Ординатура / Офтальмология / Английские материалы / Advances in Understanding Mechanisms and Treatment of Infantile Forms of Nystagmus_Leigh, Devereaux_2008

this change in relative latency would be expected to generate substantial variations in perceived location when these signals are compared. Low-pass filtering of one or both signals has the effect of smoothing these perceptual variations over time.35,43

If our suggestion that extraretinal eye movement signals are less low-pass filtered in subjects with IN is correct, then these subjects should be vulnerable to variations in the latency of retinal signals as indicated by sizeable, systematic changes in perceived stability for either high or low luminance stimuli. Tkalcevic and Abel44 found that subjects with IN most often report oscillopsia when a bright fixation stimulus (440 cd/m2) is superimposed on a much dimmer background field (ca. 0.1 cd/m2). However, the oscillopsia that these subjects reported was typically relative motion between the bright fixation stimulus and the dimmer background and is likely to reflect the Hess effect (an illusory spatial separation between moving targets of different luminance).41 It therefore remains an open question whether the perception of oscillopsia in IN varies systematically with the luminance of the stimulus.

PERCEIVED MOTION SMEAR DURING NORMAL EYE MOVEMENTS AND IN

In addition to the potential for producing oscillopsia, the rapid to-and-fro eye movements in subjects with IN would be expected, on the basis of visual persistence, to generate the perception of motion smear. However, the retinal image motion of a physically stationary object during normal observers’ voluntary45-48 and involuntary49,50 eye movements generates a smaller extent of perceived motion smear than if comparable motion of the retinal image occurs when the eyes remain stationary. Because the reduction of perceived motion smear during normal eye movements can occur when no other visual stimuli are present in the field, we attribute this reduction to the action of extraretinal signals. Recently, we found that the extent of perceived motion smear is reduced even when a smooth movement of the eye is produced by pressing on the globe, implicating a contribution from signals of eye muscle proprioception.

The reduction of perceived motion smear during normal eye movements is asymmetrical, depending on the relative direction of target motion with respect to the moving eye. Specifically, the extent of perceived smear is reduced for targets that undergo relative motion in the opposite direction of the eye movement.48,50,51 On the other hand, the extent of perceived smear for a target that moves in the same direction as, but faster than, an ongoing eye movement is the

Figure 2.3 The median duration of perceived motion smear produced by a 100-millisecond bright spot is shown during the leftward and rightward slow-phase IN eye movements of subject SS. On each trial, the bright spot moved to the left or right with respect to the moving eye. Error bars are standard errors. IN, infantile nystagmus.

same as that produced by a target that moves physically during stable fixation. Our explanation for this asymmetrical reduction of perceived motion smear is as follows. The visual system interprets the presence of “opposite” target motion during an eye movement as consistent in direction with an object that is stationary in the world. Presumably, the visual system prefers that stationary objects do not appear smeared. On the other hand, a target that moves in the “same” direction as an ongoing eye movement can be assumed to be moving physically in the world. The extent and direction of perceived motion smear has been shown to provide useful psychophysical information about the speed and direction of a target’s motion.52-55 For this reason, it is advantageous for the visual system not to reduce perceived smear for targets that are physically in motion.

The essentially incessant eye movements that occur in subjects with IN make it difficult to compare perceived motion smear when the eyes are and are not moving. Instead, we asked observers with IN to report the extent of perceived smear for a target that moved at various speeds in the “opposite” or in the “same” direction as the IN slow phase. The target was a small, bright laser spot that was triggered to occur in an otherwise dark field, after the end of a foveation period and near the start of an accelerating IN slow phase. The target was flashed for 100 milliseconds, after which the observer adjusted the length of a continuously visible line to match the extent of perceived motion smear. One observer’s nystagmus alternated periodically between jerk left and jerk right and, across trials, the target was presented in the “opposite”

