Vision and attention. II: Is visual attention a mechanism through which a deficient magnocellular pathway might cause reading disability?

Recent research in reading disability has discovered that at least some reading-disabled subjects have deficits in their magnocellular (M) visual pathways. However, the mechanism by which M pathway deficits affect reading has not been addressed. Abnormal attention has long been known to be associated with reading-disabled individuals, and new research in visual attention has determined that transient visual attention is dominated by M-stream inputs. The purpose of this study was to determine whether visual attention might be the mechanism through which a faulty M pathway could produce visual deficits in reading-disabled subjects. Spatiotemporal attentional response functions were measured using the Line Motion Illusion and compared in normal and disabled readers. Specific abnormalities in the visual attention mechanisms of disabled readers were found which might suggest mechanisms by which reading could be affected by a deficient M stream.

Vision and attention. I: Current models of visual attention.

Visual attention has been defined by different researchers and clinicians in a variety of ways that are sometimes conflicting or confusing. This paper will provide a unified definition of visual attention, as well as a putative neurophysiological mechanism that is consistent with this unified definition. In addition, recent data on the mechanisms of visual attention will be presented, including its spatial organization and temporal dependencies, as well as the role of parallel visual pathways in its activation.

Transient visual attention is dominated by the magnocellular stream.

Robust visual attentional responses are produced by the sudden onset of a visual cue, but the properties of cues that best elicit an attentional response are not fully known. We used the line-motion illusion (Hikosaka et al., 1991) to investigate the optimal cue properties that evoke visual attention. We found that visual attention is driven primarily by the luminance contrast of the cue. Furthermore, by manipulating the spatial, chromatic, and contrast properties of cues, we found that magnocellular (M) stream biased cues always override the response to parvocellular (P) stream biased cues, even when the P stream biased cues are presented first. Our data suggest that cues that preferentially excite the M pathway predominantly capture visual attention.

Motion perception deficits in glaucomatous optic neuropathy.

The mechanisms mediating impaired motion perception in glaucoma were investigated. Direction discrimination thresholds for low (4.2 deg/sec) and high (12.5 deg/sec) velocity random-dot kinematograms were measured in controls and patients with glaucoma or ocular hypertension. Thresholds were elevated significantly in glaucoma patients and individual ocular hypertensives. Threshold elevations were not due to blur or pupil size. After compensating for motion reversals, high but not low velocity thresholds remained elevated. Only high velocity thresholds correlated with differential luminance sensitivity. A hypothesis that different mechanisms mediate glaucoma-induced deficits at high and low velocities is presented.

Visual attention mechanisms show a center-surround organization.

The sudden onset of a cue triggers visual attention, which then enhances visual processing in the zone near the cue. This enhancement causes a motion illusion in subsequent stimuli presented near the cue. At greater separations from the cue, the illusory motion reverses direction, indicating prolonged processing speed. Measurements of the strength and direction of illusory motion at increasing separations from the cue reveal an attentional 'perceptive field' with an excitatory center at the locus cued and an inhibitory surround subtending the remaining visual field. These findings help explain the traditional attentional 'benefits' and 'costs' of attention.

Topography of the age-related decline in motion sensitivity.

PURPOSE: We examined whether the decline in motion sensitivity in the elderly is equivalent for different visual field locations. METHODS: High velocity (28 degrees/s) random dot kinematograms (RDK's) were used to measure direction discrimination thresholds for 5 locations in the visual field (1 position centered on fixation and 4 locations each centered 18 degrees from fixation in the nasal, temporal, superior, and inferior quadrants). Differential luminance sensitivity was assessed by automated perimetry. Younger (N = 15, mean age = 22.9 +/- 1.3 years) and older (N = 13, mean age = 66.6 +/- 4.5 years) subjects were studied. RESULTS: Motion sensitivity varied with test location for both younger and older subjects, but sensitivity was significantly lower in older individuals. The largest age-related reduction in sensitivity was in the central location, whereas the smallest decline was in the superior position. No significant correlations between motion and differential luminance sensitivity were evident. CONCLUSIONS: There is a significant age-related deterioration in visual sensitivity to motion which is more pronounced in the central visual field than in some regions of the more peripheral field. Although both motion and differential luminance sensitivity decrease with age, the rate and the magnitude of the loss differ for these two visual functions.

A sensory explanation for visual attention deficits in the elderly.

BACKGROUND: Reduced visual attention has been implicated as a major factor in age-related deficits of real-world functioning such as driving. However, the contribution of sensory factors to visual attention deficits has not been delineated. The magnocellular stream, which is diminished in aging, provides the dominant input to attention, and could provide a direct visual sensory explanation for these attentional losses. METHODS: We measured temporal response curves for transient visual attention in older and younger subjects. Attentional cues stimulated both the magnocellular and parvocellular streams. RESULTS: Older subjects exhibited slower, weaker responses than younger subjects. The time course in younger subjects agreed with that obtained for magnocellular(M)-biased stimuli, whereas that of older subjects agreed with parvocellular(P)-biased responses. CONCLUSIONS: The activation of transient visual attention is slower and weaker for elderly subjects, reflecting a reduced magnocellular input. This marks a primary visual cause for visual attention deficits of the elderly.

