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The Crowding Effect & Visual Acuity

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            My phenomenon is that patients with amblyopia often are noted to have a more severe visual acuity deficit when tested with a full line of optotypes than when tested with isolated optotypes. This phenomenon, known as the crowding effect, must be considered when testing visual acuity in patients who may have amblyopia (Harley, Nelson, & Olitsky, 2005). The crowding phenomenon relates to the fact that patients with amblyopia have better visual acuity reading s ingle optotype than reading multiple optotypes in a row (linear optotypes). Often, patients with amblyopia will perform 1 or 2 Snellen lines better when presented with single optotypes versus linear optotypes (Wright & Spiegel, 2003).  This is the phenomenon whereby the visual acuity when looking at a letter surrounded by other contours (e.g. in a letter chart) is worse than when looking at individual letters (because of ‘contour interaction). The complementary effect, better acuity with single letters, is called the ‘separation phenomenon’. The crowding effecting is unusually marked in strabismic amblyopia (Harley et al., 2005).

Crowding is also seen in cases of cerebral impairment, whether due to neonatal encephalopathy, meningitis or other causes. Here is described as the inability to pick single objects out of complex scenes, when the single object can be recognized in isolation. So crowding seems to be a normal feature of the developing visual system, which persists in amblyopia and cerebral visual impairment (Koch & Davis, 1994).

            It is important to recognize this phenomenon clinically because the use of single or widely spaced optotypes when testing visual acuity may underestimate the problem of amblyopia. Better vision may be obtained with single or widely spaced optotypes than with Snellen rows of optotypes. This is sometimes an indication of the minimum level of linear acuity likely to be achieved by occlusion (Pratt-Johnson & Tillson, 2001).

The visual acuity of the amblyopic eye is greater for isolated letters than for whole lines of letters. The acuity drops according to the degree that the letters are crowded together and is called the crowding phenomenon. As amblyopia responds to therapy, this phenomenon will be reduced or disappear altogether. Because the visual acuity obtained on the reading of entire lines of letters is a more sensitive indicator of the depth of amblyopia than the acuity obtained with isolated letters, linear Snellen or crowding should be used to follow the amblyopic patient’s response to testing (Langston, 2007).

Furthermore, in crowding, there is a drastic impairment of the ability to discriminate and recognize figures if they are surrounded with other contours. Signals conveyed by the deviating eye are less salient than those from the normal eye, as the former signals cannot be perceived as long as both eyes are open even if attention is directed to them. Identification of neuronal correlates of these deficits in animal models of amblyopia is inconclusive because the contrast sensitivity and the spatial resolution capacity of neurons in the retina and the lateral geniculate nucleus are normal (Llinás & Churchland, 1996).


Theory of the crowding of visual targets can have a significant impact on visual acuity testing of the amblyopic eye. This is because ignoring the crowding phenomenon in an amblyopic child by presenting only one single optotypes often results in erroneous good acuity scores. Thus, using a row of visual targets provides a more accurate assessment of visual acuity and improved detection of amblyopia (Harley et al., 2005). Furthermore, relatively large receptive field associated with amplyopia may have something to do with this crowding phenomenon. Crowding bars are often used around a single optotype to provide a more sensitive test for amblyopia (Wright & Spiegel, 2003).

In the visual cortex identification of neurons with reduced spatial resolution or otherwise abnormal receptive field properties remained controversial. However, multi-electrode recordings from striate cortex of cats exhibiting behaviorally verified amblyopia have revealed highly significant differences in the synchronization behavior of cells driven by the normal and the amblyopic eye, respectively. The responses to single moving bars that were recorded simultaneously from spatially segregated neurons connected to the amblyopic eye were much less well synchronized with one another than the responses recorded from neuron pairs driven through the normal eye. The difference was even more pronounced for responses elicited by gratings of different spatial frequency. For responses of cell pairs activated through the normal eye the strength of synchronization tended to increase with increasing spatial frequency while it tended to decrease further for cell pairs activated through the amblyopic eye.

Apart from these highly significant differences between the synchronization behavior of cells driven through the normal and the amblyopic eye no other differences were found in the commonly determined response properties of these cells. Thus, cells connected to the amblyopic eye continued to respond vigorously to gratings whose spatial frequency had been too high to be discriminated with the amblyopic eye in the preceding behavioral tests. These results suggest that disturbed temporal coordination of responses such as reduced synchrony may be one of the neuronal correlates of the amblyopic deficit. Indeed, if synchronization of responses at a millisecond time scale is used by the system for feature binding and perceptual grouping, disturbance of this temporal patterning could be the use for the crowding phenomenon, as this can be regarded as a consequence of impaired perceptual grouping (Koch & Davis, 1994).