and ”same” direction as the rightward and leftward IN slow phases. Consistent with the results described for normal observers, the extent of perceived motion smear is reduced asymmetrically in this subject with IN—it is less when relative target motion is to the left during rightward slow phases and when relative target motion is to the right during leftward slow phases (Fig. 2.3). To allow the measurements obtained for different velocities of retinal image motion to be combined and compared, Figure 2.3 expresses the extent of perceived motion smear as duration in milliseconds. The average duration of perceived motion smear for targets that move in the “same” direction as the leftward and rightward IN slow-phase eye movements is approximately 105 milliseconds, indicating no reduction in the extent of perceived smear. However, the average duration of perceived motion smear for targets that move “opposite” the direction of the IN slow phases is less than 35 milliseconds, which is less than the duration of perceived smear reported by normal observers for a 100-millisecond target that moves “opposite” the direction of pursuit.45,47,51 Results for a second observer with IN, whose predominant slow-phase direction is to the right, show a similar reduction of perceived smear for relative target motion to the left. These data are consistent with a previous report that a physically stationary, continuously visible target generates little or no perceived motion smear in subjects with IN,9 and suggest that the extraretinal eye movement signals for IN may be more effective in reducing the extent of perceived motion smear than the extraretinal signals of normal observers. Although not tested, we anticipate that the perception of motion smear is also reduced during the quick phases of IN, as such a reduction was shown to occur during normal saccades.46

A Possible Mechanism for the Reduction of Perceived Motion Smear

In normal observers, the duration of visible persistence and, therefore, the perception of motion smear can be accounted for by the relatively sluggish temporal response of the visual system.56 The response of the visual system in time is described by the temporal contrast sensitivity function or, alternatively, by the inverse Fourier transformation of this function, the TIRf. Consequently, the duration of perceived motion smear should be reduced during eye movement if extraretinal signals act to reduce the duration of the TIRf. Indeed, previous physiological57 and psychophysical results58 are consistent with an increase in speed of the TIRf during normal saccades.

We measured temporal contrast sensitivity functions for 6 normal observers during fixation at the center of a 10º vertical sine-wave grating (mean luminance

= 65 cd/m2; spatial frequency = 1 cpd), which drifted leftward or rightward to produce temporal frequencies between 6 and 30 Hz. For comparison, we measured the temporal contrast sensitivity for the same sine-wave stimulus during horizontal pursuit at 8 deg/s. During pursuit, the physical drift rate of the grating was varied to generate retinal temporal frequencies between 6 and 30 Hz, both in the “opposite” and “same” directions as the observers’ rightward pursuit movement. Each presentation of the grating target occurred within a smoothed temporal contrast window with a duration of approximately 400 milliseconds. Horizontal eye movements were monitored by infrared limbal reflection, and trials were accepted in the absence of blinks or saccades and only if the calculated temporal frequency of retinal image motion was within ±15% of the grating’s nominal temporal frequency. In fact, calculated temporal frequencies were highly similar for motion of the grating in the “opposite” and the “same” directions as pursuit, as the average pursuit gains of the 6 observers ranged from 0.96 to 1.03 (overall mean gain = 0.99).

To convert the temporal contrast sensitivity data to TIRfs, we assumed that the visual system can be described at threshold by a linear second-order, low-pass temporal filter (Eq. 2.2):

R(t) = (A × W/ Sqrt[1–D2]) × exp(−D × W × t) × sin(W × Sqrt[(1–D2) × t])

In this equation, R is the response of the visual system in time (t) to a brief pulse, A is the response amplitude, W is the temporal frequency of the response in radians/s, and D is the damping ratio (0 ≤ D < 1). Based on this assumption, we iteratively determined the TIRf that, after Fourier analysis, yielded the best fit to the temporal contrast sensitivity data for grating motion to the left and right during fixation, and for grating motion in the “opposite” and “same” direction as rightward pursuit. As shown in Figure 2.4, the average TIRf fit to the contrast sensitivity for motion “opposite” the direction of pursuit has the shortest duration. Consistent with our results for perceived motion smear,48,50 the TIRfs fit to contrast sensitivities for gratings presented during fixation and for gratings that move in the “same” direction during pursuit have similar but longer durations. The duration of the TIRfs can be described in terms of the inverse of their natural temporal frequencies, which range from 10.7 Hz during fixation to 11.6 Hz during “opposite” motion of the grating during pursuit. Statistical analysis indicates that the natural frequency of the TIRf determined for “opposite” grating motion is significantly higher (p < 0.05) than in either of the other two conditions.

We also compared estimates of normal observers’ TIRfs during fixation to the TIRfs of subjects with IN.

0.2

0

Figure 2.4 Temporal impulse response functions (TIRfs) are presented, estimated from the temporal contrast sensitivity data of 6 normal observers during fixation and rightward pursuit. During pursuit, the relative motion of a 1 cpd grating target was either in the “opposite” or the “same” direction as the observer’s eye movement. Sensitivities measured during rightward and leftward motion of the grating during fixation were averaged. The peak response of each calculated TIRf is normalized to a value of 1 on the y-axis. Note that the TIRfs estimated for fixation and for the “same” direction of motion during pursuit are virtually superimposed.