Visual impairment in Parkinson's disease: deficits in orientation and motion discrimination.

PURPOSE: In order to obtain a better understanding of the pathophysiology of visual impairment in Parkinson's disease (PD), orientation and motion discrimination were investigated in PD patients. METHODS: Orientation and motion discrimination thresholds were assessed psychophysically in PD patients with normal cognition (N = 11) and compared to results for age-matched controls (N = 22). RESULTS: PD patients exhibited a significant deficit in orientation discrimination for horizontal but not for vertical gratings. Also, motion discrimination thresholds were elevated significantly in PD patients. No significant correlations were found between motion and orientation discrimination thresholds. CONCLUSIONS: Our results indicate that PD patients exhibit significant visual dysfunction in the absence of cognitive impairment supporting the contention that neural dysfunction in PD is more generalized than previously realized. The results provide further evidence of an orientation selective visual deficit in PD and show that the ability to discriminate global motion is also attenuated in PD.

Topography of the evoked potential to spatial localization cues.

Visual tasks that are perceptually diverse might be expected to elicit unique evoked-potential waveforms that exhibit differing topographic maps. To investigate this possibility, multichannel visual-evoked potentials (VEPs) were recorded in response to several dot spatial localization stimuli that are physically similar yet produce different percepts (vernier offsets, steroscopic disparity, bisection, orientation, and relative displacement) to determine if the unique percepts arising from these stimuli reflect the activation of different cortical neural populations. The resulting evoked potentials were all similar in waveform, although the stereoscopic VEPs were relatively delayed. Topographic maps of the evoked-potential activity to each stimulus revealed a late major component with two independent foci: one 7 or more centimeters above the inion lateral to the midline, and the other at least 6 cm lateral to OZ. The scalp localization of both peaks was independent of both the position of the stimulus in the visual field and the particular stimulus cue presented. An asymmetric response to pattern appearance vs. disappearance indicated strong pattern specificity for each stimulus type except unreferenced motion. The timing of the VEP responses and relative insensitivity to retinal locus of stimulation suggest the involvement of higher cortical areas. The two map foci might be interpreted as activation of inferotemporal and parietal cortices whose roles are thought to be visual object interpretation and spatial attention and localization, respectively.

Monocular accommodative fluctuations associated with temporomandibular joint syndrome.

A young scientist reported severe headaches emanating from behind one eye with an associated intermittent monocular blur. The monocular near cross-cylinder test revealed an accommodative fluctuation of one eye alone with a range of approximately 3 D. Referral to a neuro-ophthalmologist failed to produce a diagnosis for the disorder. A subsequent routine visit to a dentist provided a diagnosis of Costen's temporomandibular joint (TMJ) syndrome, a condition that is commonly misdiagnosed as a variety of neurological and vascular disorders.

Serial and parallel search in pattern vision?

The nature of the processing of combinations of stimulus dimensions in human vision has recently been investigated. A study is reported in which visual search for suprathreshold positional information--vernier offsets, stereoscopic disparity, lateral separation, and orientation--was examined. The initial results showed that reaction times for visual search for conjunctions of stereoscopic disparity and either vernier offsets or orientation were independent of the number of distracting stimuli displayed, suggesting that disparity was searched in parallel with vernier offsets or orientation. Conversely, reaction times for detection of conjunctions of vernier offsets and orientation, or lateral separation and each of the other positional judgements, were related linearly to the number of distractors, suggesting serial search. However, practice has a significant effect upon the results, indicative of a shift in the mode of search from serial to parallel for all conjunctions tested as well as for single features. This suggests a reinterpretation of these and perhaps other studies that use the Treisman visual search paradigm, in terms of perceptual segregation of the visual field by disparity, motion, color, and pattern features such as colinearity, orientation, lateral separation, or size.

Selectivity of the evoked potential for vernier offset.

Hyperacuity thresholds of a few arc seconds can be achieved psychophysically for a variety of spatial localization tasks. The present experiments show that evoked potentials can be elicited in response to the introduction of vernier offsets, but not by the introduction of other cues to hyperacuity such as bisection or relative pattern motion, although each of these cues is equally salient psychophysically. Moreover, vernier acuity measurements and the evoked potentials elicited in response to vernier offsets are strongly degraded by the introduction of flanking stimuli 2-4 min from the vernier target. This suggests that the hyperacuity VEP is a cortical correlate of a very specific type of hyperacuity, that produced by vernier offsets (colinearity failure).

Electrophysiological correlates of hyperacuity in the human visual cortex.

The human visual system is capable of detecting a vernier misalignment with extraordinary accuracy. Since this remarkable precision in spatial localization is better than can be naively predicted by simple optical or anatomical considerations it has been termed a hyperacuity. So far no single neurone model seems capable of accounting for hyperacuity, and the retinal image might require reconstitution in a finer grained from in the visual cortex. We report here an electrophysiological correlate of hyperacuity recorded from the human visual cortex. The amplitude of the visually evoked potentials (v.e.ps) elicited by the appearance of a vernier offset varied systematically with the magnitude of the offset. Extrapolation of the function relating v.e.p. amplitude and log offset to zero voltage resulted in an electrophysiological estimate of vernier acuity that was similar to the observer's psychophysical threshold.