The effect of lid elevation on the cross-cover test.

OBJECTIVE: To discern the effect of manual lid elevation on muscle balance using the cross-cover test. MATERIALS AND METHODS: One hundred consecutive patients who were orthophoric in all fields of gaze were prospectively studied. A repeat cross-cover test was performed with the eyes looking down and to either side while pulling the lid ipsilateral to the abducting eye up and to either side. The presence and type of any phoria was noted. RESULTS: Eighty-four percent of patients and 76% of eyes developed a phoria with lifting a lid. Vertical phorias developed in 79 patients and 136 eyes, whereas horizontal phorias were seen in 51 patients and 83 eyes. In all but one case, the vertical deviation was a hyperphoria ipsilateral to the elevated lid. Horizontal deviations were esophoric in 63 eyes of 39 subjects. Induced phorias were most commonly symmetric from side to side. CONCLUSIONS: Lifting the lid produces an iatrogenic phoria that mimics a skew or IV nerve paresis. We hypothesize that stretching the lid causes traction on the levator-superior rectus complex, thereby increasing its tone. Although it makes testing more difficult, we recommend that the lids not be manipulated while performing phoria testing.

Neurosarcoidosis presenting as a tumour of the basal ganglia and brainstem: sequential MRI.

Neurosarcoidosis may spread from the basal leptomeninges via the Virchow-Robin spaces to form intraparenchymal masses. We present a case of sarcoidosis whose first presentation was that of secondary amenorrhoea without other neurological symptoms. Discovery of a mass invading the basal ganglia, hypothalamus, pituitary stalk and midbrain led to a search for systemic involvement. After the diagnosis was proven by mediastinal biopsy, steroids were used effectively to shrink the tumour. Sequential magnetic resonance imaging (MRI) studies demonstrate dramatic reduction in the mass over a six month period. A high index of suspicion for sarcoidosis in intracranial masses, particularly in young adults, is advocated.

Spatial and temporal analysis by neurons in the representation of the central visual field in the cat's lateral suprasylvian visual cortex.

We studied quantitatively the receptive-field properties of 74 units recorded from the representation of the central visual fields in the cat's lateral suprasylvian (LS) visual cortex. In agreement with previous workers, we found that LS receptive fields tended to be large and to lack discernible spatial structure. They resembled the complex receptive fields of areas 17 and 18 in their general organization. We examined the responses of these neurons to moving optimally oriented sinusoidal gratings that varied in spatial and temporal frequency of drift. Most LS neurons were selective for the spatial frequency of sinusoidal gratings; 7% responded to all spatial frequencies below a cutoff value. In agreement with previous reports, the optimal spatial frequencies for LS neurons covered a wider range than is seen in either area 17 or 18 alone (0.05-1 cycle/deg), but are certainly included in the range covered by both these afferent areas. Individual neurons in LS responded to a range of spatial frequencies broader than is typical for neurons in areas 17 and 18. The effect of varying the drift rate of otherwise optimal gratings was similar in LS to that reported for areas 17 and 18. Most neurons were optimally responsive to drift rates between 0.5 and 4 Hz, and resolved frequencies as high as 10-30 Hz. A few neurons had optima higher than 6 Hz and resolved frequencies in excess of 30 Hz. We conclude that the receptive fields of LS neurons reflect rather closely the properties of their afferents from areas 17 and 18.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectivity for orientation and direction of motion of single neurons in cat striate and extrastriate visual cortex.

1. We consider the consequences of the orientation selectivity shown by most cortical neurons for the nature of the signals they can convey about the direction of stimulus movement. On theoretical grounds we distinguish component direction selectivity, in which cells are selective for the direction of movement of oriented components of a complex stimulus, from pattern direction selectivity, or selectivity for the overall direction of movement of a pattern irrespective of the directions of its components. We employed a novel test using grating and plaid targets to distinguish these forms of direction selectivity. 2. We studied the responses of 280 cells from the striate cortex and 107 cells from the lateral suprasylvian cortex (LS) to single sinusoidal gratings to determine their orientation preference and directional selectivity. We tested 73 of these with sinusoidal plaids, composed of two sinusoidal gratings at different orientations, to study the organization of the directional mechanisms within the receptive field. 3. When tested with single gratings, the directional tuning of 277 oriented cells in area 17 had a mean half width of 20.6 degrees, a mode near 13 degrees, and a range of 3.8-58 degrees. Simple cells were slightly more narrowly tuned than complex cells. The selectivity of LS neurons for the direction of moving gratings is not markedly different from that of neurons in area 17. The mean direction half width was 20.7 degrees. 4. We evaluated the directional selectivity of these neurons by comparing responses to stimuli moved in the optimal direction with those elicited by a stimulus moving in the opposite direction. In area 17 about two-thirds of the neurons responded less than half as well to the non-preferred direction as to the preferred direction; two-fifths of the units responded less than one-fifth as well. Complex cells showed a somewhat greater tendency to directional bias than simple cells. LS neurons tended to have stronger directional asymmetries in their response to moving gratings: 83% of LS neurons showed a significant directional asymmetry. 5. Neurons in both areas responded independently to each component of the plaid. Thus cells giving single-lobed directional-tuning curves to gratings showed bilobed plaid tuning curves, with each lobe corresponding to movement in an effective direction by one of the two component gratings within the plaid. The two best directions for the plaids were those at which one or other single grating would have produced an optimal response when presented alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of early unilateral blur on the macaque's visual system. I. Behavioral observations.