The crowding phenomenon may arise because the eye is not fixating centrally with the fovea, but is using a region just to one side of it. The reduced acuity is governed by the amount of eccentricity. The precise position in space corresponding to the true foveal centre may often be determined by utilizing Haldinger’s brushes, and entoptic phenomenon. Alternatively, the patient may be asked to fixate the centre of the smallest field of the ophthalmoscope. The practitioner can then observe the position of the foveal reflex, relative to the illuminated area. More accurate results are obtained if an instrument is used to project a graticule image on to the patient’s retina (Agarwal, 2006).

Theory of abnormal visual experience during early life may also be another cause. This is because it can alter the functional domains of the 2 eyes in the visual cortex and result in a reduction in the number of cortical cells receiving input from the deprived eye as well as in the number of binocularly driven visual cortical cells. Even cells that remain may show significant functional deficiencies. It is thought that the receptive fields of neurons in the amblyopic visual system are abnormally large and this is responsible for the crowding phenomenon (Langston, 2007).

Sometimes, single letters or other characters are used instead of a line of letters, as with the E-cube or Sheridan-Gardner test. This is also known as ‘angular acuity’ and it occasionally the only available method of measuring minimum recognizable acuity in very young children as the task is easier for the child. There is usually a higher acuity with angular than with morphoscopic acuity and this has been called the ‘separation phenomenon’. Put another way, there is a ‘crowding phenomenon’ from adjacent letters and this is an example of contour interaction. When single letter acuity is measured, the fact should be recorded with the acuity. Because the crowding phenomenon is enhanced in strabismic amblyopia, angular acuity should only be used when all attempts to measure morphospic acuity have failed. There are several methods of measuring morphospic acuity in young children and the Glasgow acuity Cards or, for young children, the Kay crowded book appear to be most appropriate to the assessment of amblyopia (Evans, 2002).

Visual evoked responses to checkerboard stimuli and preferential looking responses to sinusoidal or square wave gratings may be used to screen for profound amblyopia in infants and preverbal children, but these tests have poor sensitivity for detecting amblyopia. The ideal method for detecting amblyopia is by visual acuity testing using a line of Snellen letters or other optotypes. Amblyopic patients read letters more easily when they are isolated than when they are by other letters or figures (crowding phenomenon) (Evans, 2002).

Testing of isolated letters may have some value in selected situations. Some examiners believe that isolated letter acuity predicts ultimate visual potential after amblyopia therapy. Others have suggested that failure of acuity to improve with conversion to isolated letters is indicative of an organic cause for visual impairment (Harley et al., 2005).

When evaluating for amblyopia, linear acuity is more desirable than single optotype presentation because single optotype presentation underestimates the degree of amblyopia. Surround bars have been used to create crowding in a single optotype and are useful in children who get confused with the multiple optotypes used in linear acuity testing (Wright & Spiegel, 2003).

If possible, it is better to use a linear test type. The patient may demonstrate the crowding phenomenon where reduced visual acuity is detected on linear testing despite better visual acuity on single optotype testing. This is due to additional stimulation from contours of adjacent letters in a linear test type and a reduction of lateral retinal inhibition which produces confision.

Assessment of fixation aids determination of visual acuity. Free alteration indicates equal visual acuity. Holding fixation beyond a blink indicates good visual acuity. String objection to acclusion of the good eye and no objection to occlusion of the deviating eye suggest severely reduced visual acuity. This is also indicated with searching eye movements of the deviating eye (Llinás & Churchland, 1996).


Agarwal, A. (2006). Handbook of Ophthalmology. Thorofare, NJ: SLACK Incorporated.

Evans, B. J. W. (2002). Pickwell’s Binocular Vision Anomalies: Investigation and Treatment. New York: Elsevier Health Sciences.

Harley, R. D., Nelson, L. B., & Olitsky, S. E. (2005). Harley’s Pediatric Ophthalmology. New York: Lippincott Williams & Wilkins.

Koch, C., & Davis, J. L. (1994). Large-Scale Neuronal Theories of the Brain. New York: MIT Press.

Langston, D. P. (2007). Manual of Ocular Diagnosis and Therapy (6th ed.). New York: Lippincott Williams & Wilkins.

Llinás, R. R., & Churchland, P. S. (1996). The Mind-brain Continuum: Sensory Processes. Massachusetts: MIT Press.

Pratt-Johnson, J. A., & Tillson, G. (2001). Management of Strabismus and Amblyopia: A Practical Guide (2nd ed.). New York: Thieme.

Wright, K. W., & Spiegel, P. H. (2003). Pediatric Ophthalmology and Strabismus. New York: Springer.

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