These TIRfs were fit to temporal contrast sensitivity functions for a 16º horizontal 3 cpd square-wave grating, which underwent temporal frequencies of counterphase flicker between 1 and 40 Hz. Because the normal TIRf is known to speed up during normal saccades,58 we included only subjects with IN whose waveforms were predominantly pendular. The TIRf fitted to the contrast-sensitivity data of the 4 normal observers has a natural temporal frequency of 8.7 ± 0.6 (SE) Hz. The lower natural frequency of this function than for the TIRf shown for fixation in Figure 2.4 is expected from the increase in the grating’s spatial frequency.59,60 Compared to the results of the normal observers, the TIRfs for 3 subjects with pendular IN all have considerably shorter durations, with natural frequencies that range from 11.6 to 14.9 Hz (Fig. 2.5).

ISSUES FOR VISUAL PERCEPTION

AND FUNCTIONING IN IN

In addition to the “cancellation” of retinal image motion by extraretinal eye movement signals, several studies conducted on normal observers indicate that an extended visual frame of reference can mediate perceived stability during changes in eye position.61-64 For example, Murakami64 proposed recently that the visual system “dismisses” the common motion that occurs in the retinal image, which is interpreted to be a

Figure 2.5 Temporal impulse response functions (TIRfs) estimated from temporal contrast sensitivity data are shown for 4 normal observers (averaged) and 2 individual subjects with pendular IN. The normal observers’ contrast sensitivity was measured during fixation. As in Figure 2.4, the peak of each calculated TIRf is normalized to a value of 1. In contrast to the results shown in Figure 2.4, the TIRfs in this figure are based on contrast sensitivities for a 3 cpd counterphase flickering grating. IN, infantile nystagmus.

consequence of ongoing fixational eye movements. Clearly, this “dismissing” hypothesis cannot completely account for the perception of stability in subjects with IN, as these subjects report motion of the visual scene when the retinal image is stabilized artificially.14,17 Further, Abadi et al.17 reported that subjects with IN perceive a small target to be stable over a greater range of nystagmus-induced retinal image motion than a considerably larger target. Similarly, Tkalcevic and

found that the perception of oscillopsia in subjects with IN is unrelated to the size of the visual target. These data contradict the tendency of normal observers to perceive a large visual frame of reference as stable. However, they may be accounted for by the higher temporal frequencies of retinal image motion in subjects with IN, compared to the temporal characteristics of the image motion in most of the studies that examined visual frame-of-reference effects in normal observers.61,62

Possibly, the visual system sets an upper limit on the amplitude of the common retinal image motion that it is willing to “dismiss,” based on its integrated long-term experience. If so, the upper limit for “dismissing” retinal image motion might be higher in persons with IN, whose visual experience during and after development includes substantially greater amounts of retinal image motion than that of normal observers. This hypothesis could account for why motion thresholds are elevated in subjects with IN, even when the nystagmus is temporarily in abeyance,65 in patients with acquired vestibular66,67 or ocular motor68 deficits,

and in normal observers who are exposed to retinal image motion to simulate the motion present in IN.69

Our data indicate that extraretinal eye movement signals associated with the slow phase of IN reduce the extent of perceived motion smear for the relative motion of a target in the direction opposite to the movement of the eye. In everyday viewing, objects that are stationary in space generate “opposite” motion during eye movements and therefore should be perceived as relatively clear. Our results so far for subjects with IN are qualitatively similar to results found in normal observers for relative target motion in the opposite direction of pursuit, smooth vergence, and the VOR.45,47-51 In normal observers, the extent of perceived motion smear is reduced also for targets that move in the direction opposite to a head movement— for example, during visual suppression of the VOR when the eyes undergo no movement with respect to the head.50,51 Indeed, during the VOR and the visually enhanced VOR, normal observers report a reduced extent of smear for two directions of target motion — the directions opposite the eye and the head movements, respectively.50 In some patients, IN is accompanied by rhythmic head movements, which typically do not reduce the magnitude of the retinal image motion that results from their nystagmus.70-72 The observation that perceived motion smear is reduced during normal head movements raises the possibility that extraretinal signals associated with these head movements in subjects with IN might supplement the effect of extraretinal eye movement signals to further reduce the extent of perceived motion smear.