We raised 8 macaque monkeys with chronic atropinization of one eye throughout the first 6-10 months after birth. This rearing procedure produces retinal image blur, with the most pronounced contrast attenuation occurring at high spatial frequencies. Measurements of contrast sensitivity were made using behavioral methods in 6 monkeys and evoked potential methods in 2 monkeys. The results showed that this rearing procedure produced long-term deficits in the contrast sensitivity and spatial resolution of the atropinized eye, which were not due to residual losses in accommodative capacity. There was considerable interanimal variation in the magnitude of the effects on visual performance. Similar losses in visual performance are seen in some forms of human amblyopia. Rearing monkeys with chronic instillation of atropine therefore provides a nonhuman primate model for studying the underlying neural mechanisms of anisometropic amblyopia.

Effects of early unilateral blur on the macaque's visual system. III. Physiological observations.

We studied the properties of visual cortical and lateral geniculate neurons in 5 macaque monkeys raised with the vision of one eye blurred by daily instillation of atropine. This rearing reduced the degree of binocular interaction in striate cortical neurons and caused a modest shift in eye dominance away from the atropine-treated eye. It also led to a difference in the spatial properties of neurons driven by the 2 eyes: neurons driven by the treated eye tended to have lower optimal spatial frequencies, poorer spatial resolution, and lower contrast sensitivity than neurons driven by the untreated eye. Some of the few binocularly driven neurons had receptive fields with sharply different spatial properties in the 2 eyes, with the treated eye's receptive field always having poorer spatial resolution. In striate cortex, the effects on neuronal spatial properties were less marked in layer 4 than in more superficial or deeper layers; there was no difference in the spatial properties of lateral geniculate neurons driven by the 2 eyes. A small sample of extrastriate cortical neurons from a single animal showed effects similar to those seen in striate cortex. The striate cortical changes varied consistently from animal to animal: The less affected animals had no discernible eye dominance shift and relatively small differences in spatial properties between the eyes; the more affected animals had substantial eye dominance shifts and larger interocular spatial differences. These variations were also reflected in, and consistent with, behavioral and anatomical measurements performed in the same monkeys.

Effects of early unilateral blur on the macaque's visual system. II. Anatomical observations.

We studied the effects of early unilateral blur on the anatomical organization of the visual pathways in 8 macaque monkeys. Blur was induced in one eye, beginning 2-14 d after birth, by 0.5% atropine twice a day. Atropinization was stopped at 6-8 months of age, and the animals were studied for anatomy 3-24 months later. The retina and all other eye tissues showed normal histology. In the dorsal lateral geniculate nucleus (LGN), cells in parvocellular layers receiving input from the atropine-treated eye were 9-32% smaller and were more lightly stained than those in layers innervated by the untreated eye. These changes were generally larger in the LGN ipsilateral to the treated eye. LGN cell size changes were absent or much smaller in the magnocellular layers. In the striate cortex, the distribution of the oxidative enzyme cytochrome oxidase (CO) was markedly altered in layer 4C beta. Layer 4C beta is uniformly stained in normal animals, but showed a distinct pattern of alternating high and low CO bands in the atropine-treated animals; the bands of higher CO activity were narrower than the bands of lower activity and had a 857-1050 micron repeat. Fainter banding was seen in layers 4A, 4C alpha, and 6, but the density of the rows of dark CO-stained dots in layer 3 was unaffected. Double-labeling revealed that the narrow dark CO bands were associated with the centers of the ocular dominance columns devoted to the atropine-treated eye. The distribution of 14C-2-deoxyglucose uptake in visual cortex produced by 4.5-9 c/deg spatial frequency stimulation was strongly biased toward the untreated eye. The treated eye could, however, elicit reasonably strong uptake when stimulated with patterns containing lower spatial frequencies. These results suggest that unilateral neonatal blur preferentially affects the parvocellular layers of the LGN and layer 4C beta of striate cortex, which are the portions of the central visual system associated with the processing of information concerning fine spatial detail. These anatomical changes are consistent with the high spatial frequency loss of vision demonstrated behaviorally and electrophysiologically in the atropine eye-driven visual system of these same animals.

Abolition of visual cortical direction selectivity affects visual behavior in cats.

We reared cats in an environment illuminated stroboscopically at 8 Hz, and studied their ability to detect and discriminate the direction of motion of sinusoidal gratings. Normal cats, like humans, could discriminate the direction of a grating's motion at contrasts that are just barely visible. Strobe-reared cats could detect the grating at contrasts similar to those required by normal cats, but required contrasts that were about 10 times threshold to identify the direction of motion. We subsequently studied the activity of single units in the striate cortex in these cats, and found that directional motion selectivity--normally a prominent feature of striate cortical neurons--was almost absent; other cortical receptive field properties were roughly normal. These results suggest that directionally selective neurons are involved in visual discriminations based on the direction of motion.