotted in random positions in each new frame; dots with a set displacement but following a random walk from frame to frame; or dots moving in random directions which remain constant for a given dot over a sequence of displacements. In each case, the signal dots may either remain the same throughout the display sequence, or the signal dots may be re-selected afresh on each frame ("different"). With our display (3 deg square, 120 msec exposure, velocity = 5 or 10 deg sec-1), all these different noise conditions yielded similar thresholds around 5-8%. There were some small but systematic differences between conditions. Thresholds in random-direction displays were consistently higher than those in random-walk or random-position displays, especially at the lower velocity. However, this effect is much smaller than would be expected from the increased standard error of the noise mean in random direction, perhaps because the motion system integrates information most effectively over a local region of space and/or time. Subjects" performance could not be explained by a strategy of identifying individual signal dots with extended trajectories. The similarity between random-walk and random-position thresholds implies that subjects do not exploit the marked differences in speed distribution between signal and noise dots in the latter case. The practical message for the design and interpretation of experiments using coherence thresholds is that the results are not much affected by the choice of noise, at least within the range of stimuli tested here.

Effect of short term changes in blood glucose on visual pathway function in insulin dependent diabetes.

BACKGROUND: Visual pathway function is abnormal in patients with insulin dependent diabetes mellitus (IDDM) without retinopathy, yet the mechanism underlying this abnormality is unknown. It is hypothesised that short term changes in blood glucose level affect visual pathway function in IDDM. METHODS: Colour discrimination was measured in 10 uncomplicated aretinopathic IDDM patients during hyperinsulinaemic clamp, with the Farnsworth Munsell 100 hue test (100 hue test). After stable euglycaemia, patients were made hyperglycaemic (14 mmol/l), maintained euglycaemic (5 mmol/l), and rendered hypoglycaemic (2.5 mmol/l), in random order, on separate occasions at least 1 week apart. RESULTS: Short term (1-2 hours) changes in blood glucose did not affect colour discrimination: mean (SD) 100 hue error score at 2.5 mmol/l was 34 (22) compared with 35 (33) at 5 mmol/l, and 39 (28) at 14 mmol/l. CONCLUSION: These data suggest that short term (1-2 hours) changes in blood glucose are not the mechanism for visual pathway dysfunction in aretinopathic IDDM patients.

Abnormalities of visual function in hereditary motor and sensory neuropathy.

Visual function was studied in a group of 15 patients with hereditary motor and sensory neuropathy (HMSN). Psychophysical measures of luminance and chromatic threshold and temporal contrast sensitivity were undertaken, together with visual evoked potentials (VEPs), visual fields and clinical neuro-ophthalmological examination. A patchy loss of visual function was found in individual cases of HMSN. In the group analysis there was evidence of a selective loss of luminance threshold and temporal contrast sensitivity at low temporal frequencies; the VEP P100 latency was not significantly prolonged. The losses of visual function in HMSN were discussed and compared with visual losses in multiple sclerosis, which had been detected using identical experimental techniques.

Detection of colour vision abnormalities in uncomplicated type 1 diabetic patients with angiographically normal retinas.

Colour vision function was assessed in 38 non-complicated type 1 diabetic patients in whom fluorescein angiography was normal, and was compared with that in 36 age-matched, non-diabetic controls. All of the patients were healthy and none were taking medication except insulin. The eye examination, which was normal in every patient, included the Ishihara and City University tests, measurement of Snellen acuity, slit-lamp examination, tonometry, and fundal photography as well as fluorescein angiography. Colour discrimination ability was measured with the Farnsworth-Munsell 100-hue test. Mean (SE) 100-hue test error score for the diabetic group was 86.8 (8.1) compared with 28.2 (3.3) for controls, p<<0.001. There was no relation between colour vision abnormalities and diabetes duration (r = 0, p>0.05), blood glucose at the time the colour tests were performed (r = 0.4, p > 0.05), most recent glycated haemoglobin result (r = 0.3, p>0.05), or the mean of all previous glycated haemoglobin results (r = 0, p>0.05). It is concluded that colour discrimination may be abnormal in uncomplicated type 1 diabetic patients before the onset of retinopathy, and that colour discrimination losses in diabetes may not be of vascular aetiology.

Visual loss in multiple sclerosis and its relation to previous optic neuritis, disease duration and clinical classification.

Visual function was investigated in a group of 58 clinically classified cases of multiple sclerosis (MS). Psychophysical measures of luminance and chromatic threshold sensitivity and temporal contrast sensitivity were undertaken, together with visual evoked potentials and Bjerrum screen perimetry. The patient group was divided on the basis of optic neuritis (ON), clinical disease duration and clinical classification. A comparison of the results of all visual measures suggested a nonuniform loss of function in the patient group without ON and a more consistent loss of function in the group with ON. The various measures were equally efficient in detecting abnormal function, albeit from different areas of the central visual field. Clinical disease duration was not a significant independent factor in predicting visual dysfunction. In contrast, a comparison of clinical classification categories revealed significantly fewer abnormalities of visual function in the suspected MS category (31%) than in the ON, early probably and clinically definite categories (75-100%), a result which indicated the importance of clinical classification as a predictor of visual dysfunction.

Anomalous loss in blue-green wavelength discrimination with very brief monochromatic stimuli presented to the normal human eye.

The wavelength-discrimination curve of the normal human eye shows minima in discrimination thresholds at about 490 and 580 nm for viewing times of 1 s or longer. A reduction in viewing time was found to yield non-uniform increases in discrimination thresholds in the blue-green region of the spectrum, and these findings were quantified in objective, two-interval, forced-choice discrimination measurements. Monochromatic stimuli were presented foveally in a circular, horizontally oriented, bipartite field of 100 Td and angular subtense 2 degrees. When viewing time was decreased to 3 ms there was a sharp increase in discrimination threshold over 490-520 nm, maximizing near 500 nm. In this region, the fields appeared markedly desaturated. The loss in discrimination was distinct from that exhibited by tritanopes, and a control experiment showed that the effect was not attributable to the reduced energy of the short flash.

Isolation of opponent-colour mechanisms at increment threshold.

An experimental examination was made of some paradigms designed to isolate the opponent-colour system at increment threshold. The effectiveness of a uniform white conditioning field spatially coincident with a 1.05-deg uniform test field was assessed by measuring intensity thresholds for simple detection and for colour discrimination. Values were obtained both by a method of adjustment and by a two-interval forced-choice procedure. For sufficiently high luminances of the conditioning field (3000 td or greater) little or no difference was found between simple-detection and colour-discrimination thresholds over the critical test-flash spectral range 520-620 nm, implying that the paradigm produced almost complete isolation of the opponent-colour system at increment threshold. A control experiment in which thresholds were obtained for a conditioning field larger than the test field gave less satisfactory isolation; near 580 nm the luminance system was found to be at least 0.3 log unit more sensitive than the opponent-colour system. A comparison was also made of the spatially coincident field paradigm with a paradigm in which a modified test stimulus of low temporal and spatial frequency content was presented on a large conditioning field. Test spectral sensitivity curves for simple detection obtained by a method of adjustment showed little difference in effectiveness in opponent-colour isolation.