The results we obtained during passive eye rotation indicate that extraretinal signals from eye muscle proprioception are sufficient to reduce the extent of perceived motion smear in normal observers. It remains unclear whether proprioceptive signals are necessary to reduce the perception of motion smear during eye movements, or whether the reduction found during normal eye movements can be mediated by efference-copy signals alone. A contribution of eye muscle proprioception to perceived clarity during eye movements is relevant because of the recent introduction of tenotomy as a surgical treatment to reduce the severity of eye movements in IN.73-75 Tenotomy disrupts the proprioceptive information that comes from extraocular muscles and could conceivably lead to some compromise of perceptual clarity in patients with IN.

Finally, an important and as yet unanswered question is what is the contribution of perceived motion smear, and its reduction during eye (and perhaps head) movements, to visual functioning in IN. To address the influence of perceived motion smear on visual acuity, we compared the acuity of normal subjects in two experimental conditions. In one condition, the motion

of the acuity target simulated that during the complete IN waveform. In the second condition, the acuity target was visible only during simulated foveation periods, and was blanked during all other phases of the waveform. Measured visual acuity was uniformly 0.1 to 0.15 logMAR poorer during the completewaveform condition for simulated foveation durations that ranged from 20 to 120 milliseconds.76 These data indicate that the presence of motion smear during simulated IN slow phases impairs visual acuity, despite the availability to the visual system of a stationary retinal image during simulated foveation periods. Nevertheless, it remains unclear whether the retinal image motion during slow phases contributes also to a reduction of visual acuity in subjects with IN. One alternative possibility is that the reduction of perceived motion smear by extraretinal signals for IN might protect against an impairment of acuity. A second possibility is that visual acuity is limited in subjects with IN by a sensory abnormality, such as amblyopia, rather than by the instantaneous parameters of the retinal image motion.5,8,77 These alternatives can be evaluated by comparing the visual acuity of normal observers and subjects with IN under comparable conditions of retinal image stability and motion.

ACKNOWLEDGMENTS We thank Susana Chung, Thanh Loan Nguyen, Spencer Obie, Mahalakshmi Ramamurthy, Shobana Subramaniam, and Lan-Phuong Vu-Yu for assistance in collecting, analyzing, and interpreting some of the data presented in this chapter. Support for these studies was provided by grants R01-EY05068, P30-EY07551, and T35-EY07088 from the National Eye Institute and by award 003652-0185-2001 from the Texas Advanced Research Program.

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RICHARD V. ABADI

ABSTRACT

Fixation behavior has an enormous influence on perception. Too little image motion and the scene fades; too much, and blurring and oscillopsia are experienced. To keep within an optimal operating range, a number of feedback control systems counter drifts and suppress unwanted saccades. Vision, which is driven by both bottom-up and topdown processing, is an important component. Thus physiological microsaccades and saccadic intrusions are modulated by exogenous and endogenous attention, while early onset afferent defects often lead to strabismus and nystagmus. The visual consequences of such fixation failures depend on the onset time of the visual loss, the nature of any attendant afferent defect, and the retinal-image dynamics. This chapter describes psychophysical studies that examine the spatial (contrast sensitivity, visual acuity, vernier acuity, and stereopsis) and temporal (absolute and relative detection and discrimination motion thresholds) visual performance of individuals with idiopathic and nonidiopathic congenital nystagmus. Experiments that investigate the strongly visually driven behavior of manifest latent nystagmus and issues relating to spatial

constancy, form deprivation, and emmetropization are addressed.

Visual perception is strongly dependent on the optical quality of the retinal image and its subsequent neuronal processing, and is also intimately linked to fixation control. Ideally, focused images should rest on the fovea and be held relatively steady. Thus retinal-image quality is dependent on both position and velocity.

The purpose of this chapter is to describe and explore the bonds that link perception and oculomotor control. Poor retinal-image quality can modify the efficiency of gaze-holding, while unstable fixation can degrade visual performance. These symbiotic relationships are all the more critical when failure occurs early in life. This chapter examines the link between involuntary physiological fixation movements and perception, the effects of early fixation failure (infantile nystagmus [IN]) on visual performance, and the effects of early visual loss on the development of normal fixation behavior. Measures of sensory performance in congenital nystagmus (CN) and manifest latent nystagmus (MLN)* include visual and vernier acuity, spatial and temporal thresholds, and spatial constancy. Fixation stability, described in terms of ocular and retinal-image dynamics, is linked with other issues,

including emmetropization, form deprivation, and ocular alignment.

NORMAL FIXATION AND VISION

Normal Physiological Fixation Behavior

If retinal-image slip velocities exceed 4 deg/s, blurring and the illusionary movement of the visual world (oscillopsia) can occur.2,3 The amount of retinal-image motion that can be tolerated before vision deteriorates depends on what is being viewed and the nature of the task. On the other hand, when retinal-image velocities are dramatically reduced or even stabilized, then fragmentation and the eventual perceptual loss of a scene are experienced due to neural adaptation.4 Clearly, in the interest of achieving high visual acuity (VA), fixation eye movements must be limited to a specific range by a set of control systems.5 One such system, the fixation system, has three distinct components: (1) the visual system’s ability to detect retinal-image drift and program corrective eye movements, (2) the ability to attend to or “engage” a particular target of interest, and (3) the suppression of unwanted saccades that would otherwise take the eye off target.

Whenever there is a change in gaze away from the primary position, the neural integrator is recruited to sustain the desired eye position. Moreover, during locomotion the vestibulo-ocular and optokinetic systems act in unison to reduce retinal-image slip. Thus gaze-holding has both visual (smooth pursuit and optokinetic) and motor (vestibular) inputs that are underpinned by neuronal processes (fixation cells in the superior colliculus, pause cells in the brainstem, and the neural integrators in the brainstem and cerebellum). The desired outcome of steady fixation is also dependent on critical cognitive factors such as attention, alertness, and the saliency of the visual task.6

Under normal circumstances, involuntary fixation eye movements, made up of small disconjugate drifts (1 to 3 minutes of arc) and small multiplanar conjugate microsaccades (5 to 10 minutes of arc, 1 to 2 per second), are ever present.7-10 In addition, a further class of larger-amplitude involuntary physiological eye movements known as saccadic intrusions are also present. Saccadic intrusions consist of conjugate horizontal saccadic eye movements that take the form of an initial fast eye movement away from the desired eye position and are followed, after a variable duration, by a return saccade or drift.5,11-13 Saccadic intrusion amplitude mean and range have been reported to be 0.60˚ ± 0.50˚ and 0.10˚ to 4.10˚ with a saccadic intrusion frequency mean and range of 18.0 ±14.4 per minute and 1.0 to 54.8 per minute, respectively.

A key motor property of both fixation microsaccades and saccadic intrusions is that, like the fast phases of vestibular (optokinetic and IN), they all lie on the main sequence.13 Recent studies indicate that microsaccades and saccadic intrusions are modulated by exogenous and endogenous attention in a similar manner, thereby suggesting that microsaccades and saccadic intrusions lie on a continuum of involuntary fixation instabilities.14-18 Evidence for the effects of attention on saccadic intrusion behavior has been found by varying the “bottom-up” target viewing conditions (target presence, servo control of the target, target background, target size). Saccadic intrusion amplitude has been found to be significantly higher when the target is abolished in the dark, and saccadic intrusion frequency is lower during open-loop conditions.16 Saccadic intrusion frequency decreases during the “hold eyes steady” command, and saccadic intrusions are more frequently directed away from exogenous cues during cue-target tasks (i.e., modulating top-down attention).16,17

In contrast to viewing a stationary target in photopic conditions, when subjects sit in darkness and attempt to view the remembered location of a target, the mean velocity of the fixation slow-drifts increases about fourfold. This implies that during active fixation of a stationary target, retinal-image slip is under the control of a feedback system that counters drift and holds gaze steady. This response has been called slowcontrol, or a field-holding reflex.6 It is pertinent to note that, invariably, reported ranges of fixation stability refer to fixation tasks undertaken in a laboratory using head restraints and bite bars. However, under natural viewing conditions (e.g., standing), when the head is free to move, mean drift velocity can increase by up to a factor of 10.19

In summary, visual inputs have an enormous influence on physiological fixation behavior. Too little image motion causes the scene to fade, while too great a movement leads to blurring and oscillopsia. Cognitive factors have strong and important influences on involuntary fixation behavior. Finally, if visual inputs are not appropriate during the early neonatal period, it is highly likely that fixation stability will become compromised.20-24

ABNORMAL FIXATION AND VISION

Infantile Nystagmus

Unstable fixation in the form of a nystagmus may occur at birth or soon after. The most common type of infantile fixation instability is IN, in which the oscillations are typically involuntary, conjugate, horizontal, and jerky. The oscillations may consist

entirely of slow phases, as in the case of pendular nystagmus.13,25-29

There are three major manifestations of IN: CN, MLN, and latent nystagmus (LN).13,25-32 The principal differences between these three lie in the form of their slow phases. In CN, the slow phases are typically of an increasing exponential velocity form, whereas in MLN and LN the slow-phase velocity is decreasing or linear.13,25,26,30-35 An additional distinguishing feature relates to the fast phase, which in MLN and LN always beats toward the viewing eye. Many different IN waveform shapes have been described, and their spatial and temporal characteristics have been examined in detail.13,25,26,28,35

Both CN and MLN are associated with a variety of afferent sensory disorders, including albinism, optic nerve hypoplasia, and congenital cataracts. On the other hand, CN and MLN may occur without any detectable ocular or sensory system abnormalities, in which case the nystagmus is designated an idiopathic CN. (The idiopathic category is clearly a clinical convenience rather than an absolute category.)

In a recent report, Abadi and Bjerre28 noted that while exclusively conjugate horizontal oscillations were found in 78% of an IN sample (n = 224), 14% (n = 32) also displayed a recordable torsional component to their IN. Neither CN nor MLN waveforms were related to any of the three subject groups (idiopaths, albinos, and ocular anomalies), although MLN was found to occur most frequently in the ocular anomaly group.

Visual performance in IN is, for the most part, dependent on three factors: (1) the range of retinalimage slip velocities, (2) foveation, and (3) the state of the eye and visual pathways. Most clinicians are likely to measure the spatial resolution limit (that is, VA) and stereoacuity, and to note the presence of any associated smear or motion of the visual scene. Not surprisingly, the distribution of the retinal-image slip velocities within the slow phase and the duration of the period corresponding to the low-slip velocities are important components in determining visual per-

formance.25,31,36-41

CONGENITAL NYSTAGMUS

Waveform and Visual Performance

For those individuals with an idiopathic CN, Snellen VA generally ranges between 20/20 and 20/60, and the retinal-image movement is an important cause of the visual degradation.28 Peak slow-phase velocities can reach 180 deg/s, and large portions of the slow phase can exceed velocities greater than 10 deg/s.39 However, periods of low retinal-image velocity (i.e., ≤ 4 deg/s)

can also be present during the slow phase, and if the duration of these low-velocity intervals is long and coincides with the fovea (±0.5°), VA can reach reasonable levels (better than 20/30). This situation is referred to as foveation.

Not all subjects with CN foveate, nor should they be assumed to do so. Abadi and Bjerre28 reported that, based on fundus video-oculography, only 41 of their 74 subjects (55%) exhibited foveation. Modification of CN waveforms, encountered when subjects make use of their gaze nulls, often improves VA and can minimize oscillopsia.42 However, there appears to be no change in foveation (and thereby VA) with increasing visual task demand,43 although the authors suggest that if the visual task is seen to be personally important, then the additional stress brought on by the motivation to perform well could increase the nystagmus intensity and thereby reduce VA.26,28,44

The length of the foveation period has been shown to be a better predictor of visual performance compared with nystagmus intensity (amplitude × frequency),25,28,45 and VA is also strongly correlated with the duration and beat-to-beat position variability of the foveation periods.46-50 Using a mirror galvanometer arrangement to simulate comparable image motion in normal observers, Currie et al.51 have shown that their control subjects performed better than individuals with IN.

Visual Acuity and Contrast

Detection Thresholds

Traditional optotype charts (Snellen, LogMAR) measure the ability to recognize high-contrast targets, whereas contrast sensitivity functions describe the threshold detection of sinusoidally modulated gratings. The IN spatial contrast sensitivity function has several distinguishing features compared with the normal function: (a) the low spatial frequency roll-off is either absent or greatly reduced, (b) the peak contrast sensitivities are shifted toward the lower spatial frequencies, and (c) the high spatial frequency cutoff point is significantly reduced (Fig. 3.1).38,52,53 That is, the IN contrast sensitivity function has been shifted down and to the left compared with the normal function. This attenuation of the medium-to-high spatial frequency detection threshold does not linearly extrapolate for suprathreshold contrast perception.54 For gratings of the same orientation but different spatial frequency, CN subjects demonstrate suprathreshold contrast matching, which depends on the physical contrast of the gratings rather than the characteristics of the contrast sensitivity function. In this case, the compensation across spatial frequency mechanisms for a threshold difference in sensitivity is as effective as that